Because of the potential for anthelmintic resistance, deworming decisions should be based on fecal egg counts, not just the calendar.

Parasites in horse establishments have most likely been overtreated in recent decades. The advent of highly efficacious drug formulations has led the equine industry to rely too much on anthelmintic treatment. We now know this excessive reliance has led to increasing levels of resistance to the drugs available, and recommendations now are based upon systematic surveillance and a much less intensive treatment regimen.

In this article I will highlight the most important points and concepts in parasite control. For more details watch the  archived Webinar on this topic. Also, watch our April 27 Ask The Vet Live event on parasite basics and deworming on (sponsored by Intervet/Schering-Plough Animal Health, featuring this author).

Exposure to Parasites

All horses are exposed to parasites, regardless of how often we treat them. Even in horses not receiving anthelmintic treatment at all, the risk of actual parasitic disease is very low. No single parasite species is a horrible horse killer, and your horse has most likely been infected with most of them without anybody noticing. In more than 99% of horses harboring considerable parasite burdens, there are no symptoms of parasitic disease.

A relevant analogy is bacteria. Some bacteria, such as Salmonella or Clostridium, can kill a horse, yet they can be found in healthy horses. Despite this, we don't treat all horses with antibiotics at regular intervals because of the risk of antibiotic resistance and unwanted reactions.

So why are we treating horses so aggressively for parasites?

Parasites should be viewed as a natural state, and we should recognize that even frequent treatments will not prevent them completely. Our goal is not to eliminate parasites, as history has shown us this is impossible and only leads to anthelmintic resistance. Instead, our goal should be to reduce parasite contamination levels in the environment and thereby keep parasite burdens under control.

It's not the parasites' interest to kill their hosts or even make them sick. Without a thriving host, they have no existence. However, parasites can cause disease in individual cases. The mechanisms behind this are not well-understood, but overstocking and subsequent buildup of too-large parasite burdens or compromised immune systems in individuals play major roles. Horses can become immunocompromised due to bacterial or viral infection or high stress levels. With weakened immune systems, horses can become more vulnerable to parasites.

Keeping Horses Parasite-Free?

An often encountered misconception is that horses and pastures can be kept parasite-free. It is important to recognize that wherever horses graze there will be parasites, and if we start looking for worms we will find them. The finding of any particular parasite should, therefore, not be alarming.

In addition, no drug is 100% effective. Even in the absence of resistance, there will always be worms surviving treatments. No drugs efficiently affect all parasite stages within the horse. Moreover, the greater part of the parasite population is not residing inside the horse. Eggs and larvae live in the fecal piles on pasture and can survive about a year if winters are cold. Even after a seemingly effective deworming of all horses on the premises, the majority of the parasite population will be left unaffected on the pasture.

The parasites will never disappear, no matter what we do to them. We can affect the levels of parasite contamination, but we cannot eradicate the parasites.

Do Horses Benefit From Parasites?

Recent evidence strongly suggests that horses benefit widely from moderate to large parasite burdens. A constant presence of parasites will serve as a vaccination, rendering horses less susceptible to future infections due to a well-developed immune response. Studies have shown horses exposed to excessive anthelmintic treatments for one grazing season were more vulnerable to parasite infections later in life.

Even more interesting is the apparent relationship between allergies/ autoimmune disease and worm burdens. It appears that in humans the general lack of intestinal parasites in the Western world might be responsible for the high levels of allergic disease we see. People in developing countries are exposed to all kinds of parasites, but they do not have allergic problems.

The possible explanation is that intestinal parasites stimulate the same branch of the immune system that is overreacting in cases of allergic disease. Similar hypotheses exist in horses and scientists are currently investigating them. If this explanation turns out to be true, frequent anthelmintic treatments might cause more harm than good.

Designing a Parasite Program

In brief, you must consider three questions to develop a good strategy:

Which anthelmintics are still effective? It is important to make sure that you use effective drugs only. A fecal egg count reduction test is the only method to determine this. This requires two fecal samples from a number of horses; one taken the day of treatment and one taken 14 days later.

Which individuals require minimal, moderate, or intensive anthelmintic treatment? Horses tend to maintain the same level of egg shedding over time, even without anthelmintic intervention. Around 20% will shed high egg numbers, often more than 1,000 eggs per gram (EPG) of feces. Around 30% will be moderate egg shedders in the range of 200-1,000 EPG. The remainder (50% or more) are low egg shedders, shedding 0-200 EPG. These three groups can be treated differently.

