Epizootic of equine protozoal myeloencephalitis on a farm.

OBJECTIVE: To determine the clinical findings, course of treatment, and long-term outcome of horses on a farm in central Kentucky during an epizootic of equine protozoal myeloencephalitis (EPM). DESIGN: Cohort study. ANIMALS: 21 horses on a farm in central Kentucky, 12 of which developed clinical signs of EPM. PROCEDURE: Horses on the farm were serially examined for signs of neurologic disease and serum and CSF antibodies to Sarcocystis neurona. Horses were considered to have EPM if they had neurologic signs and positive test results for antibodies to S neurona in CSF. Blood values were monitored for evidence of abnormalities resulting from long-term pyrimethamine and trimethoprim-sulfamethoxazole administration Physical, neurologic, and fetal necropsy examinations were performed as needed. Horses were treated for EPM until they had negative test results for CSF antibodies to S neurona. RESULTS: Of 21 horses on the farm, 12 had EPM over the course of 6 months. The duration of treatment ranged from 45 to 211 days, excluding 1 horse that persistently had CSF antibodies to S neurona. Adverse effects from pyrimethamine and trimethoprim-sulfamethoxazole administration included transient fever, anorexia, and depression (n = 2); acute worsening of ataxia (2); mild anemia (4); and abortions (3). CLINICAL IMPLICATIONS: EPM may develop as an epizootic. In the horses of this report subtle clinical signs that were originally considered unimportant ultimately progressed to obvious neurologic signs. Adverse effects associated with EPM treatment included worsening of neurologic signs, anemia, abortion, and leukopenic and febrile episodes.

Seroprevalence of antibodies to Sarcocystis neurona in horses residing in a county of southeastern Pennsylvania.

OBJECTIVE: To determine seroprevalence of Sarcocystis neurona-specific antibodies in a population of horses residing in Chester County, Pa. DESIGN: Prevalence survey. SAMPLE POPULATION: 117 serum samples from selected members of a population of 580 Thoroughbred horses. PROCEDURE: Serum was analyzed for antibodies to Sarcocystic neurona, using a western blot. Information regarding age, sex, and housing of horse was obtained by questionnaire. Data were analyzed, using multivariable logistic regression. RESULTS: Seroprevalence was 45.3% (95% CI, 36.3 to 54.3%). A relationship was not found between seroprevalence and sex of horse. Seroprevalence was greater in older horses (logistic regression; P = 0.16). CLINICAL IMPLICATIONS: High seroprevalence of antibodies to S neurona and the lack of neurologic deficits among horses sampled indicate that positive results of serologic examination alone are of limited value for diagnosis of equine protozoal myeloencephalitis. Clinical signs consistent with the disease are the most important consideration in accurate diagnosis.

Seroprevalence of antibodies to Sarcocystis neurona in horses residing in Ohio.

OBJECTIVE: To determine the seroprevalence of serum antibodies to Sarcocystis neurona in horses residing in Ohio. DESIGN: Prevalence survey. SAMPLE POPULATION: Serum from samples from 1,056 horses. Serum was collected on every 36th sample submitted to the Ohio State Diagnostic Laboratory for testing for equine infectious anemia. PROCEDURE: Serum was frozen at -80 C and analyzed for antibodies to S neurona, using a western blot. Information regarding blood sample collection, age, breed, sex, and geographic location was recorded for each horse. Data were analyzed, using multivariable logistic regression. RESULTS: Horses of 37 breeds from 81 of Ohio's 88 counties were included in the study population. There were 481 females, 133 males, and 442 geldings ranging in age from 3 months to 27 years; > 48% were < 5.6 years old. More than 53% of samples were seropositive for antibodies to S neurona. A gender or breed effect on seroprevalence was not identified. There was a significant effect of age (P < or = 0.0001; with older horses more likely to be affected), and of location (statistical and extension districts; P = 0.02 and P = 0.03, respectively) on seroprevalence. Location effects appeared to be correlated to the number of days with temperatures below freezing (P < 0.05). CLINICAL IMPLICATIONS: The high seroprevalence of antibodies to S neurona found in the sample population emphasizes the importance of examining CSF for S neurona-specific antibodies when establishing a diagnosis of equn protozoal myeloencephalitis.

