Haemonchus contortus, also known as the barberpole worm, is a very common parasite and one of the most pathogenic nematodes of ruminants. Adult worms attach to abomasal mucosa and feed on the blood. This parasite is responsible for anemia, oedema, and death of infected sheep and goats, mainly during summer in warm, humid climates. Females may lay over 10,000 eggs a day, which pass from the host animal in the faeces. After hatching from their eggs, H. contortus larvae molt several times, resulting in an L3 form that is infectious for the animals. The host ingests these larvae when grazing. The L4 larvae, formed after another molt, and adult worms suck blood in the abomasum of the animal, potentially giving rise to anaemia and oedema, which eventually can lead to death. The infection, called haemonchosis, causes large economic losses for farmers around the world, especially for those living in warmer climates. Anthelminthics are used to prevent and treat these, and other, worm infections, but resistance of the parasites against these chemicals is growing. Some breeds, such as the West African Dwarf goat and N'Dama cattle, are more resistant than other breeds to H. contortus (haemonchotolerance).
Morphology The ova is yellowish in color. The egg is about 70–85 μm long by 44 μm wide, and the early stages of cleavage contain between 16 and 32 cells. The adult female is 18–30 millimetres (3⁄4–1+1⁄8 in) long and is easily recognized by its trademark "barber pole" coloration. The red and white appearance is because H. contortus is a blood feeder, and the white ovaries can be seen coiled around the blood-filled intestine. The male adult worm is much smaller at 10–20 millimetres (3⁄8–13⁄16 in) long, and displays the distinct feature of a well-developed copulatory bursa, containing an asymmetrical dorsal lobe and a Y-shaped dorsal ray.
Life cycle The adult female worm can release between 5,000 and 10,000 eggs, which are passed out in the faeces. Eggs then develop in moist conditions in the faeces and continue to develop into the L1 (rhabditiform), and L2 juvenile stages by feeding on bacteria in the dung. The L1 stage usually occurs within four to six days under the optimal conditions of 24–29 °C (75–84 °F). The L2 rhabditform sheds its cuticle and then develops into the L3 filiariform infective larvae. The L3 form has a protective cuticle, but under dry, hot conditions, survival is reduced. Sheep, goats, and other ruminants become infected when they graze and ingest the L3 infective larvae. The infective larvae pass through the first three stomach chambers to reach the abomasum. There, the L3 shed their cuticles and burrow into the internal layer of the abomasum, where they develop into L4, usually within 48 hours, or preadult larvae. The L4 larvae then molt and develop into the L5 adult form. The male and female adults mate and live in the abomasum, where they feed on blood.
Genetics The H. contortus draft genome was published in 2013. Further work to complete the reference genome is underway at the Wellcome Trust Sanger Institute in collaboration with the University of Calgary, the University of Glasgow, and the Moredun Research Institute. Developing genetic and genomic resources for this parasite will facilitate the identification of the genetic changes conferring anthelmintic resistance and may help design new drugs or vaccines to combat disease and improve animal health. The H. contortus reference genome was published at chromosome-scale in 2020. 5 autosomes and one X chromosome were assembled. Quite remarkably, each chromosome had a very similar gene content compared to the corresponding chromosome in C. elegans, yet there was little conservation of gene order. This genome is the fourth version from Wellcome Sanger.
Pathogenicity Clinical signs are largely due to blood loss. Sudden death may be the only observation in acute infection, while other common clinical signs include pallor, anemia, oedema, ill thrift, lethargy, and depression. The accumulation of fluid in the submandibular tissue, a phenomenon commonly called "bottle jaw", may be seen. Growth and production are significantly reduced.
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