Protecting your horse from intestinal parasites involves more than timed dewormings or a daily dose of dewormer. Maintaining a healthy immune system, understanding high-risk situations, and correct choice of deworming drugs are all-important.
Your horse's immune system does much of the work in defending him against parasites. This has important implications for when, and how often, you should deworm.
Healthy adult horses often have a strong resistance to infestation by intestinal parasites. They develop this over time through exposure to the parasites. Their immunity can be so strong that many healthy adults will have extremely low worm burdens.
Before the FDA grants approval for the sale of a dewormer, the drug must be tested to make sure it is effective. Most deworming agents work only for the time the drug and the parasites are in contact. One exception is moxidectin (Quest), which may remain in the horse's body tissues longer than the other drugs.
Defending Against Parasites
- Determine your horse's risk factor.
- Follow farm management practices to reduce parasites.
- Pay particular attention to very young and very old horses because they may have poor parasite immunity.
- Do regular fecal egg counts - at least annually.
- Be aware of drug resistances that parasites can develop and choose your dewormer accordingly.
However, deworming a horse that really doesn't need it is not only a waste of money, but parasitologists are now warning that it can be contributing unnecessarily to the growing number of drug-resistant parasites. Totally eliminating parasites simply isn't realistic. They are too good at surviving and produce eggs by the thousands. (See the table on page 12.) Even if you are meticulous about maintaining a clean environment for your horse, you won't be able to reduce his risk to zero. Every time he puts his head to the ground away from home, he runs the risk of picking up parasites.
Parasitologists suggest that instead of deworming healthy adults at set intervals, we should be checking fecal egg counts to determine if the horse really needs it. The reason is that every time a population of parasites is exposed to a deworming drug, we run the risk of strains emerging that are resistant to the drug. Many parasites are already resistant to dewormers currently on the market. (See the table on page 14.)
A very different situation exists with young horses. Because they have never been exposed to parasites, they lack the strong immunity of healthy adults. Until they are at least 2 years of age, deworming schedules that work well for adults may not be as effective with youngsters, and the presence of any drug-resistant parasites in the environment complicates things even further. Heavy parasite burdens in young horses result in poor growth, poor coats, a distended belly, and damage to the lungs because the immature larvae of some parasites migrate through lung tissue.
Older horses also often have poor immunity to intestinal parasites. What constitutes "older" will vary from horse to horse, so you need to be on the lookout for the telltale signs of not holding weight well, a bloated appearance, poor coat, and possibly digestive upset or even colic. Horses that have suffered severe illnesses or injuries, surgery, shipping, or are under a lot of stress from competitions, as well as horses in a poor nutritional state, are also likely to be more susceptible to intestinal parasites.
Most Worrisome Worms
Though we like to protect our horses against all parasites, which ones are the worst? The most dangerous species used to be the large strongyles, also called bloodworms. These caused extensive damage to the intestine and even the blood vessels.
The advent of easy access to highly effective paste dewormers, especially ivermectin, has all but eliminated that problem. The two biggest troublemakers now are small strongyles and tapeworms.
Small strongyles, like bloodworms, tunnel into the intestinal wall, where they can become dormant inside of protective cysts. The only drugs effective against these forms are moxidectin and double-dose fenbendazole, with fenbendazole resistance a growing problem in some areas. The later life stages may also be resistant to several drugs.
Horses that accumulate large numbers of these encysted early life stages can become severely ill. They can suffer from weight loss, diarrhea and colic if the larvae mature and emerge from their cysts en masse. Small strongyles tend to do that in winter because it allows the parasites to mature to egg-laying stages in time for best pasture conditions in the spring.
As paste dewormers became more effective against other types of parasites, tapeworms, which are not sensitive to most dewormers, became more important. They rarely cause any obvious problems when present in small numbers, but large amounts can cause impaction and a telescoping of one section of the bowel inside another, called intussusception.
