The findings of a new report suggest that the increase in temperature caused by global warming could cause a higher outbreak of haemonchosis, a variety of intestinal worms in sheep.

Haemonchosis is a gastrointestinal worm infection of sheep and goats in regions where conditions of high humidity coincide with high temperature.

The parasitology section at the Agri-Food and Biosciences Institute (AFBI) has published a report that predicts global warming could mean the incidence of haemonchosis might increase in Ireland and the UK.

The life-cycle of the worm consists of the larvae of haemonchus contortus, also called ‘Barber’s pole worm’, hatching and maturing in faeces on the ground before migrating to fresh grass for intake by grazing animals.

This migration requires warm, moist conditions, and the larvae are quite susceptible to dryness and low temperatures.

In countries where a suitable climate occurs seasonally rather than year-round, arrested (‘hypobiotic’) larvae survive in the stomach lining of infected animals, maturing to egg-producing adults when conditions again become suitable for transmission of infection.

While cases have not been widespread across Ireland and the UK, outbreaks of haemonchosis may occur in the summer months if rainfall is sufficient to enable the larvae to survive on pasture.

This year, following the particularly wet conditions of mid-summer, an unusual increase in the number of cases of haemonchosis diagnosed has been noted by staff at AFBI.

Signs of haemonchosis

The veterinary sciences division at the AFBI said that clinical diagnosis of haemonchosis in the field relies on the observation of very pale mucous membranes, including the conjunctivae of the eyes.

Dead lambs submitted to the AFBI for post-mortem examination showed signs of severe scour, often with flystrike, and severe anaemia, with large amounts of clear fluid in the abdomen and chest.

Anaemia due to blood loss is the usual cause of death in these cases.

It has been determined by the AFBI that each worm removes about 0.05ml of blood daily from its host, so a burden of 2,000 worms would result in a daily blood loss of 100ml.

Faecal egg counts on samples submitted to the laboratory from flocks where haemonchosis is suspected usually show high levels of strongyle-type eggs.

Low counts may be encountered if the majority of the intestinal worms are immature or if anthelmintic was administered recently, according to the findings of the report.

Treatment of intestinal worms

The AFBI report suggests that if worms persist in lambs, particularly after dosing with benzimidazole (white drench), anthelmintic resistance may be an issue and administration of moxidectin, together with an iron-containing tonic, may be advisable.

Anthelmintic resistance, the AFBI concluded, is less problematic in areas where infection is encountered only sporadically, compared to the situation in warmer countries where haemonchus infection is highly endemic.

The source of haemonchus infection in flocks is often bought-in sheep from an area where infection is more common.

Due to the high reproductive potential of the worm, the AFBI said it is likely that the pasture will rapidly become contaminated with haemonchus eggs.

However, the AFBI added that the main risk for lambs in the next grazing season is from hypobiotic larvae that may remain in the replacement stock reared this year.

The report suggested that early-season anthelmintic dosing of replacement stock can help to reduce the risk of pasture contamination for naive lambs, while careful attention to quarantine and dosing of bought-in stock is essential.