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.

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.

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.

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.

Macrocyclic lactones for parasite control in equids.

Macrocyclic lactones (MLs) revolutionized parasite control in horses and other animals. They are unique in that they are effective against arthropods and nematodes. The first of the widely used avermectins was ivermectin. In 1983, it was marketed for use in horses as an injectable formulation but was withdrawn in 1984 after about a year and half on the market because of adverse problems. It was replaced by a paste formulation and an oral/stomach tube liquid formulation. Ivermectin is highly active on bots, ascarids, large and small strongyles, pinworms, strongyloides, stomach worms, and some other internal parasite species. Another ML, moxidectin, became available in 1997 as a gel formulation for oral administration. The parasiticidal activity of this compound is similar to ivermectin except efficacy is less on the common bot (Gastrophilus intestinalis) but high on encysted small strongyles. Recently however lower than initial activity on ascarids and small strongyles has been found for both ivermectin and moxidectin.

Analysis of multiyear studies in horses in Kentucky to ascertain whether counts of eggs and larvae per gram of feces are reliable indicators of numbers of strongyles and ascarids present.

Increasing levels of anthelmintic resistance in equine nematodes have led to recommendations of more sustainable anthelmintic treatment protocols with emphasis on parasite surveillance and diagnosis, rather than prophylactic calendar-based treatments. This requires knowledge of the diagnostic test performance of techniques for counts of eggs per gram of feces (EPG) as well as methods for culturing, counting and identifying third stage (L(3)) strongyle larvae per gram of feces (LPG). For horses, such information does not exist in the published literature. The aim of this study was to examine the relationship between worm count and fecal egg count (FEC) data for strongyle and Parascaris equorum infections as well as larval culture counts for diagnosing Strongylus spp. infections. Necropsy data from 693 horses used for critical or controlled tests, including information on total worm counts, fecal egg counts (FEC) and larval culture results collected at the University of Kentucky over a period of 50 years were analyzed. Sensitivity, specificity, positive and negative predictive values, and receiver operator characteristic (ROC) curves were generated for the larval cultures and ascarid egg counts. For the strongyle egg counts, potential FEC cutoff values for treatment were evaluated statistically by comparing the total strongyle worm counts below and above chosen cutoff values. All tests had high positive predictive values (>0.95), but moderate negative predictive values (<0.70). The negative predictive values of the larval counts were negatively affected by increasing egg count levels. Strongyle FEC cutoff values up to the level of 500 EPG yielded significantly higher strongyle worm counts in the treatment group, whereas no differences were found at higher cutoffs. This supports usage of cutoffs for treatment in the 0-500 EPG range. Altogether, the present study yields unique and useful information of widely used methods for parasite surveillance and diagnosis in equine establishments.

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 microg/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 3237 (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 L(4) 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 (L(4) 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.

Hookworms (Uncinaria lucasi) and acanthocephalans (Corynosoma spp. and Bolbosoma spp.) found in dead northern fur seals (Callorhinus ursinus) on St. Paul Island, Alaska in 2007.

Intestines of dead northern fur seals (Callorhinus ursinus) on St. Paul Island (SPI), Alaska were inspected for specific internal parasites (hookworms and acanthocephalans) in July and August, 2007. Pups (n=64) were examined for adult hookworms (Uncinaria lucasi) and four (6.25%) were infected. The number of specimens per infected pup was 1, 2, 2, or 408. Low prevalence was similar to that determined in the last investigation (2001) in dead fur seal pups on SPI by two of the present authors (Lyons and Spraker). Subadult males (SAMS-3-4 years old, n=115) were examined for acanthocephalans and 25 (21.7.0%) were infected. Adult Corynosoma (C. obtuscens, C. strumosum, and C. validum) (n=56) and immature Bolbosoma spp. (n=4) were found. Apparently, this is a new host record for C. obtuscens and C. validum. Notes were made on finding tapeworms. in 114 of the 115 SAMs. Examination of some specimens revealed scolices characteristic of Diphyllobothrium spp.

Evaluation of parasiticidal activity of fenbendazole, ivermectin, oxibendazole, and pyrantel pamoate in horse foals with emphasis on ascarids (Parascaris equorum) in field studies on five farms in Central Kentucky in 2007.

Horse foals on five farms in Central Kentucky were used in field studies in 2007 evaluating activity of paste formulations of four compounds (fenbendazole-FBZ, ivermectin-IVM, oxibendazole-OBZ, and pyrantel pamoate-PRT) against internal parasites with emphasis on ascarids (Parascaris equorum). It has been well established the last few years that there is widespread resistance of P. equorum to ivermectin. The main purpose of the present research was to obtain current data on ascaridicidal activity of FBZ, OBZ, and PRT; also, to acquire further information on ascarid resistance to ivermectin. Additionally, data were documented on drug activity on small strongyles. Detection of ascarid and strongyle eggs in feces of foals was by a qualitative method (presence or absence) or a quantitative method (eggs per gram of feces). Strongyle eggs all were assumed to be from small strongyles. This is based on fecal cultures from horses on one farm and historic records from horses in this area on excellent deworming programs. A girth tape was used to estimate the body weight of each foal so that the appropriate dose rate of each drug could be given. Many of the foals were used in more than one cycle of treatments. Efficacy of the drugs, administered intraorally, was determined by calculating the average percentage reduction (% red.) of the number of foals passing eggs after vs. before treatment: (1) FBZ at 10 mg/kg was tested on four farms; 76 foals were examined, 50 with ascarid eggs (84% red.) and 62 with strongyle eggs (0% red.); (2) IVM at 200 microg/kg was tested on three farms; 58 foals were examined, 18 with ascarid eggs (0% red.) and 48 with strongyle eggs (100% red.); (3) OBZ at 10 mg/kg was tested on three farms; 181 foals were examined, 78 with ascarid eggs (94% red.) and 79 with strongyle eggs (0% red.); (4) PRT was tested on two farms, one farm at 1x (6.6 mg base/kg); 42 were foals examined, 16 with ascarid eggs (0% red.) and 33 with strongyle eggs (12% red.) and one farm at 2x (13.2 mg base/kg); 18 foals were examined, 13 with ascarid eggs (23% red.) and 15 with strongyle eggs (27% red.).

