Internal Parasites: Strategies for Effective Parasite Control
Internal parasites, or worms, are silent thieves and killers. They can cause extensive internal damage without you even realizing your animals are heavily infected. The effects of internal parasites on a horse range from a dull haircoat and unthriftiness to colic and death. Internal parasites lower the horse's resistance to infection, rob the horse of valuable nutrients , and in some cases, cause permanent damage to the internal organs. In terms of management priorities, establishing an effective parasite control program is probably second only to supplying the horse with clean, plentiful water and high- quality feed. It's that important! TYPES OF INTERNAL PARASITES There are more than 150 species of internal parasites that can infect horses. The most common and troublesome are these:
Probably the most important , in terms of health risk , are the first four: large and small strongyles, roundworms , and tapeworms. The life cycle of most internal parasites involves eggs, larvae (immature worms), and adults (mature worms). Eggs or larvae are deposited onto the ground in the manure of an infected horse. They are swallowed while the horse is grazing, and the larvae mature into adults within the horse's digestive tract (stomach or intestines). With some species of parasite, the larvae migrate out of the intestine, into other tissues or organs, before returning to the intestine and maturing into egg-laying adults. Large strongyles Large strongyles are also known as bloodworms or redworms because the larvae penetrate the lining of the bowel and migrate along the blood vessels that supply the intestines. In large numbers, these larvae can cause extensive damage, and even death. Infection with large strongyles can cause unthriftiness, weight loss, poor growth in young horses, anemia (low numbers of red blood cells), and colic. In most cases, colic caused by these parasites is relatively mild, but severe infections can result in loss of blood supply to a portion of the intestine, leading to severe and potentially fatal colic. Fortunately, large strongyles can be effectively controlled with ivermectin and related dewormers. Small strongyles Small strongyles have become a group of major importance. Unlike the large strongyles, small strongyle larvae do not penetrate the intestinal wall nor migrate through the tissues. Instead, they burrow into the lining of the intestine and remain dormant, or "encysted" (enclosed in a cyst-like structure), for several months before completing their life cycle. During this time the larvae are resistant to most dewormers. Small strongyle larvae can cause severe damage to the lining of the intestine, especially when large numbers of larvae emerge from the encysted stage all at once. Colic and diarrhea are common in heavily infected horses. These parasites also cause weight loss, slowed growth in young horses, poor coat condition, and lethargy or lack of energy. While lighter infections are not obvious, it is common for a horse's general health and performance to improve after treatment for these parasites. The early and late larval stages (before and after they burrow into the lining of the intestine) and the adult parasites are susceptible to several dewormers. But currently there are only two types of dewormer that are effective against the encysted larval stage—the stage that causes the most damage. Strategic use of these products is called larvicidal therapy, as it is targeted at the encysted larvae. Roundworms Roundworms, or ascarids, are most often a problem in young horses (especially foals, weanlings, and yearlings). Adult roundworms are several inches long and almost the width of a pencil; in large numbers they can cause blockage (or impaction) of the intestine. In addition, roundworm larvae migrate through the internal organs until they reach the lungs. They are then coughed up and swallowed back into the digestive tract to complete their life cycle. Expectant mares should be dewormed 30 days before foaling and/or at foaling to reduce the new foal’s exposure to these parasites. Roundworm infection in young horses can cause coughing, poor body condition and growth, rough coat, pot belly, and colic. Colic is most likely in older foals (over 3 months of age) that are heavily parasitized with roundworms when dewormed for the first time. By this stage the roundworms can have matured into adults that could cause an impaction. In this situation, it is a good idea to have your veterinarian deworm the foal or recommend a deworming plan for the foal. Tapeworms Until recently, tapeworms weren't considered to be a significant
problem in horses. We now know that tapeworms can cause colic, ranging
from mild cramping to severe colic that requires surgical treatment. The
tapeworm life cycle involves a tiny pasture mite, so horses with access
to pasture are at greatest risk of developing tapeworm infection. Other internal parasites Lungworms cause chronic coughing in horses, ponies, and mules. Donkeys are the natural host of this parasite, so typically they don't show any obvious signs of infection. Pinworms lay their eggs on the skin around the horse's anus. The irritation they cause makes the horse repeatedly rub its tail. Bots don't usually cause major health problems, although they can damage the lining of the stomach where they attach. Since ivermectin has become such an easy deworming medication to obtain, bots are rarely found in properly dewormed horses. They may also cause small areas of ulceration in the mouth, where the larvae burrow into the tissues for a time after the eggs are taken into the mouth. Threadworms are mostly a problem in young foals, in which they can cause diarrhea. SIGNS OF PARASITISM Contrary to popular belief, horses can have potentially dangerous numbers of internal parasites while still appearing to be relatively healthy. But in some individuals, especially young horses, parasites can take a visible toll. Common signs of parasitism include the following:
