Prevalence of patent Strongyloides westeri infections in Thoroughbred foals in 2014.

The prevalence of patent Strongyloides westeri infections was determined by examination for eggs in fecal samples collected from 513 Thoroughbred (TB) foals between February 25 and June 3, 2014. The study was conducted with 244 colts and 269 fillies from 11 well-managed farms in Central Kentucky. Foals ranged from 17 to 117 days of age and had never been dewormed. The mean prevalence of patent S. westeri infection was 30% (156/513 foals). This prevalence was substantially greater than historical reports from the same sampling area during the late 1990s and early 2000s. It is hypothesized that this change is attributable to diminished use of ivermectin in foals due to concerns about macrocyclic lactone resistance in Parascaris equorum.

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 (L(4)) 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.

Investigation of strongyle EPG values in horse mares relative to known age, number positive, and level of egg shedding in field studies on 26 farms in Central Kentucky (2010-2011).

A parasite study was done in 1,300 horse mares on 26 farms in Central Kentucky over a 5-month period in 2010 and 2011. The mares included 1,114 Thoroughbreds (TB) on 24 farms, 64 Standardbreds (SB) on 1 farm, and 122 mixed light horse types (MLH) on 1 farm. The objective of this research was to determine strongyle eggs per gram of feces (EPG) counts for evaluation by known age, number positive, and level of egg shedding by the mares. This was done to establish strongyle EPG profiles for the mares to aid in determining whether antiparasitic treatment was necessary. Eggs found were considered those of small strongyles. (A) For the Thoroughbred mares, (1) 362 (32%) were EPG-positive, and (2) the average (percentage) of EPG-positive mares by year of age was 3-5 (54%), 6-10 (36%), 11-15 (24%), 16-20 (17%), and >20 (21%); (3) EPG average counts were similar for all age categories except for the 6- to 10-year-olds, which were higher; (4) and the average (percentage) of positive mares by 100 units of EPG counts was ≤100 (50%), ≤200 (62%), ≤300 (70%), ≤400 (76%), ≤500 (80%), and >500 (20%). (B) For the Standardbred mares, 31 (48%) were EPG-positive; (2) the average (percentage) of EPG-positive mares by year of age (no >20 sampled) was lowest for the 3-5 and 16-20 categories and highest for the 6-10 and 11-15 groups; (3) EPG average counts by years of age were lowest for 3-5, 11-15, and 16-20 groups and highest for the 6-10 group; and (4) the average (percentage) of positive mares by 100 units of EPGs was 62% for the ≤100 category, 71-84% for ≤200 to ≤400 units, and the highest (97%) for the ≤500 unit. (C) For the mixed light horse type mares: (1) 94 (77%) were EPG-positive, (2) the average (percentage) of EPG-positive mares by age was lowest for the two oldest age groups, higher for the 11 to 15-year-old age group, and highest for the two youngest age groups; (3) EPG average counts by year of age were lowest for the 16-20 group, higher for the 6-10 and >20 groups, and highest for 3-5 and 11-15 groups; (4) the average (percentage) of positive mares by 100 units of EPG counts was lowest for the ≤100 category (23%), increasing about 10% progressively in ≤200 to ≤500 categories, but lower (37%) for the >500 category. (D) For all the three mare types (TB, SB, and MLH), 37% of the mares were EPG-positive, and 63% of the mares were EPG-negative; for the age (years) of positive mares, about one half belonged to the 3-5 category and a progressive decrease was seen for the 6-10, 11-15, and 16-20 groups, and 36% for the >20-year-olds; the mean strongyle EPGs highest range was seen in the 11- to 15-year-olds; the highest mean was in the 6- to 10-year-olds, and the lowest mean in the 3- to 5- and 16- to 20-year-olds. For the grouping of the strongyle EPG values by units of 100, three-fourths were in the ≤500 category, and the lowest percentage was for the >500 category. This research showed the value of strongyle EPG profiling for the mares. It was most useful for TBs where data from a large number of horses showed that over two-thirds were EPG-negative, indicating that there would be no known problem in deciding not to deworm them. While data were not as clear-cut on the SB and MLH mares, several of those which were negative and with "low" EPG values could be excluded from antiparasitic treatment.

