Conium maculatum, known as hemlock (British English) or poison hemlock (American English), is a highly poisonous flowering plant in the carrot family Apiaceae, native to Europe and North Africa. It is herbaceous without woody parts and has a biennial lifecycle. A hardy plant capable of living in a variety of environments, hemlock is widely naturalised in locations outside its native range, such as parts of Australia, West Asia, and North and South America, to which it has been introduced. It is capable of spreading and thereby becoming an invasiveweed.
All parts of the plant are toxic, especially the seeds and roots, and especially when ingested. Under the right conditions the plant grows quite rapidly during the growing season and can reach heights of 2.4 metres (8 feet), with a long penetrating root. The plant has a distinctive odour usually considered unpleasant that carries with the wind. The hollow stems are usually spotted with a dark maroon colour and become dry and brown after completing its biennial lifecycle. The hollow stems of the plant are deadly for up to three years after the plant has died.[2]
Description
Conium maculatum is a herbaceous flowering plant that grows to 1.5–2.5 metres (5–8 feet) tall, exceptionally 3.6 m (12 ft).[3] All parts of the plant are hairless (glabrous). Hemlock has a smooth, green, hollow stem, usually spotted or streaked with red or purple. The leaves are two- to four-pinnate, finely divided and lacy, overall triangular in shape, up to 50 centimetres (20 inches) long and 40 cm (16 in) broad.[4] Hemlock's flower is small and white; they are loosely clustered and each flower has five petals.[5]
A biennial plant, hemlock produces leaves at its base the first year but no flowers. In its second year it produces white flowers in umbrella-shaped clusters.[6]
Hemlock can be confused with the wild carrot plant (Daucus carota, sometimes called Queen Anne's lace). Wild carrot has a hairy stem without purple markings, and grows less than 1 m (3+1⁄2 ft) tall.[7] One can distinguish the two from each other by hemlock's smooth texture, vivid mid-green colour, purple spotting of stems and petioles and typical height of the flowering stems being at least 1.5 m (5 ft), twice the maximum for wild carrot. Wild carrots have hairy stems that lack the purple blotches.[8][9]
The species can also be confused with harmless cow parsley (Anthriscus sylvestris, also sometimes called Queen Anne's lace).[6][10]
The plant should not be visually confused with the North American-native Tsuga, a coniferous tree sometimes called the hemlock, hemlock fir, or hemlock spruce, from a slight similarity in the leaf smell. The ambiguous shorthand of 'hemlock' for this tree is more common in the US dialect than the plant it is actually named after.[citation needed] Similarly, the plant should not be confused with Cicuta (commonly known as water hemlock).[10]
Taxonomy
The genus name "Conium" refers to koneios, the Greek word for 'spin' or 'whirl', alluding to the dizzying effects of the plant's poison after ingestion. In the vernacular, "hemlock" most commonly refers to the species C. maculatum. Conium comes from the Ancient Greekκώνειον – kṓneion: "hemlock". This may be related to konas (meaning to whirl), in reference to vertigo, one of the symptoms of ingesting the plant.[11]
C. maculatum, also known as poison hemlock, was the first species within the genus to be described. It was identified by Carl Linnaeus in his 1753 publication, Species Plantarum. Maculatum means 'spotted', in reference to the purple blotches characteristic of the stalks of the species.[12]
Names
In British and Australian English the most prominent vernacular name is hemlock.[13][14] In American English it is typically called poison hemlock, though this name is also used elsewhere.[15][14] Less frequent names used in both America and Australia include spotted hemlock and poison parsley.[16][14] Other local or infrequent names in the US include bunk, California-ferm, cashes, herb-bonnet, kill-cow, Nebraska-fern, poisonroot, poison-snakeweed, St. Bennet's-herb, snakeweed, stinkweed, and wode-whistle.[16] In Australia it is occasionally called wild carrot or wild parsnip.[14] In Hiberno English it may be called devil's bread or devil's porridge.
