Theories and rumors abound about deworming, covering anything from worms developing resistance to newer dewormers to the old-timer's belief that chewing tobacco kills worms. (It doesn't.) Rotational deworming seems to be the latest concept causing owners to scratch their heads. "Should I rotate dewormers? If so, how often? Do worms develop resistance over time?"
Actually, deworming is not difficult to understand. It's important to know the strengths and weaknesses of available dewormers. And it involves learning about the complicated life cycles of some unpleasant parasites. But if you know what worms your horse can have and understand how they affect it, deworming makes sense.
Let's start with one of the parasites a horse gets as a foal -- the large round-worm, or ascarid. It's a problem because its eggs have a thick, protective shell that helps them remain in the environment for several years. Once a foal ingests an egg it takes 10 to 12 weeks of the parasite to develop. The immature worms (larvae) migrate through the liver and lungs, where they are coughed up, reswallowed and passed to the small intestine. During this migration, the ascarid can damage the lungs, thus making the foal susceptible to pneumonia.
Once the ascarids reach the adult stage in the intestine -- they can be 20 inches long and 1/4 inch thick -- they lay eggs that are passed in the feces. The risk at this stage is that they rob nutrients from the foal, leading to unthriftiness and poor growth, and can lead to intestinal blockage and even rupture.
Interestingly enough, after exposure to the large roundworm, horses develop a protective immunity to this parasite; therefore horses past one year of age seldom have this infection.
Another group of parasites are large stongyles or bloodworms. Strongylus vulgaris and Strongylus edentatus are the two most common species of this group. Large strongyles differ from small strongyles in size and in life cycle.
Horses become infected with bloodworms by ingesting larvae from the environment. In the case of S. Vulgaris, the larvae migrate through blood vessels until they reach the branch of the aorta that supplies blood to the intestinal tract. The immature worms develop in the artery, and after six months return to the intestine to become egg-laying adult worms. This causes an irritation to the arterial walls, resulting inflammation and possible aneurysm, leading to colic.
The larvae of S. Edentatus also migrate out of the intestine and spend 11 months developing in the tissues of the liver, spleen and other organs.
A common parasitic infection is cyathostomes, the scientific name for small strongyles. The ingested larvae form cysts in the gut wall.
Horse owners all know what a bother it is to remove the tiny yellow botfly eggs on their horse's legs, shoulders and throatlatch. However, it is an important task. If the eggs aren't removed, with warm temperatures and the presence of moisture, the eggs hatch and the larvae enter the horse's mouth.
There, they migrate through the mouth -- often causing sores around the teeth and on the tongue -- to the stomach, where they attach to the stomach lining. The bot larvae remain in the stomach for the rest of the summer, fall and winter, where they may cause gastric ulcers and, in extreme cases, rupture the stomach wall.
As spring approaches, the larvae detach and pass out of the horse in the manure. The bot larvae then pupate and emerge as adult flies.
Tapeworms have become more prevalent in recent years. Recent postmortem observations in Kentucky revealed an equine tapeworm incidence of 54 percent in contrast to earlier reports of 18 percent and 26 percent incidence.
If you see your horse rubbing his tail on the fence, there's a good chance it has pinworms. Pinworms are located in the large and small colon of the horse, and once the female worms migrate out of the anus, they deposit eggs in a sticky liquid in the perineal area of the horse. This causes irritation, and, as a result, the horse rubs its rear end against solid objects. Intense rubbing causes a roughened tail head that is cosmetically objectionable and undesirable.
The intestinal threadworm, common in foals, is the first intestinal parasite to appear in foals; eggs may appear in the feces at about two weeks old.
Diarrhea is the most common symptom of threadworm infection, and, at this age, can be mistaken for foal-heat diarrhea (and vice versa).
Lungworm infections are rare, but young horses can be seriously affected. This worm lives in the bronchi and trachea, causing a persistent cough and respiratory complications.
There are several skin problems your horse can get from biting insects.
One is summer sores, which are caused by part of the cycle of the stomach worm. Flies deposit stomach worm larvae in skin lesions and some body areas, thus causing summers sores.
Another internal parasite, the neck threadworm, requires gnats for transmission of infection. Its microscopic larvae are found in the skin, usually on the horse's chest. This causes dermatitis, or summer itch. Biting gnats pick up the larvae from the affect skin and transmit the infection to other horses.
Specific parasites require specific action. You may need ivermectin for bots, but a pyrantel product for small strongyles.
Since ascarids are a problem in young horses, be sure ascarid control is in place for sucklings, weanlings, and yearlings.
If intestinal threadworms have been diagnosed as a problem, incorporate a product labeled for Strongyloides westeri in the first deworming at two months old and possible again at four months.
Where tapeworms are a concern, include a tapeworm treatment at the end of the grazing season. There are no products labeled for tapeworm control, so this treatment must be prescribed by a veterinarian.
If the fly season is long, or if bots have been a particular problem, consider another bot treatment in mid-summer.
The larvae of strongyles are known to be a threat to equine health, so use a larvacidal treatment in late fall (November or December).
With all of these parasites, rotational deworming is the best way to keep your horse worm-free. But there is controversy on how to rotate. One recommendation is to rotate from one chemical class to another each year; another is to rotate from one class to another with each deworming.
A fast rotation -- changing the class with each deworming -- builds on the strengths and minimizes the weaknesses of each class of dewormers.
Have your veterinarian recommend which rotation and which classes of dewormers are best for your horse.