Fecundity of various species of strongylids (Nematoda: Strongylidae)--parasites of domestic horses.

The aims of the study were to determine fecundity of several strongylid species parasitizing domestic horses and analyze possible relations between numbers of eggs in female uteri and size of both the eggs and the nematodes as well as the influence of fecundity on proportion of species in the strongylid community. Twenty-five specimens from each of 15 strongylid species (Strongylus vulgaris, Strongylus edentatus, Triodontophorus serratus, Triodontophorus brevicauda, Triodontophorus tenuicollis, Cyathostomum catinatum, Coronocyclus coronatus, Cylicocyclus nassatus, Cylicocyclus insigne, Cylicocyclus leptostomus, Cylicostephanus calicatus, Cylicostephanus goldi, Cylicostephanus longibursatus, Cylicostephanus minutus, and Poteriostomum imparidentatum) collected after necropsy were studied. The reproductive system was extracted from the female body; all eggs were removed, counted, and measured under a light microscope. Significant differences in number of eggs in female uteri of various strongylid species were observed (Kruskal-Wallis test, p < 0.001); the least numbers of eggs were registered in C. longibursatus (average = 49) and C. leptostomus (63) and the largest number in S. edentatus (5,918). Significant correlation between nematode body size and number of eggs was observed (p < 0.001). Correlation between size of eggs and body size was insignificant (Spearman R = 0.11, p = 0.70). Negative correlation was observed between number of eggs in female uteri and proportion of these species in strongylid community (Spearman R = -0.78, p < 0.001). Multiple linear regression of species proportion in the community on three predictors (number of eggs, body size, and egg size) was not significant (p > 0.05). However, the question on influence of fecundity on proportion of species in strongylid community needs further studies.

Acanthocephalans in northern fur seals (Callorhinus ursinus) and a harbor seal (Phoca vitulina) on St. Paul Island, Alaska: species, prevalence, and biodiversity in four fur seal subpopulations.

Monitoring studies of acanthocephalans in northern fur seals (Callorhinus ursinus Linnaeus, 1758) (NFSs) and a harbor seal (Phoca vitulina Linnaeus, 1758) were performed on St. Paul Island, Alaska, in July-August 2011. Gastrointestinal tracts of 105 humanely harvested NFS subadult males (SAMs) (3-4 years old) were collected during the annual Aleut subsistence harvest at four haul-out areas (HOAS): Lukanin (n = 26 NFSs), Polovina (n = 28), Gorbatch (n = 30), and Morzhovyi (n = 21). One gastrointestinal tract collected from a harbor seal (about 3-4 years old) found dead at Morzhovyi HOAS was also examined. The total prevalence of infection in NFSs with acanthocephalans was 29.52 % with variations from 7.69 % to 47.62 % between the four different HOAS. Eight acanthocephalan species of two genera-Corynosoma Lühe, 1904 (Corynosoma strumosum, Corynosoma alaskensis, Corynosoma cameroni, Corynosoma semerme, Corynosoma similis, Corynosoma validum, and Corynosoma villosum), and Bolbosoma Porta, 1908 (Bolbosoma nipponicum)-were found in the NFSs and a harbor seal. This is a new record of C. alaskensis for the NFSs. Short biological notes of the species found are presented. Differences in species composition as well as in prevalence of acanthocephalans parasitizing NFSs were observed in subpopulations from four different HOAS on St. Paul Island. The highest biodiversity of acanthocephalans and infection were found in subpopulations on Polovina and Morzhovyj HOAS, the lowest was on Lukanin HOAS. From 3.2 % (for C. validum) to 19.4 % (for C. villosum) of NFSs were infected by one acanthocephalan species; two species were found in 22.6 %; three in 9.7 %; and four in 3.2 %. Further studies of NFS parasites are necessary to follow the trends in parasitic infection rates and diversity in NFS population on the Pribilov Islands and for monitoring the influence of various ecological factors on NFS populations in Alaska.

