Equine Internal Parasites

Historically, it was too arid and too hot for internal parasites to survive and multiply to numbers that would be of concern for horses living in Arizona and other desert climates. Microclimates including lawns, increased irrigation, crowded housing/pens and an increase in the number of “backyard horses” has, in recent years, exponentially increased the parasite populations affecting desert horses. Once only a minor concern, desert horse owners need to become aware of these potentially significant threats.

The recommended deworming schedule established by the AAEP (American Association of Equine Practitioners) about forty years ago had been nationally accepted and was, until very recently, still the schedule that most veterinarians followed. That schedule included a rotational deworming system of three different classes of dewormer (ivermectin, pyrantel, fenbendazole) given once every 2 months, year-round. Forty years ago, the primary offender was the large strongyle worm (Strongylus vulgaris). The idea behind the rotational system was to destroy the various life-stages of this worm with different drugs aimed at different stages and this system did effectively eliminate the large strongyle from being a serious threat. However, what the rotational system left behind is a group of worms that are more drug- resistant, thus requiring an overhaul of the AAEP’s recommendations. These new recommendations were released in 2010.

To best understand the recommendations, we have to review the most common internal parasites. And we need to understand that geography, environment, life-stage and individuality may necessitate changes to the posted recommendations. For example, we know through several studies, that in any given horse population, 20% of the horses shed over 80% of the parasite eggs. Therefore, each individual should be tested via fecal exam.

Though the large strongyle is of little concern now, several newcomers have taken its place on the list of more significant pathogens. The Small Strongyle worm burrows into the lining of the intestinal wall and remain dormant while they complete their life cycle. At this stage, they are resistant to most dewormers. When a mass of adult worms emerge at once, serious damage can result and the horse may colic.

Roundworms (ascarids) are most common in foals, weanlings and yearlings. Adult roundworms migrate throughout the gastrointestinal tract, the lungs, liver and potentially cause significant damage tissue damage if the infection is severe. Other signs of infection can include coughing, poor body condition and growth, rough coat, pot belly, diarrhea, impaction (of worms in the intestine) and colic.

Tapeworms are also a more recent growing concern. They are now recognized as a cause of colic. The severity ranges from mild cramping to severe colic due to impaction requiring surgical treatment.

Other internal parasites include lungworms that cause chronic coughing, threadworms that are mostly a concern in foals (diarrhea) and pinworms which lay their eggs around the horse’s anus and cause intense itching resulting in the horse repeatedly rubbing its tail. Bots (fly larvae) are very common but tend to cause minimal health concerns.

General signs of parasitism include a dull, rough haircoat, lethargy or decreased stamina, unthriftiness, slowed growth, colic, diarrhea and in young horses, a pot belly. The most effective way to diagnose parasites is via a fecal egg count. This involves collection of fresh feces and submission of the sample to your veterinarian who will have it analyzed at a laboratory. The fecal egg count will identify and quantify the types of parasites with which your horse is infected and how severely.

Most horses have some parasite eggs shed into the feces, but an egg count will let your veterinarian know which dewormer is most appropriate. Tapeworms, specifically, do not shed a high number of eggs into the feces (as compared to other worms.) Therefore, your horse may be infected with tapeworms without a positive fecal sample and it is recommended that all horses are dewormed once yearly for tapeworms, in addition to whatever is required based on the fecal egg count.
Though you should discuss your individual horse’s deworming protocol with your veterinarian, in general, your horse should be dewormed a minimum of two times per year following a fecal exam and depending on the fecal egg counts. At least one of these dewormings should include praziquantal (for tapeworms). Per the AAEP, pregnant mares should also be dewormed with ivermectin the day after foaling. Foals should be dewormed at one, three and five months of age with a fecal exam performed at the first deworming. Any horse with a chronic illness or severe stress may need a modified deworming schedule. Your veterinarian will help you to determine an appropriate protocol.

Remember that while no dewormer is a guarantee to rid your horse of all internal parasites, even the lessening of the parasite load will improve your horse’s general health, lower the risk of serious medical conditions, and decrease the pasture/pen contamination of parasite eggs and immature parasite stages

Maintaining excellent husbandry will help decrease parasite levels within your horse’s environment. Disposing of manure, rather than allowing it to pile up or use it to spread over other pastures that livestock will use, has been shown to greatly reduce parasite levels. Prevent overcrowding in pens or pastures, segregate foals and dams, and weanlings from yearlings and other adults, use feeders rather than feeding on the ground and regularly remove visible bot eggs from the hair coat. Ask your veterinarian if you have any questions about establishing a deworming protocol into your wellness program.

Robin Paterson, D.V.M.
Cerbat Cliffs Animal Hospital
4110 Stockton Hill Road
Kingman, AZ 86409