Probable reason why small strongyle EPG counts are returning “early” after ivermectin treatment of horses on a farm in Central Kentucky

Critical tests were carried out in 2008 in four yearling horses (H-2, H-4, H-10, and H-11) born and raised together on a farm (MC) in Central Kentucky. These horses were treated intraorally with ivermectin paste at the dose rate of 200 μg/kg. The main interest was to try and determine more precisely, from posttreatment (PT) worm count data, the current activity of ivermectin against small strongyles in a horse herd. These horses had been treated repeatedly with this compound and counts of small strongyle eggs per gram of feces (EPGs) of these parasites have been returning sooner than previously in field tests (Lyons et al. Parasitol Res 103:209-215, 2008a). Data from the four horses revealed that a total of 3,237 (nonfactored number) specimens of small strongyles was recovered from aliquot samples of feces passed PT and in the large intestinal contents at necropsy; all specimens were examined and identified. Thirteen species of adult small strongyles were recorded. Cylicocyclus (Cyc.) insigne was the predominant species. Three of the yearlings (H-2, H-4, and H-11) were necropsied at 6 days PT with ivermectin, and removals of small strongyles were: 50%, 80%, and 36% of fourth stages (L₄), respectively, and 100%, 99%, and 100% of adults, respectively. As indicated, the only incomplete removal of adults from the three horses was for H-4. They consisted of two species: (1) young Cyc. insigne (those passed in the feces were fully developed); removal of this species was 89% and (2) fully developed Cylicostephanus longibursatus; 99% were removed. The fourth yearling (H-10), necropsied at 25 days PT, harbored 19,150 adult small strongyles in the large intestinal contents. Most of the species were Cyc. insigne; all were fifth stage but not sexually mature. Comparing the percentage of adult small strongyles found at necropsy relative to the total number present (those passed in the feces and at necropsy), only 0% to 1% were in the contents of the large intestines of the three horses at 6 days PT but in 26% for horse H-10 at 25 days PT. As mentioned earlier, only a few adults were found in one horse and several L₄ in the three horses at necropsy at 6 days PT. Therefore, in horse H-10, most adults found at 25 days PT presumably developed from “young” specimens not removed by ivermectin. Thus, data from the present critical tests indicate the probable cause of the “early” return of small strongyle EPG values after ivermectin treatment in the horses in field tests on Farm MC. It seems this was the result of incomplete removal of luminal specimens (L₄ and possibly young adults), some of which matured and began laying eggs by about 4 weeks PT (Lyons et al. Parasitol Res 103:209-215, 2008a). The research also showed that ivermectin was highly effective on adult small strongyles. At necropsy, the following other species of parasites (adult) were found, but none was recovered from the feces. These were (n = number of horses infected): (1) ascarids (Parascaris equorum--n = 1), (2) tapeworms (Anoplocephala perfoliata--n = 4), and (3) pinworms (Oxyuris equi--n = 3). Immature (L₄) O. equi were present in two horses and removals were 0% in one horse and 39% in the other. Eyeworms (Thelazia lacrymalis) were found in one horse at necropsy. Even though a small number of horses were used in the present research, the commonality of their background made them ideal candidates as a group for this study. This aspect helps strengthen the validity of the interpretation of the findings.