Parasite Control in Horses

Globally, anthelmintic resistance is widespread in parasites harbored by managed horses.

• Cyathostomins (small strongyles) are widely resistant to two of the three major drug classes (benzimidazoles and pyrimidines). Resistance to the third drug class, the macrocyclic lactones, is spreading rapidly as well.

• Ascarids (large roundworms, Parascaris spp) are widely resistant to macrocyclic lactones, and resistance to the two other anthelmintic classes is also developing.

• Pinworms (Oxyuris equi) are widely resistant to macrocyclic lactones, and some veterinarians report apparent treatment failure of the two other anthelmintic classes as well.

• Equine tapeworms (Anoplocephala perfoliata) have been found to be resistant to both anthelmintic classes available for tapeworm control—praziquantel and pyrantel pamoate—and this resistance might be developing into a global phenomenon.

Equine parasites have developed resistance to all anthelmintic classes available for horses, and multidrug resistance is becoming increasingly common in several parasite categories. Thus, no single products can be assumed to be broadly effective against a range of important equine parasites, and routine efficacy testing is required to ensure that effective products are used.

The main intestinal parasite targets of an equine parasite control program are tapeworms, strongyles (large and small), and ascarids.

Tapeworms are common in grazing horses around the world and are documented causes of colic related to the ileocecal region, the junction between the small and large intestines where these parasites tend to cluster. Accumulation of parasites in this region can cause impactions of the ileum and intussusception (telescoping) of the ileum into the cecum. Both conditions lead to colic and often require surgery.

All grazing horses harbor cyathostomins (small strongyles), and no anthelmintic product is capable of completely eliminating all parasitic stages of these nematodes. The disease caused by their larval stages (larval cyathostominosis) is characterized by protein-losing typhlocolitis and diarrhea; however, this disease is very rare, considering the ubiquitous occurrence of these parasites.

All foals are exposed to ascarid infection; typically, patent infection can be detected in approximately 50% of a given foal crop 3–7 months old. The primary health concern associated with these parasites is small intestinal impaction, which often requires surgery and is associated with a guarded prognosis.

The equine bloodworm, Strongylus vulgaris, is a large strongyle considered to be the most pathogenic GI helminth that infects horses, causing nonstrangulating intestinal infarctions. This parasite has become rare and is most often completely absent from managed equine populations. However, a reemergence of S vulgaris and associated pathology and clinical signs has been documented in countries where anthelmintics are used by prescription only, demonstrating the importance of continued awareness of this parasite when anthelmintic treatment use is decreased.

Fecal egg counts (FECs) provide important and necessary information about resident parasite populations in horses.

It is imperative to routinely evaluate anthelmintic treatment efficacy using the fecal egg count reduction test (FECRT). Detailed instructions for carrying out an FECRT can be found in the AAEP's Internal Parasite Control Guidelines.

FECs also provide crucial information about the presence of patent ascarid infections in young horses and the strongylid shedding levels of mature horses. Modified techniques enable reliable detection of equine tapeworm eggs in fecal samples; in light of findings of apparent anthelmintic resistance in these parasites, FEC monitoring is becoming important for tapeworms as well.

It should be emphasized that FECs are not clinical diagnostic tools. Egg counts do not correlate with adult worm burdens, and pathogenic larval stages do not produce eggs. Furthermore, horses can harbor relatively large parasite burdens and display positive FECs without being clinically or subclinically affected.

Thus, a positive FEC does not provide useful information in clinical cases. It does not suggest possible parasitic involvement in observed disease manifestations. Nor does a positive FEC predict the risk of disease developing in the near or distant future. Rather, the FEC helps to identify individuals that might be persistent strongyle shedders, is useful for monitoring patient ascarid infections, and provides information about whether any applied treatment has been effective.

