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Ricin: Epidemiological Overview for Clinicians

Agent

  • Ricin is one of the most toxic biological agents known—a Category B bioterrorism agent and a Schedule number 1 chemical warfare agent.
  • Ricin toxin can be extracted from castor beans, purified and treated to form a pellet, a white powder, or dissolved in water or weak acid to be released as a liquid.
  • Ricin is stable under ambient conditions.
  • Ricin particles of <10 µm have been used for animal studies characterizing inhalational exposure.
  • Ricin particles can remain suspended in undisturbed air for several hours.
  • Re-suspension of settled ricin from disturbed surfaces also may occur.

Transmission

  • Although ricin may adhere to clothing or be present on surfaces, potential for transmission via contact with contaminated clothing or contaminated surfaces is low.
  • Although ricin may adhere to skin, person-to-person transmission through casual contact has not been reported.
  • Ricin can be transmitted
    • by skin-to-skin contact (although ricin poisoning is not contagious).
    • by the airborne route through release of ricin in the form of a powder, or a mist, or re-suspension of ricin into the air from disturbed surfaces.
  • Ricin would need to be dispersed in particles with diameters smaller than 10 µm to be used as an effective weapon by the airborne route. It is very difficult to prepare particles of this size.
  • Routes of exposure include inhalation, parenteral (injection), ingestion, dermal contact (exposure risk is low; absorption through non-intact skin or via a solvent carrier), or ocular contact.

Risk Groups

  • Castor plants are distributed worldwide. Ricin is produced when castor oil is made from castor beans; however, the general public is not considered at risk for exposure.
  • Persons in or around castor oil processing plants are at risk for exposure.
  • It takes a deliberate act to make ricin and use it as a poison. Those at risk for ricin poisoning include
    • Persons in the dispersal area of a ricin aerosol release, if ricin is used as an agent of terrorism.
    • Persons who are victims of parenteral injection with ricin.
    • Persons who ingest castor beans, or food or water contaminated with ricin.
  • It is not known whether certain populations are more vulnerable to the health effects of ricin exposure (e.g., children, pregnant women, the elderly, those with immunosuppression or underlying respiratory or gastrointestinal tract disease); however, persons with pre-existing tissue irritation or damage who are exposed to ricin may sustain further injury and greater absorption of ricin toxin.

Pathogenesis

  • Ricin is a toxalbumin, a biological toxin whose mechanism of action is inhibition of protein synthesis (manufacture of proteins) in eukaryotic cells; cell death results from the absence of proteins.
  • The effects of ricin poisoning depend upon the amount of ricin exposure, the route of exposure and the person's premorbid condition.
  • Ingestion and mastication of three to eight castor beans is the estimated fatal dose in adults. The fatal dose in children is not known, but likely is less.
  • Most cases of castor bean ingestion do not result in poisoning, because:
    • It is difficult for ricin to be released from ingested castor beans, because ricin release requires mastication, and the degree of mastication is likely to be important in determining the extent of poisoning.
    • Ricin is not as well absorbed into the body via ingestion when compared to injection or inhalation.
  • Inhalation or injection of ricin would be expected to lead to a more rapid onset of signs and symptoms of ricin poisoning and a more rapid progression of poisoning compared to ingestion, given the same exposure amount.
  • Data on inhalational exposure to ricin in humans is extremely limited.
  • Animal studies suggest that inhalation is one of the most lethal forms of ricin poisoning.
  • Following severe ricin poisoning, damage to vital organs may be permanent or have lasting effects.
  • No long-term effects are known to exist from ricin exposure that did not acutely result in signs and symptoms of ricin poisoning.

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