Equine Newsletter – September 2018
INTRO TO DEWORMING
Treating horses for gastrointestinal parasites is one of the most commonly debated topics of horse ownership. This is likely compounded by the fact that the recommendations that horse owners receive from their veterinarians have changed over the last several years as we learn more about resistance in equine parasites. Deworming is a vast topic with many different aspects that should be considered prior to deciding on a deworming program that is right for your horse. To help our clients to become better informed we have decided to make common parasites and dewormers the topic of our first Equine Newsletter.
SMALL STRONGYLES
Horses will become infected by small strongyles by eating larvae out on pasture. Clinical disease associated with small strongyle infestation is related to these larval life stages. Horses will consume early stage larvae on pasture. Once these larvae have been ingested they will burrow into the horse’s large intestine. Within the wall of the large intestine the larvae will create a cyst in which they continue to develop. When the larvae are mature they will erupt from the intestinal wall to continue their development into adults. Clinical disease is most often associated with the encysted larvae and their subsequent eruption from the intestinal wall.
Only the adult parasite will produce the eggs that we observe while performing a fecal egg count. As I said above, the adults are not the life stage that we are concerned about causing disease, but we must use the number of adult parasites as a rough estimate of the number of larval parasites because the only way to accurately determine the number of larval parasites is to take a biopsy of the intestinal wall.
TAPEWORMS
Horses also become infected by tapeworms while out on pasture. We aren’t 100% sure of what disease, if any, tapeworms cause in horses but one study in the UK did demonstrate an association between tapeworm infestation and colic. It is difficult to determine the presence of a tapeworm infestation as a routine fecal egg count cannot detect tapeworms. As a result, it is currently my recommendation that horses are dewormed for tapeworms once a year. Adult tapeworms are not sensitive to the most commonly used dewormer, so we must ensure that your horse’s deworming protocol is specifically designed to address the possibility of tapeworm infestations. The best time of year to deworm your horse for tapeworms is in the late fall after the first solid frost.
SOME AVAILABLE DEWORMERS
IVERMECTIN
A very commonly used dewormer that has been on the market for a long time. It is sold under a variety of brand names, including Eqvalan, Bimectin and Panomec. Ivermectin will kill the late larval and adult stages of small strongyles. After one treatment with ivermectin the number of strongyle eggs in a horse’s fecal egg count should remain low for 6-8 weeks. Resistance to ivermectin has been documented in a number of different equine parasites, but resistance is not yet wide spread in small strongyles.
MOXIDECTIN
The same class of drug as ivermectin but it is a newer product. It is currently sold under the brand name Quest. Moxidectin will kill multiple larval and adult stages of small strongyles. As it affects a wider range of life stages it will keep small strongyle egg counts down for longer than ivermectin. After one treatment with moxidectin you can expect small strongyle eggs in a fecal egg count to be decreased for 12 weeks.
PYRANTEL PAMOATE
Another medication that has been on the market for a long time. It is sold under the brand names Strongid and Exodus. Pyrantel only has efficacy against small strongyle adults. As such it will only keep small strongyle egg counts down for 4-5 weeks following treatment. In addition to its shorter duration of action there is documented evidence of small strongyle resistance to pyrantel in Ontario. Fortunately, Pyrantel also has activity against adult tapeworms. It is important to note that if you are giving Pyrantel to address potential tapeworm infestation it has to be given at twice the standard dose.
PRAZIQUANTEL
Most often sold in combination with ivermectin or moxidectin such as in Eqvalan Gold or Quest Plus. Praziquantel has activity against adult tapeworms but not small strongyles.
You might also be interested in our Fact Sheets about Pre-Purchase Examinations and Equine Emergencies.
For more helpful resources, check out our Spinal Manipulation Therapy and How to Prepare pages.