The effect of immunosuppression with dexamethasone on Haemonchus contortus infections in genetically resistant merino sheep.

The effect of immunosuppression with the glucocorticosteroid, dexamethasone, on the susceptibility to Haemonchus contortus infection of a highly resistant Merino genotype was investigated. Higher faecal egg counts, larger worm burdens and heavier worms were recorded in immunosuppressed wethers. The characteristic globule leucocyte infiltrate in the abomasum of resistant wethers (12-month-old castrated males) was absent in immunosuppressed animals. Treatment with dexamethasone abolished differences between resistant and susceptible genotypes in faecal egg counts, worm weights, thymus weights and globule leucocyte responses to infection with H. contortus. These results suggest that an immunological basis underlies the high level of resistance to infection in the resistant genotype.

Kinetics of endotoxin-induced inflammation in ovine mammary gland.

The inflammatory response to stimulation with endotoxin of lactating and nonlactating mammary glands of sheep was examined. Similar numbers of neutrophils and mononuclear cells were recovered from nonlactating glands sampled every 2 h for 8 h and in glands sampled once at 8 h. Thus the inflammatory response was initiated by 2 h and repeated sampling did not modify the time course of the response. In contrast, in lactating ewes, fewer cells were recovered from glands sampled every 2 h than from glands sampled once at 8 h. Fewer neutrophils were also recovered when glands were serially sampled from 4 h to 8 h. Thus, removal of milk and inflammatory exudate modified the time course of the leukocyte influx into lactating glands. Significant accumulation of neutrophils occurred by 2 h in dose-response experiments in nonlactating glands. Peak accumulation of neutrophils occurred between 2 and 6 h, and a marked decline occurred after 8 h. In lactating glands, a slower onset and longer duration of neutrophil accumulation occurred. Twenty- to 30-fold more neutrophils were recovered by 8 h in lactating than nonlactating glands. This difference was not due to a lower threshold of sensitivity to endotoxin. Infusion of milk into nonlactating glands did not modify the intensity or time course of the inflammatory response to endotoxin. Thus, the physiological state of resident cells within the lactating gland, rather than the interaction of inflammatory exudate with milk, can account for the different reaction pattern in lactating glands. Inflammation in the nonlactating gland closely resembles inflammatory responses in skin and provides a convenient model for investigating the initiation and regulation of inflammatory processes.

Efficacy of oxfendazole as administered by intraruminal injection to naturally infected calves.

Oxfendazole, formulated into a 22.5% suspension, was administered by intraruminal injection to cattle at the rates of 0, 1.125, 2.25, 4.50, and 6.75 mg/kg of body weight. In total, 50 naturally infected calves were used, with 10 animals being allocated to each of the 5 treatment groups on the basis of pretreatment counts of nematode eggs per gram of feces. All animals were necropsied 7 days after treatment. The trial was done between December 1983 and January 1984, with the animals kept on concrete for a minimum of 35 days immediately before necropsy. For all nematodes and stages combined, efficacies were 97.4%, 98.8%, 99.5%, and 99.8% for oxfendazole at dosages of 1.125, 2.25, 4.50, and 6.75 mg/kg of body weight, respectively. Early 4th-stage larvae of Ostertagia and Nematodirus helvetianus adults were eliminated at rates greater than 93% only at the dosages of 4.50 mg/kg and above. Local or systemic adverse reactions were not observed in any of the animals.

Efficacy of morantel against nematode populations in calves exposed on a pasture stocked for two years with morantel sustained-release bolus-treated calves.

Two groups of 10 parasite-free calves were maintained either for 2 weeks on a pasture grazed by nonmedicated cattle (pasture A) or for 3 weeks on a pasture grazed by morantel sustained-release bolus-treated cattle (pasture B) for the preceding 2 years. After a 4-week holding period to allow for maturation of acquired gastrointestinal nematodes, 5 calves from each group were administered a therapeutic dose (10 mg/kg of body weight) of morantel tartrate. All calves were necropsied 1 week later, and the abomasal and small intestinal nematodes were isolated, identified, and enumerated. A comparison of efficacies between nonmedicated and morantel tartrate-treated calves of each pasture demonstrated that morantel was equally effective against the gastrointestinal nematode infections, regardless of infection source (ie, pasture A vs pasture B). The overall nematode reductions due to morantel tartrate treatment of calves that grazed pastures A and B were 98% and 96%, respectively. It was concluded that the sensitivity of gastrointestinal nematodes to morantel tartrate was not diminished in calves maintained on pasture B, which had been stocked with morantel sustained-release bolus-treated calves for the preceding 2 grazing seasons.

Effect of the morantel sustained-release bolus, used during one grazing season, on the sensitivity of Ostertagia and Cooperia to morantel tartrate in calves.

