Field studies indicating reduced activity of ivermectin on small strongyles in horses on a farm in Central Kentucky

Field studies (n = 6) were completed on evaluation of activity of ivermectin (200 μg/kg) paste formulation against small strongyles in horses (foals, yearlings, and older animals) on a farm (Farm MC) in Central Kentucky in late 2006 and during 2007. A girth tape was used to estimate body weights which were then used to calculate the proper dose rate of ivermectin. The foals, yearlings, and some of the older horses were born and raised on the farm. However, most of the older horses which were not raised on the farm had been there for several years. The horse herd was given ivermectin exclusively, usually four times a year, since 1990. An exception was that during the foal’s period of life fenbendazole, pyrantel pamoate, and oxibendazole were given occasionally besides ivermectin. Efficacy of drug activity was determined by pretreatment and posttreatment counts of strongyle eggs per gram of feces (EPGs). Culture of strongyle eggs in feces from some of the horses showed that only small strongyle larvae were present. The research included two studies (A and B) in foals (n = 24) and four studies (C, D, E, and F) in yearlings (n = 13) alone or with older horses (n = 10). For each of the studies (B through F), there was a treated and a nontreated group. These groups were switched for each treatment, i.e., the treated group in one study was the nontreated group in the next study and vice versa. Eggs per gram of feces counts were determined at 1- or 2-week posttreatment intervals for 4 weeks for study A and 6 weeks for studies B through F. Also, for studies B, E, and F, counts of EPGs were done either two or three times during the third week posttreatment. The studies showed a similar posttreatment pattern of strongyle EPG counts beginning to return at about 4 weeks and increasing at 5 and 6 weeks posttreatment. Two horses in study E and one in study F had low EPG values toward the end of the third week posttreatment. The results of this ivermectin investigation showed that the strongyle EPG counts started returning about twice as quickly post-ivermectin-treatment of horses than when the drug was first marketed in the early 1980s.     

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.     

Prevalence of large endoparasites at necropsy in horses infected with Population B small strongyles in a herd established in Kentucky in 1966

Two closed horse herds (Old Lot 4 and Field 24), infected since 1966 with Population B small strongyles resistant to thiabendazole (TBZ) and phenothiazine (PTZ), were terminated in February, March, and May, 2005. At necropsy, only the large endoparasites were identified and counted. The number of horses on pasture was 14 (239 days of age to 23 years old) for Old Lot 4 and two (3 to 20 years old) for Field 24. The time of the last antiparasitic treatment, relative to the year (2005) of necropsy, was 26 years for Old Lot 4 and 9 years for Field 24 horses. Gasterophilus intestinalis third instars (three to 113 specimens/horse) were found in all 16 horses and second instars (one to two) in two horses. Gasterophilus nasalis third instars (one to three) were recovered from five horses. Parascaris equorum infections (23 to 144) were in four horses (239 days to 4 years old). Strongylus vulgaris were present in the large intestine (one to 155) of 13 horses from 239 days to 23 years old and in the cranial mesenteric artery (two to 79) in 10 horses from 239 days to 23 years old. Strongylus edentatus were in the large intestine (two to 101) of 12 horses, ranging in age from 2.5 to 23 years old and in the ventral abdominal wall (one to 53) of six horses from 239 days to 21 years old. Specimens (seven to 872) of Anoplocephala perfoliata were in all horses. Oxyuris equi (one to 129) were recovered from seven horses (330 days to 23 years old). Thelazia lacrymalis (one to 85) infected the eyes of five horses (317 days to 11 years old).     

