ricin

ricin, toxic protein (toxalbumin) occurring in the beanlike seeds of the castor-oil plant (Ricinus communis). Ricin, discovered in 1888 by German scientist Peter Hermann Stillmark, is one of the most toxic substances known. It is of special concern because of its potential use as a biological weapon. Accidental exposure to ricin is rare and results primarily from the ingestion of castor seeds.

Ricin toxicity

Purified ricin occurs in the form of a soluble white powder that is extracted from castor seeds or from the waste materials generated during the production of castor oil. Purified ricin can enter the body through ingestion, inhalation, or injection. The early symptoms of poisoning following ingestion include diarrhea and vomiting, which can lead to dehydration. In cases of severe poisoning by ingestion, these symptoms are followed by seizures, hallucinations, and liver and renal failure, usually within 72 hours. If ricin poisoning occurs by inhalation, symptoms may include difficulty breathing, tightness in the chest, cough, and nausea. Severe poisoning by inhalation causes fluid accumulation in the lungs and respiratory failure, leading to death within 36 to 72 hours. When ricin is injected, it causes red blood cells to clump together (agglutination), which in turn leads to the destruction of red cells (hemolysis) and produces symptoms similar to poisoning by ingestion. Very small doses of ricin can be lethal if inhaled or injected, since these routes of exposure enable the toxin to immediately enter the bloodstream, resulting in its rapid distribution throughout the body.

Ricin toxicity is based on the substance’s ability to inhibit protein synthesis and to stimulate cells to undergo programmed cell death (apoptosis). Upon entry into the body, the toxin is readily taken up into cells and rapidly induces apoptosis, giving rise to the onset of symptoms of poisoning within hours. Studies of ricin have indicated that at low doses it is capable of selectively inducing apoptosis in cancer cells, suggesting that it may have potential for development as an anticancer drug.

Toxicology tests may be used to detect ricin in the blood or urine; however, these tests are generally impractical in an emergency situation, since they require more time than is available to confirm poisoning. In addition, there is no antidote available for ricin poisoning, and as a result treatment is supportive. If less than an hour has passed since ingestion, gastric lavage may be performed to remove the poison from the stomach. Intravenous fluids are administered to prevent dehydration, and activated charcoal may be given to absorb the poison from the gastrointestinal tract. In some cases, poisoned individuals recover.

Scientists are working to develop a toxin-neutralizing antidote that could be used in emergency cases of ricin poisoning. Vaccines to prevent poisoning in individuals such as military personnel are also under development.

Use as a biological weapon

Because of the potentially lethal effects of ricin, the relative ease of preparing it, and the ready availability of the castor-oil plant, ricin has long been considered for use as a weapon. During both World Wars the United States, Britain, and other countries experimented with artillery shells and bombs that would disperse clouds of finely ground ricin powder. Authorities concluded that ricin (known in the United States as Agent W or Compound W) would make a poor military weapon, since the compound degraded under the heat of firing and exploding, and very large quantities of it would have to be dispersed over a single area in order to have any effect on massed troops.

Research by the major powers continued during the Cold War, but stockpiles were destroyed under the terms of the Biological Weapons Convention of 1972 (though it was later determined that Iraq experimented with ricin weapons through the 1980s). Ricin is still considered to be a possible weapon of terror and assassination, however. Given the practical limitations of the toxin, some possible terrorist scenarios might be the inoculation of ricin into a food or water supply or the dispersal of a ricin aerosol in a confined indoor space. Such attacks would cause limited casualties but might sow broad panic and trigger major emergency responses. On occasion, isolated individuals and members of terrorist groups have been arrested and convicted of possessing small amounts of ricin powder that were evidently intended for use as weapons, and small quantities of ricin have been sent anonymously through postal systems, with the evident intent of causing harm. No attempted use of ricin as a weapon of mass destruction has occurred, however.

The most famous use of ricin as a weapon of assassination occurred in 1978, when Georgi Markov, a Bulgarian exile living in London, was stabbed in the leg with the tip of an umbrella that injected a tiny hollow sphere containing ricin into his body. Within three days Markov was dead. During the 1990s defectors from the KGB (the former Soviet secret police agency) confirmed that he had been poisoned by a device designed in the Soviet Union and given to an agent hired by the Bulgarian government.