Seroprevalence of antibodies to Sarcocystis neurona in horses residing in Oregon.

OBJECTIVE: To determine seroprevalence of antibodies to Sarcocystis neurona in neurologically normal horses residing in 4 regions of Oregon and to describe the effects of age, gender, breed, and housing on seroprevalence within each region. DESIGN: Prevalence survey. SAMPLE POPULATION: Serum samples from 334 horses systematically selected by practicing veterinarians. PROCEDURE: Antibodies to S neurona were measured in sera, using a western blot. Information including age, gender, breed, housing, geographic location, and duration of residence was obtained for each horse. Data were analyzed, using descriptive statistics. RESULTS: 45% (149/334) of horses evaluated were seropositive for antibodies to S neurona with significant differences in the percentage of seropositive horses from different regions of the state. Seroprevalances of antibodies to S neurona in horses in regions I and II, west of the Cascade Range, were 65 and 60%, respectively; whereas seroprevalances in central and eastern Oregon, regions III and IV, were 43 and 22%, respectively. Seroprevalence consistently increased with age of horse for each region. Gender, breed, and housing were not associated with significant differences in seroprevalence of antibodies to S neurona in the overall sample population, or in comparisons of samples obtained from horses within a particular region, or among samples obtained from horses residing in different regions. CLINICAL IMPLICATIONS: The high seroprevalence of antibodies to S neurona in neurologically normal horses indicates that analysis of serum alone would not be useful for definitive diagnosis of equine protozoal myeloencephalitis in horses in Oregon.

Experimental induction of equine protozoal myeloencephalitis in horses using Sarcocystis sp. sporocysts from the opossum (Didelphis virginiana).

Sarcocystis sp. sporocysts isolated from eight feral opossums (Didelphis virginiana) were pooled and fed to 18 commercially reared budgerigars (Melopsittacus undulatus), 14 wild-caught sparrows (Passer domesticus), one wild-caught slate-colored Junco (Junco hyemalis) and five weanling horses (Equus caballus). All budgerigars died within 5 weeks post inoculation (wpi). Histologic examination revealed meronts within the pulmonary epithelia and typical Sarcocystis falcatula sarcocysts developing in the leg muscles. Sparrows were euthanized 13 and 17 wpi and their carcasses were fed to four laboratory raised opossums. Sporocysts were detected in the feces of two opossums on 15 days post inoculation (dpi) and in a third opossum on 40 dpi. Fecal samples from the fourth opossum remained negative; however, sporocysts were found in intestinal digests from all four opossums. Sporocysts were not found in feces or intestinal digest of an additional opossum that was fed three uninoculated sparrows. Five foals were fed sporocysts (Foals 2, 3, 4, 5, and 7) and two foals were maintained as uninoculated controls (Foals 1 and 6). Sporocysts from two additional feral opossums also were fed to foals. Foal 5 was given 0.05 mg kg-1 dexamethasone sodium phosphate daily beginning 2 days before inoculation for a total of 2 weeks. Horse sera were tested three times per week, and cerebrospinal fluid (CSF) samples were tested biweekly for anti-Sarcocystis neurona antibodies by Western blot analysis. No foals had any S. neurona-specific antibodies by Western blot analysis prior to sporocysts ingestion. Seroconversion occurred in Foals 3, 5, and 7 by 24 dpi, followed by positive CSF tests on 28 dpi. Foals 2 and 4 seroconverted by 40 dpi. Cerebrospinal fluid from Foal 2 tested positive by 42 dpi, but Foal 4 remained seronegative throughout the study. Sera and CSF from control Foals 1 and 6 remained seronegative. All foals with positive CSF developed neurologic clinical signs. Neurologic disease was evident in Foals 2 and 3 by 42 dpi and in Foal 7 by 28 dpi. The severity of clinical signs progressed to marked spasticity, hypermetria and ataxia in Foal 7 by the end of the trial. Necropsy examination of inoculated foals did not reveal gross lesions; however, microscopic lesions consistent with equine protozoal myeloencephalitis (EPM) were found in Foals 2, 3, and 7. Protozoa were not observed in the tissue sections. Microscopic lesions consistent with EPM were not found in Foals 4 and 5 or in uninoculated control Foals 1 and 6. Foal 5 had unilateral non-inflammatory lesions in the cervical and thoracic spinal cord consistent with cord compression. These data indicate that the opossum is a definitive host of S. neurona.

