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Nematode © SLAGMULDER Christian

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

The social behaviour of animals counts

In the 2000s, researchers showed that the bulk of ivermectin applied topically on cattle ends up in the digestive system and faeces, since the animals lick each other, and therefore ingest the product. In herds that are treated, the result is over-exposure for the licking animals and under-exposure for the animals that are licked.

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

A herd of heifers out to pasture is first and foremost a social group, with its conflicts and affinities, as shown by frequent licking (alpine pasture in the Bauges Mountains, Savoie). © INRA, MEURET Michel
A herd of heifers out to pasture is first and foremost a social group, with its conflicts and affinities, as shown by frequent licking (alpine pasture in the Bauges Mountains, Savoie) © INRA, MEURET Michel

Topical, or pour-on, administration of avermectins is widely used for the treatment of parasitic disease in cows. This method consists of simply pouring the dose of medicine directly onto the backs of animals. Researchers have studied how effective ivermectins are when administered in this way on cattle.

High levels of ivermectin lost in faeces

Researchers observed very high levels of ivermectin in faeces, although the product, applied on the skin, is meant to pass first and foremost into the blood stream, not into the digestive tract. According to their hypothesis, the cows ingest the ivermectin by licking. Licking is indeed a natural behaviour in cows, necessary for the health of their skin, and establishing social ties within a herd.  

Individual licking…

Experiments carried out on cows kept in individual stalls have confirmed this hypothesis: in the group free to engage in licking behaviour, 70% of the dose of ivermectin administered using the pour-on method was eliminated in faeces. Conversely, in the group of cows wearing an anti-licking collar, only 7% of the dose ended up in faeces.

Group licking…

Other experiments carried out on grazing cows have shown that the animals transmit the products between them through licking. Researchers administered three different drugs separately using the pour-on method: ivermectin, doramectin and moxidectin. They found traces of all three drugs in the blood and faeces of all the animals, including those that were not treated. Total quantities ingested sometimes surpassed 20% of the therapeutic dose administered using the pour-on method.

The benefits of getting under the skin!

These results have led scientists to reassess the pour-on application method used in cattle rearing. On the one hand, those animals who are licked do not get the full dose of avermectin, and it is known that under-exposure fosters the onset of resistance to parasites, and may be the reason why some treatments fail. On the other hand, over-exposure in licking animals leads to unwanted excessive residue of drugs in meat sold on the market. And high levels of avermectins in animal faeces are harmful to the environment (see box 2).

These studies highlight the merit of sub-cutaneous injections to treat animals. Indeed, this mode of administration offers the advantage of higher bio-availability of avermectins, which allows doses to be reduced by half with less of it ending up in faeces.

Scientific contact(s):

Associated Division(s):
Nutrition, Chemical Food Safety and Consumer Behaviour, Animal Health
Associated Centre(s):


Laffont et al. 2001. Licking behaviour and environmental contamination arising from pour-on ivermectin for cattle. International Journal for Parasitology, 31-14, 1687-1692.

Bousquet-Melou et al. 2004. Endectocide exchanges between grazing cattle after pour-on administration of doramectin, ivermectin and moxidectin. International Journal for Parasitology 34 -11, 1299-1307.

Escarabajos.. © INRA

Protecting dung beetles

High rates of avermectins in animal faeces are not without consequences for the environment. Because these products also act as an insecticide, they harm insects that break down and transform cow dung, helping organic material to be recycled in grazing soils. That is why it is important to limit the excretion of the drug, both to boost its beneficial effects on animals and to preserve ecosystems.