Selection for resistance to macrocyclic lactones by Haemonchus contortus in sheep.

An anthelmintic-sensitive Haemonchus contortus strain was selected for moxidectin and ivermectin resistance concurrently for 22 generations. Treatment with 0.002 mg moxidectin/kg BW or 0.02 mg ivermectin/kg BW produced >99% efficacy against the susceptible parent strain passaged for 22 generations without any anthelmintic exposure. However, to obtain similar efficacy the moxidectin-selected and the ivermectin-selected strains of H. contortus required 0.05 mg moxidectin/kg BW or 0.4 mg ivermectin/kg BW. These results indicate that development of resistance to one macrocyclic lactone, simultaneously results in resistance to another macrocyclic lactone. However, rates of resistance development differ between compounds and occurs more slowly with moxidectin than with ivermectin.

Efficacy of moxidectin, nemadectin and ivermectin against an ivermectin-resistant strain of Haemonchus contortus in sheep.

The efficacies of ivermectin, nemadectin and moxidectin were evaluated when administered orally to lambs infected with either a susceptible laboratory strain of Haemonchus contortus or a strain reported to be resistant to ivermectin. Groups of 24 Dorset cross Cheviot cross Suffolk lambs were infected with either the susceptible or resistant strain of H contortus and allocated to treatment groups according to their faecal egg counts 27 days after infection. One day later the lambs were dosed orally with one of the three anthelmintics at 0.2 mg/kg bodyweight, and they were killed and surviving worms were recovered 13 or 14 days after treatment. Against the ivermectin resistant strain, ivermectin did not significantly reduce the egg count or the numbers of adult H contortus; however, both nemadectin and moxidectin reduced the nematode egg counts and the numbers of H contortus by 99 and 100 per cent, respectively. Against the susceptible strain, all the anthelmintics reduced the egg counts by 100 per cent as early as four days after treatment and reduced the numbers of susceptible H contortus by 100 per cent. No adverse reactions to any of the drugs were observed.

Comparison of therapeutic versus subtherapeutic levels of chlortetracycline in the diet for selection of resistant salmonella in experimentally challenged chickens.

Sixteen broiler chicks per group were fed chlortetracycline (CTC) at 0 (control) and 55, 110, and 220 ppm (subtherapeutic levels) continuously for 44 days or 550 ppm (therapeutic level) for three 5-day periods from 1 to 19 days of age. All birds were challenged at 4 days of age with a 10:1 mixture of CTC-sensitive and resistant (CTCr) Salmonella typhimurium. Chicks were sampled periodically through postchallenge day (PCD) 41, half in each pen by cloacal swabbing and the remainder by collection of droppings. Escherichia coli was monitored at PCD 6 and 34. Salmonella recovery from cloacal swabs indicated increased (P less than .05) prevalence of CTCr salmonella-positive birds in the 550 ppm treatment at PCD 6 and 13 compared to all other treatments, and at PCD 27 compared to 0, 55, and 110 ppm CTC. Mean recovery scores followed a similar pattern. Area under the curve analysis of CTCr salmonella scores from cloacal swabs for PCD 3 to 41 confirmed increased (P less than .05) selection for CTCr salmonella by 550 ppm CTC. Isolations from droppings showed increased (P less than .05) CTCr salmonella prevalence at PCD 20 for the 100 ppm treatment group compared to control, and at PCD 34 for the 110 ppm groups compared to all other groups and for 220 and 550 ppm groups compared to 0 and 55 ppm CTC birds. The CTCr salmonella counts in droppings were higher (P less than .05) at PCD 34 for the 110, 220, and 550 ppm groups compared to the control.(ABSTRACT TRUNCATED AT 250 WORDS)

Efficacy, safety, and residue evaluation of levamisole gel formulation in sows.

Anthelmintic efficacy, safety, and residue studies were conducted in sows and gilts with a levamisole gel containing 11.5% levamisole HCl. In 12 sows and 12 gilts, 8 mg of levamisole HCl equivalent/kg of body weight orally was 100% (resinate) and 91.1% (gel) effective against 55-day-old Ascaris suum and 100% (gel) and 96.1% (resinate) effective against Oesophagostomum dentatum. In 20 sows given levamisole gel (8 mg of levamisole HCl/kg) orally just before breeding, 4 to 6 weeks after breeding, 4 to 6 weeks before farrowing, and just before farrowing, there were no adverse effects. Transient salivation was noticed in five sows after treatment. In 4 groups of 4 sows each given levamisole gel orally to provide 8, 24, 40, or 80 mg of levamisole HCl/kg, adverse clinical signs were not observed in sows treated with 8 mg/kg. Transient salivation was noticed in one sow given 24 mg/kg, two sows given 40 mg/kg, and four sows given 80 mg/kg. Multiple emesis and chomping occurred in one sow given 80 mg/kg. Levamisole residues in edible tissues from sows given 8 mg of levamisole gel/kg orally were less than 0.1 mg/kg of muscle and fat in sows killed on posttreatment day (PTD) 3 and less than 0.1 mg/kg of kidney in sows killed on PTD 5. Liver residues averaged 0.78 mg/kg in sows killed on PTD 3 and were reduced to 0.31 mg/kg in sows killed on PTD 5. The 99% upper tolerance limit with 95% confidence on the withdrawal time to assure levamisole residues of less than 0.10 mg/kg in liver tissue was 11 days.

