Controlling Strongyle Parasites of Horses

Commonly used strategies for parasite control in adult horses are based largely on knowledge and concepts that are more than 40 years old. However, much has changed over this time necessitating a re-examination of recommendations for parasite control. In response to this need, the AAEP has formed a Task Force charged with producing a comprehensive set of recommendations for helping veterinarians develop improved strategies and programs for parasite control in horses of all ages. Guidelines will be specified separately for adult and young horses (less than 3 years). Recommendations developed in this document are based on the following: 1. Important changes in the parasitic fauna of horses have occurred such that Strongylus vulgaris and other large strongyles are now rare, and cyathostomins (small strongyles) and tapeworms are now the major parasites of concern in adult horses, while Parascaris spp. remains the most important parasite infecting foals and weanlings. 2. Anthelmintic resistance is highly prevalent in cyathostomins and Parascaris spp., and this must be factored into treatment decisions (Kaplan and Nielsen, 2010). 3. Adult horses vary greatly in their innate susceptibility to infection with cyathostomins and their level of strongyle egg shedding and thus, require individualized attention to their parasite control needs. 4. Horses less than about 3 years of age require special attention as they are more susceptible to parasite infection, and are more at risk for developing disease. This article will detail the separate approach taken for parasite control in this age group. AAEP Parasite Control Guidelines Introduction Traditional parasite control programs involving rotational treatment with anthelmintics at regular intervals are commonly recommended by veterinarians. However, this approach is based on concepts and strategies developed more than 40 years ago when Strongylus vulgaris (large strongyle bloodworm) was the most important parasitic pathogen of horses (Drudge and Lyons, 1966). The rationale for this parasite control scheme was rather simple: to kill S. vulgaris worms before they could mature and lay eggs that would contaminate the environment. Since it took about two months for strongyle eggs to reappear after treatment, treatment every two months prevented S. vulgaris eggs from being shed on pastures. This approach was very successful in controlling S. vulgaris infections, and disease from S. vulgaris is now very rare in managed horse populations. It is noteworthy that cyathostomins (small strongyles), were not considered important pathogens at that time, as their pathogenic potential was over-shadowed by S. vulgaris. However, that situation has changed and currently, cyathostomins (small strongyles), are recognized as a primary equine parasite pathogen (Love et al., 1999). Similarly, Parascaris spp. is recognized as a major parasitic pathogen in foals and weanlings, and Anoplocephala perfoliata has been recognized as a cause of ileal colic in the horse (Nielsen, 2016a). The biology, life-cycles and host-parasite dynamics of the cyathostomins, A. perfoliata and Parascaris spp. are very different from S. vulgaris, thus strategies designed for controlling S. vulgaris will not be appropriate or very effective for controlling these parasites. Decades of frequent anthelmintic use have selected for high levels of anthelmintic drug resistance in cyathostomin and Parascaris spp. populations (Peregrine et al., 2014), which emphasizes that the traditional approaches for parasite control are not sustainable and that new strategies are needed. Cyathostomins are truly ubiquitous, and all grazing horses are infected. But they are relatively mild pathogens and only produce disease when infections reach extremely high levels. Thus disease from strongyle parasites is much less of a concern in adult horses today than it was decades ago when S. vulgaris was highly prevalent. Frequent anthelmintic treatments are therefore not needed to keep adult horses healthy. What is needed are properly timed treatments with effective anthelmintics administered at the appropriate time of the year, which correspond to the epidemiological cycles of transmission and the relative parasite burdens in individual horses. In this document we aim to provide the information necessary to implement parasite control programs for adult horses based on the best available evidence.

