Instead of just deworming all the time, it’s a good idea for equine practitioners to perform periodic fecal egg counts to determine how badly—or whether—a horse is affected by parasites prior to being dewormed. However, fecal egg counts do not necessarily reflect the horse’s status regarding four other important internal parasites: tapeworms, bots, ascarids, and pinworms.
These “other” worms are a bit different than the worms for which fecal egg counts typically are recommended. They have different life cycles and do not lay as many eggs, so fecal counts are less useful.
1. Bots (Gasterophilus spp.)
Depending on the part of the country, big, annoying, bee-looking flies may be found buzzing around horses and driving everyone crazy. Bot flies lay their yellow eggs on the horse’s lower limbs in the summer, the horse licks the eggs off, and adult bots develop and attach inside the horse’s stomach.
Bots, while annoying, rarely are associated with any real health problems for horses. However, when bot flies enter the horse’s rectum, they can cause itching, straining, colic, increased frequency of defecation, and even prolapse of the rectum. Clipping the hair off horses’ lower legs and scrubbing the limbs to remove bot eggs can help prevent infestations, but this practice requires persistence.
Most commonly, bot treatment is recommended once a year, in late fall or early winter (or as is commonly said, 30 days after the first frost). Ivermectin and moxidectin work well in this regard, and deworming at this time also helps decrease transmission in the following year. Of course, other parasites may be killed, as well.
2. Tapeworms (Anoplocephala perfoliate)
It’s difficult to estimate the true prevalence of tapeworms in horses. Some areas of the U.S. have reported a prevalence of as much as 50 percent, but surveys haven’t been conducted across the country, so the true incidence of infection is a bit of a question mark. Tapeworms pass through mites that live in moist pastures, so horses that live in arid climates are unlikely to be affected.
Only fairly recently has it been reported that tapeworms can cause colic in horses; studies in Sweden and the Netherlands concluded that the association is likely. Even so, when horses colic as a result of tapeworm infestations, it’s due to heavy parasite loads. Conversely, light infestations are unlikely to cause much harm.
Diagnosis of tapeworm infestations can be problematic. Equine tapeworms lay relatively few eggs, and, as a result, tapeworm infections are difficult to diagnose using common egg-count methods. Still, if tapeworm eggs are found in fecal testing, the test should be interpreted as being positive, and treatment should be initiated accordingly. Serum testing is available; however, horses test positive in serum even after parasites have been expelled.
Deworming products that contain praziquantel or a double dose of pyrantel pamoate are recommended treatments for tapeworms. Because infections are so difficult to diagnose, a single annual treatment after it gets cold may not be a bad idea, especially if a horse lives in a moist pasture. If serum testing is elected, and results are low or negative titer, treatment may not be necessary.
3. Pinworms (Oxyuris equi)
Pinworms are not a common problem, and they don’t really cause serious health problems in most horses. However, in more severe cases, when the adult worm crawls out of the rectum and lays its eggs on the horse’s perineal area, the eggs can cause an intense itching. When infected horses rub, they spread their eggs onto walls, feeders, or anything else they use to scratch the itch. Parasite transmission occurs through contact with these surfaces, as well as through contact with brushes, tail wraps, and the like. These eggs are pretty tough and can survive in the environment for a long time.
Pinworm eggs don’t commonly show up in fecal exams, either. The more sensitive tests are the “Scotch tape test,” where examiners wrap Scotch tape around their fingers, sticky side out, and try to affix eggs to the tape (similar to a lint roller picking up lint or hair from clothing). Scrapings of the area around the horse’s rectum may be useful, as well.
While reports exist of pinworm resistance to deworming agents, most of the common dewormers retain their effectiveness. Deworming pastes also may be applied directly into the rectum in an attempt to get higher levels of dewormer concentrated in the area of infection. Thorough scrubbing of the horse’s hindquarters helps to remove eggs that the female worm may deposit. Afterward, thoroughly clean the items used to wash affected horses.
4. Ascarids (Parascaris equorum)
Ascarids cause the most mischief in foals. After these eggs are ingested, they hatch and develop into larvae, which can migrate through the foal, causing the animal to appear sick and grow poorly. Larvae in the foal’s lungs cause inflammation, cough, and nasal discharge; larvae in the intestines can cause small intestine blockage, as well as colic, and even lead to death (usually from intestinal rupture). Deworming schedules for ascarids are based on the foal’s environment and how many foals are present in a population.
Benzimidazole-type drugs are preferred to those that paralyze worms (such as ivermectin), as it is possible to block the foal’s gut with paralyzed worms. In addition, ivermectin and moxidectin simply don’t work well on ascarids. Ascarid eggs can remain infectious for a few years.
The importance of ascarids in young horses, their rapid life cycle, and their low level of egg shedding mean that many owners may choose not to bother with doing fecal exams on foals, particular in breeding operations. Ascarids can be seen in adults occasionally; however, they are rare, because adult horses become immune to them.
When it comes to parasite control, of course, every situation differs. Check with local university parasitologists to secure specific information.
Dr. David W. Ramey is a Southern California equine practitioner who specializes in the care and treatment of pleasure horses. Visit his website at doctorramey.com. Columnists’ opinions do not necessarily reflect those of Veterinary Practice News.