The Changing Distribution of Ticks and Tick-borne Infections
Webinar: The Changing Distribution of Ticks and Tick-borne Infections
Moderator: Nikki Grimsley
Presenter: Susan Little, DVM, PhD, DACVM (Parasit.)
Date/Time: December 7, 2:00 – 3:00 PM ET
Nikki Grimsley:
Good afternoon, everyone. Welcome and thank you for standing by. All participants are in a listen-only mode until the question and answer session of today’s webinar. Today’s call is being recorded. If you have any objections, please disconnect at this time.
I’m Nikki Grimsley with the Clinician Outreach and Communication Activity, or COCA, with the Division of Emergency Operations at the Centers for Disease Control and Prevention. We are delighted to welcome you to today’s webinar, The Changing Distribution of Ticks and Tickborne Infections. COCA is excited to offer this call in partnership with the American College of Veterinary Preventive Medicine. We are pleased to have with us today Dr. Susan Little. Free continuing education is offered for this COCA call. Instructions on how to earn continuing education will be provided at the end of the call.
CDC, our planners, and their spouses and partners, wish to disclose they have no financial interest or other relationships with the manufacturers of commercial products, buyers of commercial services, or commercial supporters, except Dr. Little would like to disclose that she has received research funding in the past 12 months from multiple animal health companies that manufacture tick control products for dogs and cats. Planners have reviewed content to ensure there is no bias.
At the end of the presentation, you will have the opportunity to ask the presenter questions. You may send in questions through the webinar system at any time during the presentation by clicking on the Q&A button at the bottom of your screen and then typing your question. For those who have media questions, please contact CDC Media Relations at 404-639-3286 or send an email to media@cdc.gov. If you are a patient, please refer your questions to your healthcare provider.
At the conclusion of today’s session, you will be able to describe some of the drivers that have resulted in the increase in geographic expansion of tick populations in North America, identify regions where certain tickborne infections are likely to emerge or increase in prevalence, and describe strategies to prevent infections in individuals and communities. Lastly, you will be able to list examples that highlight the recognition of new tickborne agents and spread of established tickborne infections.
Our presenter today is Dr. Susan Little. Dr. Little is a Regents Professor and the Krull-Ewing Endowed Chair in Veterinary Parasitology at Oklahoma State University, where she teaches veterinary students and oversees the research program focused on ticks and tickborne infections. She has been active in research on ticks and tickborne infections in North America for more than 20 years and has received numerous awards for teaching and research. In 2017, she received the Distinguished Veterinary Parasitologist Award from the American Association of Veterinary Parasitologists. Welcome, Dr. Little. Please go ahead.
Susan Little:
Well, thank you. I appreciate you hosting me and this webinar today, and I want to thank everybody for calling in, as well. I really enjoy the opportunity to be part of the COCA outreach, and I was delighted when I was invited to talk about changing distribution of ticks and tickborne disease agents, because this is something we’ve really had to come to terms with in North America. That there has been changes to where ticks are located, and also, what they transmit, or at least what we know about what they transmit. And the reasons for these changes are diverse. There’s not any one reason that we’re seeing this shift, but we certainly know about drivers for tick populations and how that influences just the number of ticks that are out there in the environment. So things like climate and seasonality play a large role in contributing to tick populations, tick questing behavior, and therefore, transmission to people. Habitat, and we’ve changed the habitat in North America in many areas in a way that is more tick friendly.
So a nice wooded understory with a dense vegetation on the ground really provides some cover for some of the major tick species that we have. And so habitat makes a big difference in terms of just tick numbers. Then wildlife populations, of course, influence tick numbers. A few of the tick species, the adults, really prefer to feed on white tail deer. So any time there’s really heavy deer numbers, we’re going to see more tick numbers, and then another driver may be more for the transmission or the likelihood of people encountering the ticks than the tick populations themselves is the lifestyle of the pets and the people, and the likelihood that they’re going to encounter ticks. And so I’ll speak a little bit to that, as well, but we’ll touch on each of these factors for the different tick species that are important in the United States and in North America in general.
And the one I want to start with this maybe one you weren’t expecting to hear about first, and that’s the brown dog tick, Rhipicephalus sanguineus, but it is a very important tick, and its present throughout North America, throughout the US and Canada. Its present throughout the world anywhere that there are dogs, because this tick has a strong preference for dogs as a host. The larvae, the nymphs, the adult, all stages of the tick will feed on dogs, and so we find it in homes and kennels where there’s one or more dogs that the ticks can subsist on. And the problem with this tick is that it sets up these indoor premise infestations that can be really challenging to eradicate. And so, because the tick will tolerate low humidity and high temperatures that are higher than outside temperatures that are found indoors, they really thrive inside homes and kennels. And we tell people once they have a home infestation with brown dog ticks, you know, that you have to expect about six months of environmental treatment effort in order to eliminate that infestation, and in the meantime, the ticks will just keep coming. And they come out of the baseboards and the drop ceiling tiles, and they crawl up the walls, and it’s really horrific for pet owners who end up with a brown dog tick infestation. It’s also, of course, a public health concern.
Historically, we would say that brown dog ticks in North America don’t really feed on people, and that’s still largely true, although most would now modify that statement to say brown dog ticks don’t feed on people except when they do, because sometimes, these ticks will move to people, and they can transmit very severe disease agents, like Rocky Mountain spotted fever, Rickettsia rickettsii, as well as other spotted fever groups of Rickettsia. Things like Rickettsia montanensis and some of the most well-known Rickettsia species, as well as several infections to dogs, bacterial and protozoal infections to dogs. And so that’s the public health risk that’s created when you have this home infestation with brown dog ticks. And you know, the ticks will survive outside of homes, as well, and so high temperatures, they do really well, even over 100 degrees Fahrenheit, they still thrive. And a lot of tick species, most of the common hard tick species will be killed by those high temperatures and dry environments. But brown dog, don’t seem to mind.
