Persistent efficacy of doramectin injectable against artificially induced infections with Cooperia punctata and Dictyocaulus viviparus in cattle.

Three studies were conducted to evaluate the persistent efficacy of doramectin injectable solution against experimental challenges with infective larvae of Cooperia punctata and Dictyocaulus viviparus. In each study, four groups of ten randomly-assigned calves, negative for trichostrongyle-type eggs on fecal examination, were treated subcutaneously in the midline of the neck with saline (1 ml/50 kg) on Day 0 or doramectin (200 microg/kg = 1 ml/50 kg) on Day 0, 7, or 14. Two additional calves from the same pool of animals were randomly assigned as larval-viability monitors and received no treatment. On Days 14-28, approximately 1000 and 50 infective larvae of Cooperia spp. and D. viviparus, respectively, were administered daily by gavage to each animal in Groups T1-T4. On Day 28, the two larval-viability monitor calves were inoculated in a similar manner with a single dose of approximately 30000 and 2000 larvae of Cooperia spp. and D. viviparus, respectively. Equal numbers of calves from each treatment group were killed on Days 42-45, as well as the two viability monitor animals to enumerate worm numbers. A 2% or 5% aliquot of small intestinal contents and washings were examined for worm quantification and identification, while 100% of the lung recoveries were quantified and identified. For each study and across the three studies, geometric mean worm recoveries for each treatment group were calculated from the natural log transformed data (worm count + 1) and were used to estimate percentage reduction. In the three studies, doramectin injectable solution was 97.5% efficacious against lungworms for up to 28 days and was 99.8% efficacious in reducing infection resulting from challenge with infective larvae of C. punctata for at least 28 days post-treatment.     

Persistent efficacy of doramectin pour-on against artificially induced infections of nematodes in cattle.

Two studies were conducted to determine the persistent efficacy of doramectin pour-on against an artificial, trickle challenge of mixed nematodes in calves. In each study, 42, 4-8 months old calves were randomly assigned into four groups of 10 animals each (T1-T4), plus two larval-viability monitor animals. All animals were treated with fenbendazole (10 mg kg(-1)) 14 days prior to the start of the study to clear any existing infection. Doramectin pour-on at 500 microg kg(-1) was used on each animal in Groups T2, T3, and T4 with intervals of 1 week (Day 0, 7, and 14, respectively). Calves in Group T1 were treated with saline solution on Day 0 and at the same volumetric rate (1 ml 10 kg(-1)) as the doramectin treated animals. All treatments were applied in a single passage along the midline of the back, from the withers to the tailhead. Subsequently, trickle inoculations with infective larvae were administered to all calves for 22 consecutive days (Days 14-35). Doramectin pour-on provided > or = 91.9% efficacy against challenge with Dictyocaulus viviparus, Haemonchus spp., and Ostertagia ostertagi for up to 35 days post-treatment and against challenge with Cooperia oncophora, Cooperia punctata, and Oesophagostomum radiatum for up to 28 days post-treatment.     

Persistent activity of doramectin injectable formulation against experimental challenge with Haemonchus placei in cattle.

Two studies were conducted in North America to evaluate the persistent activity of doramectin injectable formulation against experimental challenge with Haemonchus placei. In both studies, calves were randomly assigned to 1 of 4 treatment groups (n = 10 per group) or a larval viability group (n = 2). Calves were treated subcutaneously in the lateral midline of the neck with saline (1 ml/50 kg) on Day 0, or with doramectin (200 mg/kg = 1 ml/50 kg) on Day 0, 7, or 14. Animals used to assess larval viability did not receive any treatment. Beginning on Day 14 and continuing through Day 28, each of the 40 treated calves were given approximately 300 infective larvae of H. placei per os. The two larval viability animals received approximately 10,000 larvae as a single dose on Day 28. Approximately two weeks later, all animals were slaughtered and the abomasum from each calf processed for nematode recovery. A 2% aliquot of abomasal contents plus wash was examined for enumeration and identification of nematodes. Geometric mean H. placei counts were calculated from the log (H. placei count +1) and used to estimate percentage reduction. Overall, doramectin was > or =96.9% efficacious in reducing infection with H. placei when challenged daily 14-28 days after treatment.     

