Pathophysiologic effects of Ostertagia ostertagi infection and their prevention by strategic anthelmintic treatments were studied in 3 groups each of 6 steer calves. Group-1 calves were noninfected controls. Group-2 calves were inoculated with 100,000 third-stage larvae on the 1st and 28th days of the experiment and grazed on pasture initially free of contamination. Group-3 calves were on a similar regimen as those in group 2, but were also treated with ivermectin 9 days after each larval inoculation. Group-2 calves had increased plasma pepsinogen and gastrin values and decreased weight gains, and total serum protein and albumin concentrations from the 2nd week of infection onward. They were anemic at 10 to 12 weeks and had lower carcass and meat quality at slaughter. Strategic anthelmintic treatments were effective in preventing these effects and calves in groups 1 and 3 had similar performances. On the basis of our findings, high pepsinogen values were related to worm burdens, whereas high gastrin concentrations were related to gastric lesions.

Previous work had indicated that the 2 canine hookworms, Ancylostoma caninum and Uncinaria stenocephala, may differ in their susceptibility to treatment with milbemycin oxime. Thus, the study reported here was to examine the effects of this drug on concomitant infections in experimentally infected dogs. Twenty specific-pathogen-free Beagles were inoculated orally with 500 infective-stage larvae from a mixture of larval A caninum and U stenocephala. Quantitative fecal examinations were performed weekly, beginning the day of infection. The dogs were assigned to 2 equal groups, 1 group that received the compound and 1 that received a placebo. The dogs were treated on postinoculation days 30, 60, and 90. For A caninum, egg counts dropped precipitously after the first treatment, and no eggs of this species were found in the feces of any of the treated dogs after the second treatment. The treatments had no significant effect on the mean egg counts made on U stenocephala, although 2 dogs stopped passing eggs entirely after the second treatment. At necropsy, no A caninum were found in any of the treated dogs; the mean number recovered from the control-group dogs was 56.1. Significant difference was not found in the mean number of adult U stenocephala recovered from the treated and control groups (27.0 and 21.7, respectively).

The efficacy of netobimin against digestive tract strongyles and the small lungworm Muellerius capillaris was tested in a dairy goat herd. The drenches were given orally at the rate of 20 mg/kg of body weight once, 10 mg/kg on 2 successive days, or 7.5 mg/kg on 3 successive days. Fecal egg counts and larval counts were done 8 days before and on the day of drenching; further counts were performed on postdrenching days 8 and 18. Two goats were necropsied, 1 on postdrenching day 5 and 1 on day 10 in the group treated on 3 successive days. The fecal egg counts were reduced by 44 to 79% depending on dosage on postdrenching day 18; the remaining worms were Trichostrongylus colubriformis. The larval counts of M capillaris were reduced by 72 to 92%, depending on dose. The reduction was significant in all the treated groups, compared with that in the control group. The use of netobimin at the dosage of 10 mg/kg given on 2 successive days or 7.5 mg/kg given on 3 successive days might be recommended to treat goats infected with Muellerius spp and digestive tract strongyles.

Inoculation of swine with a sylvatic isolate of Trichinella spiralis, designated T s nativa, resulted in low numbers of muscle larvae, compared with muscle larvae accumulation in swine inoculated with a pig type of T s spiralis. Despite low infectivity of T s nativa for swine, primary inoculation resulted in high levels of immunity against challenge infection with T s spiralis. This immunity was expressed in accelerated expulsion of challenge adults from the intestine and reduced numbers of muscle larvae. Pigs inoculated with T s nativa developed cellular and humoral responses similar to those in pigs inoculated with T s spiralis. However, in immunoblots, sera from pigs inoculated with T s nativa recognized additional proteins in muscle larvae excretory-secretory (ES) products, compared with sera from pigs inoculated with T s spiralis. Active immunization of pigs with ES products from T s nativa resulted in numerically higher, but not significantly different levels of immunity, compared with pigs immunized with ES from T s spiralis. The highest levels of immunity were obtained in pigs immunized with a T s spiralis newborn larval extract. The combination of ES products and newborn larval extract did not result in additive levels of immunity. These results indicate that the major immune effector response to Trichinella sp in pigs is against the newborn larvae, regardless of the genetic type of Trichinella sp.

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.