Parasitological examination for presence of hookworms (Uncinaria spp.) in northern elephant seals (Mirounga angustirostris) at Año Nuevo State Reserve, California (2012).

Northern elephant seals (Mirounga angustirostris Gill, 1866), inhabiting rookeries on the mainland of Año Nuevo State Reserve in central California, were investigated in 2012 for presence of hookworms (Uncinaria spp.). Material collected and examined for hookworms included: blubber (n = 15), stomach and intestines (n = 21) from dead pups; feces from the rectum of weaned pups (n = 23); sand containing apparent feces in areas of weaned pups (n = 28) and sand without apparent feces in areas of weaned pups (n = 54); milk from females (n =23) at 5 days and about 23 to 26 days postpartum; and placenta from one female. Evidence of hookworm presence was not detected in any of the samples examined. Possible reasons why hookworms were not found in northern elephant seals on the mainland of Año Nuevo State Reserve are discussed.

Efficacy of moxidectin and an ivermectin-praziquantel combination against ascarids, strongyles, and tapeworms in Thoroughbred yearlings in field tests on a farm in Central Kentucky in 2016.

Field tests were performed on Thoroughbred yearlings (n=143), evaluating efficacy of moxidectin (MOX) against ascarids and strongyles and the efficacy of an ivermectin (IVM)-praziquantel (PRAZ) combination against those nematodes and tapeworms on a farm in Central Kentucky. The study was started on March 1, 2016 and completed on August 23, 2016. Fecal samples were collected from yearlings every two weeks for counts of ascarid and strongyle eggs per gram of feces (EPGs) and for determining the presence of tapeworm eggs. MOX was given to 88 yearlings on March 1; 55 yearlings were nontreated controls on this date but were treated two weeks later with MOX. On June 15, 2016, all yearlings (n=87) remaining on the farm were treated with IVM-PRAZ combination. Ascarids were present in low numbers for MOX-treated horses, and the drug had virtually no effect. The low number of ascarid infected horses before any treatment during the study was most likely due to age related immunity. Efficacy against strongyles was alike for both MOX and the IVM- PRAZ combination. At two weeks post-treatment, strongyle EPG values were reduced over 90%, started increasing at four weeks post-treatment, and returned to near pre-treatment values at six weeks post-treatment. Tapeworm efficacy of the IVM-PRAZ treatment was 96% at two weeks and was maintained for the entire ten-week post-treatment examination period.

A multicenter evaluation of the effectiveness of Quest Gel (2% moxidectin) against parasites infecting equids.

