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Nematode © SLAGMULDER Christian

Anti-parasitic drugs: humans and animals united in the same fight

Smart breeding to slow resistance to avermectins

Integrated management of grazing animals helps decrease the amount of anti-parasitic drugs used.

By Pascale Mollier, translated by Inge Laino
Updated on 06/20/2017
Published on 12/07/2015

Herd of heifers grazing on permanent grasslands in Normandy. © INRA, CAUVIN Brigitte
Herd of heifers grazing on permanent grasslands in Normandy © INRA, CAUVIN Brigitte

When and how should drugs be used to treat disease? Whether it be for animal or plant health, these questions are key when it comes to combatting diseases that are capable of developing resistance to the drugs used against them.“When it comes to intestinal parasitic diseases (strongyles) in farm animals, the goal is not total eradication”, explains Jacques Cabaret. “Those are common parasites that pose no real threat in limited quantities, but that must be monitored and controlled. It’s all about reducing selection pressure to avoid the emergence and development of resistant parasites”.

Toward greener pastures

During a grazing season, which lasts about six months, animals graze on a different parcel of land roughly every month. Treating animals systematically before they change parcels allows farmers to ensure the presence of “clean” animals in “clean” pastures. However, this strategy fosters the rapid development of resistant parasites as soon as they appear, since they alone can thrive in the presence of the drug. By maintaining a stock of non-resistant parasites in a parcel, healthy competition between non-resistant and resistant parasites is preserved, creating dynamic populations. That is why researchers suggest treating animals as they leave a parcel, rather than when they enter a new parcel. In other words, “move and treat” rather than “treat and move”.

Using statistics to boost treatment

Simulation studies have shown that random treatment of only 20% of animals each month represents the best compromise to keep parasites in check while delaying the onset of disease. This method has proven efficient in experiments with intestinal strongyles in sheep.

A diagnosis issue

“Keep in mind that in a herd, only 20% of the animals carry 80% of the parasites”, explains Jacques Cabaret. If only those infected animals are treated, the quantity of medicine can be decreased accordingly. But then the problem of diagnosis arises, and in this field, “we are still in the stone age”, says the researcher. Methods of quantitative diagnosis are needed, which would allow experts to estimate the degree of infection and the relevance of treatment. For now, diagnoses are essentially qualitative, based on observations of faeces under the microscope, where the presence of parasite eggs can be detected. Faeces analyses are costly. To reduce cost, they can be run on the faeces of one group of animals. There are also some low-cost diagnostic methods available, such as a diarrhoea score(1), or observation of animal eyes and eyelids (2). A pale eye or eyelid is the sign of anaemia, and therefore high levels of parasites, since parasites feed on the blood of an animal, making them anaemic.

(1) The diarrhoea score (Disco), developed at INRA.

(2) Famacha anaemia index, tested with success in tropical regions.

Scientific contact(s):

Associated Division(s):
Animal Health , Microbiology and the Food Chain
Associated Centre(s):
Val de Loire


Cabaret, J et al. 2006. Indicators for internal parasitic infections in organic flocks: the diarrhoea score (Disco) proposal for lambs. In: Organic Farming and European Rural Development (p. 552-553). Presented at Organic Farming and European Rural Development, Odense, DNK (2006-05-30 - 2006-05-31).