As I said, they’re found anywhere there’s dogs. So we always see them with that association with dogs. Wildlife really aren’t involved with supporting the host, and so the kind of homes that are likely to develop a brown dog tick infestation is often a multi-dog household, but it’s also dogs that get to see other dogs. So they might be allowed outside and kind of roam around the neighborhood where they’re going to come in contact with other dogs, or they may be regularly taken to a boarding kennel or a dog daycare, which is very popular now, where dogs are taken into play groups a few times a week, so they can interact with other dogs. Or they go to a groomer and come back with ticks. And so that’s the concern is anytime the dog contacts other dogs where ticks may have been deposited, they can bring back a brown dog tick infestation, and that’s the reason that groups like the Companion Animal Parasite Council recommend — one of the reasons the CAPC recommends that all dogs should be on tick control year-round, because that’s the other risk factor for dogs or cats acquiring ticks is just the absence of control.
And so that really speaks to the role that veterinarians have to play in public health, and that by recommending routine tick control for dogs and cats, we’re not just recommending something to keep the pets comfortable and make sure they aren’t gross and covered with ticks. It’s really a health recommendation for the pets, because we want to interrupt that transmission of pathogens to the pets. But it’s also a public health recommendation, in that we want to make sure that the pets don’t bring ticks into the home that will then create a risk for the family. And so a treated dog is a safe dog. It’s the non-treated dogs that might be higher risks for bringing ticks into the home. And when it happens with brown dog ticks, it’s pretty catastrophic.
This is the sheepskin blanket. You can see on the left-hand side of the screen here that was lining an airline kennel for a patient that was brought into the teaching hospital here in Oklahoma. You can see a few of the adult female ticks, and then a large number of the immature engorged ticks that fell off the dog just as it was being transported to the teaching hospital. This dog had a packed cell volume of 8, required two blood transfusions to recover it, and it was just severely anemic, even exsanguinated, by the sheer number of ticks that were infesting the dog.
You can see here on the right this mass of female ticks in a little bit of dog hair mixed in, and that’s what clogged the drain in the treatment tub when the dog was taken in the back to start removing the ticks. There were just so many ticks on the dog that the drain clogged up, and the sink had to be bailed out. So that’s what happens with a home infestation, because the dogs can’t get away from the ticks, and you can imagine just seeing this what the carpet at home was like and what the family was confronting in terms of just sheer numbers.
So brown dog ticks are unusual. This is the brown dog tick here with the short mouth parts and sort of the solid henna brown color. They’re unusual in that they don’t subsist on wildlife. It truly is a domestic dog phenomenon.
The other ticks shown here, and then one other that I’ll mention, Amblyomma maculatum. They all live on wildlife species. So whether it’s Lone Star ticks, or Ixodes scapularis, Ixodes pacificus, in the West, scapularis in the East, that black legged or deer tick, or the Dermacentor, the American dog tick or wood tick, they’re all going to be existing and thriving on wildlife in natural habitats, and so their populations can spiral up quite high when the wildlife populations increase, when habitat and climate changes happen, and when there’s increased humidity and increased temperature that will support many of the tick populations, but brown dog ticks are the one that will be even inside the home.
And so we have a lot of different tick species active that are willing to feed on people, Ixodes and Amblyomma and Dermacentor very enthusiastically attach to feed on people and transmit infections, and then Rhipicephalus the brown dog tick, occasionally does, but we also have more ticks than we have had ever before, and so the last few decades, we’ve seen a dramatic increase in tick populations, increase in number where the ticks have been historically, but also, this expanding geographic distribution where we used to see the tick species, and therefore, the infections that they transmit, confined to a smaller relative geographic area, and the pattern in recent decades has been one of geographic spread. The ticks have moved out from their historic foci, and there’s a few different species I’ll give you as an example, and that means that they’re able to introduce infections to people in new areas, and it kind of catches local physicians off guard, because they’re not used to diagnosing that infection and that disease in their geographic practice area. But because the ticks have slowly spread and moved over time, the cases start to show up, and they realize that there is an infection that’s sort of newly arrived in an area.
The other thing about ticks is they transmit a lot of different pathogens. And so most of the focus from the media is on Lyme disease, which is a very important bacterial infection transmitted by Ixodes species, scapularis, and pacificus. But ticks also transmit viral infections, other bacterial infections like Ehrlichia and I mentioned Rickettsia already or Anaplasma. Protozoal infections like the Babesia to humans, and then different Babesia species to dogs, but then dogs and cats will also get Hepatozoon and Cytauxzoon and other protozoal infections and even helminths, even nematodes can be transmitted by ticks. And so there’s a certain type of malaria, a subcutaneous nematode of dogs in Europe that transmitted by brown dog ticks. So it’s really a wide array.
We’re learning more, especially in recent months, about viral infections transmitted by ticks. So we’ve known about for Powassan virus for many years, but just in the last few years – Powassan is transmitted by Ixodes, but in the last few years, Amblyomma transmission of the Heartland virus and Bourbon virus has become a very active area of research, and there’s a lot of interest in seeing if that system is, indeed, responsible for the transmission of those very serious viral pathogens to people.
So it’s almost a depressing concept to talk about. Because there’s all these different pathogens that are transmitted by all these different ticks, and it feels just insurmountable, but the good news is, the one thing they’ll have in common is the ticks.
And so comprehensive tick control focused on making sure that pets aren’t introducing ticks to the home, making sure people are aware of ticks and protecting themselves from ticks when they’re in a tick-infested environment, is really the best way to minimize the risk of infection with all these different pathogens. So it’s not just one disease that we should focus on or one tick, but rather, this whole complex system of ticks and tick-borne pathogens.
