Parasites & Pests
Worms in Horses
Internal parasites, or worms, are silent thieves and killers. They can cause extensive internal damage without you even realizing that your horses are heavily infected. The effects of internal parasites 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 the following:
- Large strongyles (bloodworms or redworms)
- Small strongyles
- Roundworms (ascarids)
Probably the most important in terms of health risk are the first four: large and small strongyles, roundworms and tapeworms.
The lifecycle 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, as larvae, penetrate the lining of the bowel and migrate along the blood vessels that supply the intestines. Even small numbers of these larvae can cause extensive damage and possibly 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 by most available dewormers for horses.
Small strongyles have become a group of major importance. Unlike the large strongyles, small strongyle larvae do not penetrate the intestinal wall or 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 lifecycle. 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 dewormers 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, 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 lifecycle. Large infections can lead to damage to the liver or lungs due to migration of these larval forms.
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.
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 severity requiring surgical treatment. The tapeworm lifecycle involves a tiny mite as an intermediate host, and horses are at risk of developing tapeworm infection when they eat this mite in grass, hay or grain.
Until recently, no equine dewormer was approved for use against tapeworms. Praziquantel has been demonstrated to be highly effective against tapeworms. Several pharmaceutical companies have developed combination products that offer a complete antiparasitic spectrum of activity. Horses should be dewormed for tapeworms annually. Consult your veterinarian for advice on the best product to use for your situation.
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. Threadworms are mostly a problem in young foals, in which they can cause diarrhea. 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.
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:
- Dull, rough haircoat
- Lethargy (decreased energy) or depression
- Decreased stamina
- Unthriftiness or loss of condition
- Slowed growth in young horses
- Pot belly (especially in young horses)
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. High fecal egg counts of 500—1,000 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.
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 percent 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.
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.
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:
- Oral paste syringe
- Feed additive (powder, liquid or pellets)
- Nasogastric (stomach) tube
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 two basic types of deworming programs:
- Interval—deworming at regular intervals of one, two or three months, depending upon the product and the management system
- Strategic—deworming only at certain times of the year or when fecal egg counts rise
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.
Whichever deworming program or product you use (e.g., monthly, vs. two or three monthly dewormings; 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 via manure, good management is essential.
- Keep the number of horses per acre to a minimum to prevent overgrazing and reduce pasture contamination with parasite eggs and larvae.
- Pick up and dispose of manure regularly (at least twice a week, even in dirt or sand yards).
- Do not spread manure on fields to be grazed by horses; instead, compost it in a pile away from the pasture.
- Mow and harrow pastures periodically to break up manure piles and expose parasite larvae to the elements. Larvae can survive freezing, but they cannot tolerate extreme heat and drying for very long.
- Consider rotating pastures by allowing sheep or cattle to graze them, thereby interrupting the lifecycles of equine parasites.
- Keep foals and weanlings separate from yearlings and older horses to minimize the foals’ exposure to ascarids and other parasites.
- Use a feeder for hay and grain rather than feeding on the ground.
- Remove bot eggs regularly from the horse’s haircoat (flea combs work well in some instances).
- Consult your veterinarian to set up an effective deworming program for your horse(s) and monitor its effectiveness.