Comparative efficacy of sulfamethazine and sulfathiazole in feed for control of Bordetella bronchiseptica infection in swine.

Seventy-two conventionally raised pigs were challenge exposed intranasally when approximately 3.5 weeks old with yolk-grown Bordetella bronchiseptica. Twenty-four pigs acted as noninfected, nonmedicated controls. Feed containing sulfamethazine or sulfathiazole (110 mg/kg of feed) was initiated in 2 groups of 24 infected pigs each 3 days after challenge exposure and was fed continuously for 56 days. Twenty-four infected pigs were given nonmedicated feed. Challenge exposure with the B bronchiseptica resulted in nasal bordetellosis characterized by isolations of the test organism from nasal cavities of infected control pigs at greater than 90% frequency through 28 days and from at least 50% of the pigs through 56 days. Moderate turbinate atrophy developed with a 48% increase in mean turbinate space in infected control pigs at necropsy. Performance was not affected by the infection which was confined to the nasal cavity. The B bronchiseptica isolation rate decreased faster (P less than 0.01) in the sulfamethazine group than in the sulfathiazole group. By day 42, sulfamethazine-medicated pigs were negative for B bronchiseptica in nasal swab samples; whereas 8% to 17% of sulfathiazole-medicated pigs were positive from days 42 to 56. Turbinate spacing measurements averaged 11% less in the sulfamethazine group than in the sulfathiazole group.

Anthelmintic efficacy, safety, and residue evaluation of levamisole gel formulation in cattle.

Anthelmintic efficacy, safety, and residue studies were conducted in beef calves using a levamisole gel formulation. The effectiveness of levamisole gel and tablet formulations was studied in 30 calves with experimental infection of Ostertagia ostertagi. Removal of 33- to 34-day-old O ostertagi was 99.6% and 97.7%, for the gel and tablet formulation, respectively, when administered orally to supply 8 mg of levamisole HCl equivalent/kg of body weight. A comparative blood concentration study was also used to demonstrate bioequivalence of the levamisole gel to the tablet formulation. In a cross-over-designed test, 5 cattle/treatment group were dosed orally with levamisole 11.5% gel or levamisole tablets at the dosage rate of 8 mg of levamisole HCl/kg. Blood levamisole values were similar with the levamisole gel and tablet formulations. In a safety study, 3 groups of 6 calves each were given levamisole gel orally to provide 8, 24, or 40 mg of levamisole HCl/kg. A 4th group served as controls. Adverse clinical signs were not observed in cattle treated with the recommended dosage level of 8 mg/kg. Transient salivation was noticed in 1 placebo control calf, 2 calves given 24 mg/kg, and 4 calves given 40 mg/kg. Edible tissues from cattle given a single oral dose of levamisole gel (8 mg/kg) were analyzed for drug residues 2 hours and 2, 3, 5, and 7 days after treatment.(ABSTRACT TRUNCATED AT 250 WORDS)

Efficacy of nithiamide in preventing and treating swine dysentery in experimentally inoculated pigs.

In a series of experiments, weanling pigs (12 to 18 kg av weight) were inoculated by means of feed with scrapings and contents of dysenteric colons. Mucohemorrhagic diarrhea developed in 97% of the infected, nonmedicated pigs, and 80% of these pigs died. Prophylactic feed medication with nithiamide at dose levels of 600, 300, or 150 ppm beginning 2 days before inoculation to 28 days after inoculation prevented the development of swine dysentery and permitted performance equal to that of the noninfected nonmedicated pigs. Morbidity and mortality were reduced by medication with 100 ppm of nithiamide but these were not influenced by medication with 50 ppm. Therapeutic feed medication with nithiamide at a dose level of 150 ppm beginning the day after diarrhea first occurred was not effective. A single oral dose of nithiamide (25 mg/kg of body weight), when diarrhea first occurred in each pig, produced rapid remission of the signs of dysentery; relapses occurred 1 to 2 weeks later, however, and mortality followed. The combination of oral dosage and feed medication was highly effective in treating dysentery, preventing its recurrence, and maintaining performance.

Excretion of levamisole in milk from cows treated with various formulations.

The rate of disappearance of levamisole in milk from cows given levamisole hydrochloride drench, levamisole resinate in feed pellets, levamisole hydrochloride boluses, or levamisole phosphate injectable was determined. Each formulation was given as a single treatment to each of five cows at a rate equivalent to 8 mg of levamisole hydrochloride/kg of body weight. Levamisole hydrochloride residues in milk averaged .50, .55, .58, and .32 ppm at 12 hr after the administration of levamisole drench, feed, bolus, and injectable formulations. Levamisole hydrochloride residues were below .01 ppm in milk at 48 h after drench treatment and at 60 h after treatment with other three formulations. Toxicity symptoms were not observed in any cows following treatment.