So if we look at the distribution of ticks, these are the CDC maps that are posted at CDC.gov that show us the different distribution in the United States, and you can see brown dog ticks is everywhere, because they’re everywhere there’s dogs. That’s not to say that, of course, every day of the year you could walk into any county in the country and pick brown dog ticks off dogs, but we do have the potential to have Rhipicephalus infestations just about anywhere, and in fact, I’ve dealt with the home infestations in Ontario, in February, and in Alaska in the colder months, because it doesn’t really matter what the temperature is outside, since this tick is happy to thrive inside.
We do see more brown dog tick problems in the southern US where the tick will survive and do very well outdoors, as well as indoors. And then Ixodes ticks, the scapularis, is focus, of course, in the eastern half of the United States, that Midwest focus is almost starting to fuse with the traditional Northeastern focus as this geographic spread of that species has continued, and there’s a lot of Ixodes subscapularis in the southern US, and sometimes that surprises people, because there’s not evidence of autochthonous transmission of Borrelia burgdorferi or Anaplasma phagocytophilum in the southern US, certainly not down in Oklahoma or Texas, Louisiana, across the Deep South, but it has spread down to Southwest Virginia, some evidence in North Carolina now. A lot of — West Virginia, of course, is endemic. So there has been a spread to the maybe the middle South the upper South, more so than in recent years, and then Western blacked legged tick, pacificus, of course, picks up disease transmission in the western US.
And then we have the Dermacentor ticks, American dog tick present throughout the eastern two-thirds of the United States and in the higher altitudes, the Rocky Mountain Wood tick is more common. And then the Lone Star tick, which historically was the tick of the South, and still is the most common tick that we find on people, dogs, horses, cattle, cats throughout the southern US, the Lone Star tick has moved northward. And so we see a lot more Lone Star tick problems and issues in the northern part of the eastern US then we did just a few decades ago.
So the story of distribution is really one of change, and what I want to do is go through each of the tick species, the major tick species and just speak to habitat and when they’re active and what they transmit, and then we’ll circle back and kind of put it together, in terms of what we can do and what we can expect in the future.
So the Ixodes species are found in wooded areas, but it doesn’t have to be wilderness woods. It couldn’t be just a little bit of tree cover and understory right around the home. That dense, thick understory is important for the Ixodes ticks, because it keeps them from desiccating. And so they’re able to survive longer, and therefore, they’re more likely to find a host. It also provides a little shelter habitat for the rodent populations that support the immature species or immature stages, and then it provides some browse for the deer that support the adult stages.
One thing that surprises some folks about Ixodes is that the adult Ixodes are actually active in the cooler months. October and November are usually peak times when we find the most Ixodes on dogs and cats and horses and people, October/November, but they’ll be active even through into February and sometimes we still find a few in March. And it’s not that they’re active every day, and certainly not when there’s deep snow cover or a really hard freeze, but as soon as it warms up to right about 38, 40 degrees Fahrenheit, the Ixodes will start becoming active. So it’s right about 4 degrees Celsius, the Ixodes will start questing, looking for a host, and then can be acquired by a person or via pets.
The nymphs, which are the primary stages that transmit Borrelia burgdorferi to people, are active in the warmer months, May to June, and estimates vary for what percent of infections are transmitted by nymphs versus by adults, but both stages can transmit infection. So there is a risk throughout the year. Just that people forget about the risk, I think, in the cooler months, because they’re not focused on ticks being out and active. Then of course, Lyme disease is a primary concern with Ixodes, but also in Anaplasma, human Babesia. There’s a newer Ehrlichia species been described from the upper Midwest Ehrlichia muris or Ehrlichia muris-like agent, and then I mentioned Powassan virus transmitted by Ixodes, as well.
And if we think about, again, when the Ixodes are out, that fall/winter season is an unexpected time when people can encounter the adult Ixodes, because they do survive the cold very well. They’ll often be found questing right above the snow cover, where the vegetation is just poking up through the snow. They’ll be looking for a host there, because as you’re starting to get a snow melt, as it gets up to about 40 degrees Fahrenheit ts to melt, it creates a nice layer of humidity to keep them from drying out. And then deer, of course, are a cornerstone host for feeding the adults, but it’s the rodents that feed the immatures that then create that risk of Borrelia infection. And interestingly, in the southern US, the immature Ixodes ticks will often feet on lizards rather than rodents. They’re much less likely, if at all, to be infected Borrelia burgdorferi sensu stricto, and that contributes to the absence of autochthonous transmission in the southern US. And then, so lifestyle, really, if dogs are going to have Ixodes and bring it into the home, or if people are, is just outdoor access. You don’t have a home infestation with them, but if you go out, they will, of course, find you.
So when we track, when we try and look for evidence of infection in dogs and evidence of ticks and monitor the spread of Ixodes subscapularis and Ixodes pacificus and monitor infection, one source we use for that is this national data that’s available from veterinarians in practice testing dogs for Borrelia burgdorferi infection. And so there are some tests in clinic and diagnostic lab tests that veterinarians can run to look at what is the risk that a dog has been infected with vector-borne disease agents, and one of the most common tests that’s used looks for evidence of heartworm infection, because that’s an important disease to screen for in dogs, but then also, antibodies to Borrelia burgdorferi Anaplasma, Phagocytophilum, and Ehrlichia. So in tapping into that data, we actually have access to over a million data points a year from testing dogs for these vector-born agents, and it gives us a nice geographic distribution, and we can look at where the organisms are most likely being transmitted. There’s of course issues with the data. You can have a dog be infected in one area of the country and then move to another area of the country before he’s diagnosed with that infection. The antibodies persist, but overall, as you look at trends, and as the N, as the number increases, you do get valuable data.
