Common equine parasites in Louisiana an update on problems and control programs

Abstract

Internal parasites of horses continue to be a major obstacle to successful horse breeding throughout the world. Parasitic infections play major roles in the clinical conditions of colic, enteritis, respiratory tract disease, dermatitis and poor growth and development. Control programs have been relatively standardized throughout the equine industry, with recommendations of anthelmintic treatment administered every 6 to 8 weeks for control of strongyles, ascarids and bots. However, many questions arise regarding internal parasites and their control with regard to the impact of parasites on horse health, recommended rotational programs and the efficiency of anthelmintics. The most common parasite associated with colic is the large strongyle (strongylus vulgaris). However, recent studies of colic cases reveal that alow percentage of cases are directly attributable to large strongyles. Studies conducted at Louisiana State University indicate that S. vulgaris does cause vascular impairment in chronic cases. Ponies with S. vulgaris infections have impaired activity of the digestive system that could predispose it to colic. Other large strongyles may have importance in colic cases also. Strongylus edentatus larvae may migrate extensively throughout the abdominal organs, especially the liver, and cause extensive hemorrhage, eosinophilic, granulomas and thickened, fibrosed nodules. The typical signs associated with parasitic infections such as this would be mild to moderate intermittent bouts of colic. Ivermectin is the only anthelmintic which at the recommended dose kills migrating strongyles. Death of these migrating strongyles has caused concern because of the effects the dead larvae might have on the horse. Studies conducted by Turk, Klei and Chapman at LSU suggest that repeated killing ofS. vulgaris larvae by ivermectin does not increase the severity of verminous arteritis nor induce lesions. Thiabendazole at 10 times the recommended dose on two successive days, and fenbendazole (Panacur) attwo times the recommended dose on three successive days have been used in treating migrating strongyles. Small strongyles are the most common of internal parasites in the horse. They may also be important in the production of low grade intermittent colics. There are 26 species that are known to infect horses and the small strongyles are the worms that have developed resistant strains against benzimidazole deworming agents such as TBZ and mebendazole (Telmin). Impaired growth and development brought about by internal parasites is a great concern within the equine industry. In a study conducted over the past year at LSU, ivermectin was used in 16 mares and their foals at eight week intervals. When compared to horses which did not receive anthelmintics, weight gains, back fat values and condition scores were significantly higher in treated foals than those in non-treated foals. This study clearly indicates the importance of routine parasite control to maximize the growth and development of young horses. The most common concern throughout the horse industry over the past years, has been the efficacy of currently available drugs. Benzimidazole resistance has been a widely discussed issue. In studies conducted on various horse farms across south Louisiana, only two benzimidazoles have been found to reduce egg counts 80% or more. Oxibendazole (Anthelcide) has consistently shown greater than 90% reduction in fecal egg counts and oxfendazole (Benzelmin) has consistently reduced fecal egg counts by greater than 80%. Recent studies on the effects of ivermectin demonstrated that it had 91% effectiveness against larval stages and 100% efficacy against adult parasites. The majority of parasite associated health problems occur in pastured horses where they are continually exposed to both small and large strongyle larvae. Studies conducted on ponies pastured in south Louisiana have shown two seasonal peaks of strongyle eggs in feces, one in the spring and the second during the early fall months. Infective larvae on pasture reach high levels approximately two to four weeks after peaks in fecal egg counts. Preliminary data on pasture strongyle larval burdens in south Louisiana have demonstrated a marked decrease of infective larvae on herbage in the hot months of June, July and August. Marked differences are normally observed in concentrations of larvae on different areas of pasture. Apparently, horses have learned not to eat grass in areas of fecal contamination. Pastures therefore develop into areas of short, grazed grass (lawns) and high, ungrazed grass around fecal piles (roughs). STudies conducted in Ohio have shown concentrations of infective larvae in roughs to be 15 times higher than in lawns. Regular mechanical removal of feces from pastures by eliminating the roughs may be practical under specific management conditions. Because of the new information and anthelmintics which have become available in the past five years, control programs f o r parasites must be reconsidered. New approaches are to target seasonal treatments in order to break up the worm cycle more effectively, the rotation of anthelmintics is even being questioned, with some indicating thata slow rotation of one to two years so that each generation of worms is exposed to only one class of anthelmintic, may slow the build up of resistance. Currently, recommendations for the Gulf Coast climates based on all available information and in progress studies; are to maintain an eight week interval anthelmintic schedule using a rotation of oxibendazole or oxibendazole-piperazine combinations, pyrantel pamoate and ivermectin. Ivermectin should be