STRONGYLE EGG SHEDDING/CONTAMINATION POTENTIAL Although horses grazing together share the same parasite population, they demonstrate huge differences in their levels of strongyle egg shedding. Within any group of mature horses (> 3 yrs. of age), strongyle egg counts are highly concentrated in certain horses, such that 15 – 30% of adult horses usually shed approximately 80% of the eggs. This distribution of parasite egg shedding among hosts is common to all species and is referred to as over-dispersion. This characteristic for a horse is very stable over time, when it is otherwise in good health, pasture management practices are sound, and the horse has not recently moved from one farm to another. Thus, a healthy pastured horse with a low egg shedding potential will tend to always have a low FEC, while a healthy pastured horse with a high egg shedding potential will tend to always have a high FEC (Nielsen et al., 2006; Becher et al., 2010). In order to determine the egg shedding potential for an individual horse, it is necessary to collect a fecal sample and perform a fecal egg count (FEC) after the effects of the last dewormer administered are completely gone. If you do not wait a suitable period of time following treatment, then the results of the FEC will only reflect the efficacy of the last dewormer used, rather than measuring the innate ability of the horse’s immune system to regulate levels of cyathostomin egg shedding. Studies have illustrated that parasites reduce their egg shedding outside the grazing season, where conditions are less favorable for parasite transmission (Poynter, 1954). This indicates that FECs may be less reliable in cold winter months (northern climates) and during hot, dry summers (southern climates). To evaluate the egg shedding status in adult horses (> 3 yrs. of age) a fecal sample should be collected a minimum of 4 weeks beyond the Egg Reappearance Period (ERP) for the last drug used. After Moxidectin (ERP = 10-12 weeks): Wait ≥ 16 weeks to collect a fecal After Ivermectin (ERP = 6-8 weeks): Wait ≥ 12 weeks to collect a fecal. After benzimidazoles (fenbendazole/oxibendazole) or pyrantel (ERP = 4-5 wks): Wait ≥ 9 weeks to collect a fecal. There are little data available for scientifically setting the FEC thresholds used for dividing adult horses into low, moderate and high categories for egg shedding. However, one study reported that strongyle FEC cutoff values up to the level of 500 EPG yielded significantly different strongyle worm counts, whereas no differences were found at higher cutoffs. These data support usage of cutoffs for treatment in the 0-500 EPG range (Nielsen et al., 2010a). Nonetheless, currently recommended thresholds are based largely on the opinions of a majority of equine parasitologists, and as such could change as more data are collected and analyzed. Guidelines for classifying horses on the basis of egg contamination potential are presented in table 4. Table 4. Suggested guidelines for classifying horses into different levels of strongyle egg shedding and the expected percentage of the horse population belonging to each group (Kaplan and Nielsen, 2010). Egg count level Percentage of adult populationa Low contaminators: 0-200 EPG 50-75 Moderate contaminators: 200-500 EPG 10-20 High contaminators: >500 EPG 15-30 a These values are only estimates and the actual percentage of horses in each category will vary among farms depending on a multitude of factors It is generally advised to classify adult horses to the three strongyle contaminative groups based on more than just one egg count performed at one point in time. In a Danish study where FEC were performed every six months over three years, greater than 90% of horses with FEC < 200 epg on two consecutive fecal exams had a FEC of less than 200 EPG on the third (Nielsen et al., 2006). Thus, it appears that egg shedding categories for most horses remain consistent, but some horses may switch categories, particularly those with FEC near the cutoff values. GOALS OF PARASITE CONTROL The true goal of parasite control in horses (and other equids) is to limit parasite infections so animals remain healthy and clinical illness does not develop. The goal is NOT to eradicate all parasites from a particular individual. Not only is eradication impossible to achieve, the inevitable result is accelerated development of parasite drug resistance. In addition, both small and large strongyles cause the greatest disease during their larval stages, which are refractory to most anthelmintic treatments. Consequently, most treatments that kill only adult worms yield limited direct benefit to the horse. However, treatments effective against adult stages have an indirect benefit in that they prevent further contamination of the environment with infective stages. The resulting corollary is that to achieve good parasite control, one must prevent contamination of the living environment of a horse or horses with high numbers of parasite eggs and larvae. Thus, treatments should be timed to control the level of egg shedding into the environment. This relies on the use of deworming medications that are effective for their intended use. But treatments are only necessary when the environmental conditions are conducive to egg and larval development and survival. If strongyle eggs and developing larvae will be rapidly killed by the adverse environmental conditions (such as hot summers) (Nielsen et al., 2007), then little is gained by deworming the horse if the horse is not showing any clinical symptoms of parasitic disease. The goal of any parasite control program can therefore be summarized as follows: 1. To minimize the risk of parasitic disease. 2. To control parasite egg shedding. 3. To maintain efficacious drugs and avoid further development of anthelmintic resistance as much as possible. To achieve these goals, it is important to know the magnitude of egg shedding of individual horses. This information can only be generated by performing periodic FEC surveillance. As noted above, the acceptable limits of strongyle EPG for a horse remain debated, and the egg shedding status of a horse may change over time as a result of changes in the horse’s immune status and level of parasite exposure. In addition, no exact guidelines have been published regarding the “acceptable” number of Parascaris spp. eggs in young horses. However, even with these limitations in our knowledge, the magnitude of the FEC is the only means available to estimate the worm burden and egg contamination potential of a horse, and determine the effectiveness of anthelmintics. Consequently performing FEC surveillance is necessary to properly develop and monitor any parasite control program. FECAL SAMPLING AND FECAL EXAMINATION There is a large number of techniques available for generating fecal egg counts in equines, and Appendix A provides protocols for two of the most widely used techniques. Automated smartphone-based egg counting systems are currently under development and will be made commercially available to veterinarians in 2016.