It has become naturalised in Asia, North America, Australia and New Zealand.[20][21][14] It is sometimes encountered around rivers in southeast Australia and Tasmania.[22] Infestations and human contact with the plant are sometimes newsworthy events in the U.S. due to its extreme toxicity.[23][24]
Ecology
The plant is often found in poorly drained soil, particularly near streams, ditches, and other watery surfaces. It also appears on roadsides, edges of cultivated fields, and waste areas.[20]Conium maculatum grows in quite damp soil,[13] but also on drier rough grassland, roadsides and disturbed ground. It is used as a food plant by the larvae of some lepidoptera, including silver-ground carpet moths and particularly the poison hemlock moth (Agonopterix alstroemeriana). The latter has been widely used as a biological control agent for the plant.[25] Hemlock grows in the spring, when much undergrowth is not in flower and may not be in leaf. All parts of the plant are poisonous.[15]
Toxicity
Hemlock contains coniine and some similar poisonous alkaloids, and is poisonous to all mammals (and many other organisms) that eat it. Intoxication has been reported in cattle, pigs, sheep, goats, donkeys, rabbits, and horses. Ingesting more than 150–300 milligrams of coniine, approximately equivalent to six to eight hemlock leaves, can be fatal for adult humans.[26] The seeds and roots are more toxic than the leaves.[27] Farmers also need to ensure that the hay fed to their animals does not contain hemlock. Hemlock is most poisonous in the spring when the concentration of γ-coniceine (the precursor to other toxins) is at its peak.[28][29]
Alkaloids
Chemical structure of one of the two enantiomers, the (S)-(+) isomer, of coniine, where natural mixtures are considered likely racemates (equal mixtures) of this and the (R)-(–) isomer.[30]
C. maculatum is known for being extremely poisonous. Its tissues contain a number of different alkaloids. In flower buds, the major alkaloid found is γ-coniceine. This molecule is transformed into coniine later during the fruit development.[31] The alkaloids are volatile; as such, researchers assume that these alkaloids play an important role in attracting pollinators such as butterflies and bees.[32]
Coniine has pharmacological properties and a chemical structure similar to nicotine.[20][36] Coniine acts directly on the central nervous system through inhibitory action on nicotinic acetylcholine receptors. Coniine can be dangerous to humans and livestock.[34] With its high potency, the ingestion of seemingly small doses can easily result in respiratory collapse and death.[37]
The alkaloid content in C. maculatum also affects the thermoregulatory centre by a phenomenon called peripheral vasoconstriction, resulting in hypothermia in calves.[38] In addition, the alkaloid content was also found to stimulate the sympathetic ganglia and reduce the influence of the parasympathetic ganglia in rats and rabbits, causing an increased heart rate.[39]
Coniine also has significant toxic effects on the kidneys. The presence of rhabdomyolysis and acute tubular necrosis has been shown in patients who died from hemlock poisoning. A fraction of these patients were also found to have acute kidney injury.[40] Coniine is toxic for the kidneys because it leads to the constriction of the urinary bladder sphincter and eventually the accumulation of urine.[41]
Toxicology
A short time after ingestion, the alkaloids induce potentially fatal neuromuscular dysfunction due to failure of the respiratory muscles. Acute toxicity, if not lethal, may resolve in spontaneous recovery, provided further exposure is avoided. Death can be prevented by artificial ventilation until the effects have worn off 48–72 hours later.[20] For an adult, the ingestion of more than 100 mg (0.1 gram) of coniine (about six to eight fresh leaves, or a smaller dose of the seeds or root) may be fatal. Narcotic-like effects can be observed as soon as 30 minutes after ingestion of green leaf matter of the plant, with victims falling asleep and unconsciousness gradually deepening until death a few hours later.[42]
The onset of symptoms is similar to that caused by curare, with an ascending muscular paralysis leading to paralysis of the respiratory muscles, causing death from oxygen deprivation.[43]
It has been observed that poisoned animals return to feed on the plant after initial poisoning. Chronic toxicity affects only pregnant animals when they are poisoned at low levels by C. maculatum during the fetus' organ-formation period; in such cases the offspring is born with malformations, mainly palatoschisis and multiple congenital contractures (arthrogryposis). The damage to the fetus due to chronic toxicity is irreversible. Though arthrogryposis may be surgically corrected in some cases, most of the malformed animals die.
