Bovis ISSN 2398-2993

Hemlock poisoning

Synonym(s): Hemlock, Spotted hemlock, Poison hemlock, Conium maculatum

Contributor(s): Nicola Bates, Alan Murphy


  • Cause: ingestion of hemlock (Conium maculatum).
  • Signs: gastrointestinal and neurological signs, abortion and teratogenic effects (chronic ingestion).
  • Diagnosis: based on history of exposure and evidence of plants grazed.
  • Treatment: supportive.
  • Prognosis: poor in animals with pronounced signs.



  • Fetid, biennial plant growing up to 3 m high. It grows in damp places, meadows, open woods, river, stream and canal margins, roadsides and disturbed ground .
  • The stems are usually purple-spotted or blotched and hollow. The leaves have a feathery appearance, with a pungent smell when crushed. The unpleasant, fetid odor of Conium maculatum is described as 'mousy' or resembling mouse or cat urine. The flowers are white and borne in umbels up to 10 cm across. The fruits are 2-celled, rounded and about 2.5-4 mm across.
  • The related plant Cicuta virosa (water hemlock) does not have the purple spotted stem or mousy odor .
  • Most animals find the plant unpalatable and will only eat it when other forage it unavailable; however, there are reports of some animals actively seeking the plant.
  • Dried plant material is less likely to be toxic as the alkaloids are volatile, although poisoning from hemlock in hay has been reported.

Predisposing factors


  • Lack of availability of other vegetation.


  • The toxic nature of Conium maculatum has been recognized for centuries.
  • The toxins are piperidine alkaloids and all parts of the plant are toxic.
  • The main toxic compound is coniine (2-propylpiperidine), which is similar in structure and function to nicotine.
  • Coniine exerts a nicotinic effect on autonomic ganglia with a characteristic biphasic pattern, an initial stimulant and secondary depressant effect.
  • It also has a curare-like action at the neuromuscular junction, causing flaccid paralysis of the skeletal muscles, which can progress to the respiratory muscles.
  • The concentration of coniine within the roots, fruit and other plant parts is variable and is thought to be dependent on local climate, geographical location and age of the plant.


  • Signs can be rapid in onset and occur within a few minutes or up to 3 h.
  • In some cases, effects can occur later, eg in an outbreak of poisoning involving contaminated hay effects occurred 36 hours after its introduction.
  • Recovery can also be rapid and occur within a few hours.
  • Death can occur within a few hours of ingestion or up to 2 days later.


  • Poisoning can occur wherever the plant grows.


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Further Reading


Refereed Papers

  • Recent references from PubMed and VetMedResource.
  • Swerczek T W & Swerczek S J (2012) Spotted hemlock poisoning in a herd of Angus cattle. JAVMA 240 (11), 1280-1 PubMed.
  • Binev R, Mitex J & Miteva T (2007) Intoxication with poison hemlock (Conium maculatum L.) in calves. Trakia J Sci 5 (3-4), 40-50.
  • Vetter J (2004) Poison hemlock (Conium maculatum L.). Food Chem Toxicol 42 (9), 373-82 PubMed.
  • Lopez T A, Cid M S & Bianchini M L (1999) Biochemistry of hemlock (Conium maculatum L.) alkaloids and their acute and chronic toxicity in livestockToxicon 7 (6), 841-865 PubMed.
  • Scatizzi A, Di Maggio A, Rizzi D, Sebastio A M & Basile C (1993) Acute renal failure due to tubular necrosis caused by wildfowl-mediated hemlock poisoning. Renal Failure 15 (1), 93-96 PubMed.
  • Galey R D, Holstege D M & Fisher E G (1992 Toxicosis in dairy cattle exposed to poison hemlock (Conium maculatum) in hay: isolation of Conium alkaloids in plants, hay, and urine. J Vet Diag Invest 4 (1), 60-64 PubMed.
  • Panter K E, Keeler R F, Bunch T D & Callan R J (1990) Congenital skeletal malformations and cleft palate induced in goats by ingestion of Lupinus, Conium and Nicotiana species. Toxicon 28 (12), 1377-1385 PubMed.
  • Panter K E, Bunch T D, Keeler R F, Sisson D V & Callan R J (1990) Multiple congenital contractures (MCC) and cleft palate induced in goats by ingestion of piperidine alkaloids-containing plants: reduction of fetal movements as the probable cause. Clin Toxicol 28 (1), 69-83 PubMed.
  • Panter K E & James L F (1990) Natural plant toxicants in milk: a review. J Anim Sci 68, 892-904 PubMed.
  • Panter K E, Keeler R F & Baker D C (1988) Toxicoses in livestock from the hemlocks (Conium and Cicuta spp.). J Anim Sci 66 (9), 2407-2413 PubMed.
  • Jessup D A & Boermans H J (1986) Toxicosis in tule elk caused by ingestion of poison hemlock. JAVMA 189 (9), 1173-1175 PubMed.
  • Keeler R F, Balls L D, Shupe J L & Crowe M W (1980) Teratogenicity and toxicity of coniine in cows, ewes and mares. Cornell Vet 70, 19-26 PubMed.
  • Keeler R F & Balls L D (1978) Teratogenic effects in cattle of Conium maculatum and conium alkaloids and analogs. Clin Toxicol 12 (1), 49-64 PubMed.
  • Keeler R F (1974) Coniine, a teratogenic principle from Conium maculatum producing congenital malformations in calves. Clin Toxicol 7 (2), 195-206 PubMed.
  • Bowman W C & Sanghvi I S (1963) Pharmacological actions of hemlock (Conium maculatum) alkaloids. J Pharm Pharmacol 15, 1-25 PubMed.

Other sources of information

  • Burrows G E & Tyrl R J (2013) Toxic Plants of North America. 2nd edn. Wiley Blackwell, USA.
  • Cooper M R & Johnson A W (1998) Poisonous Plants and Fungi in Britain. 2nd edn. The Stationery Office, UK.


  • ASPCA Animal Poison Control Center. Tel: +1 (888) 426 4435; Website:
  • Veterinary Poisons Information Service (VPIS). Tel: +44 (0)2073 055 055; Website: