Haemonchus contortus in lamb in Athlone, September 2021



Haemonchus contortus has been diagnosed as the cause of death of a 5-month-old lamb from a flock in County Cavan. The lamb carcase was submitted to Athlone Regional Veterinary Laboratory for post-mortem examination. The lamb had a history of pining for the previous 3 weeks and having a slight scour, was treated with antibiotics but died a few days later. The main finding on post-mortem examination was an extremely pale carcase. Large numbers of Haemonchus contortus worms were found in the abomasum (fourth stomach) of the lamb. It is often called the ‘barber’s pole worm’ due to its resemblance to a red and white barber’s pole when the intestine is full following a blood meal. The red intestine spirals round the pale uterus of the female giving the ‘barber’s pole effect’.  A faecal sample from the lamb contained an estimated 10,400 strongyle eggs per gram of faeces. H. contortus are prolific egg-layers and the high FEC and gross post-mortem findings were consistent with haemonchosis as the cause of death.


Haemonchus contortus most commonly affects sheep and goats but can also affect cattle. The adult worms have a piercing lancet which facilitates extraction of blood from the blood vessels of the abomasum. The host becomes ill due to the blood feeding activities of the adult and developing larvae. Each worm can remove about 0.05ml of blood per day so a sheep or a goat with 5000 H. contortus worms may lose about 250ml of blood daily; but even much lower worm burdens can cause enough blood loss to cause disease. Clinical signs can vary from sudden death in very acute cases to ill-thrift in chronic cases. Typical signs of infestation include paleness of gums and conjunctivae, swelling under the jaw (bottle-jaw), increased rate and depth of breathing, and an increased heart rate. Diarrhoea is not a typical feature of the disease.


Adults are also at risk of Haemonchosis. Contrary to previous conclusions that adult sheep do not develop a strong acquired immunity to H. contortus, current thinking is that suppressive treatment regimes and/or the sporadic nature of the challenge reduce the ability of sheep to develop immunity. Control and prevention depend on regular faecal egg counts, pasture management and strategic anthelmintic use. In the past, the disease was generally not that prevalent in temperate climates like Ireland’s as the larvae develop more rapidly and have better survivability over winter at warm temperatures in tropical and subtropical areas therefore it did not tend to survive the winter in large numbers on pasture in this country. The principal way in which it survives in flocks over winter is through the persistence of larvae in a hypobiotic (dormant) state in the gut of the host. This feature lends itself to the development of anthelmintic resistance in H. contortus worms, and so resistance is a common feature in flocks affected by the parasite. With the prospect of warmer temperatures and milder winters in Ireland due to climate change, it is to be expected that the prevalence of haemonchosis will increase in the years ahead, and that outbreaks causing substantial losses will become more common. It is also noteworthy that the parasite was traditionally considered a risk in the south of the country, but that also will need to be reconsidered as the climate evolves.


Farmers are advised to contact their private veterinary practitioner to discuss diagnosis, treatment and control in greater detail if they suspect haemonchosis in their flocks or herds.

Species: Ovine
3:36 PM on Tue, 5 October

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