If you prevent overgrazing and allow grass to grow at optimal levels you'll help minimize parasite infestations in your herd.

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.

How to Take a Fecal Egg Count

If you only own one or two horses, are low risk, and you don’t use drugs known to have resistance problems, a yearly fecal count will let you know if your program is adequate. If you do use dewormers known to possibly induce resistance, are higher than low-risk status, or have more than two horses, a yearly fecal count is almost a must to prevent problems from developing. To be of any use, samples must be collected properly, stored properly, and examined shortly after collection. Otherwise, eggs may hatch and the larvae can be destroyed by many commonly used fecal egg count procedures, which could give you an incorrect result. Never send fecal samples through the mail. It’s best to collect the samples yourself (plus it saves a farm-call fee) and take them to your vet’s office to minimize delays in examining the sample. Using a disposable latex exam glove, available in any drug store, pick up a fecal ball from a pile of freshly passed manure. Take the glove off, turning it inside out and trapping the fecal ball inside in the process. Place the glove containing the fecal ball into either a small container with a lid or a zippered sandwich bag, and keep it cool on the way to the vet’s office.

What’s Your Risk?
Because parasites can’t be completely eliminated, every horse is at risk. How great that risk is depends on the strength of the horse’s immune response and the level of exposure. Even a healthy horse can be overwhelmed by exposure to a large number of parasites.

While zero parasite infestation isn’t realistic, it is important to do what you can to avoid heavy exposure, such as:

Pick up manure piles in stalls and paddocks regularly.
Avoid overcrowding on turnout (minimum of 1 acre per horse).
Do not allow pastures to become overgrazed. Horses eating short grass are more likely to pick up parasite larvae, and hungry horses will graze closer to manure piles, where larvae are concentrated.
Don’t allow your horse to graze along heavily traveled trails, or when away from home in areas where there have been a lot of horses, such as showgrounds.
Feed hay and grain from feeders and bunkers rather than off the ground. Whenever possible, clean up spilled grain and hay pulled out of feeders before the horses do.

In general, your horse is at low risk if none of the following factors apply, moderate risk if one is true, and high risk if more than one is true.

Horse is under the age of 2, older than mid-teens, or is under stress.

Pastures are crowded or overgrazed.

Paddocks or stalls are not kept clean of manure.

New horses are introduced into the herd without knowing their parasite status.

Horses are rotated through paddocks used by other horses of unknown parasite status.

Horse is allowed to graze or eat from the ground when in areas heavily traveled by horses.

Horse is dewormed using drugs other than ivermectin or moxidectin without doing periodic fecal egg counts.

Deworming Strategies
Since resistance develops after exposure to deworming drugs, it’s time to take a hard look at practices like routinely deworming on a set schedule.

Even the common practice of rotating dewormers frequently, which was originally thought to help

prevent resistance, is coming into question. For example, worms can become resistant to drugs in the benzimidazole family (fenbendazole, oxibendazole currently on the market) after only a few exposures. This has led to more heavy reliance on drugs like pyrantels, but resistance to them is now being found also.

Be aware also that rotating dewormers isn’t as simple as changing brands. As the table on page 14 shows, many different brand names can contain exactly the same active ingredient.

Your vet is the best person to give you detailed advice on your specific situation, but below are some examples of deworming programs.

Low Risk

Plan A

Check fecal egg counts two to four times/year, deworm only if positive.

Deworm once a year, after grazing season, for bots and tapeworms (which often don’t show up on fecals) using a combination of praziquantel with moxidectin or ivermectin.

Sample: November – deworm with praziquantel product. March — fecal. June — fecal. September or October — fecal.

If using dewormer other than ivermectin or moxidectin, recheck a fecal after any needed deworming to be sure the drug is working.

Plan B

Deworm once a year, after grazing season, for bots and tapeworms using a combination of praziquantel with moxidectin or ivermectin.

Cluster other dewormings around time of greatest risk, which is grazing season.

Sample: November – deworm with praziquantel product. April — deworm. Mid-June — deworm. End-September — deworm. (Note: Infective strongyle larvae have a longer lifespan in spring and fall than they do in the peak of summer heat. If you live in an area that supports year-round grazing, speak with your vet about modifying this schedule.)

Check fecal at least once a year, before the November deworming, to be sure this schedule is working well.

A good parasite control program is important at any age, but is especially important in foals and weanlings. Their underdeveloped immune systems make them especially vulnerable to infection.