FECAL EGG COUNTS One of the most useful tools in a parasite control program is the fecal egg count—microscopic examination of fresh manure for parasite eggs. This simple test allows the veterinarian to determine which parasites are present and whether the infection is light, moderate, or heavy. This information is important in developing a deworming program for your horse or farm, and in monitoring the effectiveness of the program. Fecal egg count involves collecting two or three fresh manure balls from the horse to be tested and sending the manure sample to a veterinary laboratory. Results are expressed as eggs per gram (epg) of manure. A fecal egg count of less than 200 epg suggests a light parasite load. Horses with high fecal egg counts of 500-1000 epg suggest the interval between deworming is too long. It is important to note that a negative fecal examination does not mean the horse is free of internal parasites. Some types of parasites produce eggs only intermittently. Larvae do not produce eggs at all, and may be present in large numbers in a horse with a fecal egg count of zero. And tapeworm eggs may be missed with routine fecal egg count techniques. The results are most useful when several horses on a farm are tested on the same day. This information gives the veterinarian and farm manager a good idea of the level of parasitism on the property. DEWORMERS There are several different dewormers, or anthelmintics, currently available. Most are broad-spectrum, meaning that they are effective against several different types of parasites. It is generally best to use a broad-spectrum dewormer as the basis of your deworming program. If a specific problem is identified, such as tapeworms or encysted small strongyles, a more specific dewormer can be used. No deworming product is 100% effective in ridding every horse of all internal parasites. However, it is not necessary for a product to kill every worm in order to improve the horse's health, minimize the risk of serious disease, improve feed efficiency, and reduce pasture contamination with parasite eggs and larvae. Daily dewormers Daily dewormers can be worthwhile in grazing horses. With these products, a small quantity of dewormer is fed to the horse each day, usually in a small amount of feed. They effectively prevent new infections by larvae picked up during grazing. But they may not resolve existing infections and they do not kill bots, so they should not be relied upon as the sole method of parasite control. Treatment interval When paste or liquid dewormers are used, how often you should deworm your horse depends on the circumstances. With some products, or in some situations, it may be best to deworm your horse every month; in other circumstances, every 2 months may be adequate. With certain dewormers in good management systems, the deworming interval may be as long as 3–4 months. It is a good idea to have your veterinarian help you determine the best deworming interval for your horse. Fecal egg counts can be very useful in this regard, as well as in evaluating the effectiveness of the product you're using. Rotating products Whether or not to rotate dewormers (switch to a chemically different product every few months or every year) is a controversial topic. When the same or a chemically similar dewormer is used repeatedly for years, the parasites may become resistant to it and the product then becomes less effective, or even ineffective. However, rotating products too often could create strains of parasites that are resistant to multiple products. Whether or how often to rotate classes of dewormer is something you should discuss with your veterinarian. Methods of administration There are three main ways of administering dewormers:
All three methods are effective, provided the proper dose is given at the right time, and the horse receives the full dose. The dose must be calculated based on the horse's body weight. Weight tapes are an accurate enough way of estimating a horse's body weight for this purpose. Deworming pastes and feed additives are convenient and easy to administer. However, some horses find them unpalatable and spit them out or refuse to eat them. So be sure that all of the dose you've given is actually consumed by the horse. Tube deworming is a highly effective means of ensuring that the horse receives the proper dose because the dewormer is delivered directly into the horse's stomach. However, with the range of dewormers now available, it is seldom necessary for a veterinarian to deworm a horse by this method. DESIGNING A DEWORMING PROGRAM There are three basic types of deworming program:
Combination programs can also be used. For example, continuous deworming can be supplemented with strategic deworming for bots. There is no single deworming program that suits all horses and all situations. The ideal program for your horse(s) depends on the type, number and ages of the horses on your farm, pasture management, and your geographic location. It is best to have your regular veterinarian help you devise an appropriate deworming program for your horse or farm. Monitoring Whichever deworming product or program you use (e.g. daily vs. monthly, vs. 2- or 3-monthly deworming; single product vs. rotation of products; etc.), have your veterinarian perform fecal egg counts every 6–12 months to make sure your program is effective. The outlay of time and money will be well worth it. A COMPLETE MANAGEMENT PROGRAM Chemical control using dewormers is just one part of a complete parasite control plan. As parasites are primarily transferred through manure, good management is essential:
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