Government officials in Washington have shut down mail delivery to the US Senate after detecting ricin in a letter addressed to Mississippi senator Roger Wicker, a Republican, on 16 April. Here are some facts about the toxin.

What is ricin?
According to the US Centers for Disease Control and Prevention (CDC), ricin is a poison found naturally in castor beans, and can be derived from the waste product, called 'mash', left over when castor beans are processed to make castor oil.

How deadly is ricin?
According to the CDC, ricin is “very toxic.” Data from tests in monkeys suggest that just 3 milligrams of inhaled ricin can kill an adult.

How does it work?
Ricin inactivates ribosomes, components responsible for manufacturing proteins within human cells. Cells stop making proteins essential to life and die.

What are the symptoms of ricing poisoning?
That depends on how ricin enters the body. Inhaled ricin can cause breathing difficulties, fever, cough and nausea. Ingested ricin can cause vomiting, diarrhea, dehydration, and seizures. Symptoms can appear as early as 4 hours and as late as 24 hours after exposure. Death can occur between three and six days after exposure.

How does one become exposed?
It’s possible to become poisoned with ricin by eating large quantities of castor beans or by ingesting the poison itself, but ricin is a more potent and deadly bioterror agent when inhaled as aerosol particles. Simply touching ricin is not likely to kill a person unless he or she ingests it from the skin.

Has ricin been used as an agent of warfare and bioterrorism?
Ricin has a long history as an agent of biological warfare. The US War Department first considered using ricin in 1918 and worked with British scientists to develop a ricin bomb that appears never to have been used in combat. The US military experimented with inhalable ricin powders in the 1940s and the Iraqi military packed it into artillery shells in the 1980s. Ricin was most likely used to kill Bulgarian journalist Georgi Markov in Great Britain in 1978. Ricin was also detected in 2003 and 2004 in a South Carolina postal facility, in a mailroom serving the office of the US Senate's then-majority leader Bill Frist, and in a letter sent to the White House, though it did not cause any illnesses or deaths in those cases. In the mid-1990s, members of a militia group, the Minnesota Patriots Council, were convicted of conspiring to kill law enforcement officials using ricin. It has also been found in the possession of suspected terrorist groups such as al Qaeda.

How is ricin detected?
Sensors at locations around the country, such as mail sorting facilities, check routinely for the presence of ricin and other pathogens. If a sample tests positive, it is transferred to a lab that performs follow-up tests using antibodies targeted to ricin proteins or DNA from the castor bean plant. In this instance, ricin was initially detected at a Washington DC mail sorting facility and its presence was confirmed by a laboratory in Maryland.

Can ricin poisoning be treated?
There is no antidote to ricin toxin. One company, Soligenix of Princeton, New Jersey, is developing a vaccine against ricin, but it has only progressed through very early-stage clinical trials and has not been approved by the US Food and Drug Administration. It could theoretically be given to poisoning victims under an “emergency use authorization” that permits the use of unapproved treatments and vaccines if no alternative exists. However, the vaccine works by stimulating the body in advance of an attack to produce protective antibodies against ricin. Since the symptoms of ricin poisoning are irreversible four hours after exposure,  the vaccine is not likely to help soon enough to save lives after the discovery of a ricin release. The US National Institute of Allergy and Infectious Diseases is funding research into drugs to treat ricin poisoning.

This article is reproduced with permission from the magazine Nature. The article was first published on April 17, 2013.