Poison Hemlock (Conium maculatum), Water Hemlock (Cicuta maculata) and Hemlock Water Dropwort (Oenantha crocata) are three species of the family Apiaceae that are found in Australia. All are highly toxic to humans and livestock, but are typically not consumed.
They contain a variety of toxins that affect nerves, ultimately leading to respiratory failure and death. In humans it is capable of causing serious illness or death, and all parts of the plant are poisonous when ingested. Symptoms include nervousness, trembling and then respiratory distress. Water hemlock contains the potent neurotoxin, cicutoxin. Poison hemlock contains a piperidine alkaloid neurotoxin, called coniine and γ-coniceine. Hemlock water droplet contains the toxin oenathetoxin.
What does it look like?
Hemlock is a robust biennial herb growing 1-2.5 metres high, with hollow stems that have distinct reddish-brown or purple spots on the outer surface. The leaves are large (up to 50 cm long and 40 cm wide), hairless and carrot-like and alternatively fixed along the plant stem. They have a sharp offensive odour when crushed. This is often reported as a ‘mousey’ smell. A similar odour can be noticed on the breath of animals that have been poisoned. The plant flowers in spring and summer. The flowers have 5 petals each and are arranged in clusters at the end of the stalk.
Water hemlock is also a biennial herb that can also grow to 2m in height. The flower colour is similar to poison hemlock (white to greenish-white). Water hemlock has compound, serrated leaves that are sharply toothed.
Hemlock and water hemlock are primarily differentiated by the leaves (see below).
Hemlock water dropwort is also a member of the carrot family. It therefore also has flowers that grow in umbrella-shaped clusters. It has shiny, fern like leaves that resemble parsley.
Where is it found?
Poison Hemlock is a weed of disturbed areas, often occurring near stockyards and along roadsides and riverbanks. It is found coastal and sub-coastal regions in Victoria, Eastern NSW, ACT, Tasmania, SE Queensland, SE South Australia, and SW WA. It is a declared weed in NSW, Victoria and WA. Water Hemlock is commonly found in wet areas in south eastern Australia, including Victoria, NSW, ACT, and Tasmania. Hemlock water dropwort can be found in the Adelaide hills.
What are the signs of poisoning?
There are scant reports of horses affected by hemlock. Water hemlock poisoning was responsible for the death of 4 miniature horses in Japan. The animals were found recumbent, unable to stand and experienced seizures before death. Reported signs of poison hemlock muscular weakness, ataxia, salivation, dilated pupils, and respiratory paralysis. Hemlock was thought to be the poison that led to the death of Socrates.
Treatment and prognosis
There is no treatment for hemlock poisoning, irrespective of the plant species involved. As with most toxicities not all cases are fatal, but most of the reports in horses suggest a rapid onset of signs and death.