Moderate Risk

Same as Plan B, above, but add another fecal egg count check eight weeks after your November deworming if the dewormer was an ivermectin/praziquantel or 12 weeks after if a moxidectin/praziquantel dewormer was used. This is to check for emergence and maturation of any very early stage small strongyles, which neither moxidectin nor ivermectin will kill.

High Risk

Check fecals, or deworm, at intervals appropriate for the last deworming drug used, i.e., six to eight weeks after ivermectin, 10 to 12 weeks  after moxidectin, four weeks for all other drugs. If using a daily dewormer, check fecals at least twice a year.

Deworm twice a year, at the beginning and end of grazing season, using a praziquantel and ivermectin or a moxidectin product.

If rotating using drugs that can induce resistance, check fecals twice a year, four weeks after the horse has been dewormed with that product, right before your next scheduled deworming.

Sample A: November — deworm with ivermectin/praziquantel. January — fecal or deworm with ivermectin. March – fecal or deworm with ivermectin. May — deworm with praziquantel/ivermectin. July — fecal or deworm with ivermectin. September — fecal or deworm with ivermectin. Note: If you substitute a non-ivermectin product (other than moxidectin) for any of these dewormings, you will have to do a fecal check, or deworm, in four weeks instead of eight.

Sample B: November — deworm with moxidectin/praziquantel. February — fecal or deworm with moxidectin. May — deworm with moxidectin/praziquantel. August — fecal or deworm with moxidectin. Note: If you substitute ivermectin for any of the moxidectin dewormings, you will need to do fecal or deworm in eight weeks instead of 12. If you substitute a non-ivermectin product, you will need to do a fecal or repeat in four weeks instead of 12.

Any horse showing clinical signs that could indicate parasitism, such as poor coat, poor hooves, slow growth, trouble holding weight, a pot-bellied appearance, colic or change in manure, should be checked by fecal egg counts.

To Drag or Not to Drag

Dragging pastures — the practice of pulling a chain or harrow through the field to break up manure piles — definitely has pros and cons. Early developmental stages of strongyles are very sensitive to drying, so disrupting piles of feces can result in their death. However, the stage that is actually capable of infecting a horse (called L3) has a protective coating and won’t be bothered by low moisture. This means that dragging can actually spread these infective larvae around. Whether killing off younger forms before they can mature to infective larvae outweighs the risk of spreading infective ones is debatable. However, since horses instinctively will not graze around manure anyway, it may be wiser to just leave nature as is. If dragging pastures to spread manure as fertilizer is part of your pasture management plan, simply take some precautions. The best time to drag pastures is when temperatures are high. Under those conditions, larvae mature to the infective stage rapidly, quickly use up their stored energy supplies, and die. Do your dragging in late summer, and remove horses from dragged pastures for 10 days to two weeks if you live below the Mason-Dixon line; four weeks for states north of this line.

Also, you need to take special care with foals because of their high susceptibility to parasites. Begin by deworming the mare a month before foaling to minimize the parasites she is carrying. Move her to her foaling stall only after that deworming. Within 24 hours of foaling, deworm her with ivermectin to prevent strongyloides larvae in the milk.

Whenever possible, turn mares and foals out in separate fields/paddocks that have not been used for high-risk horses for at least a year. Start foals on a regular deworming schedule between 1 and 2 months of age. If using drugs known to have resistance problems, check fecals regularly.

Another important part of your deworming strategy it to forgo turning out new horses with your present herd until you know the new horses’ parasite status. Have a fecal egg count done on any new horse if he was not dewormed recently, or if he was dewormed with anything except ivermectin or moxidectin. (No resistant strains have emerged to these two drugs yet.) If the horse has parasites, deworm him with whatever drug you are currently using. Be sure to repeat the fecal egg count after deworming to make sure the parasites he is carrying aren’t resistant. Your vet will advise you how long to wait after deworming to recheck, which depends on the product you are using.

If the horse had recently been dewormed with ivermectin or moxidectin, find out the date of the treatment and do a fecal egg count six weeks after treatment with ivermectin, or 10 to 12 weeks after moxidectin. If the egg count is positive, follow the procedure above — deworming the horse with the drug you are currently using, but being sure to recheck the fecal if you use anything except ivermectin or moxidectin.

With parasites, it’s the right strategy for your situation that will best serve you and your horse.