Serum- or saliva-based assays are available in some countries to detect antibodies specific to tapeworms and cyathostomins. Antibodies generally reflect exposure rather than current infection, and these assays are not quantitative, so they do not provide information about infection severity. However, horses testing negative for either tapeworms or cyathostomins on serum- or saliva-based assays most likely harbor very small parasite burdens, if any at all, which can be useful information. Furthermore, antibody levels can reflect the overall parasite infection pressure in a herd—information that can be useful in the design of parasite control programs.

The equine parasites discussed above are all pasture-borne; ascarids are potentially capable of being transmitted in paddocks and barns as well. For these parasites to become infectious, the external stages of the life cycle must develop, and this process is highly dependent on climate and weather.

Very good information describing conditions favorable for the development and survival of strongyle eggs and larvae in horses is available, as summarized in the AAEP Guidelines. Thus, parasite transmission seasons can be defined for any region/locality, and parasite control programs can be designed accordingly.

In general, strongyle eggs hatch at temperatures > 6°C (> approximately 43°F), and development to the third infective larval stage occurs at increasing rates as temperatures rise. Optimal temperatures for the development of pasture stages are in the range of 25–30°C (77–86°F). At temperatures above this range, survival is negatively affected. Neither eggs nor larvae survive temperatures > 40°C (> 104°F) for more than a couple of days.

Decisions about when to conduct FECs and treat horses for parasites should take into consideration climate and season, as well as possible effects of pasture management interventions such as dragging, mowing, rotating, and pasture hygiene. For example, mowing and resting a pasture can markedly decrease pasture infectivity of parasites in warm and hot conditions, whereas beneficial effects of these management practices might be minimal under cool and moist conditions.

Current guidelines for equine parasite control take a two-tiered approach. A baseline of annual treatments is defined for all horses, and additional treatments for individual horses are based on parasite surveillance findings.

Surveillance-based treatments are aimed primarily at controlling strongyle parasite contamination (ie, egg output) on pasture. Baseline treatments are applied strategically to control noncyathostomin parasites such as large strongyles and tapeworms. In addition, routine evaluation of anthelmintic efficacy to avoid using ineffective classes is strongly recommended.

Baseline treatments typically consist of one or two annual treatments administered strategically in relation to the parasite transmission season. In mature horses, macrocyclic lactones are often the most appropriate anthelmintic class, with the optional addition of praziquantel to treat tapeworms.

Surveillance-based treatments typically target high strongylid shedders over the course of the transmission season. In areas with relatively short transmission seasons (eg, May–September), one additional treatment targeting these individuals might suffice. In regions with substantially longer seasons (eg, March–November or longer), treating high strongylid shedders 3–4 times a year might be appropriate.

For foals and weanlings, parasite control recommendations are substantially different. The baseline treatments target ascarids rather than strongyles, and the timing of these treatments is based on age rather than time of year. Parasite surveillance in foals/weanlings to monitor the presence of ascarids is recommended while foals develop immunity to these parasites and ultimately eliminate them.

General guidelines for foals and weanlings are to deworm for ascarids by administering a benzimidazole at approximately 2 and 5 months of age, and then to monitor the relative presence of ascarids versus strongyles in the typical transitional period (5–8 months of age) between these two types of parasites. Similarly, short/young yearlings should be monitored for ascarids and, if these parasites are found, treated with a benzimidazole. 

Strongyle parasites should be targeted in young horses starting at approximately 5–6 months of age, when they are becoming the dominant parasite. Generally, macrocyclic lactones are recommended for strongyle treatment; however, routine efficacy testing is necessary. As a general rule of thumb, weanlings with pasture access should receive their first tapeworm treatment (praziquantel or pyrantel pamoate) by their first winter, at the latest.

• CANTER (Controlling Antiparasitic Resistance in Equines Responsibly).

• Internal Parasite Control Guidelines. American Association of Equine Practitioners (AAEP).

• Nielsen MK, Reinemeyer CR. Handbook of Equine Parasite Control. 2nd ed. John Wiley and Sons; 2018.

• Also see pet owner content regarding GI parasites of horses.