Live and let die

Tight quarters, infrequent access to pastures and crowded stables all increase the risk of parasites

By Amy Hayek Jones, D.V.M.

When it comes to parasites in horses, domestication is a mixed blessing. To be sure, the companion horse may be provided complete nutrition, veterinary care and shelter, but some of those creature comforts can actually work to increase the animal's parasite load. Stresses such as work, tight quarters, infrequent access to plentiful pastures and crowded stable conditions all increase your horse's risk of being infected with various types of parasites.

Deworming is the best way to keep these infections at a manageable level-the rate at which the horse can live with parasites without a severe health risk.

How you accomplish the goal of keeping parasites to a manageable level depends on the horse's environment. Horses living where temperatures are cold enough to kill some overwintering worms have lower rates of infection the following summer, when worms thrive. Horses living in warmer climates have the added challenge of being infected by worm eggs and larvae year-round, and therefore require a completely different medical protocol.

Bear in mind, however, that horses-no matter how northerly the climate-living in crowded conditions will have higher infection rates because the amount of worm eggs and larvae being deposited on the ground increases with each horse.

The issue of crowding isn't confined only to stabled animals. Even horses in a pasture with plenty of thick green grass might be too crowded if they can't eat in one section of the pasture and defecate in another. While not all horses are neat in the pasture, most are so inclined; they will defecate in a designated area of the pasture and eat elsewhere.

When this is not possible, infection and re-infection with worms increases. Dragging the pasture regularly (weekly or every two weeks) helps to break up piles of manure, effectively exposing worm eggs to sunlight, which kills a significant percentage of the larvae.

Daily stall cleaning likewise removes worm eggs and larvae, thereby reducing the chance for infection.

If your horse is stabled in an environment where its risk of parasitical infection is increased, you should deworm frequently and rotate deworming medications on a set schedule.

For instance, if you live in the North with freezing winters, but your horse is stabled in a confined area with several other horses, you need to deworm every three to four months; fewer horses means spring and fall dosings should suffice.

Dosages increase with geography: If your horse is stabled in the South with many horses, then you should deworm every month. Stabled with fewer horses? Every six weeks should do the job.

Some decisions regarding de-worming schedules depend on the type of medication you choose. Since the goal is to keep the worm population at a manageable number, sometimes it is better to use a dewormer that targets one type of parasite rather than try to kill all of them in a single dose.

Foals are not born with immunity to parasites, so it's important to deworm foals regularly and more frequently than adult horses. Deworm-ing the mare soon after the foal's birth reduces the number of eggs in the immediate environment and reduces the risk of infection.

The foal should be dewormed monthly, beginning when it is a month old. Use a dewormer that contains oxibendazole; Anthelcide is a good choice. Be sure to dose according to the foal's weight, keeping in mind foals are mostly legs and don't weigh more than 100 to 150 pounds at this age.

Increase the dose as the foal grows, rotating dewormers each month and using boticides at the appropriate times of year. Continue this schedule until the foal is six months old, when it can be deworm-ed on the same schedule as the adult horses. Check with your veterinarian for his or her recommendations before deworming these youngsters: Some dewormers are not appropriate for immature horses.

Immunosuppressed horses, horses that have lost large amounts of weight, horses with dry, dull, poorly shed coats or those exhibiting pot bellies are probably supporting a large number of parasites. Your vet can confirm such a pasture diagnosis. You can pinpoint which type of anthelmin-tic to use by having him do a fecal exam to isolate the type of parasite causing the problem.

Vets generally begin with a de-wormer that kills a broad spectrum of parasites; it makes sense to deworm heavily infected horses gradually, as large loads of dead parasites can cause worm impactions resulting in colic and septicemia, which can cause founder.

Several dewormings will be needed. Horses in poor condition should be given fenbendazole (Panacur) for encysted strongyles, as well. The recommended dosage for this is five days in a row. Follow this procedure with one dose of ivermectin or moxidectin to kill the remaining worms and bots. Read the label for complete instructions before each dose.

Anytime you add a new horse to your herd, you should deworm it before it arrives to reduce the number of worms it brings on the property.

Daily dewormers are effective in helping underweight horses gain weight. They are also effective in reducing the amount of feed needed to maintain a healthy weight for normal horses. In situations where crowding is a problem, daily dewormers help reduce the risks of continual re-infection.

Even when horses are dewormed on a regular basis, owners often find white, segmented worms in the feces. These are tapeworms. The only effective treatment known for tapes is a double dose of pyrantel pamoate (Strongid T, also known as Strongid paste). A one-time treatment generally is effective in removing tapeworms. Daily dewormers are effective against tapeworms, too.

Pinworms can cause horses to rub their tails until the hair falls out. Most paste wormers are effective in killing pinworms, but if the problem persists following deworming with the appropriate dose, the cause may not be pinworms. Biting gnats and midges can cause a similar problem.

Parasites don't just sit in the horse's intestines. Many migrate through the tissues during their stay with your horse, which is why certain deworming medications are recommended at specific times of year. Migrations are related to season, weather and sunlight duration. When deworming with paste wormers, rotating medications on a set schedule is important.

As with any medical program, check with your vet first. He will help you develop an effective, safe deworming program.