Fecal Egg Count Exams offer critical information

Knowing How to Interpret a Horse's Results is an Important First Step in Designing an Effective Parasite Protection Plan

For many horse owners and managers, a fecal egg count exam is just one part of the usual equine healthcare services provided by their veterinarians. This one simple, inexpensive test can provide a wealth of information about general health and the effectiveness of a deworming program. However, even with the critical facts it provides, the fecal egg count exam also has significant limitations; knowing how to correctly interpret results is a key step in using them to design, or alter, a farm's parasite protection plan.

"The fecal egg count exam (FEC) is really the gold standard when it comes to testing for parasitic infection in the horse," said John Donecker, VMD, MS, Dipl ABVP (equine) -- Senior Veterinarian at Pfizer Animal Health. "Unfortunately, however, a number of factors can influence the test results, making it a less than completely precise measure of the parasite burden in any one horse, or in the herd."

Why do horses shed parasite eggs?
In the middle of the 20th century -- before the invention of many modern classes of dewormers -- horse owners were primarily concerned with large strongyles. Known by their scientific name, Strongylus vulgaris, or more commonly as "bloodworms", these then prevalent parasites could cause massive damage to the horse's internal organs -- even creating a condition known as thromboembolic colic, which resulted in death.

Today, however, large strongyle infections are rare in properly managed horse herds because the deworming options we have now are more than capable of killing adult, and immature migrating larval stages of this parasite. As a result, most of the parasite eggs seen on an FEC exam come from one of many small strongyle species -- collectively known as cyathostomins.

There are 52 known species of equine cyathostomins (small strongyles). However, only 12 species are widely prevalent and they represent the largest percentage of the parasite burden carried by most managed horses in the U.S. today. As a result, a positive fecal egg count is a probable indicator of infection with one or more of these 12 species. In the case of horses that have not been regularly treated for parasites, the positive result may also indicate infection with ascarids, large strongyles or resistant types of cyathostomins (small strongyles).

The FEC measures only the presence of eggs from any of these -- or many other -- parasites in the horse's manure. It cannot, however, specifically reveal what species of strongyle parasite is living inside the horse. More precise tests for different species of parasites require veterinary techniques such as DNA probes and PCR-ELISA analysis. However, this is unnecessary for most circumstances.

Interpreting the Fecal Egg Count Exam
When a veterinarian performs a fecal egg count (FEC) exam, they are looking to establish how many parasite eggs are present in each gram of manure tested -- yielding a result called "eggs per gram", or EPG.

There are no universal standard EPG measures considered "good" or "bad". However, traditionally a horse with an EPG value of 200 Ð 500 would be considered a candidate for treatment. In a herd, an average EPG value between all horses of 100 Ð 300 has long been considered the threshold between normal and unusual levels of parasite infestation.

It's important to realize, however, that these measures were established more than 50 years ago when large strongyles were common and a 200 EPG rating could indicate a dangerous infection level with this potentially deadly parasite. Today, FEC results showing 200 EPG are sometimes considered low and relatively insignificant because they represent a limited amount of cyathostomin (small strongyle) infestation, which is not always associated with clinical disease.

A veterinarian might also use an FEC to decide how effective a deworming program is for a horse or a whole herd. For example, if a horse starts with an EPG value of 200 - 500 before treatment is given, the veterinarian will perform a second FEC test several days after a deworming is given (there is a different interval between these two tests based on the dewormer used) and will expect to see a significant reduction in the second EPG value to indicate that the deworming was effective.

Unfortunately, over reliance on the results of fecal egg count exams can be misleading. For example, heavily parasitized horses with certain types of infections may have a negative or very low FEC. By contrast, adult horses receiving good management may be very healthy and normal despite an FEC in the hundreds of EPG.

Limitations of the FEC
When interpreting the results of a fecal egg count exam, it's critical to keep in mind the limitations of the test itself, and the factors that may influence its outcome. For example, cyathostomin (small strongyles) in North America shed eggs in a very seasonal pattern. As a result, EPG values will increase in the spring for horses living in the northern tier state climates. These readings will then peak in the late summer and autumn, and decline in the winter. In states south of the northern tier states , this pattern is precisely the opposite, with the peak occurring in the winter months.

Another important factor that affects FEC results is something known as the "hypobiotic cycle". Cyathostomin (small strongyle) larvae infest pastures and horses eat them when grazing. After they enter the horse, these parasites invade the tissue of the large intestine and burrow in -- they become "encysted" cyathostomin (small strongyles). Once they are encysted, small strongyles can become dormant, sitting in the intestinal tissue for months or even years. This arrested or inhibited development is called "hypobiosis".

A horse with a substantial number of cyathostomin (small strongyles) in hypobiosis may still have a negative or low EPG because these inhibited parasites are not shedding eggs. Additionally, when a horse is given an effective dose of a dewormer, it will kill off the adult strongyles. The absence of the adults created in the intestines, however, will send a signal to the inhibited cyathostomin (small strongyles) to break out of their cysts, become adults and start producing eggs. As a result, effective parasite treatment may occasionally increase a horse's EPG.

Increasing a horse's, or a herd's, exposure to infective cyathostomin (small strongyle) larvae is one sure-fire way to increase the likelihood of high EPG values. Strongyle exposure can occur in a number of ways, including overstocking horses on pastures too small for their numbers, poor pasture management, inadequate sanitation, failure to treat parasitized horses, failure to isolate at-risk horses (such as yearlings and incoming horses), and suboptimal anthelmintic dosing.

Implications of FEC for Parasite Protection
Fortunately, total eradication of fecal egg shedding in horses on pastures is not a necessary goal for a parasite protection plan. This is good news, because it's also nearly an impossible goal to reach. In fact, there is little evidence to show that a continuous zero-EPG standard is needed for the normal development of foals or the well-being of adult horses.

Parasitologists generally agree that the proper objective of parasite control is to maintain the parasite burden at a low level, rather than to eliminate parasites entirely. This middle ground avoids over-treatment, limits the cost of parasite control and helps horses maintain partial immunity to overwhelming infection. In other words, it's a good idea to allow a very low level of parasite infection so that horses' immune systems can learn to deal with these invaders if they occur in larger numbers.

Daily administration of Strongid® C or C 2X(TM) (pyrantel tartrate) works by preventing the build-up of intestinal parasites that often occurs between dosings of traditional, purge dewormers. At the same time, Strongid C/C 2X allows the very low levels of parasite infection that maintain natural immunity in the horse. This method is particularly useful in herds of horses where exposure to cyathostomin (small strongyles) occurs frequently.

For more information on effective parasite protection, and educational articles about parasitism, visit Pfizer Animal Health online at www.pfizer.com/equine.

Pfizer Animal Health, a world leader in animal health, is passionate about the health and wellness of animals, and is dedicated to improving the safety, quality and productivity of the food supply, enhancing the health of livestock and poultry, and helping pets live longer, healthier lives. For additional information on Pfizer's portfolio of equine products, visit www.pfizer.com/equine.

Article information sourced from Pfizer Animal Health Technical Bulletin: Significance of Fecal Egg Counts in Managing Equine Intestinal Parasites. Donecker, Holland and Conder. May 2007.