• To evaluate the anthelmintic efficacy using the FECRT.
• To evaluate and monitor the egg reappearance period (ERP) of the most recently administered dewormer.
• To determine the shedding status of the horse at the time of sampling.
• To determine whether parasite burdens in foals and weanlings are primarily Parascaris spp. or strongyle. LIMITATIONS OF FEC
• They do not accurately reflect the total adult strongyle or Parascaris spp. burden of the horse.
• They do not detect immature or larval stages of parasites including migrating large strongyles and ascarids, and/or encysted cyathostomins.
• Tapeworm infections are often missed or underestimated by fecal techniques.
• Pinworm eggs are usually missed since they are adhered as egg packets around the anus rather than being shed in the feces.


Environment-based approaches Equine strongyle parasites begin life as an egg in a manure pile, which then must develop to infective larvae in the feces, get out onto the pasture, and then be ingested by a horse. Thus, infection of horses could be prevented if all feces were promptly removed from the pasture. In a bygone era, the most elite stables employed pasture grooms, who followed grazing horses with a scoop shovel and a broom. Their job was to remove manure as quickly as it was dropped. In the 1980s, a similar approach was evaluated using updated technology. Studies at Newmarket in Great Britain examined the efficacy of cleaning horse pastures with a large commercial vacuum unit that was originally designed for golf course maintenance. Twice weekly vacuuming was demonstrated to control pasture infectivity more effectively than routine deworming (Herd, 1986). However, the cost of the vacuum units was prohibitively expensive for the average horse owner, and the process only worked well on level, relatively dry pastures. Despite this, several commercial devices are now available for cleaning pastures, and these have found use on many horse farms. Environmental Control Eggs hatch and develop into infective larvae under conditions of moderate temperature and moisture. Cold slows the rate of development or stops it altogether, and excessive heat kills eggs and larvae. It is possible to heat manure sufficiently to kill the parasites, including even ascarid eggs (Gould et al., 2012). Proper composting of manure and soiled bedding will generate relatively high internal temperatures, and strongyle larvae in manure are virtually eradicated by exposure to temperatures over 40 ºC for a minimum of one week (Gould et al., 2012). Composting is a practice that should already be in place at any stable. Non-composted horse manure should never be spread on pastures as this will increase the level of parasite contamination. This practice has been associated with higher Parascaris spp. prevalences in Germany (Fritzen et al., 2010). Leaving pastures unoccupied for several months of the year may or may not reduce the risk of infection depending on the time of the year and the climate where the farm is located. Infective strongyle larvae (L3) can survive for only a few days to a few weeks in hot weather (temperatures around 40˚C), but for as many as six to nine months during colder weather (Nielsen et al., 2007). Consequently, L3 survival in the environment will vary greatly from region to region and season to season. Thus, strategies for environmental control must be made based on local conditions. Strongyle infective third-stage (L3) larvae can survive in wide extremes of weather and climate, but there are sets of conditions that are optimal and sets of conditions where development and/or survival are poor (Table 5). Therefore, it is recommended to focus anthelmintic treatments at times of the year that are most optimal for larval development, i.e. when transmission of strongyles is most likely. Doing so will reduce pasture contamination with infective stages, thereby decreasing the acquisition of new infections. In addition, a time when the transmission is likely is also the time of year when adequate refugia are present, thus selection pressure for anthelmintic resistance is theoretically lessened. Conversely, it is recommended to avoid or limit treatments of equine strongyles during the winter months in cold temperate climates and during summer months in warm/hot climates (times of low refugia), in order to reduce the development of anthelmintic resistance.