Critical test evaluation (1977-1992) of drug efficacy against endoparasites featuring benzimidazole-resistant small strongyles (population S) in Shetland ponies.

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.

A study (1977-1992) of population dynamics of endoparasites featuring benzimidazole-resistant small strongyles (population S) in Shetland ponies.

Critical tests (91) were done between 1977 and 1992 in Shetland ponies to evaluate drug susceptibility and population dynamics (present paper) of endoparasites. The test ponies, most less than 1 year old, were from a herd where older animals were treated every 8 weeks initially with cambendazole (CBZ) (1974-1978) and then with oxibendazole (OBZ) (1978-1992). Previous field test data (1974-1992) on older ponies in the breeding herd indicated the presence of benzimidazole (BZ) resistant small strongyles. Data on population dynamics from the present critical tests indicated that 28 species of small strongyles persisted over the study period in spite of initial susceptibility and later refractiveness of six species to both CBZ and OBZ. Changes in intensities and other aspects were observed for the six BZ-resistant species (Cyathostomum catinatium, Cyathostomum coronatum, Cylicocyclus nassatus, Cylicostephanus calicatus, Cylicostephanus goldi, and Cylicostephanus longibursatus). Variabilities, some striking, were found in prevalence and intensity in bots, stomach worms, ascarids, eyeworms, large strongyles, pinworms and tapeworms.

Identification of opossums (Didelphis virginiana) as the putative definitive host of Sarcocystis neurona.

Sarcocystis neurona is an apicomplexan that causes equine protozoal myeloencephalitis (EPM) in North and South America. Horses appear to be an aberrant host, because the merozoites continually divide in the central nervous system, without encysting. The natural host species has not previously been identified. The small subunit ribosomal RNA (SSURNA) gene of S. neurona was compared to those of Sarcocystis muris, Sarcocystis cruzi, Toxoplasma gondii, and Cryptosporidium parvum to identify a unique region suitable for a species-specific amplification primer. The S. neurona SSURNA primer was used in a polymerase chain reaction (PCR) assay for the purpose of identifying this organism in feces and intestinal digest of wildlife specimens. Sporocysts were isolated from 4 raccoons (Procyon lotor), 2 opossums (Didelphis virginiana), 7 skunks (Mephitis mephitis), 6 cats (Felis catus), 1 hawk (Accipiter sp.), and 1 coyote (Canis latrans). The S. neurona SSURNA PCR assay and a control PCR assay using protist-specific primers were applied to all sporocyst DNA samples. All sporocyst DNA samples tested positive on the control assay. The SSURNA PCR assay yielded a 484-bp product only when applied to opossum samples. The SSURNA gene of both opossum sporocyst samples was sequenced to determine its relationship to the S. neurona SSURNA gene. The sequence had 99.89% similarity with S. neurona. This suggests that opossums are the definitive host of S. neurona.

Survey for internal parasites in cattle in Kentucky (1993).

Fecal samples were examined between 17 September and 1 November 1993 from 1765 cattle (one bull, 533 cows, 474 heifers, 22 steers, and 735 calves) on pasture on 15 farms in 11 counties in Kentucky for eggs or larvae of internal parasites. All of the cattle were beef-type except for 22 which were dairy-type. In fecal samples from the bull, cows, heifers, steers, and calves, the types of helminth eggs present were trichostrongyles (excluding Nematodirus) in 0%, 25%, 31%, 86%, and 93%, Nematodirus in 0%, 0%, < 1%, 0%, and 34%, Strongyloides in 0%, 0%, < 1%, 0%, and 7%, Trichuris in 0%, 0%, 0%, 0%, and 2%, Capillaria in 0%, 0%, 0%, 0%, and < 1%, and Moniezia in 0%, 1%, 8%, 5%, and 21%, respectively. Dictyocaulus viviparus larvae were present in feces of calves (7%) on one farm and heifers (< 1%) on another farm.

