What Is Water Hemlock?

The USDA Agricultural Research Service calls water hemlock the most violently toxic plant that grows in North America, says only a small amount of its toxic substance is needed to poison livestock or humans, and describes that toxin, cicutoxin, as a violent convulsant acting directly on the central nervous system. CDC's MMWR says eating a 2 to 3 centimeter portion of the root can be fatal in adults.

It is easy to mistake. The MMWR says water hemlock is similar in appearance to parsnips, smells like fresh turnips and tastes sweet, but is the most toxic indigenous plant in North America. The Agricultural Research Service says it may be confused with poison hemlock because of the similar names, though they are different plants that cause different types of poisoning, and that it has also been confused with wild parsnips, other herbs and medicinal plants. The MMWR says people who forage for edible wild plants need to be aware of and able to recognize the poisonous plants in their area.

Seizures are the danger. The MMWR describes a man who began having seizures and delirium 2 hours after eating a root later identified as water hemlock, and says that in fatal poisonings severe seizures follow the first symptoms and death usually results from status epilepticus, with a case fatality rate of 30 percent for poisonings reported from 1900 through 1975.

The Agricultural Research Service advises contacting a poison control center and getting emergency medical help as quickly as possible, because the severe seizures and convulsions must be controlled to preserve normal breathing and cardiovascular function, and the MMWR says no antidotes exist and treatment is supportive.