Controlled trials with a common protocol were conducted in Idaho, Illinois and Tennessee to evaluate anthelmintic effectiveness of Quest Gel (QG; 2% moxidectin) against lumenal parasites in horses. Candidate horses were required to have naturally acquired nematode infections, as confirmed by presence of strongylid eggs in feces. At each site, 24 equids were blocked on the basis of pretreatment strongyle fecal egg counts (EPG) and randomly assigned to treatments within blocks. Within each block of two animals, one received QG on Day 0 at a dosage of 0.4 mg moxidectin/kg b.w. and one was an untreated control. Body weights measured the day before treatment served as the basis for calculating treatment doses. Horses assigned to treatment with QG received the prescribed dose administered orally with the commercially packaged Sure Dial syringe. Horses were necropsied 12-14 days after treatment, and lumenal parasites and digesta were harvested separately from each of five organs, including the stomach, small intestine, cecum, ventral colon and dorsal colon. Parasites from stomachs and small intestines were identified to genus, species and stage. Micro- (i.e., < 1.5 cm) and macroparasites (i.e., > 1.5 cm) in aliquots from the cecum, ventral colon and dorsal colon were examined in aliquots of approximately 200 parasites until at least 600 parasites had been identified to genus, species and stage or until all parasites in the 5% aliquot were examined, whichever occurred first. Data were combined across sites and analyzed by mixed model analysis of variance to assess the fixed effect of treatment and random effects of site and block within site. Because QG does not contain a cestocide, efficacy of QG against tapeworms was not significant (P > 0.05). Based on geometric means, however, efficacy of QG was greater than 90% (P < 0.05) against 38 species and developmental stages of cyathostomes, strongyles, bots, larval pinworms and ascarids encountered in at least 6 of 36 control horses in the combined data set. None of the horses treated with moxidectin exhibited evidence of adverse effects. Study results demonstrate QG, administered to horses with naturally acquired endoparasite infections at a dosage of 0.4 mg moxidectin/kg b.w., was highly effective against a broad range of equine parasitic infections.

Effect of moxidectin selection on the genetic variation within Cylicocyclus nassatus based on amplified fragment length polymorphism (AFLP).

Cyathostomins are among the most important intestinal nematodes of horses, yet, the literature on the molecular genetics of these worms is scarce. In this study, the technique of amplified fragment length polymorphism (AFLP) was applied to study the genetic diversity as well as to determine the effect of moxidectin selection on the population genetic diversity for Cylicocyclus nassatus, one of the most common cyathostomin species. Genomic DNAs from 30 individual male worms were used from each of two populations: an avermectin-milbemycin (AM)-naive population (Population-S) and a population derived from Population-S following 21 treatments with moxidectin (Population-Mox). Three selective primer pairs were used for each worm, yielding a total of 229 AFLP markers. Calculation of average pair wise Jaccard indices revealed a high degree of genetic variation within both populations using all three primer combinations. In addition, selection by moxidectin during a 3-year period caused a significant decrease in the level of genetic diversity as evidenced by analysis of AFLP markers for two primer combinations but not for the third. A dendrogram of relationships among individuals based on AFLP markers did not show a clear classification of individuals in separate groups. It was concluded that a high degree of genetic intrapopulation variation exists in C. nassatus and that moxidectin selection has a significant effect on the genetic composition of C. nassatus.

Ascaridoid nematodes and associated lesions in stomachs of subadult male northern fur seals (Callorhinus ursinus) on St. Paul Island, Alaska: (1987-1999).

Stomachs from subadult male northern fur seals (SAMs) (Callorhinus ursinus) (n = 5,950) from St. Paul Island, Alaska, were examined from 1987 to 1999 for lesions and parasites. On gross examination of these stomachs, parasite nodules were evident in 92% of the stomachs and active and healing ulcers were found in 14% and 10%, respectively. Prevalence of nematodes from 1987 to 1997 was 88% for 5,700 SAMs for which numbers of parasites were estimated but not identified to the genus level. All nematodes recovered from 250 SAMs examined in 1998 (n = 124; 99% infected) and in 1999 (n = 126; 91% infected) were identified and counted. Prevalences in 1998 and 1999 were 5% and 0% for Anisakis spp., 52% and 18% for Contracaecum spp., and 96% and 89% for Pseudoterranova spp., respectively.

Prevalence of internal parasites in horses in critical tests of activity of parasiticides over a 28-year period (1956-1983) in Kentucky.