Activity of praziquantel (0.5 mg kg-1) against Anoplocephala perfoliata (Cestoda) in equids.

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%.

Phylogenetic relationship of Sarcocystis neurona to other members of the family Sarcocystidae based on small subunit ribosomal RNA gene sequence.

Sarcocystis neurona is a coccidial parasite that causes a neurologic disease of horses in North and South America. The natural host species are not known and classification is based on ultrastructural analysis. The small subunit ribosomal RNA (SSURNA) gene of S. neurona was amplified using polymerase chain reaction techniques and sequenced by Sanger sequencing reactions. The sequence was compared with partial sequences of S. muris, S. gigantea, S. tenella, S. cruzi, S. arieticanis, S. capracanis, Toxoplasma gondii, Eimeria tenella, and Cryptosporidium parvum. Alignments of available sites for all 10 species and alignments of the entire SSURNA sequence of S. neurona, S. muris, S. cruzi, T. gondii, and C. parvum were performed. Alignments were analyzed using maximum parsimony and maximum likelihood methods to determine relative phylogeny of these organisms. These analyses confirmed placement of S. neurona in the genus Sarcocystis and suggested a close relationship to S. muris, S. gigantea, and T. gondii. Molecular phylogeny suggests that Sarcocystis spp., which utilize the dog (Canis familiaris) as the definitive host, evolved from a common ancestor, whereas those species (including T. gondii) that utilize the cat (Felis domesticus) as the definitive host evolved from another common ancestor. This suggests a possible definitive host for S. neurona.

Differentiation of Sarcocystis neurona from eight related coccidia by random amplified polymorphic DNA assay.

Four isolates of Sarcocystis neurona from horses with equine protozoal myeloencephalitis and eight species of coccidia from the genera Sarcocystis, Toxoplasma or Eimeria were differentiated using the random amplified polymorphic DNA assay. A single, common, 550-bp DNA fragment was amplified from the DNA of each S. neurona isolate using a 16-nucleotide primer. Crosshybridization analyses among S. neurona isolates showed that DNA fragments had at least partial sequence homology. The primer generated several DNA fragments, including a 550-bp DNA fragment, from S. cruzi, Eimeria falciformis, E. neischulzi, E. ahsata and E. bovis. DNA hybridization analyses indicated no sequence homology between these fragments and the 550-bp DNA fragment generated from S. neurona. The S. neurona 550-bp DNA fragment also did not hybridize with genomic blots of various other coccidia. These results suggest that the S. neurona DNA fragment may be exploited as a species-specific probe for this parasite.

Transmission of some species of internal parasites in horses born in 1990, 1991, and 1992 in the same pasture on a farm in central Kentucky.

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 of infections in the horses are discussed. The value of this type of research is mentioned.

Control of cambendazole-resistant small strongyles (Population S) with oxibendazole in a pony band: an 8 year field test (1984-1992).

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 seventh 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.

Critical tests of thiabendazole, oxibendazole, and oxfendazole for drug resistance of population-B equine small strongyles (1989 and 1990).

