Haemonchosis - Sheep

Introduction

Haemonchus contortusis a tropical and sub tropical parasite of the abomasum, that may cause problems in temperate regions. It is around 2 cm long and has a 'barbers pole' appearance. It is a blood-sucking nematode and hence, clinical signs may be quite severe. Together with Ostertagia, Nematodirus and Trichostrongylus it makes up the parasitic gastroenteritis complex in sheep.

Haemonchus has a high biotic potential and egg to L3 development occurs very fast in hot humid conditions. In tropical and sub-tropical areas, disease outbreaks depend on local rainfall. The parasite survives prolonged dry periods by arrested development and development is resumed just prior to rainy season e.g. Australia, Brazil, Middle East and Nigeria. Arrested development is less common in areas with more frequent rainfall e.g. East Africa. There are several cycles of infection annually.

In temperate regions, the ewe is the primary source of infection (relatively few L3 overwinter on pasture) and eggs hatch and develop to L3. Most L3 ingested by ewes and lambs in late summer become arrested (EL4, abomasal wall) to then resume development following spring and cause acute haemonchosis (epidemiology equivalent to type 2 ostertagiosis). A few L3 ingested by lambs do not become arrested and immediately cause acute haemonchosis (if sufficient L3 ingested; equivalent to Type 1 ostertagiosis). This is most likely to happen in hot thundery summer periods. There is normally, only one cycle of infection annually in temperate areas.

A moderate infection (2000 worms) leads to increased red blood cell turnover (from 4 months to 3 weeks) and a loss of haemoglobin and iron (normally recycled). Depleted iron reserves leading to anaemia and death.

Clinical Signs

There are three forms of Haemonchosis. The hyperacute form, occurs mainly in the tropics where the biotic potential is very high and worm burdens can suddenly become huge over a preiod of a few days. Signs in this form of the disease will include sudden death in apparently healthy sheep and severe anaemia. In temperate regions, an acute form can arise. This will consist of oedema, loss of condition, diarrhoea, lethargy and death. The sheep will also be anaemic and hypoalbuminaemic. The chronic form can arise over two months or so and signs will include progressive weight loss or reduced weight gain and diarrhoea and will appear similar to signs of poor nutrition.

Diagnosis

Clinical signs and time of year are characteristic of the disease.

Blood tests may reveal anaemia (low blood haemoglobin, PCV and red blood cell count), elevated blood pepsinogen and hypoalbuminaemia, which would lead to a presumptive diagnosis as no other nematode of sheep is blood-sucking to that degree.

A faecal egg count to assess for presence of haemonchus eggs should be performed. If infection is present, this sample will probably be very high.

Post-mortem examination may reveal an extremely high proportion of adult worms (>2000 adult worms in some cases). There will also be numerous haemorrhagic lesions on gastric mucosa and there may be brown fluid in the abomasum. The carcasses of sheep that may have succumbed to the disease will appear pale and oedematous. There may be expansion of red marrow from epiphysis into medullary cavity of long bones.

Treatment and Control

If infection is discovered, treat with an anthelmintic. Give electrolyte solution if diarrhoea is severe.

Temperate Areas: Dose ewes around lambing

Tropical and Sub-Tropical Areas: Dose all stock every two to four weeks during the rainy season.

There is a hidden antigen vaccine currently under development .

References

Blood, D.C. and Studdert, V. P. (1999) Saunders Comprehensive Veterinary Dictionary (2nd Edition) Elsevier Science
Fox, M and Jacobs, D. (2007) Parasitology Study Guide Part 2: Helminths Royal Veterinary College
Merck & Co (2008) The Merck Veterinary Manual (Eighth Edition) Merial
Radostits, O.M, Arundel, J.H, and Gay, C.C. (2000) Veterinary Medicine: a textbook of the diseases of cattle, sheep, pigs, goats and horses Elsevier Health Sciences

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Small Ruminant Nematodes Flashcards