Oh, Those Worms

While there's no new science on parasites, it pays to review your deworming program.

We all know that controlling parasites through appropriate deworming is just one part of an overall management program. Still, it is a major part. In addition, resistance built up by “bugs” is not only a concern; it’s a reality. Regardless of your level of experience, it is helpful to review your parasite control program with your veterinarian and stay up to date on what is current.

Parasite Review

According to the American Association of Equine Practitioners (AAEP), there are more than 150 species of internal parasites. Robert Holland, DVM, senior veterinarian at Pfizer Animal Health, points out that bots, pinworms, small strongyles, roundworms and tapeworms pose the greatest risk to horses. There has been a reduction in large strongyles, also called bloodworms or redworms, due to widespread use of deworming products. However, they can be seen occasionally in horses that are not being dewormed, so it is still important to consider them. Large strongyles penetrate the bowel lining and travel through the intestine’s blood vessels. In large numbers, they can cause extensive damage, but ivermectin products effectively control them.

Small strongyles burrow into the intestinal lining and remain dormant, or “encysted,” for several months. Left unchecked, small strongyles can cause severe damage to the intestinal lining. In the encysted stage they are resistant to most dewormers, particularly benzimidazol, but they are susceptible in early and late larval stages. Products containing fenbendazole and moxidectin are the only ones effective against encysted small strongyles.

Roundworms, or ascarids, are most often a problem in young horses. The larvae migrate into the lungs where they are coughed up, swallowed and complete their life cycle. In large numbers, they can block the intestines. To decrease a foal’s exposure to roundworms, the AAEP suggests deworming pregnant mares 30 days before foaling or right at foaling.

Tapeworms are most common in horses with access to pasture. They settle in the ileocecal junction, which is where the small intestine, cecum and colon meet. Dr. Craig Reinemeyer of East Tennessee Clinical Research has written that an average of 54 percent of horses surveyed in the U.S. have been exposed to tapeworms. This ranges from 12.7 percent of the horses on the Pacific coast to 95.8 percent of horses in the upper Midwest. A product containing praziquantel is effective against tapeworms, and it is recommended that it be used annually.

Signs of Parasitism

It may come as a surprise to many that horses can have a dangerously high parasite load and still look relatively healthy. However, some horses (especially young ones) will show visible signs such as:

• dull, rough haircoat

• lethargy or depression

• decreased stamina

• unthriftiness

• slowed growth in young horses

• pot belly (especially in young horses)

• colic

• diarrhea

Parasite Prevalence

“Parasite resistance is a concern for all domestic animals and should be monitored,” says Holland. One way to determine the type and level of infection is through a fecal egg count done by your veterinarian. Holland recommends the McMaster’s Fecal Egg Per Gram Test over regular fecal floats, which he feels can be inaccurate. However, it is important to note that a negative count doesn’t mean the horse is parasite free. Some produce eggs intermittently, larvae don’t produce them at all, and tapeworm eggs are particularly difficult to find in horse manure.

Proper Deworming Program

You will want to consult with your veterinarian to help design a specific program to meet your farm’s needs. At what interval you deworm will depend on such factors as the number of animals, pasture management, weather conditions, and geography. Typically, using a broad-spectrum dewormer works well unless a specific problem is determined, and then that one should be targeted. The use of a daily dewormer can prevent new infections, but it may not resolve existing ones and it does not kill bots or tapeworms. Using ivermectin, moxidectin and praziquantel twice a year will take care of those.

Holland points out that they have seen some potential resistance among ascarids and small strongyles to some of the deworming products. “This is one reason that veterinarians recommend rotating among chemical classes of dewormers,” he says.

At present, the three main chemical classes of deworming products are pyrantel, benzimidazol and ivermectin. Holland is impressed with the relatively new group of products that combine ivermectin and praziquantel. “They provide safe, broad spectrum control for tapeworms, through the use of praziquantel, as well as parasites such as ascarids, bots, lung worms, pinworms, intestinal threadworms, small strongyles and large strongyles through the use of ivermectin,” he notes.

What’s New in Research?

A study conducted by German scientists, lead by Horst Zahner, PhD, investigated the effects of dewormers on the bowel walls. Results showed that the bowel walls of small strongyle-infected ponies became inflamed after using fenbendazole. It was not directly caused by the product, but rather by toxins excreted by the dying and dead larvae. Surprisingly, moxidectin ­didn’t show the same effects. They concluded there are drug-related dif­ferences in dead larvae, but they don’t know why. However, it is important to note that this reaction only occurred with a heavy infestation, and no illness was seen in any of the ponies. Craig Reinemeyer, DVM, PhD, in Tennessee, has conducted similar studies. He stressed that while there may be some inflammation, the bowel tissue of treated horses is still healthier than that of untreated horses.

Wrap Up

By working with your veterinarian on proper management, performing appropriate fecal tests and timing specific dewormers, you should be able to keep those nasty parasites at bay.






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