Production responses following strategic parasite control in a beef cow/calf herd.

Strategic parasite control has been reported to improve performance in a beef cow/calf herd that does not demonstrate clinical parasitism. This study was conducted to determine if strategic anthelmintic treatment at turnout and at midsummer would improve growth and reproduction. Two cow/calf groups of similar composition were grazed on separate, but equivalent pastures in the spring, and at midsummer, they were moved to new pastures. The treated group was given fenbendazole at tumout (cows) and at midsummer (cows and calves). The control group was not treated. The study was repeated in the following year. Across both years of the study, parasite egg counts were substantially reduced in both the treated cows (P < 0.005) and treated calves (P < 0.0001). The calves in the treated group significantly (P < 0.0001) outgained the control calves in both years of the study by 19.3 kg and 13.2 kg, respectively. Average daily gain (ADG) after adjusting for birth weight and birth date for treated calves was 0.13 kg greater than for control calves (0.83 vs 0.70) for the 2-year study (P < 0.0001) There was also a significant (P = 0.0357) increase in the reproductive performance of the cows. The pregnancy rate averaged across years was 94% for the treated cows compared to 82% for the control animals.

Epizootiology of helminth parasitism in a beef cow/calf herd in Minnesota.

To test the effect of a parasite control program for cattle, 2 groups of similar composition were grazed on separate, but equivalent, improved pastures. Cattle in 1 group were treated with fenbendazole at 5.0 mg/kg of body weight at the time they were turned out on pasture in the spring and again at midsummer, when the cattle were moved to a new pasture. The control group was not treated. Parasite egg counts were significantly (P less than 0.04) lower in the treated group. Trichostrongyle-type eggs were the most prevalent throughout the year, except in the month of May, when Strongyloides papillosus eggs were predominant. The number of worms recovered from tracer calves was lower for those on pastures where the treated group grazed than for those on the control group's pasture. The most consistently recovered parasite was Ostertagia ostertagi, and hypobiosis was observed.

Efficacy of clorsulon against Fascioloides magna infection in sheep.

In a study to evaluate the efficacy of clorsulon against Fascioloides magna infection in sheep, 12 ewes were inoculated orally with 100 metacercariae of F magna, and 6 were treated with clorsulon (15 mg/kg of body weight) 8 weeks after inoculation. The sheep were euthanatized 16 weeks after inoculation, flukes were recovered, and the liver and other tissues were subjectively scored for the severity of lesions (0 to 4+). The number of flukes recovered from the clorsulon-treated group (3.8 +/- 1.2 flukes) was significantly (P = 0.025) lower than the number of flukes recovered from the group of untreated controls (10.0 +/- 6.6 flukes). The severity of lesions was significantly (P = 0.004) reduced (45.9%) in the treated group (2.0 +/- 1.1), compared with that in the untreated controls (3.7 +/- 0.5). In the untreated group, 3 sheep died and 1 became moribund 14 to 16 weeks after inoculation. The data suggested that a single treatment with clorsulon at a dosage of 15 mg/kg 8 weeks after inoculation was not effective in preventing F magna infection in sheep, because the survival of only a few F magna is potentially fatal in sheep within 6 months after infection.

Experimental bovine coccidiosis: control with monensin.

Young Holstein-Friesian bull calves were used in a controlled experiment to evaluate the efficacy of monensin against coccidiosis. The calves were given oocysts of Eimeria bovis and/or E. zurnii. Medication was started 3 days prior to inoculation and continued during the 30-day experimental period. Oocyst shedding was quantified prior to and throughout the experiment and demonstrated that monensin at the rate of 20 or 30 g ton-1 of feed significantly reduced oocyst shedding and clinical coccidiosis. Clinical infection with E. zurnii was very difficult to establish, even when calves were treated with 20 mg dexamethasone IM on Days 12, 15, and 16 post-inoculation.

Activity of closantel against experimentally induced Fascioloides magna infection in sheep.

The efficacy of closantel against experimentally induced Fascioloides magna infection in sheep was studied. In each of 3 experiments, closantel was administered 8 weeks after the sheep were given (oral inoculation) 100 metacercariae of F magna. In the 1st experiment, closantel was given orally to 5 groups of 6 sheep each at dosages of 0 (nontreated control), 5, 7.5, 10, and 15 mg/kg of body weight. In the 2nd and 3rd experiments, groups of 10 or 12 sheep were treated to confirm the efficacy of the previously determined optimal dosage of 15 mg/kg. An additional group of sheep (n = 10) was used in the 3rd experiment to evaluate the efficacy of closantel given IM at a dosage of 7.5 mg/kg. Closantel given orally at a dosage level of 15 mg/kg was highly effective (94.6% to 97.7%) in reducing F magna burdens. Also, pathologic scores associated with the F magna infection were reduced by 81.3% to 92.6% in sheep given this dosage of closantel. Efficacy of the IM administered dosage of 7.5 mg of drug/kg was equivalent to that of the 15 mg/kg oral dosage. Other than mild, transient lameness of the limbs which were injected with the drug (group 10), side effects were not observed.

