Haemonchus contortus

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Haemonchus contortus
Haemonchus contortus.jpg
Haemonchus contortus egg
Scientific classification
Kingdom: Animalia
Phylum: Nematoda
Class: Secernentea
Subclass: Rhabditia
Order: Strongylida
Family: Trichostrongylidae
Genus: Haemonchus
Species: H. contortus
Binomial name
Haemonchus contortus
(Rudolphi, 1803) Cobb, 1898

Haemonchus contortus, also known as the barber's pole worm, is 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 months in warm, humid climates.[1]

Females may lay over 10,000 eggs a day,[2] 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.[3]

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.

Morphology[edit]

The oocyte is yellowish in color. The egg is approximately 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 mm 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 mm 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[edit]

The adult female worm can release between 5,000 and 10,000 eggs, which are passed out in the feces. 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. 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 infecting larvae. The infecting larvae pass through the first three stomachs 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[edit]

The H. contortus draft genome was published in 2013.[4] Further work to complete the reference genome is underway at the Wellcome Trust Sanger Institute [5] 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.

Pathogenicity[edit]

Clinical signs are largely due to blood loss. Sudden death may be the only observation in acute infection, 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.

Prevention[edit]

Prophylactic anthelmintic treatment necessary to prevent infection in endemic regions, but wherever possible, a reduction on reliance on chemical treatment is warranted given the rapid rise of anthelmintic resistance. Targeted selective treatment methods such as the FAMACHA method may be valuable in reducing the number of dosing intervals, selecting for more worm resistant sheep and increasing the percentage of parasites on pasture that are still susceptible to anthelmintics.

References[edit]

Footnotes[edit]

Other sources[edit]

  1. Newton, S. 1995. Progress on vaccination of Haemonchus contortus. International Journal of Parasitology, 25: 1281–1289.
  2. Roberts, L., J. Janovy. 2000. Foundations of Parasitology. US: The McGraw Hill Companies, Inc..
  3. Fetterer, R., M. Rhoads. 1996. The role of the sheath in resistance of Haemonchus contortus infective stage larvae to proteolytic digestion. Veterinary Parasitology, 64: 267–276.
  4. Dorny, P., A. Batubara, M. Iskander, V. Pandey. 1996. Helminth infections of sheep in North Sumatra, Indonesia. Veterinary Parasitology, 61: 353–358.