In June, the University of Guelph hosted an international veterinary conference on small ruminant health and productivity. One of the major topics was how to manage internal parasites in sheep and goats. Internal parasitism is believed to have one of the biggest impacts on animal health and production — ranging from lamb/kid growth to fertility in ewes and does.
One of the major concerns about internal parasite control is how to treat sheep and goats appropriately to provide the greatest benefit while maintaining the effectiveness of the parasiticide. There are a number of reasons why parasiticides may fail to work including how they are used (not administered properly or not using the correct dose) but one reason of real concern is that the parasites themselves might have become resistant to the drug. There is a real concern that this type of resistance is widespread for Haemonchus, one of the major internal parasites in small ruminants. Haemonchus is often the parasite that causes sick lambs but there are other internal parasites (worms) that are not as likely to cause overt sickness but impact health and productivity more subtly, causing what is called subclinical parasitism.
One of the strategies to try to maintain the parasiticide effectiveness is to try to ensure that not all the parasites in a group of animals have been exposed to the anti-parasitic drug. If all animals in a group are treated with the parasiticide and the drug works to eliminate the parasites that are susceptible to the drug, then the only parasites left would be the parasites that are resistant and not killed by the drug. Those resistant parasites will continue to reproduce and contaminate the animal’s environment. Other animals will be infected by those resistant parasites in the environment.
An alternative treatment strategy would be to use a treatment plan so that not all the parasites in a group will be exposed to the parasiticide. Usually this means that you don’t treat every one of the animals in the group. That way, the parasites in the untreated animals will not be exposed to the parasiticide. Some susceptible parasites will survive. Those susceptible parasites will reproduce and be shed into the environment. The idea is that these susceptible parasites will help dilute the resistant parasites.
This is called ‘the concept of refugia.’ It has been proposed as a strategy to help manage parasites and parasiticide drugs in cattle and horses in addition to sheep and goats.
A big decision in implementing a refugia strategy is determining which animals that you will not treat. The group that is most likely to benefit from the parasiticide is the young animal group. So older ewes are the animals that are least likely to be impacted if they are not dewormed going on to pasture. A percentage (15-20 %) of those ewes would be left untreated. It is recommended that even if some ewes are not dewormed, ewes nursing 3 or 4 lambs likely should be dewormed.
Farmers can monitor the parasite burden by testing manure samples. That is likely a wise idea anyway to get some assessment of whether the deworming program is working.
More details on how to implement a refugia program is posted on the website: https://www.cansheep.ca/documents/Sheet%202%20Refugia%20EN_FINAL.pdf
Because under-dosing or inappropriate dosing might also lead to an ineffective parasiticide or even leading to an increased risk that it will become ineffective, it is important to use these drugs properly. That might include re-familiarizing yourself with the instructions on the product’s label even if you’ve used the drug before.
Dr. Rob Tremblay is veterinarian for Boehringer-Ingelheim and lives near Guelph. Follow his blog at http://bovinehealth.ca.