Induction of protective immunity to Dictyocaulus viviparus in calves while under treatment with endectocides

Three groups of five parasite-naive calves were used. The treatments were: (a) Group 1 calves were weighed on Day 0 and injected with doramectin at 200 microg/kg. From Day 1 to 19 they were dosed orally with 2000 infective larvae of Dictyocaulus viviparus. On Day 28 they were again injected with doramectin, and infected with D. viviparus larvae from Days 33 to 41. They were then left untreated until Day 81 when they were infected with 20 infective larvae of D. viviparus per kg body weight. They were killed on Day 110 and lungworms were counted; (b) Group 2 calves were immunised with oral lungworm vaccine on Days 0 and 28, and infected and slaughtered as Group 1 on Days 81 and 110, respectively; (c) Group 3 calves acted as infection controls. Blood samples were taken at Days 0, 21, 49, 77 and 110 for antibody tests to D. viviparus. At autopsy there were no significant differences between the number of lungworms from Groups 1 and 2 (Means 17.4 and 31.3, respectively); Group 1 had significantly less value than Group 3 (Mean 228) (p < 0.05). Increased antibody titres to the larval sheath of the infective larvae were observed from Groups 1 and 2, showing that the larvae in Group 1 had penetrated the intestine before being killed by the circulating anthelmintic. This experiment shows that if calves are exposed to infective larvae while under systemic endectocide cover, an immune reaction is stimulated.     

Evaluation of the efficacy of doramectin against an artificial infection of Dictyocaulus viviparus in calves

Three groups of eight Friesian calves, reared parasite-free, were experimentally infected with 1000 infective larvae of Dictyocaulus viviparus. Two groups were injected subcutaneously with 1% doramectin at 0.2 mg/kg body weight, one group 5 days after infection and the other 25 days after infection. A third group served as untreated controls. Faecal samples were examined for lungworm larvae on days 28, 32, 33, 34 and 35 after infection; the calves were killed and necropsied 39 or 40 days after infection and any lungworms present recovered and counted. Doramectin proved 100% effective against both 5-day-old and mature D. viviparus infections.     

Dose confirmation studies of moxidectin 1% non-aqueous injectable and moxidectin 0.5% pour-on formulations against experimentally induced infections of larval and adult stage Oesophagostomum radiatum and Trichuris discolor in cattle

Separate controlled trials were conducted to evaluate the efficacy of two formulations of moxidectin (1% non-aqueous injectable solution and 0.5% pour-on (Cydectin) against larval or adult stages of Oesophagostomum radiatum and Trichuris discolor infecting cattle. Fifty-three strongylate-free dairy breed steer calves were obtained from commercial sources. After a brief acclimation period, calves were randomly divided into two pools to evaluate the efficacy of the moxidectin formulations against targeted larval (n = 27 calves) or adult (n = 26 calves) parasites. Calves in the larvacidal trial were inoculated on Day -16 relative to treatment with approximately 1000 embryonated Trichuris spp. eggs and approximately 640 infective Oesophagostomum spp. larvae. Calves were allocated by lottery to one of three treatment groups (n = 8 per group), which included: Group 1--moxidectin 0.5% pour-on (0.5 mg/kg body weight (BW)) applied topically; Group 2--moxidectin 1% non-aqueous injectable (0.2 mg/kg BW) administered subcutaneously; Group 3--untreated controls. Treatments were administered on Day 0 and calves were housed by group with no contact among animals of different treatments. Three sentinel calves were necropsied on Day 0 of the larvacidal trial to assess viability of larval inocula. On Days 14, 15 and 16 after treatment, calves were euthanatized (two or three from each group per day) and samples of gut contents were collected for determination of total worm counts. On Day -63 relative to treatment, calves in the adulticidal efficacy trial were inoculated with approximately 1000 embryonated Trichuris eggs and then on Day -35 with approximately 2500 infective Oesophagostomum spp. larvae. Fecal samples were collected on Day -7 and the 24 calves with the highest egg counts were assigned by lottery to the following three treatment groups (n = 8 per group): Group 4--moxidectin 0.5% pour-on; Group 5--moxidectin 1% injectable; Group 6--untreated controls. Details of experimental treatments, calf housing and necropsy scheduling were similar to the larvacidal trial. In both the larvacidal and adulticidal trials, inocula contained a variety of parasites in addition to the targeted species. Based on geometric means, both moxidectin 0.5% pour-on and moxidectin 1% non-aqueous injectable significantly reduced (P < 0.05) numbers of Oesophagostomum spp. and Trichuris spp. with anthelmintic efficacies of >99% when used against adult or larval stages of infection. In addition, both formulations of moxidectin demonstrated >95% efficacy (P < 0.05) against larval stages of Strongyloides papillosus. The pour-on formulation had >97% adulticidal and larvacidal efficacy against Cooperia spp. females, while the injectable product was effective against female Cooperia spp. larvae and Cooperia oncophora adult males.     

