Research Accomplishment Reports 2007

Molecular Mechanisms, Ecology, and Control of Natural Infections of Equids and Ruminants by Drug-Resistant Internal Parasites

E.T. Lyons
Department of Veterinary Sciences

 

Project Description

Population B drug-resistant small strongyles were studied in naturally infected horses in Kentucky for more than 40 years. These parasites first were found to be resistant to phenothiazine (Ptz) and thiabendazole (Tbz), later to other parasiticides. Antiparasitic efficacy of several compounds, especially the benzimidazoles, against population B small strongyles was evaluated in clinical (field) tests (1959-1983) on the commercial farm of origin and in clinical and critical tests (1966-2001) at the University of Kentucky (UK) research farm. Efficacies of various anthelmintics (mostly Tbz) against these and other internal parasites in critical tests done 1991-2001 are presented. These were the last critical tests in the UK horses; the herd was terminated in 2005. Population B small strongyles were established in horses on a pasture at the UK research farm on old lot 4 in 1966, and a satellite of this group was relocated to field 24 in 1987. The last treatment of any of the horses in clinical tests on pasture was 22 years for old lot 4 (mostly benzimidazoles) and 5 years for field 24 (Tbz) before the last critical test in 2001. Antiparasitic compounds (all paste formulations) administered orally in critical tests (n = 36) reported in this paper were Tbz (at 44 mg/kg), pyrantel pamoate (PRT at 6.6 mg base/kg),Ptz (at 55 mg/kg), fenbendazole (Fbz at 5 mg/kg), oxfendazole (Ofz at 10 mg/kg), and oxibendazole (Obz at 10 mg/kg). The drug given and number of horses treated from old lot 4 were Tbz (18), PRT (3), Ptz (2), Fbz (2), Ofz (1), and Obz (1) and from field 24 were Ofz (1) and Tbz (8). Removal of small strongyles in old lot 4 was excellent for PRT, Ofz, and Obz but much less for Tbz, Ptz, and Fbz. For the 16 species present in this lot, removal by Tbz was lowest for seven species (Coronocyclus (COR.) coronatus, Cyathostomum (Cya.) catinatum, Cylicocyclus (Cyc.) nassatus, Cylicostephanus (Cys.) calicatus, Cylicostephanus goldi, Cylicostephanus longibursatus, and Cylicostephanus minutus). Of these seven species, lowest activity was found for five by Ptz and Fbz. One of the five resistant species was different for each of these two drugs. In field 24, efficacy against small strongyles was excellent for the one foal treated with Ofz in 1992. Tbz initially had higher activity than in later years. Of the 12 small strongyle species present in this field, Tbz activity throughout the study was, in general, low for Cor. coronatus, Cys. goldi, and Cys. longibursatus, but it declined progressively for Cya. catinatum, Cylicocyclus leptostomus, Cyc.nassatus, and Cys. calicatus over the study period. Cys. minutus were not present in high enough numbers to evaluate drug efficacy. Overall activity of Tbz on the group of small strongyles did not change; i.e. susceptibility did not increase over time in old lot 4 where these parasites were not exposed to a benzimidazole for many years. However, in field 24, where additional Tbz pressure was put on these parasites, efficacy decreased. From the data for small strongyles in the two groups of foals, eight species were considered benzimidazole resistant in varying degrees (most research on Tbz).

Impact

Among other benefits of this research, it was established that even after over 20 years of nonuse of a parasiticidal compound, the drug-resistant small strongyles did not revert to susceptibility. In other words the drug-resistant factor of these parasites still persisted after many years of nonexposure to a chemical compound. The study on the small small strongyles (Population B) was unique in that it extended for about 40 years allowing numerous publications which contributed to a wealth of information on drug resistance of these parasites. The initial research was considered pioneer because it was one of a few research studies in the world on the drug resistant problem in small strongyles.

Publications

Lyons, E.T., Tolliver, S.C., and Collins, S.S. (2007). Study (1991 to 2001) of Drug-Resistant Population B Small Strongyles in Critical Tests in Horses in Kentucky at the Termination of a 40-Year Investigation, Parasitology Research, 101: 689-701.