Such losses may be underestimated, at least in some regions, because of the difficulty in associating malformations with the much earlier maternal poisoning.
Since no specific antidote is available, prevention is the only way to deal with the production losses caused by the plant. Control with herbicides and grazing with less-susceptible animals (such as sheep) have been suggested. It is a common myth that C. maculatum alkaloids can enter the human food chain via milk and fowl, and scientific studies have disproven these claims.[44]
In ancient Greece, hemlock was used to poison condemned prisoners. Conium maculatum is the plant that killed Theramenes, Socrates, Polemarchus, and Phocion.[45] Socrates, the most famous victim of hemlock poisoning, was accused of impiety and corrupting the minds of the young men of Athens in 399 BC, and his trial gave down his death sentence. He decided to take a potent infusion of hemlock.
^"Poison Hemlock". pierecountryweedboard.wsu.edu. Pierce County Noxious Weed Control Board. Archived from the original on 2021-12-08. Retrieved 2020-05-12.
^ ab"Poison Hemlock"(PDF). store.msuextension.org. Montana State University. Retrieved 3 May 2015.
^DNRP-WLRD-RRS Staff (November 28, 2016). "Poison-hemlock". Noxious Weeds in King County, Weed Identification Photos. Seattle, WA: Department of Natural Resources and Parks (DNRP), Water and Land Resources Division (WLRD), Rural and Regional Services (RRS) section. Retrieved January 23, 2017.
^Nyerges, Christopher (2017). Foraging Washington: Finding, Identifying, and Preparing Edible Wild Foods. Guilford, CT: Falcon Guides. ISBN978-1-4930-2534-3. OCLC965922681.
^Clapham, A.R.; Tutin, T.G.; Warburg, E.F. (1968). Excursion Flora of the British Isles (2nd ed.). Cambridge University Press. ISBN0521-04656-4.
^Hackney, P., ed. (1992). Stewart & Corry's Flora of the North-east of Ireland. Institute of Irish Studies and The Queen's University of Belfast. ISBN0-85389-446-9.
^Stephen T. Lee; Benedict T. Green; Kevin D. Welch; James A. Pfister; Kip E. Panter (2008). "Stereoselective potencies and relative toxicities of coniine enantiomers". Chemical Research in Toxicology. 21 (10): 2061–2064. doi:10.1021/tx800229w. PMID18763813.
^Forsyth, Carol S.; Frank, Anthony A. (July 1993). "Evaluation of developmental toxicity of coniine to rats and rabbits". Teratology. 48 (1): 59–64. doi:10.1002/tera.1420480110. ISSN0040-3709. PMID8351649.
^Rizzi, D; Basile, C; Di Maggio, A; et al. (1991). "Clinical spectrum of accidental hemlock poisoning: neurotoxic manifestations, rhabdomyolysis and acute tubular necrosis". Nephrol. Dial. Transplant. 6 (12): 939–43. doi:10.1093/ndt/6.12.939. PMID1798593.
^Barceloux, Donald G. (2008), "Poison Hemlock (Conium maculatum L.)", Medical Toxicology of Natural Substances: Foods, Fungi, Medicinal Herbs, Plants, and Venomous Animals, John Wiley & Sons, Inc., pp. 796–799, doi:10.1002/9780470330319.ch131, ISBN9780470330319
^"Conium maculatum L."Inchem. IPCS (International Programme on Chemical Safety). Retrieved 2012-07-06.
^Frank, A. A.; Reed, W.M. (April 1990). "Comparative Toxicity of Coniine, an Alkaloid of Conium maculatum (Poison Hemlock), in Chickens, Quails, and Turkeys". Avian Diseases. 34 (2): 433–437. doi:10.2307/1591432. JSTOR1591432. PMID2369382.
^Blamey, M.; Fitter, R.; Fitter, A. (2003). Wild flowers of Britain and Ireland: The Complete Guide to the British and Irish Flora. London: A & C Black. ISBN978-1408179505.
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