So this map was published back in 2009, and it shows the focus of Borrelia burgdorferi infection in dogs, as evidenced by antibodies to the organism. In the Northeastern US and in the Upper Midwest, and then along the West Coast, that correlates really nicely to where people are most commonly reported infected, too. So that’s the autochthonous areas of transmission. And then we repeated that looking at data from 2010, 11, 12, and you can see in the more recent map, a spread southward, sort of down into southern Appalachia, as well as along the Atlantic Seaboard into North Carolina, and also a spread from the Midwest focus around the Great Lakes and into the lower Peninsula of Michigan and down through Indiana, even into Ohio, which now there is documented evidence of infection.
As I said, just finding antibodies in dogs is not enough to show autochthonous infection. Once that antibody prevalence level, if you’re testing thousands of dogs and you broach about 1 or 2% of the dogs testing positive, that’s when it gets interesting. Usually when it goes endemic or hyperendemic, it’s 5-10% of the dogs testing positive. And then in some of the hyperendemic areas in the Northeast, as many as 20 or 30% of the dogs will test positive.
But then the next step, of course, is for entomologists and field biologists to go in and collect ticks on the ground and test them for the agent and show that there’s both ticks and Borrelia burgdorferi sensu stricto that could be transmitted to people. But the dog serology data is a great way to track spread. We’re about to take delivery on the newer data set, and so we should be updating this map paper sometime in 2018.
If we look at number of human cases, so we’re seeing this spread in the dog data and prevalence going up, and it correlates with what we see in human case reports, as catalogued by CDC. You can see the trend going up, and this is true with Lyme disease. Now the scale here is Lyme disease times 10, because there’s so many more cases of Lyme disease reported in people than any other tick-borne infection, but we see that same trend with Rocky Mountain spotted fever cases, with human Ehrlichiosis, human monocytic Ehrlichiosis cases and with human and granulocytic anaplasmosis cases. So that increasing trend, and that increase is due to, in part, geographic spread. Because there’s more people at risk if the ticks are in more places, but there’s also increased number of ticks where they’ve been historically. And so that contributes to greater risk, too. So there’s greater likelihood that a person will become infected, even in the same area, even if it’s not movement to a new area.
So we also look at transmission risk for dogs, and this is that Companion Animal Parasite Council, or CAPC site, that I mentioned. You can also click on this and switch to the Canada map and see similar data for Canada, but if we look at just Lyme’s disease, that Northeast and Midwest focus definitely comes through, but if you then look at specific US states, like Connecticut and Pennsylvania, where Lyme disease has long been endemic and just compare what percent of dogs were testing positive in 2013 versus 2015 versus 2017, and you can go on this website and do the comparison yourself, too, for fun. You’ll see that in Connecticut, you know, 15% has always been one of the highest prevalent states, but even with that high prevalence, it’s continuing to increase, and that’s in the face of excellent tick control and very effective vaccines. So dogs that are tested annually for vector-borne infections, they’re going to veterinarians. They’re also getting tick control, and they’re likely in Connecticut and Pennsylvania being vaccinated annually for Borrelia burgdorferi infection. So even with those efforts at control, there’s still this increasing prevalence of infection that’s seen.
So as I said, we have a lot of ticks. So it’s not just Ixodes scapularis although that’s the most important transmitter of Borrelia burgdorferi. It’s the only tick known to transmit Lyme disease in North America, but there are other ticks like the Lone Star ticks and the Dermacentor ticks that warrant consideration, as well. Lone Star ticks, the female, they’re called the Lone Star because the female has that white spot on the back of her scutum and the male has that pattern right around the edge of his scutum and these two little bars where his shoulder should be. They’re found in habitats similar to Ixodes species, in that they are in the wooded areas around homes, and they do tend to be associated with denser vegetation, but populations are really intense. Where they differ is that Lone Star ticks are much more dense in the southern US, especially the middle south, that band of states from the Carolinas, across to Oklahoma and Kansas. That’s where they’re going to be the most intense population, and then they’re very aggressive feeders. So Ixodes certainly, they find their hosts and they do very well, but Lone Star ticks will run after hosts in the woods, and so they’re quite aggressive at finding a host. Their activity time is a bit different than Ixodes. Instead of being active in the winter, the adults, the Lone Star ticks, they start becoming active in the early spring, which in the Deep South can be as early as February. By May or so, they’re active just about everywhere, but they sort of come out different times, based on how warm it is outside. They like — the adults like warm weather, as do the nymphs, and so the nymphs will be active in the summer and in the fall, and they transmit many different infections.Ehrlichiosis, human Ehrlichiosis is the best well known, probably the most common infection they transmit, but they do transmit other spotted fever group Rickettsia, and they’ve been shown, rarely, to transmit Rocky Mountain spotted fever agent, Rickettsia or Rickettsii. So that’s both brown dog ticks and Lone Star ticks transmitting spotted fever group Rickettsia. We think of Dermacentor ticks is the main transmitter of Rickettsia or Rickettsia, and that’s certainly true, but the others manage to succeed in creating some infections, as well.
They’re also the ticks that are associated with this STARI phenomenon. So Southern Tick-Associated Rash Illness, which is a Lone Star tick associated conditions that looks like Lyme disease, but the patients don’t seroconvert in a way that’s classic Borrelia burgdorferi infection. They’re not PCR positive for Borrelia. We don’t really know what causes it. We just know that it’s associated with the Lone Star tick.
And then in cats, Cytauxzoon is a major concern. And then this is the ticks that suspected in Heartland and Bourbon virus transmission, which are both highly fatal and serious viral infections in people.
In terms of seasonality, it is — they do prefer much higher temperatures, especially the adults, than the Ixodes ticks do, but we’ve had warmer and more humid environments in more places in North America in recent years, and so they have been able to expand their distribution accordingly. They do dry out readily, and so any time a drought hits, then this population will be knocked back, and then they like that wooded environment, because they need that shelter, and in fact, plants like invasive honeysuckle have been shown to be really good for supporting Lone Star tick populations, because they provide shelter for the ticks and browse for the deer, and deer are a key host Amblyomma.
Unlike Ixodes, it’s not just the adult Lone Star ticks that feed on deer, the larvae and the nymphs also feed on deer. The larvae and nymphs will feed on ground-dwelling birds like wild turkey, but they have that preference for deer, and so Lone Star ticks and deer populations are tightly linked.
So again, the people and the pets at risk would be those going outside at the right time of year or the wrong time of year when the Lone Star ticks are active.
So just to show you what intense populations look like, this is just me pointing a camera down at the ground in the spring time in Oklahoma and pushing record. We get these things called tick forms, and if you go out just the right day, or just the wrong day, you’ll see all over the ground, ticks crawling around just like ants looking for hosts. And so there’s no amount of tick control or protection that can really prevent all tick bites in an environment like this, when the populations are that intense. And so just instead, avoiding the environment during those tick swarm times is what’s recommended.
When we do our dog walking studies, we’ll take beagles out and walk them through tick-infested environments. The university requires that the people walking the dogs wear Tyvek, so biosafety containment to protect us from the ticks. That’s how dangerous the woods are in terms of tick infestation when the ticks are swarming. We do monitor the dogs closely. We just look at how many ticks they acquire, and then if they seroconvert to tickborne disease agents and then treat them with antibiotics and adopt them out through our university adoption program. And all beagles make wonderful pets, but research beagles especially so.
So when we do these dog walking studies, and we walk dogs through areas with ticks, we look at, you know, okay, what percent of them are going to become infected with the Rickettsial agent, spotted fever group Rickettsial agent, or with an Ehrlichial agent, either Ehrlichia chaffeensis or Ehrlichia ewingii coming from the Lone Star ticks? And just been doing eight weekly walks, one hour at a time, you know, once a week for eight weeks, they walked the dogs in the woods, and then they go back into their indoor lab animal facility, we did a study with just 10 dogs a couple years ago, and every single dog became infected with Rickettsia. Every single dog became infected with Ehrlichia. So even though the prevalence in the ticks may be low, the transmission efficiency is high when the numbers are that high. And so the risk is quite high if there’s not tick control on board.
So you can also think of Lone Star tick habitat as your own backyard, because, like Ixodes, they’ll live in that wooded area right around the home, and so this is my backyard in Stillwater. My husband and I have a fence to keep the dogs and the kids in, but it also helps keep the wildlife out, and our home is right south of campus. We’re only about a half-mile from the vet school. The neighborhood was built up in the 40s. There’s another row of houses you can just see through the trees here, and then this wooded area here is a wildlife corridor.
And so we have coyotes run through there, road runners sometimes. So it’s actually a true thing. We’ll have occasionally a deer, more foxes, raccoons, that sort of thing. But almost every month of the year, there’s about 10 months of the year when I can go back here and collect ticks without any trouble at all, because of that wildlife corridor. As long as the dogs and the kids stay inside on the mowed grassy area, then they’re not going to acquire ticks. But back here, there is Lone Star tick habitat. So it, again, doesn’t have to be wet wilderness at all. It’s usually right next to the home where there’s ticks.
So in terms of the other major tick, Dermacentor is still in important tick for dogs and for people. Habitat is a bit different, in that it likes more overgrown fields and meadows, and then it’s common throughout most of the United States. The adults do tend to be more active in the warmer months, and it’s legendary for transmitting Rocky Mountain spotted fever agent, as well as some other spotted fever group, Rickettsia. And it has been shown to transmit some agents of Anaplasma and Ehrlichia and Tularemia. So it’s concern, certainly, for disease transmission.
As I said, it’s active in the warmer months in its more grassy meadow environment. It can be right along the edge of trails, where the grass grows kind of tall, and so dogs and people will pick it up when they’re out walking.
And then in terms of the wildlife that support the major Dermacentor populations, it’s more medium-sized mammals. We’ll occasionally find Dermacentor on deer and other large ruminants, but the foxes and raccoons and coyotes are more likely to support the population, with rodents being the major supporter for the immature stages.
And then the fourth wildlife tick that I do want to mention is the Gulf Coast tick. And I know that probably most of the folks on the call don’t live on the Gulf Coast, but neither does this tick anymore. It was named the Gulf Coast tick because historically it wasn’t really found more than about 100 or 150 miles from the Gulf Coast. But it has invaded, very successfully, other areas like Oklahoma and Kansas, and there’s been reports from Nebraska and over into Virginia, North Carolina. So it’s not Gulf Coastal only anymore. It is more active in the warmer months as most of the tick species are, with the exception of Ixodes, and then when it peaks varies dramatically with where you are geographically. So that coastal populations have a different timing of peaks than these inland populations, and there’s some evidence that there’s genetic drift between the two populations.
The key thing with Gulf Coast ticks is that they also transmit spotted fever group Rickettsia, most notably Rickettsia parkeri, which is associated with clinical disease in people and eschar at the tick bite, as well as some infection to dogs, and to cattle and sheep and goats that are important for an animal disease concern in the case of Heartwater and then just dog health for Hepatozoon.
