Parasite of the month: Haemonchus contortus in small ruminants

Parasite of the month: Haemonchus Contorts in small ruminants. Read more about the parasite and how it affects sheep and goats

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Parasite of the month: Haemonchus contortus in small ruminants

Haemonchosis affects sheep and goats of all ages at pasture. It can also affect cattle, however, unlike sheep and goats, cattle develop a level of immunity from 12 months of age.

Caused by the blood sucking parasitic worm Haemonchus contortus (aka barber’s pole worm due to its red and white striped appearance), haemonchosis is generally seen from mid-spring to autumn, when the temperatures rise allowing hypobiosed (dormant) larvae to emerge from the abomasal lining and mature to the blood sucking
adults. Warmer, wetter weather also lets the earlier (L3) stage larvae & eggs develop at pasture.

H.contortus adults are found in the abomasum where they latch on to the mucosa and suck blood. When they detach, the abomasal mucosa continues to bleed leading to severe anaemia and death. A single adult H.contortus can remove 0.05ml/day so infection with 5000 worms can cause 250ml of blood loss/day1. It is also important to note that females are prolific egg layers and can produce 5000-6000 eggs/day each.

Three types of haemonchosis exist:

Acute: presenting with sudden onset anaemia +/- sudden death due to high worm burden and high blood loss. Often seen in young animals or animals with a low body condition score that cannot produce red blood cells fast enough to replace those lost.
Sub-acute: Occurs when red blood cells are produced at the same rate as they are lost to the parasite, however this puts a drain on the bone marrow supply and often progresses to acute disease. As well as the chance of progressing to acute disease, sub-acute haemonchosis causes lethargy, slow growth rates and weight
Chronic: Usually with a low burden where the affected individual(s) can produce red blood cells faster than they are lost to the parasite, although pallor and a low level of anaemia are often seen +/- weight loss due to the persistent protein drain. A ‘bottle jaw’ can be seen in some cases.

Unlike most small ruminant diseases caused by parasitic worms, diarrhoea is not a feature of haemonchosis.


Diagnosed through faecal egg counts & clinical examination, you may see us use an eye colour chart (the FAMACHA system) for grading anaemia in relation to H.contortus burden. The chart allows us to assess the degree of anaemia, and therefore H.contortus burden, by tracking changes in membrane colour. In more severe cases haemonchosis is diagnosed at post-mortem examination.

Control is through having a good parasite management system. Regular faecal egg count tests should be performed, especially through the warmer risk period and if treatment is indicated, a faecal egg count reduction test performed to check resistance status. H.contortus resistance is a particular problem, so treatment should be targeted and specific rather than blanket treatment across your entire flock.

As with all parasite control strategies, quarantine of new stock and quarantine treatment is paramount. Barbervax is a vaccine against haemonchosis that is licensed in Australia and South Africa, to help reduce haemonchosis by reducing egg counts and worm burden. In some circumstances, it may be possible to import this vaccine for use in UK flocks.

If you are worried about severe haemonchosis in your flock please do contact us to discuss control strategies further.

Header photo courtesy of CSIRO

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