The prevalence and number of naturally acquired gastrointestinal parasites were compiled for horses used in critical tests of activity of parasiticides over a 28-year period (1956-1983). Data are presented as follows: n = number of horses examined; % = mean prevalence; number in parentheses after % = aggregate mean number of parasites in infected horses. Parasites found were: bots (n = 513) - Gasterophilus intestinalis, 2nd instar, 61%(58); 3rd instar, 94%(168); G. nasalis, 2nd instar, 36%(28); 3rd instar, 81%(51); stomach worms (n = 200) - Habronema muscae, 65%(179); Draschia megastoma, 29%(95); Trichostrongylus axei, 46%(3000); ascarids (n = 513) - Parascaris equorum, mature, 50%(25); immature, 23%(33); tapeworms (n = 513), Anoplocephala perfoliata, 17%(15); A. magna, 14%(10); large strongyles (n = 487), Strongylus vulgaris, 84%(80); S. edentatus, 79%(101); S. equinus, 6%(14); small strongyles (n = 210), 100%(142,000); pinworms (Oxyuris equi), immature (n = 210), 78%(9000); mature (n = 506), 40%(62); Probstmayria vivipara (n = 210), 12%(10(7]; S. vulgaris in cranial mesenteric artery (n = 472), 89%(57). The majority of the horses examined were mixed lighthorse type but several Thoroughbreds were included. Ages varied from about 4 months to 20 years old, with most being approximately 1-3 years old. They probably had either no or infrequent previous treatment with parasiticides. Most of the horses were selected for presence of certain internal parasites, usually large strongyles, prior to usage in the critical tests.

Critical tests in equids with fenbendazole alone or combined with piperazine: particular reference to activity on benzimidazole-resistant small strongyles.

Seven critical tests in equids were conducted with single doses of fenbendazole (5 mg kg-1) alone (Panacur--American Hoechst, Somerville, NJ); (2 tests with paste and 1 with suspension formulation) or in combination with piperazine (American Hoechst); (40 mg base kg-1); (4 tests with paste formulation). The main purpose of the tests was evaluation of activity against benzimidazole-resistant small strongyles (Cyathostomum catinatum, Cyathostomum coronatum, Cylicocyclus nassatus, Cylicostephanus goldi, and Cylicostephanus longibursatus). Natural infections of 2 populations of benzimidazole-resistant small strongyles were evaluated; 1 was population B in 2 horses and the other was population S in 5 ponies. Removal of the 5 species of population B was 49-91% in the animal treated with fenbendazole paste alone and 100% (4 of these species present) in the animal treated with the combination. For population S, 2 of the 5 resistant species were present in small numbers in 1 animal treated with fenbendazole paste alone and all were removed; the 1 animal receiving fenbendazole suspension alone had removals of 0-70% for the 5 benzimidazole-resistant species. Also for population S, the 5 resistant species were present in 2 animals treated with the paste combination and removal was 98-100% and of 4 of the 5 resistant species in 1 animal, removal was 76-99%. Removal of large strongyles (Strongylus vulgaris and Strongylus edentatus) was 92-100% for fenbendazole paste alone or in combination with piperazine in the 5 infected animals. For Oxyuris equi, present in 1 animal treated with the combination, there was 91% removal of immature and 100% removal of mature specimens. There probHably was no activity by fenbendazole alone or the combination against bots, tapeworms, and parenteral stages of S. vulgaris and S. edentatus. The combination may have had some activity against immature Habronema spp. and mature abronema muscae.

Critical tests of morantel-trichlorfon paste formulation against internal parasites of the horse.

Critical tests were completed on six horses to evaluate the antiparasitic activity of a paste formulation mixture of morantel citrate and trichlorfon, administered intraorally at the dose rate of 6 mg morantel base kg-1 and trichlorfon at 30 mg kg-1. Aggregate average removals were: 78% for two horses infected with 2nd instar Gasterophilus intestinalis; 100% for one infected with 2nd instar G. nasalis; 96% for six infected with 3rd instar G. intestinalis; 100% for four infected with 3rd instar G. nasalis; 100% for five infected with Parascaris equorum; 100% for one infected with mature Oxyuris equi; 100% for five infected with Strongylus vulgaris; 72% for five infected with S. edentatus; and partial removal (25%) of Anoplocephala perfoliata infection from one infected animal. Pre- and post-treatment EPG and LPG data indicated a reduction of 97% of the mature small strongyle infections. Evidence of toxicosis was not observed in any of the horses.

