The unseasonably warm weather this winter has been a bonus for equestrians and their horses--easier stable management, and more time in the saddle. It has also been a bonus for a parasite lurking in the pastures. Warm weather and an absence of freezing temperatures make prime living conditions for oribatid (forage) mites. These tiny arthropods (related to ticks) live in the grasses and fields and are consumed by a grazing horse. Why is this important? Turns out, the oribatid mite is the natural intermediate host for tapeworms, and in particular the most notorious tapeworm species present in the United States, Anoplocephala perfoliata.
The cycle starts like this:
- A horse with tapeworms passes manure containing tapeworm eggs onto the pasture.
- Forage mites ingest the tapeworm eggs.
- The tapeworms develop into infective larvae within the body of the mite
- A horse grazing on pasture inadvertently ingests the mites along with grass.
- The infective larvae develop into adult tapeworms within the horse’s intestinal tract.
- There they wreak damage as well as excrete eggs to pass in the manure, thereby perpetuating the cycle.
The problem for the horse arises when the tapeworm larvae attach their hooked mouths near the junction of the small intestine (ileum) and the cecum. These parasites are small, about an inch long, shaped like a pumpkin seed. Damage to the lining of the cecum causes inflammation, bowel dysfunction and abnormal intestinal contractions (peristalsis), leading to possible impaction colic or an intussuseption. An intussuseption is the result of one portion of the intestine telescoping into another section, usually the ileum folding into the cecum. Blood flow is obstructed with the potential for portions of the bowel to become necrotic and die. Not only does the horse exhibit extreme colic pain but also this is a life-threatening condition, requiring immediate surgery.
Recently in Kentucky, the veterinarians at Hagyard Equine Medical Institute reported a more than usual number of these cases have presented for surgery. The forage mites are prospering in the unusually warm winter temperatures, so horses are more at risk of exposure and infection. The good news is that this is a preventable situation simply by deworming regularly for tapeworms, particularly for horses turned out on pasture. While not all horses infected with tapeworms develop such a serious surgical colic, it is estimated that tapeworms might be responsible for 22% of gas or spasmodic colic cases.
It is difficult to identify tapeworm eggs in fecal samples so deworming on a regular basis is the preventive strategy of choice. In parts of the country where the climate remains above freezing year-round, it might be necessary to deworm against tapeworms a couple of times a year. In other areas where freezing temperatures normally occur during the winter, it might only be necessary to deworm once a year. Depending on location, equine incidence of infestation ranges from 17% on the Pacific Coast to 95% in the Midwest.
Have a conversation with your veterinarian about timing and frequency to administer the appropriate deworming medication to your horses in your particular geographical location. The drug of choice against tapeworms is praziquantel. It is often combined with another dewormer like ivermectin or moxidectin. (If you own dogs or cats, you may recognize praziquantel as the drug called Droncit.) Praziquantel is 95% effective at killing Anoplocephala perfoliata. It is a safe drug to use in horses, including pregnant or nursing mares.