Internal parasite control for horses has come a long ways since the early part of the 20th Century. Veterinary texts of the early 1900s recommend concoctions of turpentine or areca palm nuts administered in linseed oil as treatments for intestinal worms. Such treatments caused their own set of problems and probably did little to kill off worm populations within the horse, yet that was all that was available to try.
In modern times, we are blessed to have several types of deworming drugs (anthelmintics) that are safe and efficacious to give to a horse. This was so much the case with newly developed safe anthelmintics in the 1980s that it was recommended to deworm every two months to attack burdens of large strongyles infecting a horse. This practice of every-two-month deworming continued on into the 21st Century.
A variety of dewormers greatly reduced the threat of large strongyles to horse health.
However, our horses currently are more impacted by small strongyles, called cyathostomins. Small strongyles encyst within the lining of the colon and set up inflammatory conditions that have the potential to wreak havoc on a horse’s intestinal tract.
Pressure from intensive drug use has stimulated both small strongyles and ascarids (round worms) to become resistant to existing dewormer products. In light of this situation, and particularly because no new classes of deworming products are in development, it is important to reduce the selection pressure for parasite resistance--this is best accomplished by limiting the frequency with which anthelmintics are given.
How Can We Minimize Resistance?
The fewer doses of dewormer administered, the less the selection pressure on worms to develop resistance. There is a simple strategy to minimize the number of doses a horse receives each year while still retaining intestinal health in your herd: Use fecal egg counts (FEC), which is a quantitative test that counts the number of internal parasite eggs in a manure sample. Fecal egg counts are best conducted at least 12-16 weeks after previously medicating with an anti-parasitic drug. This is a reasonable time interval from the last deworming dose to see re-emergence of parasite egg shedding within the horse.
The sample requires just two normally formed fecal balls as fresh as possible, preferably collected within 12 hours. If the manure is left too long, especially in warm weather, then parasite eggs will hatch and won’t be visible as eggs in the microscope--this artificially reduces the fecal egg count. The collected manure sample should be refrigerated until it is set up in the lab for evaluation.
How is a Fecal Egg Count Done and What Does it Mean?
A gram of feces is put into a precisely measured amount of concentrated salt solution and mixed thoroughly. The eggs generally float to the top of the solution. A measured sample of the manure-mixed solution is put on a special slide under the microscope to count the number of eggs per gram (EPG).
Armed with this information, we can then determine if a horse is a low shedder (less than 200 EPG), a moderate shedder (200-500 EPG), or a high shedder (greater than 500 EPG).
A moderate shedder is typically a horse that doesn’t show signs of parasitism, but it is capable of shedding moderate numbers of eggs into the environment through its feces. It is recognized that 20% of horses shed 80% of the worm eggs, so it is helpful to identify who the shedding culprits might be. The American Association of Equine Practitioners (AAEP) Parasite Control Guidelines states: “One study illustrated that if highly effective drugs are used, then treating all horses exceeding a strongyle FEC of 200 EPG only leads to treating about 50% of the horse population, but still provides about 95% reduction of the overall egg shedding.”
Worms that lay eggs as part of their life cycle include large and small strongyles and ascarids. Pinworms, bots and tapeworms won’t be represented in a fecal egg count. Because tapeworms only release eggs sporadically, they are not likely to show up in a fecal sample and testing is best done by a blood test. Pinworms remain adhered to the anus rather than being shed in the feces. Bots develop to larval stages by the time they reach the rectum and don’t lay eggs until they emerge and become flies--their eggs are visible tiny yellow spots adhered to horse hair on the face, chest and legs.
A negative (or clear) fecal egg count does not necessarily mean that your horse is free of strongyle or ascarid worms; it only means that any adult worms present are not actively shedding eggs at that point in their reproductive cycle. Furthermore, a fecal egg count doesn’t show evidence of the burden of encysted small strongyles, migrating larvae or immature ascarids within the horse.
Many horses develop decent immunity to keep their worm infestations in check. These horses are likely to be the low shedders who need deworming less frequently than a medium or high egg shedder. Once you determine which category each horse falls into, then you are able to tailor your deworming protocol specifically for each individual based on a horse’s individual susceptibility. By reducing parasite larval burdens in herd situations, particularly on pasture, you’ll reduce the amount of exposure of every horse to infective larvae.
Fecal Egg Count Reduction Testing
One of the most important elements of performing fecal egg counts is to not only perform the preliminary count before a horse is dewormed, but to also follow up with a comparative fecal egg count 10-14 days after giving deworming medication. What does this comparison tell us? The results indicate:
- Whether the frequency of dewormer given is sufficient to suppress egg-laying activity
- If there is resistance building to a particular deworming product
Ideally, there is at least a 90% reduction in fecal egg counts between the pre- and post-deworming fecal egg counts--this is known as fecal egg count reduction testing (FECRT). Your veterinarian is well equipped to offer this testing service and to interpret the results. It is possible that resistance has developed in your herd to a particular anthelmintic. It is also possible that the worms are still susceptible to anthelmintics, but their egg re-emergence period is shortened to 5-6 weeks instead of 10-12 weeks, for example. The FECRT yields information to help you develop a tailor-made deworming program for each horse.
If you have a large herd, then cost-consciousness might dictate that you don’t test every horse. Instead, test at least 5-6 members of the herd to arrive at an estimate of how well your deworming program is working.
Tailoring the Program
Parasite control programs depend in large part on the geographical location of the herd and the time of year. Warm weather is the seasonal time when the eggs develop into infective larvae and pose the greatest risk of infection. Horses in a paddock environment where the manure is cleaned up daily have a limited risk of parasite infection as compared to horses out on pasture where manure is allowed to accumulate, particularly if there is a high stocking density of horses.
Fecal egg counts arm you with the information to make necessary decisions on deworming strategies for individuals and the herd while helping to reduce the selection pressure on worms that stimulates them to develop resistance.