Haemonchosis -The Blood Sucking Worm

Date published: 28 August 2025

Haemonchosis is a disease increasingly detected in Northern Ireland in sheep, goats and alpacas. Since June 2025 AFBI has confirmed haemonchosis in 12 carcasses submitted for postmortem examination. The carcasses were all sheep and ranged in age from 4-month-old lambs to adults, with varied clinical signs including sudden death, weight loss and weakness.

Haemonchus contortus, otherwise known as the ‘barber’s pole worm’ due to the distinctive red spiral appearance of its gut, is the cause of this disease. It is a gastrointestinal roundworm with the adults residing in the ‘true’ stomach, where they feed on blood through the stomach wall. Adults can produce 5,000-15,000 eggs per day which are shed in the faeces. Larvae can then develop providing the temperature is above 9°C, with higher temperatures speeding up larval development. Larvae require warm moist conditions to migrate to fresh grass for ingestion by animals, and they are sensitive to low environmental temperatures. In countries where a suitable climate occurs seasonally, arrested (‘hypobiotic’) larvae survive in the stomach lining of infected animals, maturing to egg-producing adults when conditions become suitable.

Haemonchosis was seldom detected in Northern Ireland until recently, as prior to 2023 the maximum number of Hameonchus cases recorded in a year was 5. It is suspected that the effects of climate change causing increased temperatures, along with increased movement of animals, have provided a more favorable environment for the larvae to survive on pasture. In 2023, following the wet conditions of mid-summer, an unusual increase in the number of cases of Haemonchus was noted, with 14 cases detected, and the trend continued in 2024 with 20 cases.

Clinical signs depend on the number of worms present within the animal. Each worm can remove roughly 0.05mls of blood daily, so a burden of 2,000 worms would result in 100mls of daily blood loss. Acute disease occurs when many larvae are ingested or emerge from overwintering in a short period, this is seen as severe anaemia with weakness and death. However, ingestion of a smaller numbers of larvae over weeks to months leads to poor body condition scores, bottle jaw and milder anaemia. Diarrhoea is not often a feature of this infection, and it can affect all ages of animals, as no immunity appears to develop.

AFBI identifies haemonchosis in submitted carcasses with anaemia being the main cause of death, hence findings include pale carcasses with free fluid in the chest and abdomen, along with ‘barber pole type’ worms evident within the stomach. Faecal egg counts on samples submitted to the laboratory from flocks where haemonchosis is suspected, usually show very high levels of strongyle-type eggs. However, since strongyle eggs cannot easily be differentiated on a species basis, except by advanced techniques which are not routinely applied, these counts represent the combined output of Cooperia, Haemonchus and Telodorsagia infections. Low egg counts may be encountered if most of the worms are immature or if an anthelmintic was administered recently. It is important to interpret faecal egg results with farm history and the clinical signs of the animal including the use of FAMACHA scoring, to determine anaemia.

Treatment for haemonchosis can be achieved by the administration of anthelmintics. It is important to note, due to its high reproductive rate and rapid life cycle resistance can develop quickly. Anthelmintic resistance is less problematic in Northern Ireland, as Haemonchus infection is currently encountered only sporadically, compared to the situation in warmer countries where it is endemic. Managing grazing by removing batches of animals before grass is grazed too low, co-grazing or grazing after cattle or horses may help to reduce infections.

In Northern Ireland, the source of Haemonchus infection in naïve flocks is often bought-in sheep from an area where infection is more common. This means careful attention should be given to the quarantine procedures and dosing of bought in stock. In flocks were Haemochus is ‘hypobiotic’ in the stomach of animals’ early season anthelmintic dosing of older animals should be undertaken to reduce pasture contamination and protect naive lambs.

As in previous years, it is suspected an increasing number of cases will be seen in the coming months. It is advised to submit suspected carcasses for postmortem examinations and to undertake regular faecal egg count monitoring, along with discussing treatment plans with your vet.