Veterinary RecordVolume 164, Issue 4 p. 106-106 Surveillance Anthelmintic resistance in equids First published: 24 January 2009 https://doi.org/10.1136/vr.164.4.106Read the full textAboutPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Share a linkShare onFacebookTwitterLinked InRedditWechat Volume164, Issue4January 2009Pages 106-106 RelatedInformation

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.

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.

Clinical cyathostomiasis is a parasitic disease caused by a sudden release of huge numbers of encysted small strongly or cyathostome larvae from the lining of the large intestine of horses. Included among the clinical signs are diarrhea, ventral abdominal edema, weight loss, pyrexia, colic, hypoproteinemia, and the presence of large numbers of larval cyathostomes in the feces. The disease is reported more commonly in the winter and spring and after treatment with antiparasitic compounds effective on the luminal stages of these parasites.

A survey to determine current prevalence of several species of endoparasites in equids (n=350) at necropsy was conducted at the Livestock Disease Diagnostic Center in central Kentucky (1995–1999). Thelazia lacrymalis were found in 42% of the 1- to 4-year olds (n=132), which was the only age group examined for these parasites. Five- to 30-year olds (n=157), examined for Onchocerca spp. worms/lesions, had a prevalence of 24%. In ≥1-year olds (n=324), the infection rate for Gasterophilus intestinalis was 12% for second instars and 14% for third instars and for Gasterophilus nasalis was 2% for second instars and 2% for third instars. For Anoplocephala perfoliata (n=268 equids), prevalence was 52%. Data were compiled for the equids according to breed, age, sex, month of necropsy, and number of parasites. Arterial lesions caused by migrating Strongylus vulgaris larvae were observed in a low number (3 of 52) of equids. Specific examination for Parascaris equorum revealed that 46% of 13 weanlings and 10% of 20 older horses were infected. Anoplocephala magna were found in one of the weanlings. Compared to previous studies (about 15–25 years ago) in Kentucky, the present survey indicated the genera Onchocerca and Gasterophilus greatly declined but Thelazia, Anoplocephala, and Parascaris did not.

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.

Pyrantel pamoate paste was administered intraorally once at the therapeutic dose rate (6.6 mg base/kg) to 17 horses naturally infected with Anoplocephala perfoliata. Evaluation of drug activity by a modified critical test method indicated removals varying from 0 to 100% (aggregate average of 70%). Clearance was 0-27% (n = 2 horses), 36-44% (n = 3), 64-67% (n = 2), 75-88% (n = 4), 91-98% (n = 3), and 100% (n = 3).

A field test was done to evaluate activity on Anoplocephala perfoliata for the low dose rate (2.64 mg/kg) of pyrantel tartrate fed once daily for 30 consecutive days to Thoroughbred mares (n = 83) and yearlings (n = 58) on a farm in central Kentucky. Tapeworm eggs were found pretreatment in feces of 35% of the mares and 33% of the yearlings. Posttreat-ment fecal samples for the mares were negative for tapeworm eggs at 35 days and 153-167 days postinitial treatment day (PIT) but positive (2%) at 72-86 days PIT. For the yearlings, post-treatment fecal samples were positive for tapeworm eggs at 2 days (71%), 7 days (33%), 14 days (13%), 21 days (7%), 35 days (2%), and 75 days (2%) but negative at 28 days PIT. Also, for the yearlings, about two-thirds of the feces was collected during the first 2 days PIT for examination for A. profoliata specimens; 401 with scolices were found in 74% of these animals. Prevalence of A. perfoliata in the yearlings was 96%, as determined by findings eggs in feces pretreatment and 48 hr PIT in addition to specimens passed in feces.

As part of a study evaluating efficacy of pyrantel tartrate fed at the low dose rate (2.64 mg/kg) once daily for 30 consecutive days against Anoplocephala perfoliata in Thoroughbreds on Farm A in central Kentucky, about two-thirds of the feces passed by yearlings (n = 58) for 2 days postinitial treatment day was examined for specimens of this parasite. Efficacy data are given elsewhere (Lyons, et al., 1997), and observations on external characteristics of A. perfoliata specimens recovered from feces are presented here. Of a total of 401 A. perfoliata specimens with scolices found in feces of 43 of the yearlings, external features were typical (normal strobila and scolex with 4 suckers/4 lappets) for 97.5% (n = 391 specimens). However, 2.5% (n = 10) were atypical: (a) 1 had a normal strobila but a scolex with 6 suckers/4 lappets, (b) 7 had a triradiate strobila and a scolex with either 5 suckers/6 lappets (n = 1) or 6 suckers/6 lappets (n = 6), and (c) 2 had a tetraradiate strobila and a scolex with either 7 suckers/7 lappets or 8 suckers/8 lappets. Reexamination of 22,557 specimens of A. perfoliata from equids in previous investigations at the University of Kentucky, for comparison of external characteristics in specimens from Farm A horses, revealed 99.95% (n = 22,545) were typical and 0.05% (n = 12) were atypical (all had a triradiate strobila and a scolex with 6 suckers/6 lappets).