Infective 3rd-stage larvae of Ostertagia and Cooperia, obtained from the feces of nonmedicated and morantel sustained-release bolus (MSRB)-treated calves, were orally administered to 2 groups of parasite-free calves. After a 42-day maturation period, a therapeutic dose of morantel tartrate was administered to half of the calves from each group. All calves were necropsied 7 days after treatment. After comparing the nematode counts for the nonmedicated and morantel-treated calves of each group, morantel tartrate was demonstrated to be equally effective against the nonmedicated-derived and MSRB-derived nematode populations. The sensitivity of Ostertagia spp and Cooperia spp to morantel tartrate, therefore, was not diminished after use of the MSRB for a single grazing season.

Efficacy of oral clorsulon in the treatment of Fasciola hepatica infections in calves.

Twenty calves at each of 2 Arkansas locations were inoculated with infective Fasciola hepatica metacercariae. After 56 days, the calves at each site were randomly assigned by weight to 2 treatment groups of 10 calves/group; vehicle control or clorsulon at the rate of 7 mg/kg of body weight. All treatments were given orally as a suspension. Calves were killed 6 weeks after treatment and F hepatica counts were performed for all animals. At the 2 sites, mean levels of efficacy were 96% and 91%. Adverse reactions to clorsulon or the vehicle were not observed in the calves.

Controlled field trial on the anthelmintic effectiveness of the morantel sustained-release bolus in grazing calves.

The direct and indirect anthelmintic efficacies of the morantel sustained-release bolus given to calves were assessed in a 154-day controlled field trial. A permanent calf pasture (divided into 2 lots) and naturally parasitized calves were used. The medicated calves were given the bolus at the time they were placed on the pastures. Control calves did not receive anthelmintic therapy. The effectiveness of the bolus to control parasitic gastroenteritis was determined by monitoring various parasitologic determinants. The treated calves had significantly (P less than 0.01) reduced numbers of fecal nematode eggs for every posttreatment sampling period when compared with the control calves. Tracer calves, used periodically during the study to indicate pasture larval infectivities, had equivalent worm burdens at the beginning of the trial (treated vs control pasture). Tracer calves, added later in the study to the lot with treated calves, harbored 83% to 94% fewer nematodes than did their counterparts in the lot with the controls. Plasma pepsinogen concentrations, reflective of abomasal worm burden size and/or activity, were significantly higher (P less than 0.01) in the control than in the treated calves from day 54 until trial termination. At trial termination, the treated calves weighed an average of 27.8 kg more and harbored 80.9% fewer nematodes than the control calves. The morantel sustained-release bolus is an anthelmintic delivery device that has therapeutic and prophylactic antinematode activities. To achieve its optimum performance, the bolus must be used so that the epizootiologic patterns of the predominate parasitic nematodes are effectively disrupted. Generally, internal nematode parasitisms in the calf flourish during the animal's first springtime grazing period.(ABSTRACT TRUNCATED AT 250 WORDS)

Critical test evaluations of oxfendazole and trichlorfon: effectiveness of a paste formulation in the horse.

Critical tests were performed on 6 horses to evaluate the antiparasitic effectiveness of oxfendazole given in combination with trichlorfon in a paste formulation. Treatments were given orally as a single dose. The rates of active ingredient administration were 2.5 and 40 mg/kg of body weight for oxfendazole and trichlorfon, respectively. The combined activities of the 2 antiparasitic compounds proved 100% efficacious in the removal of adult Strongylus vulgaris, S edentatus, Oxyuris equi, and Parascaris equorum. Fourth stage O equi, and 2nd and 3rd instars of Gasterophilus nasalis also were completely removed. Second and 3rd stage instars of G intestinalis were removed at the rates of 98.1% and 98.8%, respectively. Nematodes of the "small strongyle" category were removed at the combined rate of 97.0%. Pronounced larvicidal effects of the test formulation were demonstrated via culturing fecal nematode eggs during the trial. Untoward effects of treatment were not seen in any of the trial animals.

Antiparasitic efficacy of ivermectin in naturally parasitized sheep.

Sixteen sheep harboring naturally acquired parasitisms were allocated to 1 of 2 treatment groups: (i) sheep given ivermectin in an oral solution at the dosage rate of 200 micrograms/kg of body weight, and (ii) those given the vehicle at a dosage rate of 0.25 ml/kg. All animals were necropsied at 2 weeks after treatment. Parasites and percentages of parasitic reductions, as demonstrated in this trial, were: Dictyocaulus filaria (99.4%), Oestrus ovis first stage instars (100%), Trichuris ovis (98.9%), Strongyloides papillosus (99.8%), Nematodirus spathiger (100%), arrested 4th stage Nematodirus spp (96.2%), Trichostrongylus colubriformis (100%), T axei (100%), Oster tagia circumcincta (100%), Haemonchus contortus (100%), and arrested Haemonchus spp 4th stage larvae (99.9%). The sheep showed no adverse effects due to ivermectin or vehicle administration.