What are the climatic conditions in your area? Parasite transmission is seasonal, and in most climates the calendar year can be divided into a parasite season and an offseason. In the offseasons, deworming is not indicated unless a horse is showing signs of parasitic disease. In southern warm climates, the hot summers can be considered the offseason, whereas in northern climates it is during the cold winters that no parasite transmission occurs. Thus, spring and fall will most often mark the beginning and ending of the active parasite season, and most treatments should be concentrated around these two seasons.

What Parasites Are Important?

About 100 different parasite species can infect the horse, and each horse will harbor a good representation of different types. As different drugs can be considered for different parasites, it is very useful to know which parasites are present on each premise. Some parasites are more likely to be detected in certain age groups, and below you can find a brief outline on relevant parasites to consider in each age group.

Foals/Weanlings (0-1 years old)

Parascaris equorum (roundworm, or ascarid) This is the main parasitic threat in foals. It is most prevalent in foals 1-6 months old. At the time of weaning, roundworm infection becomes less prevalent due to the development of immunity against these parasites. Roundworms can cause ill-thrift, diarrhea, and colic, so anthelmintic treatments are indicated within the first six months of life. It is very important to test for resistance, as roundworm populations have been reported resistant to at least ivermectin/moxidectin and pyrantel.

Cyathostominae (cyathostomin, or small strongyle) This parasite type can be present in foals as well, but it typically does not play a major role for the first six months. At weaning, strongyle burdens will start becoming high, and treatments targeting this parasite group should be considered. Resistance testing is required, as strongyle populations have been reported resistant to all drug types on the market.

Strongyloides westeri (threadworm) This parasite can be found in very young foals, typically less than 1 month old. Although foals of this age group often will have episodes of diarrhea, there is no evidence suggesting Strongyloides plays a role.

Adolescents (1-3 years old)

This is an age group in which large worm burdens can develop.

Cyathostominae (cyathostomin, or small strongyle) Disease caused by these parasites is most often reported in this age group, and these horses should be the major target of a deworming program. Consider three treatments during the grazing season, and test for drug resistance each year.

Anoplocephala perfoliata (tapeworm) These are reported more often in this age group than in older horses, and treatments with a tapeworm drug should be considered. The best time to administer is by the end of the grazing season. Generally, about 50% of horses harbor tapeworms.

Parascaris equorum (roundworm, or ascarid) can still be found in this age group, but prevalence is much lower than in foals. When present, roundworms are less numerous, and they only rarely constitute a major health threat.

Adults (4-20 years old)

Main parasite groups that affect these ages are strongyles and tapeworms, but these parasites tend to be less prevalent, and worm burdens are considerably smaller. As a result, many horses maintain very low or even negligible (0) egg counts. These horses will not require anything more than just one or two annual treatments, if at all. On the other hand, a few horses will harbor large worm numbers and shed large numbers of eggs. It is important to identify these shedders and treat them aggressively. Always remember to perform annual screenings for drug resistance.

Geriatrics (more than 20 years old)

It is often believed that old horses are prone to having more parasites due to loss of immune status. In my work with horses, I have never seen this documented. In fact, I have often encountered the opposite, where the oldest horses had the lowest egg counts. If your geriatric horse is in good body condition and appears to be thriving, there is no reason to believe he is more at risk for acquiring heavy parasite loads than other horses. So unless your geriatric horse is showing ill thrift, you should follow the adult horse recommendations.

Take-Home Message

Disease caused by parasites is rare in horses, but younger horses are more at risk. An immunosuppressed horse can be more vulnerable to parasites, as well. So keep an eye on your horse's well-being and consider relevant testing and treatment if he is showing symptoms of ill thrift.

About the Author

Martin Krarup Nielsen, DVM, PhD, Dipl. ACVM

Martin Krarup Nielsen, DVM, PhD, Dipl. ACVM, is an associate professor of parasitology and the Schlaikjer professor in equine infectious disease at the University of Kentucky's Maxwell H. Gluck Equine Research Center, in Lexington. His research focus includes parasite diagnostic measures and drug resistance. Known as a foremost expert in the field of equine parasites, Nielsen chaired the American Association of Equine Practitioners’ (AAEP) parasite control task force, which produced the “AAEP Parasite Control Guidelines.”

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