The effectiveness of dewormers and parasite resistance are current topics of concern in equine medicine. There are reported cases of small strongyles being resistant to many commonly used dewormers, and there are no new classes of anti-parasitic drugs on the horizon. The goal of parasite control is not to eliminate all intestinal parasites, as this is impossible. The goal of your horse farm is to keep the level of parasites to an insignificant level and decrease contamination of your farm. If we continue deworming all horses every 8 weeks, the current products will only kill the susceptible parasites, leaving the resistant ones to multiply without competition from the susceptible parasites. If we do not change our deworming strategy, eventually, the only parasites present will be only resistant ones for which we will have no drugs to kill. This has led us to change our recommendations regarding how we treat parasites in horses. The newer recommendations center around “targeted deworming” and identifying horses that are more susceptible to infection/shedding of parasites and treating them. Even horses on a deworming program or daily dewormer should be checked as they may develop parasites that are resistant to these medications requiring special treatment. Horses on the same farm may have differing levels of infection based on age, health, and immune status, so it is important to check all horses and follow up on those that appear to be infected.

HERE ARE THE BASICS:

You will need to deworm all horses twice yearly (after the last frost and again in the spring) with an ivermectin or moxidectin product to kill large strongyles and bots. In addition, these dewormers should contain praziquantel to kill tapeworms which are not always found on fecal examination.

We are recommending fecal egg counts be done on all horses in October PRIOR to giving any dewormers. This will help to identify horses who are heavily infected and may require continual deworming. It is estimated that only 20% of horses will be “heavy shedders” and need to continue on a regular deworming program every 8 – 12 weeks throughout the year. These horses should also have an additional fecal egg count 2 weeks after deworming to determine if the parasites are being effectively killed by the dewormer selected.

It is estimated that 80% of horses will fall into the not-infected or “low shedder” category. In general, these horses do not require deworming beyond twice yearly. Throughout the year, these horses should have fecal egg counts checked every 8 – 12 weeks to see if additional deworming is needed.

Horses under 24 months of age should be routinely dewormed every 8 weeks due to risk of roundworm infection.

Please start by submitting a sample (one manure ball per horse is adequate) in a plastic bag or container labeled with the horse’s name, your name & date. The sample can be refrigerated but should be <4 hours old and not frozen. The best time to get this sample would be in the spring prior to giving any dewormers. It must be a minimum of 8 weeks after giving any ivermectin product or 12 weeks after giving moxidectin. You can bring these samples to the office, or we would be happy to collect them during your horse’s spring visit.