So their habitat is more similar to Dermacentor, and that they’re more field or meadow ticks. They do particularly well in early to mid-succession fields that have an open canopy. So you can think about sort of a manicured old estate where there are mature trees, but there’s plenty of light getting through, and the wildlife that support the maculatum are the medium-sized mammals, but also the larger mammals so things like deer and cattle. Cattle are a great host to support Gulf Coast ticks, and then the immatures feed mostly on rodents and birds.
So one of the challenges, as you can tell, of the tick populations that we have is this geographic expansion that’s really being fueled, in part, by habitat change and in part by increasing temperature and humidity. So it depends on which tick species you’re thinking about and considering, which is the stronger driver, and then that increased number of ticks is also important as the environmental conditions become more ideal for the ticks. And so when someone tells me that they have a property, you know, 10 acres, 20 acres, and it’s just the ticks are horrible, well to me, that means they have a really healthy wildlife population, and they probably have a pretty healthy habitat, as well, because that is sort of the normal state of things to have a really heavy tick population.
If we removed all the vegetation and paved everything, then the ticks would abate. I mean, we’d have taken care of it. But most of us don’t want to live in that environment. And so that’s why we have that increased abundance of ticks. The other challenge we have with ticks is that we don’t really have much we can do about tick reproduction. Any veterinarian in the country, if you have a flea infestation in your home, on your pets, can take care of it. It’s really straightforward to address flea infestations. We have excellent products to control fleas on dogs and cats, and we can control their reproduction, because they’re really just completely existing on the pets. With ticks, it’s different. The only tick we have access to are the brown dog ticks in the home. All the others are wildlife ticks, and we just have no say in their population numbers. So we’re sort of victims of the same environment. People and dogs are at the whims of the tick population.
Another challenge, though, that we confront is that there’s a lot of misunderstanding about ticks and when they’re active. So I mentioned the Ixodes, the adults being active in the winter months. Some people are confused about immature ticks and how likely they are to feed on people or pets. If you’re in an area with the larval ticks or seed ticks, you realize very quickly that they’re happy to feed on people, because people can acquire hundreds of ticks in a short period of time just from one hatch of a clutch of eggs. And so I think there’s an underestimation of the risk of infestation, especially for pets, and I would say even more so for cats. And so many people that have both dogs and cats will make sure their dog is protected from ticks. They’re aware that ticks are a risk. They’ll have them vaccinated for Lyme disease, but they won’t be doing anything for their cats, and cats can bring ticks into the home just as dogs can. And so it’s really important to protect both, as I said, for their health, but also for the health of the family.
If we look at when they’re active, ticks certainly have a wider period of activity in the southern US, because the environmental conditions are hospitable to them for longer. But they do have activity in the northern US, and that does seem to be expanding every year. So in other words, the ticks come out earlier and stay longer when the conditions are supportive for them for a longer period of time. So the milder the winter, the more degree days of activity that adult Ixodes can have, and therefore, more successful at reproducing and ensuring the next generation is even higher.
And so we have to think about those immature, the larvae and the nymphal ticks, as well as the adult male and the adult female ticks. And so this cluster of photos are sort of the tick family portrait. So you can see that for all the different species, there’s the larvae, the nymph, and the male and female, and they will feed on people, as well as on dogs and cats.
And the diseases they transmit are quite severe. We deal with fatal infections in pets, just as physicians deal with fatal infections in people. I think that’s one of the reasons veterinarians are so attuned to the concerns about ticks, because we know how tragic the outcome can be in the absence of tick control.
The one question that we get a lot is how long the tick has to feed before it can transmit, and for the general public, most folks are aware that the Ixodes tick has to attach and feed for at least 24 to 48 hours before Borrelia can be transmitted. Now there were some experiments done that showed earlier transmission is possible, especially with interrupted feeding. So if the tick feeds on one mouse for a few days and then you move it to another mouse, it can transmit faster. But for the most part, there’s at least a 24 to 48-hour lag time before transmission occurs.
For the Rickettsial agents, though, it’s different. Rocky Mountain spotted fever is transmitted within a few hours. Ehrlichia canis, the canine Ehrlichial agent, has been shown to be transmitted to dogs within three hours of tick attachment, and then the data on Anaplasma phagocytophilum vary. There are some papers that say it does take more than 24 hours, but then there’s another paper that showed transmission in less than 24 hours. So faster transmission, perhaps in less than 24 hours, is possible with the Rickettsial agents.
The protozoal agents take longer. So they are your backup to the 48 hour or longer, but probably the scary group are the viral pathogens, because Powassan virus has been shown to be transmitted from an Ixodes tick in his little as 15 minutes of attachment. And so we don’t really have that grace period. I mean daily tick checks are important for sure, because you want to remove the ticks as soon as possible, but the ideal strategy would be to protect the skin from the ticks by making sure that protective clothing is worn and secured, and also just frequent tick checks. Just pick them and flick them throughout the hike, rather than waiting at the end to try and remove any ticks.
And then, of course, more than just tick control. Vaccination we have for dogs and certainly keeping pets up makes a difference, but vaccination just protects against one infection. So we have so many others out there, and it doesn’t protect against ticks, of course. It’s just the bacteria. So the dog can still bring ticks into the home, and that’s why having the dogs on year-round tick control is so critical. But also keeping the pets up. Preventing them from roaming, keeping cats indoors and dogs on a leash or behind a fence. Making sure to minimize tick habitat.
So removing leaf litter has been shown to dramatically decrease tick numbers, because you just took away the sheltered habitat maybe that the ticks were using to keep from drying out, and then having that hard fence barrier between the woods and the yard, so that the wildlife don’t enter and deposit ticks and leave again. If your deck backs right up to a wooded area, and you’re feeding deer off your deck, and the deer are coming right up, then they’re dropping ticks there. And in fact, there have been studies that show that how frequently people sight deer from their home. So if you see a deer at least once a week, it dramatically increases the likelihood that someone in that home, if you’re in an area where Lyme disease is endemic, it increases the likelihood that someone in that home is infected with Borrelia burgdorferi and has had clinical Lyme disease. Just the evidence of seeing deer. The deer, of course, aren’t the reservoir host for Borrelia, but they do support the tick populations that then bring that infection to people.
