Deworming

Deworming Strategies
One of the most common questions that we are asked is “Doc, what deworming rotation should I use?” The answer to this question is becoming more and more difficult even for veterinarians to answer. Many of the strategies that we were taught in vet school are no longer recommended due to increasing parasite resistance, and many of the common dewormers that have been around for many years are no longer very effective against many parasites.

The most common parasites in the horse are large and small strongyles, roundworms, pinworms, bots, and tapeworms. Horses are constantly exposed to parasites through the environment. The majority of adult horses with a healthy immune system will have a low parasite burden; however , there are some horses out there that are considered to be “high shedders”. These horses do not seem to have the same immune resistance to parasites and often are a source of parasites for other horses. Another factor that affects parasite burden is climate. Parasites are less infective in dry, cold or very hot climates so the times of the year when the parasites are most infective varies depending on location. In Colorado parasites are less likely to be a problem during November-March (depending on the weather that year).

There are two main objectives in parasite control- to minimize a horse’s level of exposure to parasites in the environment and to decrease the internal parasites in the horse. Minimizing exposure is through environmental control – picking up manure from pens, keeping pastures in good shape through low stocking rates or rotation. Decreasing the internal parasite load is done by deworming. Since parasite resistance is becoming a problem and some classes of dewormers are not as effective due to overuse, there is a concern that the currently most effective dewormers – ivermectin and moxidectin, will become ineffective in time as well. In fact there have been a few documented cases of some common parasites being resistant to ivermectin.

A good test your veterinarian can perform to know if your horses are high or low shedders, or if they need to be dewormed is a Fecal Egg Count (FEC). To perform a FEC, a small amount of fresh manure is obtained and processed in a specific way. The sample is looked at under a microscope and the number and type of parasite eggs are counted to get the number of eggs per gram of feces. Horses with egg counts of 200 or below are considered normal. The information obtained can also help decide which dewormer should be used and if there is parasite resistance to a specific dewormer on your property. Horses that are classified as low shedders may only need to be dewormed twice a year, where as high shedders need to be done more frequently.

Deworming schedules will vary depending on the individual horse and its environment so you should talk to your veterinarian about what is right for you and your horse.
In summary here are some of the strategies that we should be using: 1) Determine which horses need the most frequent deworming by using fecal egg counts, 2) determine which parasites are present in what amounts, 3) use targeted and effective dewormers at appropriate doses in these horses (no underdosing!), 3) Skip deworming or elongate the deworming interval in the horses with low fecal egg counts, 4) stop using ineffective dewormers, 5) time deworming with the seasons to maximize effectiveness, 6) manage manure and pastures to minimize contamination with eggs.

If you have any questions please do not hesitate to contact me or your regular veterinarian.

Shannon Harland DVM