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. (C) 1997 Elsevier Science B.V.

At 4 days of age, an orphan horse foal horn 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.

Trichostrongylus axei have been maintained in domestic rabbits (Oryctolagus cuniculus), by serial passage, at the University of Kentucky since 1953 for an equine strain (A) and 1954 for a bovine strain (O). On 17 August, 1995, this research was Terminated. Presentation here is mostly on cumulative data on number of serial passages, infectivity, and longevity of T. axei since 1984/1985. Comparison is made with earlier research, much of which has been published (Lyons et al., 1987).

Several compounds (n = 13 single or combinations; most at therapeutic dosages) were evaluated between 1977 and 1992 in critical tests (n = 91) against benzimidazole (BZ) resistant small strongyles (Population S) and several other species of internal parasites in Shetland ponies, mostly under 1 year old. The closed breeding herd, from which the test ponies were selected, had been treated every 8 weeks with cambendazole (CBZ) for 4 years (1974-1978) and oxibendazole (OBZ) for 14 years (1978-1992). Published field test data (1974-1992) on older ponies in the herd showed BZ resistance of small strongyles. Average efficacies in the present critical tests against small strongyles for OBZ (n = 59 animals) were high in early years (95% or higher), but gradually declined to a low of 1% in 1991. Side-resistance of small strongyles was evident in critical tests (n = 1-6/single drug or combination) for several other BZs and a pro-BZ; ivermectin and piperazine were highly active, but pyrantel pamoate exhibited weak activity. BZ resistance was evident for six small strongyle species (Cyathostomum catinatum, Cyathostomum coronatum, Cylicocylus nassatus, Cylicostephanus calicatus, Cylicostephanus goldi, and Cylicostephanus longibursatus). Activity on bots, ascarids, large strongyles, and pinworms was essentially as expected, indicating no drug resistance.

Praziquantel injectable formulation was administered at 0.5 mg k-1 per os to 24 equids naturally infected with 1-183 (average 40) Anoplocephala perfoliata. Drug activity was evaluated by a modified critical test method with necropsy 24 h after treatment. There was variable efficacy of 0-100% (aggregate average 85%); for 18 equids, 93-100%, for three equids, 70-85%, and for three equids, 0-20%.

Studies were conducted on transmission of natural infections of several species of internal parasites in horses born and kept on the same pasture on a farm in central Kentucky. Data for the first year (1989) of a 4 year study on this farm have been published recently. The present research represents the second (1990), third (1991), and fourth (1992) years of the investigation. The number of animals (n = 28) examined varied from eight born in 1990 to ten each born in 1991 and 1992. For each year, examination was made of one horse per month, beginning in June of the year of birth and extending through January (1990) or March (1991 and 1992) the following year. Ages of the horses at necropsy ranged from 87 to 251 days. Major parasites present and months of recovery were: bots - Gasterophilus intestinalis in the mouth September-January and in the stomach August-March; stomach worms - Trichostrongylus axei in August and November, Habronema spp. (immature) in July-November and January, and Habronema muscae in October, January, and February; ascarids - Parascaris equorum in the small intestine and lungs all months; intestinal threadworms - Strongyloides westeri in all months except February; large strongyles - Strongylus vulgaris in the large intestine in all months except July and August and in the cranial mesenteric artery in all months, and Strongylus edentatus in the large intestine in January and in the ventral abdominal wall in all months; pinworms - Oxyuris equi in June and January-March; tapeworms - Anoplocephala perfoliata in August-October and December-March; and eyeworms - Thelazia lacrymalis August-February. Yearly differences and similarities o f infections in the horses are discussed. The value of this type of research is mentioned.

Studies in a band of ponies harboring Population S benzimidazole-resistant small strongyles were initiated in 1974 and have continued for 18 years. Treatment (bimonthly) was with cambendazole for the first 4 years and with oxibendazole (OBZ) for the next 14 years. Data on the first 10 years have been published. The present investigation includes the last 8 years (4 October 1984-11 September 1992), which are the seventh through fourteenth years, of treatment with OBZ. Pre- and posttreatment mean counts of strongyle eggs (epg) and larvae (lpg) per gram of feces were determined biweekly during the current study to monitor the efficacy of OBZ. The average annual percent reductions of epg counts effected by OBZ treatments were 51%, 53%, 38%, 38%, 39%, 28%, 40%, and 19% for the seventhth through fourteenth years, respectively. Similar levels of reductions were observed for lpg counts. Although OBZ was initially highly effective on this population of small strongyles, epg and lpg counts gradually declined, but have remained more or less constant since the fifth year of research. However, reductions of the counts were the lowest for the last year of the study.