And then there are some strategies for treating wildlife. A lot of them are focused on deer understandably, and so there’s things like four-posters that are corn feeders that have upright posts so that with acaricide applied to it. So when the deer goes in to eat the corn, the acaricide is brushed across the ears, and therefore, it’s treated for the ticks.
There’s also strategies to treat mice for ticks. So there are tick tubes that are toilet paper tubes with some permethrin-treated cotton in them, and the mice, you put those out in your yard. The mice take the cotton to make their nest with, and that treats them for the ticks. And you might think, well, why not just kill the mice, because that way, the ticks won’t feed on them either, but mice are very territorial. So if you remove the mice, then more mice move in with their ticks. If instead, you treat the mice you have, they’ll exclude the other mice, and you’ll have tick-free mice on the property.
Same idea with deer. It’s not practical to try and remove them all, other than from isolated island communities, and so if you go to take care of the population you have and try and minimize the ticks there, that it decreases ticks overall for the community. It doesn’t eliminate them by any means, but it does decrease the numbers.
And then I will say, too, that veterinarians, of course, are concerned about more than just ticks, and so the products that are used for tick control have other benefits that also benefit human health. So if we think about the new isoxazolines that are available for use in dogs, either monthly or every three months, or in cats or some of the topical products that are used for ticks in dogs or cats, they also are efficacious against fleas. And some of them are labeled and effective against something like sarcoptes mites. And so those are other zoonotic risk that pets ownership can create if the pets not managed in a safe way, and that’s something that I always stress.
Having pets doesn’t, in and of itself, create a public health risk. Having pets that aren’t cared for by a veterinarian, that aren’t safe pets, that aren’t vaccinated appropriately, and that aren’t on routine parasite control is what creates the safety issue. So as long as the pets are receiving good veterinary medical care and are current on recommended health strategies like tick control, then those pets will be at very low risk to introducing ticks into the home, and in fact, the ticks they encounter will be killed, and therefore not available to bite people in that same home environment.
And so with that, I’ll wrap up so that we’ll have a few minutes for questions. If you guys have posted any questions to the Q&A. I just want to, again, thank COCA for providing me this opportunity to share some of our data with you, and of course, credit my home university, Oklahoma State, with a shout out to Go Pokes! So thanks everybody.
Nikki Grimsley:
Thank you Dr. Little for your excellent presentation. We will now open the lines for the question-and-answer session. As a reminder, you may submit questions to the webinar system by clicking on the Q&A button at the bottom of your screen and typing in your question. Dr. Little, we will go ahead with our first question. Can you mention specific, comprehensive tick control methods to best control risk in infection?
Susan Little:
Sure, so we think about tick control for the pets, and then also tick control for the people, so that everybody in the family is protected. So for the pets, there are several options available, and I always direct folks to their veterinarian to get what they recommend, because the local veterinarian will be the local expert on what’s going to be most effective, what time of the year, what the risk is.
But there are products, everything from liquid topical products that you can either spot on or spray, to oral products that are given. It’s a chewable treat that’s given once a month or once every three months but then provides long-lasting tick control for that pet to a collar. There’s a collar — collars available that last. One lasts three months. One lasts six months, and one lasts eight months of tick control. And so you just apply the collar. They’re not like the old, bad flea collars from years ago that weren’t very effective. These are highly effective, slow release pyrethroid collars that really provide good, comprehensive control. And so, there’s so many different options and routes of administration. That’s why it’s just a universal recommendation now that dogs and cats be on tick control.
And then for people, of course, is awareness is key, and then protective clothing. Light colors make it easier to spot the ticks. Taping your pant legs to your boots. If you’re in tall grass, then taping the waistband to your shirt is helpful, as well, because they’ll sometimes quest higher up on the vegetation and enter there. Ticks don’t fall from trees. So that’s not a concern. It’s commonly held folklore, but it doesn’t happen. They quest up from the vegetation, but you’ll often acquire ticks in a wooded area, because that’s where the ticks are — some of the species are most active and most likely to be found. And so that gives that perception. When people go under the trees that they’re getting the ticks.
Nikki Grimsley:
Thank you, Dr. Little. Our next question. Is there any evidence that increased risk of tick disease is due more to effective diagnosis?
Susan Little:
Yeah, yeah, that’s a really important question. And so, I think diagnosis has improved. We’ve actually been using the same strategy to diagnose Borrelia burgdorferi in people for decades. So that laboratory diagnosis hasn’t changed too much, but awareness certainly changes, especially as the infections moved to a new area, and unfortunately when spread first happens, and it’s an area that previously transmission was not known to occur and now the cases are starting to show up, sometimes physicians are caught by surprise, and is underdiagnosed. But then once awareness takes hold, diagnosis increases and reporting increases. And so that can contribute somewhat, but of course, that has to be balanced with the work that field entomologists do in documenting the infections in the field and collecting the ticks, and all that has shown there’s clearly geographic spread an increase in numbers locally, as well as, perhaps, better reporting. So it’s a combination of things.
Nikki Grimsley:
Thank you. Our next question, can you discuss safe and unsafe methods of tick removal?
