Reduced activity of moxidectin and ivermectin on small strongyles in young horses on a farm (BC) in Central Kentucky in two field tests with notes on variable counts of eggs per gram of feces (EPGs).

Two field tests (A and B) were conducted in Thoroughbred weanlings born in 2008 and 2009 on a farm (BC) in Central Kentucky. The purpose was to determine the current efficacy of moxidectin (MOX) and ivermectin (IVM) on small strongyles by pre- and posttreatment counts of eggs per gram of feces (EPGs) in this geographical area. In previous parasite studies in horses on this farm and in horses on numerous other farms in this area, cultures of feces indicated the strongyle larvae were all small strongyles. Therefore, the strongyle eggs in the EPG counts are considered small strongyles. Designation of the location of the horses in the current study is by barn number. However, the horses were kept on pastures except when gotten up in the specific barns for treatment and collection of feces. The number of horses for study A (2008-2009) was 40 in a total of ten barns (four in each location) and that for study B (2009-2010) was 49 in a total of nine barns (four each in six locations and seven to nine each in three locations). The nine barns for the study-B horses were the same ones used for the study-A horses; the tenth barn for study A was not used for study B. The number of horses treated was 20 with MOX and 20 with IVM in study A and 24 with MOX and 25 with IVM in study B. Drugs given to horses in barns in study B were switched from those used in barns in study A. That is, horses in barns where MOX was used in study A were given IVM and vice versa for those in study B. Study-A horses in some of the barns had unexplained either negative or delayed return of EPG counts after treatment with either drug, compared to horses in other barns. No reason for inconsistencies could be determined. Study B was initiated to compare results with study-A horses, relative to their location in the same barns. Study- B horses, contrary to horses in study A, had mostly consistent results between barns. Most of the MOX- and IVM-treated horses in study B and some in study A had strongyle EPG counts returning almost twice as quickly as when these compounds were first marketed.

Observations on development of natural infection and species composition of small strongyles in young equids in Kentucky.

Early development and maturation of species of small strongyles have not been studied extensively. Most information is on the first appearance of strongyle eggs in feces of foals. However, species cannot be determined in this manner because of similarity of the morphological features of the eggs. To determine more definitive knowledge on development and species composition of natural infections of small strongyles, eight equid foals were necropsied and examined at various ages (31 to 92 days of age). The entire contents of the large intestine were examined, and all worms (6,671) recovered were identified. This was done to recover small strongyle specimens in an effort to find the stage of development of the worms for different time periods. The primary interest was to obtain data on the potential prepatent period. A total of 17 species of small strongyles were recovered. The earliest gravid female was Cylicostephanus longibursatus in a 57-day-old foal; then Cylicostephanus goldi was positive at 64 days, Cyathostomum catinatum at 68 days, Coronocyclus coronatus and Cylicostephanus calicatus at 70 days, Coronocyclus labiatus, Cylicocyclus leptostomus, Cylicocyclus nassatus, Cylicostephanus hybridus, and Cylicostephanus minutus at 74 days, and Cylicostephanus bidentatus at 92 days. While it is not known exactly when the foals began eating pasture vegetation (typically they begin to graze a short time after birth) and first ate infective free-living third-stage larvae, the results give an indication of the natural prepatent period of several species of small strongyles. Additional data are presented on the number of worms per foal, distribution of the worms in the parts (cecum, ventral colon, and dorsal colon) of the large intestine, and proportion of species found.

Further evaluation in field tests of the activity of three anthelmintics (fenbendazole, oxibendazole, and pyrantel pamoate) against the ascarid Parascaris equorum in horse foals on eight farms in Central Kentucky (2009-2010).

The activity of three anthelmintics (fenbendazole-FBZ; oxibendazole-OBZ; and pyrantel pamoate-PRT) was ascertained against the ascarid Parascaris equorum in horse foals on eight farms in Central Kentucky (2009-2010) in field tests. A total of 316 foals were treated, and 168 (53.2%) were passing ascarid eggs on the day of treatment. Evaluation of drug efficacy was determined qualitatively by comparing the number of foals passing ascarid eggs in their feces before and after treatment. The main purpose was to obtain data on current activity of these compounds against ascarids. Additionally, the objective was to compare these findings with those from earlier data on the efficacy of these three compounds on nematodes in foals in this geographical area. Efficacies (average) for the foals ranged for FBZ (10 mg/kg) from 50% to 100% (80%), for OBZ (10 mg/kg) from 75% to 100% (97%), and for PRT at 1× (6.6 mg base/kg) from 0% to 71% (2%) and at 2× (13.2 mg base/kg) 0% to 0% (0%). Although the efficacy varied among the drugs, combined data for all farms indicated a significant reduction of ascarid infections for FBZ (p < 0.0001) and OBZ (p < 0.0001) but not for PRT (p = 0.0953).

Field tests demonstrating reduced activity of ivermectin and moxidectin against small strongyles in horses on 14 farms in Central Kentucky in 2007-2009.

Efficacy of ivermectin (IVM) and moxidectin (MOX) against small strongyles was evaluated in horses (n=363) in field tests on 14 farms in Central Kentucky between 2007 and 2009. Most of the horses were yearlings but a few were weanlings and mares. The number of horses treated with IVM was 255 and those treated with MOX was 108. Horses on six farms were allotted into two groups. One group was treated with each of the two drugs, whereas horses on the other eight farms were treated with only one of the two drugs--IVM on six farms and MOX on two farms. Strongyle eggs per gram of feces (EPGs) compared to initial use of IVM and MOX returned almost twice as quickly after treatment of horses on all of the farms. IVM has been used much more extensively in this geographical area than MOX. Reduced activity of MOX was evident even on farms with rare or no apparent previous use of MOX but with probable extensive use of IVM.

Critical tests evaluating efficacy of moxidectin against small strongyles in horses from a herd for which reduced activity had been found in field tests in Central Kentucky.

Critical tests were performed in 2009 and 2010 in four 2-year-old horses naturally infected with internal parasites. The horses were from a herd (Farm MC) where reduced activity of ivermectin and moxidectin on small strongyles was demonstrated previously from EPG (eggs/gram of feces) data in field tests. Also, in critical tests in horses from the same herd, ivermectin was less effective on immature small strongyles in the lumen of the large intestine than when the drug was first marketed. The main interest in the present critical tests was to determine the efficacy of moxidectin (400 µg/kg) on small strongyles. This was done to try and find indications of why there has been a return of strongyle EPG counts sooner after treatment in field tests than when moxidectin was first commercially available. Removal of adult small strongyles for the four treated horses was >99% to 100%. Efficacy on immature (L(4)) small strongyles was 82%, 96%, 98%, and >99% for the individual horses. Identification of small strongyles recovered from two of the horses revealed that three genera and 11 species were present. Specimens of Cylicocyclus ashworthi are reported for the first time in horses in Kentucky although eggs of this species have been identified. Moxidectin, in the present study, was excellent on removing adult small strongyles but was less effective on immatures (L(4)) in the intestinal contents. The question as to why moxidectin efficacy on small strongyles has declined in field tests may have been answered at least to a certain extent. It seems that a significant factor is "quick development" of a few remaining immatures in the gut lumen of horses. Also, possible activity may have decreased on encysted stages in the large intestinal lining. In any event, after treatment of some horses with moxidectin, the life cycle of small strongyles is shorter now than at the onset of usage of this compound.