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 microg/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.

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.

Parasite field study in central Kentucky on thoroughbred foals (born in 2004) treated with pyrantel tartrate daily and other parasiticides periodically.

Foals (79), born in 2004 on three thoroughbred horse farms (C, M, and S) in central Kentucky, were fed pyrantel tartrate daily, beginning at about 3 months of age. In addition, other parasiticides [fenbendazole (FBZ), ivermectin (IVM) alone or with praziquantel (PRAZ), oxibendazole (OBZ), pyrantel pamoate (PRT), and moxidectin (MOX)] were given periodically. All treatments were administered by farm personnel. Over a 14-month period, from May 2004 to July 2005, collections (n=989) of feces were made from the foals for determination of presence of internal parasite eggs/oocysts by qualitative and/or quantitative methods. Conclusions on drug activity are based necessarily on considering the combined effect of pyrantel tartrate and the other compounds. For small strongyles, this was related to which specific additional compound was given. Based on the percentage of foals with strongyle-egg-positive feces and/or the level of eggs per gram of feces (EPG) counts for the foals after treatment, drug activity on small strongyles was highest to lowest for MOX, IVM and IVM/PRAZ, FBZ, OBZ, PRT, and FBZ (2x for 5 days). The macrocyclic lactones (MOX and IVM) were highly superior to the other compounds. Some of the strongyle counts were high (over 2,000), especially on one farm (S), during periods when foals received only pyrantel tartrate, but a few days after administration of therapeutic dose rates of the drugs IVM or MOX, they were negative or very low. Ascarid eggs were present in feces of three foals after treatment with a combination of IVM and PRAZ. The qualitative method was more efficient than the quantitative method in detection of ascarid and strongyle eggs in the feces. Prevalence of eggs of ascarids (Parascaris equorum) was low (0, 4, and 31%), of strongyles high (80, 100, and 100%), of Strongyloides westeri very low (only one infected foal), and oocysts of Eimeria leuckarti medium to high (36, 41, and 85%) for the three farms, C, M, and S, respectively. It is uncertain whether the low ascarid prevalence was from activity of pyrantel tartrate and/or the other drugs or to a limited source of infective eggs.

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).

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.

Field studies on endoparasites of Thoroughbred foals on seven farms in central Kentucky in 2004.

Fecal samples (n=1,584) for a parasite study were collected from Thoroughbred foals (n=349), 28 to 330 days old, on seven farms in central Kentucky during 49 monthly trips from May to October, November, or December 2004. The main purpose was to determine possible drug resistance of ascarids, also of strongyles, to ivermectin (IVM) and other commercially available compounds [fenbendazole, moxidectin (MOX), oxibendazole (OBZ), and pyrantel pamoate]. In addition, interest was on prevalence of foal parasites. Qualitative data were obtained by recording presence or absence of ascarid and strongyle eggs in feces from 7 to 25 days after treatment of some of the foals. None of the compounds completely eliminated ascarid eggs in feces of all foals posttreatment. Activity on ascarids was significantly the highest for OBZ and lowest for IVM. Activity on strongyles was significantly higher for IVM and MOX than for the other compounds. Prevalence (mean percentage of foals infected) was recorded for eggs of Parascaris equorum (39%), of strongyles (32%), and of Strongyloides westeri (2%) and oocysts of Eimeria leuckarti (28%) in feces of foals. One or more foals had infections of P. equorum on six farms (86%), strongyles on seven farms (100%), S. westeri on two farms (29%), and E. leuckarti on six farms (86%). The oldest foal infected with E. leuckarti was 301 days of age. Comparison was made on the prevalence of parasites in foals on five of the same farms in the 2004 study, which were part of a similar 14-farm project in 2003 (Lyons and Tolliver in Parasitol Res (2004) 92:400-404).

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.

Seasonal prevalence and intensity of hookworms (Uncinaria spp.) in California sea lion (Zalophus californianus) pups born in 2002 on San Miguel Island, California.

Intestines of California sea lion (Zalophus californianus) pups (n= 204), born in 2002 on San Miguel Island, California, were examined for hookworms (Uncinaria spp.) as part of a seasonal mortality study from June through December 2002 and January 2003. The investigation was planned to coincide with most of the previously established hookworm infection period of the pups. Prevalence of hookworms in dead pups was 100% for each month of the study. The geometric mean intensity of infections per month was: 94.03 (n=30) for June, 629.09 (n=50) for July, 319.90 (n=31) for August, 159.90 (n=30) for October, 109.03 (n=30) for November, 37.84 (n=24) for December 2002 and 11.05 (n=9) for January 2003. In addition to the temporal pattern, the infection intensity was higher for pups in good condition and for male pups. An inter-year comparison of hookworm counts from dead pups collected in July of 1996, 2000, and 2002 also demonstrated higher intensity in pups in better condition but sex-differences in intensity were inconsistent across years. The inter-year comparison also demonstrated higher intensities in dead pups collected from portions of the rookery with sandy substrate versus rocky substrate. No annual differences in intensity were found after adjusting for substrate and condition.