A pilot study of the efficacy of topical ivermectin against lungworm in young red deer (Cervus elaphus)

Three groups of ten 4-month-old red deer (Cervus elaphus) calves naturally infected with lungworm (Dictyocaulus viviparus) were treated with either oral ivermectin (200 microg/kg), topical (pour-on) ivermectin (500 microg/kg) or oral oxfendazole (5 mg/kg). Faecal larval counts for lungworm were undetectable or very low for 14 days after treatment with oxfendazole, 28 days after treatment with oral ivermectin and for 49 days after treatment with topical ivermectin. This pilot study suggests that the topical formulation of ivermectin was very effective against lungworm and had a more persistent action than the oral ivermectin formulation in young red deer.     

Comparison of the efficacy of dermal formulations of ivermectin and levamisole for the treatment and prevention of Dictyocaulus viviparus infection in cattle

Four groups of six parasite-naive calves were infected at seven day intervals with three doses of infective larvae of Dictyocaulus viviparus. Twenty-one days after the first dose three of the groups were treated either with an injectable formulation of ivermectin at a dose rate of 200 micrograms/kg bodyweight, or with pour-on preparations of levamisole at 10 mg/kg or ivermectin at 500 micrograms/kg. On day 28 two calves from each group were slaughtered and their burdens of lungworms counted. On day 35 the remaining calves were reinfected with D viviparus infective larvae at a rate of 80 L3/kg. The levamisole preparation was 94.6 per cent effective and both ivermectin preparations were 100 per cent effective against the initial infections. The ivermectin-treated calves were protected from the reinfection which subsequently became patent in the levamisole-treated and control calves.     

The residual effect of treatment with ivermectin after experimental reinfection with nematodes in calves

The residual effect of treatment with ivermectin after experimental reinfection in calves was tested. Twenty-four calves were divided into 6 groups of 4 calves each. All calves received a primary infection of 50,000 larvae of both Ostertagia ostertagi and Cooperia oncophora and 1000 Dictyocaulus viviparus larvae. Calves of group 1 remained untreated, and all other calves were treated 21 days after primary infection (0.2 mg/kg injected subcutaneously). Calves of groups 1 and 2 were slaughtered 7 days later. Calves of groups 3-6 were reinfected with the same number of larvae 3 days, 1, 3 and 6 weeks after treatment respectively. Slaughter was 21 days after reinfection. Based on post-mortem worm counts the efficacy of ivermectin after primary infection was 99.7% for O. ostertagi, 95.1% for C. oncophora and 100% for D. viviparus. A residual effect was present for at least one week, but could not be observed 3 weeks after treatment.     

The effectiveness of a single dose of doramectin pour-on in the control of gastrointestinal nematodes in yearling stocker cattle

Three field studies were conducted to determine the efficacy of a single dose of doramectin pour-on in the control of gastrointestinal nematode infections in yearling stocker calves on pasture. These 140-day studies were carried out between October 1995 and March 1996 in Tennessee (TN), between January and June 1997 in Louisiana (LA), and between May and September 1997 in Wisconsin (WI). Calves with patent nematode infections were equally allocated to treatments (doramectin pour-on, at 500 microg/kg body weight or untreated control) and pastures as randomized complete-block designs (LA and TN studies) or completely at random (WI study). There were six pasture replicates per treatment at each site, with each pasture replicate accommodating six calves at the TN site (36 calves per treatment), five calves at the LA site (30 calves per treatment), and seven calves at the WI site (42 calves per treatment). Fecal samples for nematode egg counts were collected on Day 0, and at 28-day intervals thereafter. Body weights were recorded on Day 0 and at 28-day intervals until study termination. Nematode egg output of the doramectin-treated groups was reduced over the entire grazing period compared to those in the untreated control groups, resulting in average daily weight-gain advantages of 0.055 kg (p < or = 0.05) for the TN study, 0.208 kg (p < or = 0.05) for the LA study, and 0.116 kg (p < or = 0.05) for the WI study.     

