Objective--To evaluate the nematocidal effectiveness of the ivermectin sustained-release bolus throughout its 135-day delivery period. Design--Twenty-four naturally infected calves were randomly allocated to 1 of 3 equivalent experimental groups: group-T1 calves were untreated controls, group-T2 calves each received a sustained-release bolus on trial day 0 and group-T3 calves were rendered nematode-free and used at 35-day intervals during the study as tracers. One contaminated pasture was used for all principal calves for the 135-day grazing interval of the study. Calves of groups T1 and T2 were also artificially administered mixed infective nematode larvae at intervals during the grazing period, after which, all calves were confined to concrete for 21 days prior to necropsy. Animals--All calves were approximately 6 months old on trial day 0, weighed from 136 to 216 kg, and were of mixed breeding and sex. Procedure--At intervals during the study, feces from all calves were analyzed for nematode egg counts, and all calves were weighed and examined for bolus retention (T2 calves only). For nematode recovery, all calves were necropsied 21 to 22 days after removal from the contaminated pasture. Results--Parasitic populations of Haemonchus, Ostertagia, Trichostrongylus, Cooperia, Bunostomum, and Oesophagostomum spp were significantly reduced in cattle treated with the ivermectin sustained-release bolus. Conclusion--The nematocidal activity of the ivermectin sustained-release bolus proved highly effective, with > 98% efficacy for all nematode species present.

Administration of progesterone in poly(d-,l-lactide) microspheres was used to maintain pregnancy in mares after luteolysis was induced by treatment with pros F2 alpha at day 14 of pregnancy. Mares were given vehicle only (control, n = 6) or 0.75 g (n = 7), 1.5 g (n = 8), or 2.25 g (n = 5) of microencapsulated progesterone at days 12 and 22 of pregnancy. Serum progesterone concentrations were determined daily, and pregnancy was evaluated by transrectal ultrasonography on alternate days. Significantly (P < 0.05) more mares given 1.5 or 2.25 g of progesterone (6 of 8 and 4 of 5 mares, respectively), but not those given 0.75 g (3 of 7 mares), maintained pregnancy through day 32, compared with control mares (O of 6). Progesterone concentrations decreased significantly (P < 0.025) in all groups after administration of prostaglandin F2 alpha at day 14, and significant (P < 0.05) effects of time and treatment on progesterone concentrations were found between days 12 and 22, and 22 and 32. Although treatment with 1.5-g and 2.25-g doses of microencapsulated progesterone improved maintenance of pregnancy, compared with that of vehicle-treated controls, administration of 2.25 g of microencapsulated progesterone appeared to be most efficacious in maintenance of pregnancy during the study interval.

A matched case-control study design was used to assess the effects of long-term administration of a prolonged release formulation of bovine somatotropin (sometribove) on clinical lameness and limb lesions in dairy cows. Cows treated with sometribove for at least 2 lactations (cases) and nontreated dairy cows matched by herd, parity, age, and stage of lactation (controls) in 8 herds were evaluated for clinical lameness (as assessed by gait abnormality) and limb lesions by 2 observers, using a standardized scoring procedure at a single herd visit. Although a high proportion of the study cows were clinically lame (43%), an association was not detected between chronic administration of sometribove and prevalent lameness. Of 21 types of limb lesions identified, 2 were positively associated and 2 were negatively associated with long-term sometribove use. Superficial laceration of the tarsus (odds ratio [OR] = 2.1) and superficial swelling of the metatarsophalangeal joint (OR = 4.5) were positively associated with sometribove treatment, whereas femoral lesions (OR = 0.2) and superficial lacerations of the femur (OR = 0.14) were negatively associated with sometribove treatment.

Enrofloxacin was encapsulated in multilamellar liposomes composed of phosphatidylcholine and cholesterol (molar ratio, 1:1), and its potential use as sustained release formulation was evaluated. The encapsulated drug was administered IM to rabbits (n = 6). Results indicated that absorption rate was slow, compared with previous studies; additionally, peak concentration was lower (0.5 +/- 0.1 micrograms/ml), and the time to peak concentration was considerably longer for liposome-encapsulated enrofloxacin (1.5 +/- 1.08 hours) than for unencapsulated drug. Apparent elimination half-life of drug in the body was significantly (P < 0.05) increased (4.05 +/- 1.08 hours) when it was administered encapsulated in liposomes. Large-size liposomes containing enrofloxacin administered IM to rabbits gave sustained drug release from the injection site, providing therapeutic and prolonged plasma concentrations of drug in the body.