Susan Little:
Sure. So safe methods. What’s recommended if it’s just one tick or a few ticks, so three, four ticks, is just removal with forceps or gloved hands is ideal and rearward traction, and so you want to grasp the tick close to the skin by the mouth part and just steady rear retraction. So you don’t flip or pop them out, because you can break the mouthparts, right, and leave mouth parts behind. It’s not catastrophic if that happens. It’s just, there’ll be a lot more inflammation, and it’s more uncomfortable. It’s like a foreign body reaction in the host when the mouthparts are left behind.
What’s not recommended and what was done when I was a kid. I grew up in rural Kentucky, and it was common to use a lit match that had just been blown out or a lit cigarette and place it on the tick to get the tick to back out. That is not recommended for two reasons. One is, the likelihood burning the person that you’re trying to help, right, by removing the tick that way. And the other is that that trauma to the tick will cause it to regurgitate and actually possibly pass more infection back into the host. That’s why just pulling them out is recommended.
Now if an animal comes in with hundreds or thousands of ticks, then we’ll use the chemical means of removal. We’ll use the pyrethrin and then go back and remove the last few ticks. But for people, it’s usually just one tick and steady rearward retraction is the way to go. Wearing gloves is important, because the agent can be aerosolized. And so you don’t want to just pick engorged tick off an animal without gloves or protecting yourself, because there have been cases reported of Rocky Mountain spotted fever transmitted that way. Not through tick bite, but presumably, through aerosolization as the ticks were being removed.
Nikki Grimsley:
Next question. Can you talk about entomopathogens as potential control agents?
Susan Little:
So there’s been some great research done on entomopathogens, and I think it’s a really exciting area, but I have not seen any field application. It’s one of those things that works well in the lab, but it’s difficult to get the numbers right and get the distribution right for it to actually to work in the field, but the ticks do have natural micropredators or diseases that they develop.
And in fact, Rickettsia/Rickettsii does make the ticks less fit. So it creates a fitness problem for survivability of the ticks. It’s just that it’s not an absolute enough shift, in order to be effective at wiping out the population.
And then there’s often interest, too, besides the entomopathogens, there’s interest in using tick predators, and so people will talk about peafowl, guinea fowl eating ticks, and they do. It’s just, again, once the tick numbers are low, they’ll shift to a different food selection. And so you don’t ever get rid of the ticks altogether, but it can help the numbers lower, if you have tick predators around.
Fire ants are another tick predator. So if you have a heavy fire and population, they will eat the engorged females and the egg clutches, and so you’ll have less tick reproduction. But of course, I would never recommend introducing fire ants to control your ticks.
Nikki Grimsley:
Next question. Is there any data on the effectiveness of tick tubes?
Susan Little:
Is there any data on the effectiveness of tick tubes? You know, those were developed so long ago that I think if there were data, it would be back from the 80s, and they’re not papers that I’m familiar with.
I think what’s attractive about this strategy is that it’s commonsensical. And so there’s the impression that the mice are going to — we’re not going to be able to eliminate all the mice, right? We can’t remove them from the property. And so, instead, we try and manage them. And they’re not going to go to a four-poster the way a deer will. So they’re not as easy to entice through a trap. There was also, there was a run-through trap that was like a little tiny cattle chute, basically, for mice, and I don’t think it’s available any longer. But the mice would run through, basically, I have a Havahart type trap, where they would be encouraged to walk through the chute, and they would get a dose of fipronil, which is a common acaricide on the back of their neck, to treat them for the ticks, as well. So it’s not — tick tubes are not the only strategy that’s been attempted to treat rodents. The problem, of course, is penetrance in the rodent population and making sure you get all the rodents treated. And so use in that way. Mice, what I suspect is that there would be a decrease, but again, not an elimination of the tick population. And that would be true of any wildlife treatment. That you would affect the tick population but not eradicate it.
Nikki Grimsley:
Thank you, Dr. Little. We will take time for one more question. Can you talk about concerns about the selection for resistance to tick control agents?
Susan Little:
Yeah, that’s a really important question. And so it’s something that we worry about a lot in veterinary medicine, and I’m sure in human medicine, as well. The good news about ticks is that we have almost infinite refugia.
So because the Lone Star ticks and Ixodes ticks and Dermacentor ticks are feeding primarily on wildlife, they’re not all being treated for their — all the ticks out there, most of the ticks out there never see a drug. They never see a chemical at all.
The one tick species where we have selected for resistance in dogs would be brown dog ticks with the Rhipicephalus sanguineus. And if you think about that, you know, brown dog ticks, all the dogs are treated with the tick control agent. So all the ticks in that population see the drug, and they see it over and over again, or the chemical, they see it over and over again. And so we’re able to select for resistance, because there’s no refugia.
But with wildlife ticks, it’s less likely to happen. The large animal example of that would be what used to be Boophilus ticks on cattle. It’s now also a Rhipicephalus species. Very important as the transmitter of Texas cattle fever, and that tick, there are resistant populations of the Rhipicephalus ticks, because of the constant use of chemicals, but again, that’s a cattle-only tick, and so the same animals in the same ticks are exposed repeatedly to the tick control product. So I don’t expect it to happen with Ixodes, Amblyomma, or Dermacentor because of refugia.
Nikki Grimsley:
Dr. Little, thank you so much. On behalf of CDC’s Clinician Outreach and Communication Activity, I would like to thank everyone for joining us today with a very special thank you to our presenter, Dr. Little.
The recording of the call and transcript will be posted within the next few days to the COCA website at emergency.CDC.gov/coca. Free continuing education is available for this call. If you would like to receive continuing education, you should complete the online evaluation by January 7, 2018, using course code WC2286. For those who will view the archived webinar after January 7, complete the online evaluation between January 8, 2018, and December 7, 2020, using course code WD2286. All continuing education and contact hours for COCA conference calls are issued online through TCE online CDC’s Training and Continuing Education Online System at www.CDC.gov/TCEonline.
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