Historical perspective of cyathostomes: prevalence, treatment and control programs.

Small strongyles are highly prevalent in horses and can be found worldwide. Even though over 50 species of these parasites have been described, about 10 species comprise the bulk of the total number present in horses. Efforts to control small strongyles and other equine internal parasites have been undertaken for many years. Some of the early medications and control measures provided questionable activity against endoparasites; whether effective or not, they could be detrimental to the horse. Beginning in the early 1900s, scientific effort was used to evaluate activity of antiparasitic compounds. Only a relatively low number of classes of compounds has been used for parasite control. Progressively, in time, newer drugs have become broad-spectrum, safer, and available in lower dose rates than previously. With extensive use of parasiticides, small strongyles have been found to be resistant to all commercially-available classes of compounds except the macrocyclic lactones. Innovative parasite-control programs, relying less on drugs, need to be developed.

Natural infections of Strongyloides westeri: prevalence in horse foals on several farms in central Kentucky in 1992.

During the period 28 February-1 July 1992, fecal samples were collected and examined for eggs of Strongyloides westeri once from each of 382 horse foals (364 Thoroughbreds, 16 Standardbreds, one Lippizaner, and one draft-type horse) in central Kentucky. Ages of the foals at the time of sampling ranged from 7 to 63 days (mean 22 days). The nine farms in the study were considered to have overall excellent deworming programs. None of the foals had been treated with an antiparasitic compound before the study. Eggs of S. westeri were found in 6% (22 of 382) of the foals on 78% (7 of 9) of the farms. Current prevalence of this parasite in foals is discussed relative to studies before the availability and usage of effective drugs.

Transmission of some species of internal parasites in horses born in 1993, 1994, and 1995 on the same pasture on a farm in central Kentucky.

Data are presented on the last 3 years of a 7-year study (1989-1995) on transmission of natural infections of internal parasites in horse foals (n = 27) born in 1993, 1994, and 1995 on the same pasture on a farm in central Kentucky. The foals were in a closed breeding herd of horses. Research on the first 4 years (1989-1992) of the study was published earlier (Lyons et al., 1991, 1994). Thirty-five species of endoparasites were identified, including 24 species of small strongyles. Monthly, seasonal, and host-age transmission patterns were elucidated for the parasites. Comparison of data between the first 4 years and last 3 years of the study indicates similarities, but also differences, including an increase in prevalence and numbers of Thelazia lacrymalis and Anoplocephala perfoliata.

Natural superinfection of Parascaris equorum in a stall-confined orphan horse foal.

At 4 days of age, an orphan horse foal born on a pasture was placed in a parasite-free stall. It was euthanized at 205 days of age and examination revealed numerous. Parascaris equorum specimens, ranging from 3.0 mm to adult size, indicating reinfections in the stall over an extended period of time. Initial infection of the foal was from stages on pasture. The foal had been observed repeatedly eating its own feces and the question is posed as to whether "recycling' of P. equorum eggs several times by coprophagy allowed time for embryonation and reinfection to occur.

Phenothiazine in the origin of benzimidazole resistance in population-B equine strongyles.