Critical tests were conducted in horses (n = 11) with naturally acquired infections of benzimidazole (BZ)-resistant population-B small strongyles in 1989 and 1990. Anthelmintics administered were thiabendazole (44 mg/kg of body weight, n = 4), oxibendazole (10 mg/kg, n = 3), and oxfendazole (OFZ, 10 mg/kg; n = 4). All compounds were paste formulations administered orally except for 1 of the OFZ treatments, which was a suspension formulation given by stomach tube. Aggregate mean efficacy was calculated for all species of small strongyles, drug-resistant and non-resistant. The highest efficacy was for oxibendazole (98%) and OFZ (94%); efficacy for thiabendazole was 63%. Five genera and 16 species of small strongyles were recovered from the 11 horses, ranging from 7 to 13 species (mean, 11). Of these, 7 species were found to have resistance in variable degrees to most of the anthelmintics. These strongyles were Cyathostomum catinatum, Cyathostomum coronatum, Cylicocyclus nassatus, Cylicostephanus calicatus, Cylicostephanus goldi, Cylicostephanus longibursatus, and Cylicostephanus minutus. The large strongyle, Strongylus vulgaris, was present in all 11 test horses, and efficacy was 100% for all drugs. Seven of the BZ-treated foals (at least 1 horse from each BZ-treatment group), were infected with S edentatus; removal was 100%.

Controlled tests of activity of several antiparasitic compounds against natural infections of Haemonchus contortus and other helminths in lambs from a flock established in 1962.

Antiparasitic activity of several compounds was evaluated over a long period (about 25 years) in the same flock of sheep. Haemonchus contortus was of special interest, including its relation to drug resistance, especially to thiabendazole and other benzimidazoles, in addition to phenothiazine. Eleven compounds were evaluated in 15 controlled tests, done between 1966 and 1989 in naturally infected lambs (n = 145) born and raised on the same pasture. Sheep were first placed on the pasture in 1962, and a few more were added thereafter. Internal parasites in these sheep were classified in 3 general categories: indeterminate exposure to parasiticides; H contortus, resistant to thiabendazole; and H contortus, resistant to phenothiazine. The parasitic infections probably became more homogeneous after several years because of few introductions of outside sheep after initial establishment of the flock. Activity against naturally acquired internal helminths was evaluated for cambendazole (CBZ: dosage, 20 mg/kg of body weight), fenbendazole (FBZ: 5 or 7.5 mg/kg), mebendazole (MBZ: 10 mg/kg); oxfendazole (OFZ: 3.5 or 10 mg/kg), oxibendazole (OBZ: 10 mg/kg); parbendazole (PBZ: 15 mg/kg), phenothiazine (PTZ: 550 mg/kg); pyrantel pamoate (PRT: 25 mg base/kg), tetramizole (TET: 15 mg/kg); thiabendazole (TBZ: 30 or 44 mg/kg), and trichlorfon (TCF: 100 mg/kg). Thiabendazole was used more often (9 tests) than the other compounds. Thiabendazole was more active against mature H contortus in later years than when first used in 1966, although it was never 100% effective. Efficacy against immature H contortus for TBZ did not exceed 86%. Activity against immature and mature stages of this parasite was good overall for the other benzimidazoles.(ABSTRACT TRUNCATED AT 250 WORDS)

Critical and controlled tests of activity of moxidectin (CL 301,423) against natural infections of internal parasites of equids.

The activity of moxidectin was evaluated in 1988 and 1989 against natural infections of internal parasites in 20 critical tests (n = 20 equids) and three controlled tests (n = 20 equids). Two formulations, injectable administered intramuscularly (i.m.) or intraorally (i.o.) and gel i.o., were given at dose rates of 0.2, 0.3 or 0.4 mg kg-1 body weight. For the critical tests (all three dose rates evaluated), removals of second instar Gasterophilus intestinalis were 93-100%, except (89%) for the injectable formulation (i.m.) at 0.2 mg kg-1. Removals of third instar G. intestinalis were 88-100% for the injectable formulation given i.m. or i.o. and 93-100% for the gel formulation, except (53%) for one batch (0.4 mg kg-1). Activity was 100% for third instar Gasterophilus nasalis, Parascaris equorum, Strongylus vulgaris and Strongylus edentatus. For Oxyuris equi, removals were 91-100%, except (27%) for one batch of the injectable formulation given i.o. at 0.3 mg kg-1. There was apparent activity against migrating S. vulgaris and S. edentatus at various dose rates and routes of administration for both formulations. At necropsy, there were local reactions observed at the injection site of three equids. In the controlled tests, dose rates were 0.2 or 0.4 mg kg-1. Removal of third instar G. intestinalis was highest for the injectable formulation given i.m. All formulations and dose rates were highly effective against S. vulgaris and S. edentatus, but variable and incomplete against O. equi. Removal was excellent on Habronema muscae and on migrating S. vulgaris and S. edentatus, although incomplete on S. vulgaris. Gasterophilus nasalis third instars and P. equorum were present in low numbers in some non-treated equids, but none were recovered from treated equids. Toxicosis was not evident.