Twenty-six mixed-breed (14 males, 12 females) dogswere used in a double-blind study to evaluate the effect of milbemycin oxime against naturally acquired infection with Ancylostoma caninum. Dogs were ranked and paired, on the basis of number of hookworm eggs/g of feces, and treatment was randomly assigned. Each dog was given either the study drug or placebo (1 tablet/11.4 kg [0.5 mg/kg] of body weight). Eggs per gram of feces enumeration was done on days 3 and 7 after treatment, and dogs were euthanatized on day 7. On day 3, 5 of the 13 dogs in the milbemycin-treated group had hookworm eggs in the feces (results of the McMaster test). In these dogs, mean number of eggs per gram of feces had decreased markedly (from 5,289 to 452) and, by day 7, was 114. At necropsy, 16 A caninum adults were recovered from 2 of the milbemycin-treated dogs. On day 3, 12 of the 13 dogs in the placebo-treated group had hookworm eggs in the feces. Mean number of eggs per gram of feces in these dogs decreased slightly (from 5,243 to 2,646), but did not decrease further by day 7. A mean number of 54.4 A caninum adults was recovered from 12 of the 13 placebo-treated dogs at necropsy. Milbemycin oxime had 97.8% efficacy against A caninum. Results also indicated that milbemycin oxime may be effective against Trichuris vulpis, but not against Dipylidium caninum.

Pathophysiologic effects of Ostertagia ostertagi infection and their prevention by strategic anthelmintic treatments were studied in 3 groups each of 6 steer calves. Group-1 calves were noninfected controls. Group-2 calves were inoculated with 100,000 third-stage larvae on the 1st and 28th days of the experiment and grazed on pasture initially free of contamination. Group-3 calves were on a similar regimen as those in group 2, but were also treated with ivermectin 9 days after each larval inoculation. Group-2 calves had increased plasma pepsinogen and gastrin values and decreased weight gains, and total serum protein and albumin concentrations from the 2nd week of infection onward. They were anemic at 10 to 12 weeks and had lower carcass and meat quality at slaughter. Strategic anthelmintic treatments were effective in preventing these effects and calves in groups 1 and 3 had similar performances. On the basis of our findings, high pepsinogen values were related to worm burdens, whereas high gastrin concentrations were related to gastric lesions.

Three studies were conducted to determine the efficacy of milbemycin oxime in the prevention of Dirofilaria immitis infection in dogs. Dogs were given single or multiple experimental inoculations with infective third-stage D immitis larvae and were treated with milbemycin oxime at a target dosage of 0.5 mg/kg of body weight either once or at monthly intervals at various times after inoculation. The compound was effective in preventing infection when 1 dose was administered 30 or 45 days after inoculation. Significant, but incomplete, protection was achieved when single treatments were administered 60 or 90 days after inoculation. Multiple monthly treatments beginning 60 days after inoculation appeared to provide additive effects that resulted in restoration of complete efficacy.

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 < 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.

One hundred four heartworm-free Beagles < 1 year old were studied to determine the efficacy of ivermectin chewable tablets and of 2 other ivermectin tablet formulations against heartworm larvae. At 30 days after SC inoculation of dogs with infective Dirofilaria immitis larvae, all ivermectin formulations were given orally at dosage of 6 microgram/kg of body weight. The ivermectin chewable tablets also were given orally at dosage of 2 and 6 microgram/kg at 30 and 45 days, respectively, after injection of larvae. Replicates of 6 or 8 dogs in each study were formed on the basis of gender and body weight and, within replicates, were randomly allocated to treatment groups. At 30 days after injection of larvae, the additional dogs (in replicates of 8) were assigned to the control group and to the group given ivermectin chewable tablets at dosage of 6 microgram/kg. All dogs were housed individually. Necropsy was performed approximately 5 or 6 months after larvae were administered. In both trials, all control dogs had heartworms at necropsy (University of Illinois-geometric mean, 35.0; Florida-geometric mean, 26.1). In both trials, the ivermectin chewable tablet (6 microgram/kg) and both tablet formulations (6 microgram/kg) given at 30 days after larval injection, and the chewable formulation (6 microgram/kg) given at 45 days after larval injection were 100% effective (P < 0.01) in preventing development of induced infection with D immitis. Of 8 dogs at the University of Illinois that were given ivermectin chewable tablets (2 microgram/kg) at 30 days after larval injection, 6 had heartworms (geometric mean, 2.25; efficacy, 93.6%; P < 0.01) and 5 of 7 dogs treated similarly in Florida had heartworms (geometric mean, 4.4; efficacy, 83.3%; P < 0.05). Drug-related adverse reactions were not observed in either trial.