Clinical trials on a central Kentucky Thoroughbred Farm (B) during 1960-1961 involved nine test treatments of mares, yearlings and weanlings (n = 124), and compared efficacies of phenothiazine (PTZ), piperazine (PPZ), mixtures of PTZ + PPZ and thiabendazole (TBZ) for (1) percent reduction and (2) complete clearance or graded reduction of strongyle egg per gram (epg) counts. Three grades of PTZ (N.F. green, micronized purified and 2-3 microns purified) at the dose rate of 55 mg kg-1 were ineffective in six tests; reduction of strongyle epg count were low grade (26%) and complete clearance was found in only 1 of 32 horses. Piperazine alone at 88 mg base kg-1 was efficacious in two tests; epg counts were effectively reduced (98%), but complete clearance was found in only 2 of 10 horses. Several mixtures of PTZ + PPZ with various dosages of PTZ (27.5-55 mg kg-1) plus PPZ base (88 mg kg-1) were quite effective in six tests, as epg counts were reduced by 96% and complete clearance occurred in 25 of 38 horses. Four trials with TBZ at dose rates between 13 and 44 mg kg-1 in 1961 reduced epg counts by 98%, but cleared only 15 of 36 horses. These data indicated: (1) a large segment of the Farm B small strongyle population was resistant to PTZ at the outset of these observations in 1960; (2) TBZ-resistant individuals were also present in this population when TBZ was first used in 1961. During the ensuing 4-year study (1962-1965) on Farm B mares (n = 15 per year) and yearlings (n = 15-17 per year), biweekly epg and larvae per gram (lpg) counts were carried out to compare the efficacy of three treatment programs: (1) monthly doses of TBZ at 26 mg kg-1; (2) monthly doses of TBZ at 13 mg kg-1; (3) bimonthly doses of a mixture of PTZ + PPZ and carbon disulfide (CS2) at dose rates of 27.5 + 88 and 78 mg kg-1, respectively, or a mixture of PTZ + PPZ + trichlorfon (TCF) at dose rates of 27.5 + 88 + 40 mg kg-1, respectively. Emergence of a TBZ-resistant segment of the small strongyle infections was evaluated by three criteria: (1) annual mean strongyle epg counts; (2) efficacy of individual treatments for epg reductions (%); (3) complete clearance or graded reductions of strongyle epg counts.4+ PTZ-resistant small strongyles. Initial use of TBZ in 1961 produced effective epg reductions, but complete clearance data revealed the presence of TBZ-resistant small strongyles at the outset...

Transmission of endoparasites in horse foals born on the same pasture on a farm in central Kentucky (1996-1999).

Research carried out during the last 4 years (1996-1999) of an 11-year study of the prevalence of internal parasites naturally transmitted to horse foals born on the same pasture on a farm in central Kentucky is presented here. Horses in this herd were not treated with any antiparasitic compound for over 20 years except for a replacement stallion in 1994. A total of 22 species, including 12 species of small strongyles, were recovered in the 4-year period. Transmission patterns of all species (n=35) of endoparasites recovered are compared for the 11-year study. Some of the changes were an increase in number of Thelazia lacrymalis and Anoplocephala perfoliata and a decrease in Gasterophilus intestinalis, Parascaris equorum, and Strongylus vulgaris. Clinical problems associated with parasitism were not observed in any of the 92 foals in the long-term investigation.

Continuance of studies on Population S benzimidazole-resistant small strongyles in a Shetland pony herd in Kentucky: effect of pyrantel pamoate (1992-1999).

Research on benzimidazole-resistant Population S small strongyles began in a Shetland pony herd in 1974 at the University of Kentucky and has continued for over 25 years. The present update, for the period 1992-1999, evaluated activity of pyrantel pamoate (PRT) in field tests in the pony herd. Additional critical tests with PRT and oxibendazole (OBZ) were done in foals born in the herd. Activity of PRT was initially excellent in field tests, based on epg/lpg count data, but declined rapidly during the second full year of pyrantel treatments. Critical test data for small strongyles indicated efficacies of PRT were about 60% at the beginning of the present observations and this intermediate level of removal continued throughout the seven-year period except for 1994 (75%). Unusual was the finding that field test epg/lpg data on small strongyles indicated much lower activity of PRT than found in worm count data in critical tests. The previously reported ineffective activity of OBZ on this population of small strongyles continued. Data are presented on prevalence and drug activity on several species of internal parasites besides small strongyles.