Water Hemlock

Water Hemlock, also known as Poison Parsnip, and Cowbane is a small genus of four species (Cicuta bulbifera, Cicuta douglasii, Cicuta maculata, Cicuta virosa) in the carrot family (Apiaceae) that most sources consider to be among the most poisonous plants in both North America and in the United Kingdom. Specifically, the United States Department of Agriculture (USDA) considers Water Hemlock to be “the most violently toxic plant that grows in North America”. Native to nearly all of North America, from northern Canada to southern Mexico, Water Hemlock is a very common rhizomatous perennial herb occasionally (often fatally) mistaken with wild parsnips, other herbs, and medicinal plants.

Water Hemlock is a spring flowering wetland plant commonly found in wet meadows and pastures along the banks of streams and rivers. Accordingly most cases of poisoning occur in early spring. The plant grows to a height of between 3 and 5 feet with shiny green, serrated, lance shaped leaflets, growing off the rootstalk. At the end of the rootstalk, small white flowers grow from a compound umbel similar in appearance to many other species in the carrot family that grow in umbrella like clusters. The rootstalk is compartmentalized with numerous small chambers containing a highly poisonous brown or straw-colored liquid that is released when the stem is broken or cut. Unlike many toxic plants, grazing animals find Water hemlock to be quite palatable, especially the roots and green seed heads. The fact that this plant commonly grows in moist areas near a water source where grazing animals are likely to congregate makes it all the more dangerous.

The chief poison is cicutoxin, an unsaturated aliphatic alcohol that acts by severely disrupting the central nervous system (CNS). The toxin is most concentrated in the roots and rootstalk, although all parts of the plant contain a potentially lethal amount of the toxin. Other toxins in the plant include coniine, which is structurally similar to nicotine and acts as both an agonist and antagonist on nicotinic (cholinergic) receptors. Like cicutoxin, coniine plays a role in disrupting CNS function, initially acting as a stimulant and then as a CNS depressant resulting in coma and death; typically by respiratory failure.

The initial symptoms of ingestion may manifest in as little as 5 minutes or be delayed as long as an hour. Additionally death may occur in as little as 15 minutes or be delayed for hours or days as symptoms rapidly or slowly progress from bad to worse depending upon the amount ingested and the specific animals constitution; in most cases death will occur in under six hours. The specific quantity necessary to cause death varies with the season and age of the plant as well it depends upon which part of the plant was ingested; the roots being the most toxic. The dose needed to cause clinical signs and the lethal dose are nearly identical, a little more than 1 g/kg body weight. A piece of root roughly the size of a walnut is enough to cause the death of a cow; approximately 230 grams will kill a horse. However, the ingestion of even smaller amounts may result in a severe and potentially lethal intoxication. As little as 0.3% body weight for swine, 0.5% body weight for horses, 0.1% body weight for cattle, and 0.2%-0.5% body weight for sheep can be lethal. The initial symptoms of ingestion generally consist of drooling, frothing at the mouth, severe gastrointestinal upset (vomiting, diarrhea), anxiousness, confusion, and loss of coordination. As the intoxication progresses the symptoms will become more severe. In potentially lethally intoxicated animals the syndrome is typically very violent and includes tremors, muscular weakness, renal failure, severe seizures, rhabdomyolysis, cardiac dysrhythmias, respiratory failure and death. It is not uncommon for symptoms to progress so fast that death occurs as soon as 30 minutes after the initial onset of clinical signs.