Further indication of lowered activity of ivermectin on immature small strongyles in the intestinal lumen of horses on a farm in Central Kentucky

Critical tests were performed in 2011 in four weanling horses (L-1, L-2, L-29, and L-30) treated with ivermectin paste at 200 μg/kg. They were born in 2011 and raised together on a farm (MC) in Central Kentucky. The horses had not been treated previously with an antiparasitic drug. However, ivermectin had been administered repeatedly to the horse herd for several years and strongyle eggs per gram of feces (EPGs) returned sooner posttreatment than after initial usage. Critical tests in a recent previous study in this horse herd indicated that the reason for the early return of strongyle EPGs after ivermectin treatment probably was because of lowered drug activity on immature (L₄) small strongyles in the lumen of the large intestine. Therefore, the life cycle was shortened. The main purpose of the present study was to obtain further data on the activity of ivermectin on small strongyle immature stages, in addition to adults, in the intestinal lumen. Twelve species of small strongyles were present. Combined data for immature and adult small strongyles for the four ivermectin-treated horses demonstrated efficacy of 68 to 83 %. Removal of adults was 100 % for all four horses, and on immatures, it ranged from 0 to 16 %. Efficacy on immature small strongyles was even lower than in the previous study. This supported the earlier finding of apparent reduced time for maturation of the parasites because of incomplete elimination of immature small strongyles in the intestinal lumen of horses after ivermectin treatment. Data on five other helminth species were recorded.     

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.     

Field test data on small strongyles in evaluation of activity of fenbendazole given once a day for 5 consecutive days to thoroughbred yearlings on two farms in Kentucky in 2002 and 2003

Fenbendazole (FBZ) suspension was administered intraorally at the dose rate of 7.0–10.3 mg/kg once a day for 5 consecutive days to 58 thoroughbred yearlings on two farms in central Kentucky in April, 2002. The average dose rates of drug given to groups of colts and fillies on each farm were 7.8–8.5 mg/kg. Only 3 of the yearlings had negative counts of strongyle eggs per gram of feces (EPGs) after treatment which was at 8.4, 8.7, or 9.4 mg/kg; the pretreatment EPG counts were low (10–30). Reduction of EPG values at the highest dose rates was 0% (at 9.5 mg/kg) and 78% (at 10.3 mg/kg). This study was repeated in April, 2003 in 38 thoroughbred yearlings on one of the two same farms used in the 2002 research, but all horses were treated at the same dose rate (10 mg/kg) of FBZ paste once daily for 5 consecutive days. Only 1 of these yearlings had a negative EPG count after treatment, but this value was also negative before treatment. Reductions of EPG counts after treatment ranged from 0% to 85% (mean =22%) for the colts and from 0 to 63% (mean =14%) for the fillies. Examination of cultures of fecal samples from these yearlings revealed that only small strongyle larvae were present. There was obvious FBZ-resistance of the small strongyles in yearlings on both farms at the dose rates used.     

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?相似文献   

Reconstruction of intracortical whisker-evoked local field potential from electrocorticogram using a model trained for spontaneous activity in the rat barrel cortex

Electrocorticogram (ECoG) has provided neural information from the cortical surfaces, is widely used in clinical applications, and expected to be useful for brain–machine interfaces. Recent studies have defined the relationship between neural activity in deep layers of the cerebral cortex and ECoG. However, it is still unclear whether this relationship is shared across different brain states. In this study, spontaneous activity and whisker-evoked responses in the barrel cortex of anesthetized rats were recorded with a 32-channel ECoG electrode array and 32-channel linear silicon probe electrodes, respectively. Spontaneous local field potentials (LFPs) at various depths could be reconstructed with high accuracy (R > 0.9) by a linear weighted summation of spontaneous ECoG. Current source density analysis revealed that the reconstructed LFPs correctly represented laminar profiles of current sinks and sources as well as the raw LFP. Moreover, when we applied the spontaneous activity model to reconstruction of LFP from the whisker-related ECoG, high accuracy of reconstruction could be obtained (R > 0.9). Our results suggest that the ECoG carried rich information about synaptic currents in the deep layers of the cortex, and the same reconstruction model can be applied to estimate both spontaneous activity and whisker-evoked responses.