Science - November 30, 2010

Resistant parasites in sheep

The commonest sheep parasite has become resistant to the most widely used worming medicine. And former CVI researcher Fred Borgsteede is worried.

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Borgsteede examined sheep droppings two weeks after the sheep had been treated with the worming medicines ivermectin and moxidectin. He counted the number of eggs of intestinal nematodes, among them Haemonchus contortus, the most common and damaging sheep parasite. This bloodsucking parasite costs sheep farmers both money and sheep, says Borgsteede.

Tip of the iceberg
Treatment with ivermectin in three groups of sheep at different farms led to a decrease in the numbers of eggs by 90, 63 and 59 percent. Borgsteede concluded that on all three farms, H. contortus was resistant to the worming medicine. He published his finding in this month's issue of the Dutch veterinary medicine journal, Tijdschrift voor Diergeneeskunde. Although he only studied sheep droppings on three farms - 'Nobody wanted to fund large-scale research' - he guesses that the result is the tip of the iceberg. 'What is more, our results tie in with those of previous research abroad.'

Not an antibiotic
There are growing problems with of resistant parasites among farm animals, says Borgsteede, especially in sheep and horses. It is true that there are several medicines on the market for treating worm infestations, but most of them work on the same principle. And once a worm is resistant, it stays resistant. 'A whole group of drugs can no longer be used against  H. contortus. Ivermectin is used against worms in humans, horses and other animals too. 'To be clear about this: it is not an antibiotic, it is an anthelmintic.'

Parasitologists
Until the end of last year, Borgsteede was a researcher at the Central Veterinary Institute in Lelystad. He is now retired. 'All the parasitologists have disappeared from the CVI, which is cause for concern. Policy pays very little attention to parasitology, so there are no assignments coming in. There are just a handful of parasitologists left, at the University of Utrecht. And yet parasite problems are on the rise now that pigs and poultry are being kept out of doors in natural conditions.'  

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