At initiation of a 140-day postweaning weight gain test, Angus bulls were assigned in equal numbers (n = 5) to 1 of 3 treatment groups to determine effects of implantation with zeranol, an estrogenic growth promotant, on selected reproductive characteristics. The bulls, whose age (mean +/- SD) was 233 +/- 20 days at initiation of the test (day 0), were implanted with 36 mg of zeranol on day 0, on days 0 and 60, or were not implanted. At day 140, scrotal circumference and testicular consistency were unaffected by zeranol implantation. Zeranol implantation did not affect the morphologic characteristics of semen samples collected by electroejaculation on day 139. There was no effect of zeranol treatment on paired weights of testes, epididymides, or vesicular glands from bulls at slaughter 47 to 68 days after day 140. Microscopic lesions associated with estrogenic exposure were not observed in accessory sex glands or epididymides of any bull. Histopathologic changes in the seminiferous epithelium were not induced by zeranol treatment. Implantation with zeranol did not affect body weight or hip weight at day 140 or carcass weight at slaughter. Plasma concentration of luteum hormone was increased (P = 0.04), whereas testosterone concentration tended to be less (P = 0.08) in both groups of zeranol-implanted bulls after administration of gonadotropin-releasing hormone on day 140.

The anthelmintic efficacy of ivermectin (IVM) delivered from a sustained-release (SR) bolus was evaluated against natural infections with gastrointestinal tract nematodes in 12 crossbred beef heifers in spring. The 12 calves were randomly allotted to 2 groups of 6 calves each. Group-1 calves were treated with an SR bolus designed to deliver 8 mg of ivermectin/d. Group-2 calves were nontreated controls. Cattle groups were kept in separate concrete-floored pens (grass hay nutrition) and slaughter was performed at 35 days after treatment. Fecal egg counts for group-1 calves remained zero after treatment, except for detection of < 1 egg/g of feces in 1 calf at the time of slaughter; counts in nontreated calves increased. Mean and range of Ostertagia ostertagi inhibited larvae in nontreated calves were 27,093 and 10,622 to 56,368, respectively. Efficacy of the IVM SR bolus was 100% against O ostertagi developing fourth-stage larvae (L4) and inhibited early L4, Haemonchus placei adults, Cooperia punctata and C spatulata adult males, Cooperia spp adult females, Cooperia spp L4, Trichostrongylus colubriformis adults, Bunostomum phlebotomum adults, and Oesophagostomum radiatum adults. Efficacy for O ostertagi and T axei adults was 99.9%. Numbers of nontreated calves infected with C pectinata adult males and Oes radiatum L4 were too low to evaluate efficacy. Calves treated with the IVM bolus gained 10.2 kg, whereas nontreated calves lost 1.8 kg. Abomasal lesions were dearly greater in nontreated calves on the basis of index comparisons of abomasal weight and total live weight and gross pathologic features.

Twelve calves (mean weight, 175.5 kg) were used to confirm efficacy of ivermectin delivered from a prototype sustained-release bolus against naturally acquired gastrointestinal nematodes including early fourth-stage (inhibited) larvae of Ostertagia ostertagi. The calves were allocated by restricted randomization on weight to 1 of 2 groups: controls, to which a placebo bolus was given orally, and treated calves, to which a sustained-release bolus designed to deliver 8 mg of ivermectin/day at a steady rate was given orally. After treatment, the 2 groups were housed in separate pens with concrete flooring. Twenty-eight days after treatment, all calves were euthanatized and necropsied. The ivermectin-treated calves had no larval or adult Ostertagia spp and significantly (P < 0.01) fewer adult Trichostrongylus axei and adult Cooperia (C oncophora, C punctata and C surnabada) than control calves. Efficacy of ivermectin was > 99% for Cooperia spp, and 100% for other parasites. Drug-related adverse reactions were not observed.