| | Dennis D. French, left, and Thomas R. Klei, who are jointly appointed in the School of Veterinary Medicine and the Department of Veterinary Science, work cooperatively with the LSU AgCenter in research on horses. They are two of the nation’s leading experts. (Photo by John Wozniak) |
| | | Sharon R. Chirgwin, left, and Sharon U. Coleman are involved in the research on parasites in horses. Parasites are present in horses maintained under all management conditions.(Photo by John Wozniak) |
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Thomas R. Klei, Dennis D. French, Melanie R. Chapman, Sharon U. Coleman and Sharon R. Chirqwin
The most ubiquitous internal parasites in horses on well-managed farms in the United States are the nematodes known as small strongyles, cyathostomes or, more recently, cyathostomins. While total number of cyathostomins within the horse vary on different farms and are dependent on the use of anthelmintics (drugs that act against infections caused by parasitic worms) and ages of horses, these parasites are present in horses maintained under all management conditions. While more than 50 species of cyathostomins have been described, 10 common species comprise the majority of the total populations in horses surveyed throughout the world. We have identified 26 different species in Louisiana.
The cyathostomin life cycle is typical of strongylid nematodes and transmission is direct – through the ingestion of infective third stage larvae (L3) that develop on pastures. Numerous studies have measured pasture larvae at different seasons of the year in different climatic conditions, including in southern Louisiana. The peak transmission occurs in the fall, winter and spring during cooler weather. When ingested, the L3 penetrate the epithelium of the large intestine. These L3 then follow one of two developmental paths. They may continue to grow and eventually become adult male or female worms. Or, following entry into the intestinal mucosa, the L3 development is arrested and the larvae become hypobiotic L3 (EL3). Large numbers of these “hibernating” EL3 build up in the wall of the large intestine. As they grow to adulthood, they become an important cause of disease.
While the cyathostomins have been a constant companion of horses over time, it has only been since the elimination of the deadly large strongyles that the significance of cyathostomins has emerged. Larval cyathostominosis, the most common condition attributed to cyathostomins, is characterized by a variety of signs including acute or chronic diarrhea, peripheral subcutaneous edema, weight loss, general illness and, in some instances, death. The condition is generally attributed to the seasonal emergence of large numbers of larvae, previously in the hypobiotic state, from the intestinal mucosa. While this generally occurs in winter or spring, the condition has been described at other seasons of the year under a variety of conditions. Cyathostomins have also been implicated in causing colic in horses.
The most common equine parasite control programs, developed in the 1960s, involve the regular treatment of all horses on a farm at six-week to two-month intervals with an available anti-parasitic drug. The drug classes are alternated during the year in an attempt to seasonally treat some specific parasite such as bot fly larvae and to prevent development of resistance. This regimen, together with the advent of highly effective anthelmintics, resulted in the near elimination of large strongyles as problems in horses. Other control programs have been suggested with increased concerns over the emergence of drug-resistant cyathostomins.
Anti-parasitic drugs currently used in horses fall into one of three classes. The benzimidazoles include fenbendazole (FBZ), oxfendazole and oxibendazole (OBZ). The tetrahydropyrimidines include pyrantel pamoate (PP) and pyrantel tartrate. The later compound is used as a daily feed additive in a low dosage. The macrocyclic lactones that have efficacy against both bot fly larvae and nematodes are ivermectin (IVM) and moxidectin (MOX). Most recently, mixtures of macrocyclic lactones and praziquantel have been produced, which provide the horse owner with anti-parasitic drugs that control nematodes, bot fly larvae and tapeworms. While this catalogue of products is relatively large, cyathostomin resistance to all but IVM and MOX has been reported around the world.
The most comprehensive and careful survey for drug resistance in equine cyathostomins was recently carried out in Florida, Georgia, Kentucky, South Carolina and Louisiana. To demonstrate the presence of resistant cyathostomin populations, fecal egg count reduction (FECRT) tests were performed on horses on a variety of farm types and breeds. Ten of the 16 farms screened in Louisiana were used in this survey. Statistical analysis of the larger, multi-state survey indicated that there were no differences between the results of tests based on farm type, breed or state.
The anthelmintics tested in this survey were FBZ, OBZ, PP and IVM. These are commonly used and represent drugs in all of the three classes available for use against nematode parasites in horses.
Of the 10 Louisiana farms examined, all had cyathostomins resistant to FBZ. Four of the 10 had worms resistant to OBZ. Six of the farms had worms resistant to PP. Resistance to IVM was not seen on any farm. Resistance to both PP and benzimidazole anthelmintics occurred on six farms. IVM was the only effective drug on three of these farms.
These observations indicated that resistance is widespread to all but one class of anthelmintics, the macrocyclic lactones. The reason for the absence of cyathostomin resistant to IVM is unclear. Nonetheless, this is reason for concern because IVM resistant nematodes of sheep and goats are well-known and widespread. Further, resistant populations of cattle nematodes are now being identified. It is the general concern of veterinary parasitologists that IVM-resistant cyathostomins will appear in the near future, and because of the continuous movement of horses throughout the country, it is likely that resistant parasites would spread rapidly. This of particular importance because other classes of anthelmintics are not being tested for use in horses at this time, and none appear on the horizon.
It is clear that only effective anthelmintics should be used. Thus, it is important to consider performing a FECRT on horses using the available anthelmintics. It can be seen from the results of this survey that a wide range of options may exist even as resistance is common. As an example, while the findings of the survey indicate FBZ has little efficacy and that resistance to it is widespread, OBZ remains effective on five of these farms. Further, while PP resistance was more widespread than previously shown or expected, it remains highly effective on four of the 10 farms showing BZ resistance. Using multiple effective drugs in a control program will reduce the use of only macrocyclic lactones and potentially preserve efficacy of this class.
Only a small proportion of any group of mature horses produces a significant number of strongyle eggs (200 EPG). Selective treatment of those horses passing greater numbers of eggs rather than treating every horse on the farm will result in treatment of fewer horses and reduce the potential selection of resistant cyathostomins. This approach requires regular fecal egg count determinations and may not reduce the cost of parasite control.
Finally, it should be clear that internal parasite control is not simple. It varies with different farms, uses of horses and ages of horses present. It is important to include the consultation of a veterinarian in the establishment and monitoring of any program to be developed.
Thomas R. Klei, Professor, Dennis D. French, Professor, Melanie R. Chapman, Research Associate Retired, Department of Veterinary Science, LSU AgCenter, Baton Rouge, La.; Sharon U. Coleman, Research Associate and Sharon R. Chirgwin, Research Assistant Professor, Department of Pathobiological Sciences, LSU School of Veterinary Medicine.
(This article appeared in the fall 2004 edition of Louisiana Agriculture.) |