Controlled tests on activity of contemporary parasiticides on natural infections of helminths in lambs, with emphasis on strains of Haemonchus contortus isolated in 1955.

Ten controlled tests were done between 1972 and 1989, in lambs on pasture, evaluating activity of fenbendazole (FBZ; 5 mg/kg of body weight), oxfendazole (OFZ; 3.5 and 10 mg/kg), oxibendazole (OBZ; 10 mg/kg), pyrantel pamoate (PRT; 25 mg of base/kg), and thiabendazole (TBZ; 44 and 50 mg/kg) against natural infections of helminths, with emphasis on 2 strains (A and B) of Haemonchus contortus. Strain A was phenothiazine-susceptible and strain B was phenothiazine-resistant when isolated in 1955. For approximately 10 years prior to these tests, sheep infected with both strains had been treated periodically each year with several compounds, including thiabendazole, which was used many more times than the other drugs. For this study, 4 (FBZ, OFZ, OBZ, and PRT) of the 5 compounds were evaluated in either 1 or 2 controlled tests. The fifth compound, TBZ, was used for 5 tests. Strain A H contortus was resistant to TBZ for all years tested, but more susceptible to FBZ, OFZ, OBZ, and PRT. Overall, strain B was susceptible to TBZ (with a few exceptions), and also to FBZ, OFZ, OBZ (activity less on immature forms), and PRT. Other abomasal parasites (2 species of Ostertagia and 3 of Trichostrongylus) were found in low numbers, but removal overall was good for the compounds tested. Trichostrongylus axei, found in higher numbers than species of Ostertagia and other species of Trichostrongylus, were effectively removed by all compounds in most cases. Activities of TBZ and PRT were also evaluated against several species of intestinal helminths, most of which were found in low numbers. Cooperia curticei were inconsistently removed by TBZ, but activity of PRT was effective.(ABSTRACT TRUNCATED AT 250 WORDS)

Evaluation of exclusive use of ivermectin vs alternation of antiparasitic compounds for control of internal parasites of horses.

A study for about a 30-month period was done to compare strongyle control programs, using per os treatments of ivermectin (IVE) paste exclusively or alternation of 4 antiparasitic paste compounds: IVE, oxfendazole (OFZ), oxibendazole (OBZ), or pyrantel pamoate (PRT). Every 8 weeks, 1 group of horses (barn C; n = 14 to 16) was given IVE paste exclusively, and a second group (barn E; n = 16) was given the 4 antiparasitic pastes on an alternating schedule. Worm eggs and larvae per gram of feces (epg and lpg, respectively) values were determined every 2 weeks during the investigation. This study in grazing horses (mares and fillies), naturally infected with internal parasites, was conducted during the period between Oct 22, 1987 and Feb 8, 1990, with an additional observation on Mar 28, 1990. For barn-C horses, treated exclusively with IVE (200 micrograms/kg of body weight) 14 times, 2-week posttreatment mean strongyle epg and lpg (small strongyle) values were reduced 99 to 100%. Mean strongyle epg and lpg (small strongyle) values for each 2-week sample period remained low (less than 20) throughout the study period, except for 1 moderate transient increase in July 1988. For the entire study period, the aggregate mean strongyle epg value was 12 and the lpg value was 6. Two-week posttreatment mean strongyle epg and lpg (small strongyle) values for barn-E horses, treated alternately with therapeutic (approx) dosage of IVE (200 micrograms/kg; 4 times), OFZ (10 mg/kg; 5 times), OBZ (10 mg/kg; 4 times), or PRT (6.6 mg base/kg; 2 times), varied within and between compounds.(ABSTRACT TRUNCATED AT 250 WORDS)