The latest Paper of the Month for Parasitology is “First report of eprinomectin-resistant isolates of Haemonchus contortus in dairy sheep farms in France“ and is freely available for one month. 

For ewe milk to be transformed into the golden crusted, delicious, pressed cheese called Ossau Iraty, the ewes need to graze for at least 240 days a year in the hilly or mountainous lush green pastures of the Pyrénées Atlantiques, in the most southwestern part of France. The overall warm and rainy climate allows this extended grazing period, but also promotes extended survival of the free living stages of nematodes that infect the gastro-intestinal tract of ruminants. These round worms have a simple life cycle: adults reproduce in their host and eggs are shed into the environment. Given the correct temperature and humidity, eggs hatch into larvae, and upon uptake by the grazing host, they evolve into further larval stages, then to adults, and the cycle resumes. Of the strongyles, one species called Haemonchus contortus, is of particular and increasing concern in sheep and goat farms throughout the world, because adult forms of Haemonchus contortus feed off the blood of their host. To limit the impact of this killer parasite on lactating dairy ewes, farmers have a very limited choice of anthelminthic molecules belonging to the classes of benzimidazoles or macrocyclic lactones. Use of other molecules are forbidden in dairy production, their excretion in milk could affect consumer health.

Reports of lack of efficacy of different classes of molecule have been increasing in France over the last 20 years, especially regarding the benzimidazoles. Recently, dairy sheep farmers have therefore massively relied on eprinomectin, a macrocyclic lactone that is barely excreted in milk: it’s use does not entail a withdrawal period, making it economically sound. The first reports of lack of efficacy of eprinomectin in the Pyrénées Atlantiques were taken very seriously. Alarm was raised by veterinarians, when they saw the persistent anemia and death of ewes due to haemonchosis despite eprinomectin treatments. We confirmed the presence of resistant strongyles in 5 farms of this area using the official WAAVP guideline test called Fecal Egg Count Reduction Test (FECRT), enhanced with measurement of eprinomectin in serum. We also identified H. contortus as the major strongyle species still present after eprinomectin administration. In all 5 farms and treatment groups, fecal egg count reduction test showed a reduced efficacy of eprinomectin, while with the injectable and oral drench formulation drug concentration in serum of treated animals was at a level expected to be efficient. However, for ewes that received the topical formula, eprinomectin concentration was mainly under the efficacy threshold. We therefore also confirm that topical formulation of eprinomectin should not be used, because under-dosage can select for resistant strains in the strongyle population.

This work is a milestone in the description and understanding of macrocyclic lactone resistance in Haemonchus contortus affecting dairy sheep in France. This phenomenon is indeed already influencing strongyle management, which in turn can change herd management practices.

The paper “First report of eprinomectin-resistant isolates of Haemonchus contortus in dairy sheep farms in France“ by S. Jouffroy et al., published in Parasitology, is available free for a month.