Pathophysiologic effects of experimentally induced Fascioloides magna infection in sheep.

The pathophysiologic responses of 13 sheep inoculated orally with 100 metacercariae of Fascioloides magna were monitored for 4 months after inoculation. There were no differences in weight gains between these and a number of noninoculated control sheep throughout the experiment. Complete blood cell counts showed an increase only in the absolute number of eosinophils. Serum preparations (2 times a week) from 7 inoculated and 7 noninoculated sheep did not identify any significant changes in alkaline phosphatase, aspartate aminotransferase, alanine aminotransferase, and gamma-glutamyl transferase activities. There were no changes in total bilirubin, BUN, creatinine, inorganic phosphorus, calcium, albumin, chloride, potassium, and sodium values. Four months after they were inoculated, all sheep were necropsied, and flukes were recovered. Gross lesions attributed to fluke migration were found in the liver and portal lymph nodes, diaphragm, lungs, kidneys, and spleen. Microscopically, liver lesions in inoculated sheep occurred in the portal areas, veins, and Glisson's capsule and were characterized by both active and chronic forms of inflammation. Abundant infiltrates of eosinophils and plasma cells often marked the portal areas. Endophlebitis, with or without thrombosis, was the predominate vascular lesion. The flukes recovered varied greatly in size (6.5 to 48.0-mm long) and demonstrated some sexual development, but none was sexually mature.

Efficacy of albendazole against Fascioloides magna in sheep.

Sheep experimentally infected with Fascioloides magna were used in a controlled study to determine the flukicide activity of different doses of albendazole (methyl [5(propylthio)-1H-benzimidazol-2-yl] carbamate). Sheep, about 4 months old, were inoculated with 100 metacercariae of F magna and treated with the various doses of albendazole 10 weeks later. At 6 weeks after treatment, the sheep were necropsied, flukes were recovered and measured, and the pathologic changes were recorded. The sheep were given different doses of albendazole (5, 7.5 [single dose or double dose], 10, and 15 mg/kg) and were compared with both nontreated inoculated and nontreated noninoculated controls. There were up to 70% fewer worms recovered in the group given 2 doses of 7.5 mg/kg than there were in the nontreated controls. There was also a significant reduction in flukes recovered in the groups given the 7.5, 10, and 15 mg/kg doses. Because a single F magna fluke has the potential to kill a sheep, it is imperative to consider the number of sheep with no flukes after treatment. According to that criterion, 50% of the sheep were protected at dose rates of 7.5 (single dose, double dose), 10, and 15 mg/kg of body weight.

Efficacy of lasalocid sodium against coccidiosis (Eimeria zuernii and Eimeria bovis) in calves.

Calves experimentally infected with Eimeria bovis and E zuernii were used in a controlled experiment to determine the anticoccidial activity of lasalocid sodium. Eleven-week-old Holstein calves were given an inoculum of 300,000 E bovis and 200,000 E zuernii oocysts; medication was initiated on the day of inoculation and continued for a 4-week period. The progress of parasitic infection was monitored with quantitative fecal oocyst examinations for the 6 weeks before calves were inoculated, and then in the 4-week treatment period and a 3-week observation period. The calves were given different doses of lasalocid sodium (0.5, 0.75, 1.0, and 3.0 mg/kg) and were compared with both nonmedicated inoculated calves and controls (nonmedicated, noninoculated calves). There were overall numerical reductions of oocysts produced in the medicated groups when compared with the nonmedicated inoculated controls.

Natural transmission of Dirofilaria immitis by Aedes vexans.

Aedes vexans mosquitoes were reared in the laboratory from field-collected fourth instar larvae. Mosquitoes were allowed to feed on dogs which were infected with Dirofilaria immitis and then the mosquitoes were housed in an environmental chamber with an ambient air temperature of approximately 26 C and a relative humidity of approximately 80%. Incandescent lighting was used to simulate daylight. On the 17th, 19th, 21st, and 28th day after their infected blood meal, the mosquitoes were allowed to feed on five microfilaria-negative dogs which had been reared in mosquito-free environments. Blood from four of the five exposed dogs became positive for microfilariae of D immitis in an average of 272 days after the feeding. The local strain of A vexans was established as a vector of canine heartworm disease in the Minneapolis-St. Paul metropolitan area.