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

Population B, drug-resistant small strongyles have been 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. Studies have been on evaluation of antiparasitic efficacy of several compounds, especially the benzimidazoles, against Population B small strongyles 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. Research on these nematodes through 1990 has been published. The current paper presents data on efficacies of various anthelmintics (mostly TBZ) against these and other internal parasites in critical tests done between 1991 and 2001. These were the last critical tests in the UK horses; the entire 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 (@ 44 mg/kg), pyrantel pamoate (PRT @ 6.6 mg base/kg), PTZ (@ 55 mg/kg), fenbendazole (FBZ @ 5 mg/kg), oxfendazole (OFZ @ 10 mg/kg), and oxibendazole (OBZ @ 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 early (1992) in the study. 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 more or less 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; that is, 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 not only did not increase but it 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). Data on prevalence and drug activity on other internal parasite species besides small strongyles also are given.