Efficacy of topically administered doramectin against eyeworms, lungworms, and gastrointestinal nematodes of cattle.

OBJECTIVE: To evaluate efficacy of topically administered doramectin against eyeworms, lungworms, and gastrointestinal nematodes of cattle. ANIMALS: 400 cattle (20 cattle in each of 20 trials). PROCEDURE: Trials were conducted in North America; natural and experimentally induced infections were used. In each trial, cattle were allocated randomly to control (placebo [saline [0.9% NaCl] solution at 1 ml/10 kg of body weight] or untreated; n = 10) or doramectin-treated (500 microg/kg of body weight; 10) groups. Treatments were applied in a single passage along the midline of the back, from the withers to the tailhead. Cattle were euthanatized > or =14 days after treatment, and worm burdens were determined by use of standard techniques. RESULTS: Efficacy of doramectin was > or =95.3% against adults of Thelazia gulosa, T skrjabini, Dictyocaulus viviparus, Haemonchus contortus, H placei, Ostertagia lyrata, O ostertagi, Trichostrongylus axei, Bunostomum phlebotomum, Capillaria spp, Cooperia oncophora, C pectinata, C punctata, C spatulata, C surnabada, Nematodirus spathiger, Strongyloides papillosus, T colubriformis, Oesophagostomum radiatum, and Trichuris spp. Efficacy was 95.1% against fourth-stage larvae of D viviparus, H placei, O lyrata, O ostertagi, T axei, C oncophora, C punctata, C spatulata, C surnabada, N helvetianus, T colubriformis, O radiatum, and Trichuris spp. In addition, efficacy against inhibited fourth-stage larvae of O ostertagi and Ostertagia spp was > or =98.1%. CONCLUSIONS AND CLINICAL RELEVANCE: A single topical application of doramectin pour-on was efficacious against a broad range of nematode species in cattle.     

The efficacy of a pour-on formulation of moxidectin in young red and wapiti-hybrid deer

AIMS: To measure the efficacy of a pour-on formulation of moxidectin against lungworm and abomasal parasites in weaner wapiti x red deer and to compare this with its efficacy in weaner red deer. METHODS: Six red and six wapiti hybrid deer, naturally infected with lungworm and gastro-intestinal parasites, were treated with pour-on moxidectin at 500 microg/kg body weight and slaughtered 14 or 16 days later, along with six red and six wapiti hybrid untreated control deer. Total worm counts were performed on the lungs, abomasum and abomasal digest of each deer. RESULTS: The efficacy of moxidectin pour-on was 100% against adult and immature lungworms (Dictyocaulus viviparus) in red deer, and 100% and 99.7% effective against adult and immature lungworm in wapiti hybrid deer. The efficacy of moxidectin pour-on was 100, 100, 99.9 and 99.9% respectively against adult, fifth stage, late fourth stage and early fourth stage larvae of Ostertagia-type nematodes (assumed to be Ostertagia, Spiculopteragia, Skrjabinagia and Apteragia spp.) in both red and wapiti hybrid deer. CONCLUSIONS: The pour-on formulation of moxidectin, at 500 microg/kg body weight, is highly effective against mature and immature lungworms and abomasal nematodes in wapiti hybrid deer and equally effective in red deer.     

Effect of repeated doses of levamisole on grazing cattle infected with Dictyocaulus viviparus

Seventy-one worm-free Friesian calves were allocated by weight to three trial groups (1, 3 and 4) of 18 and a control group (2) of 17 animals. Calves in group 1 were vaccinated with a bovine lungworm oral vaccine on days 0 and 28, and on day 42 all groups were turned out to graze together on pasture known to be infected with Dictyocaulus viviparus larvae. Twenty-eight days after first exposure to infection one control calf died of parasitic bronchitis. Anthelmintic medication consisting of two doses of levamisole (7 . 5 mg/kg) at 14 day intervals was promptly administered to group 3 calves and three doses at the same intervals to group 4 calves. All calves were challenged with 20,000 infective D viviparus larvae on day 147. Calves were weighed every 14 days throughout the trial which ended 42 days after challenge. Pasture contamination and infectivity were monitored by pasture larval counts and tracer calves. Statistically there was no significant difference between the performances of treated and vaccinated groups before challenge but all were significantly superior to the control group. After challenge the productivity of all experimental groups was temporarily depressed but the levamisole treated cattle recovered more rapidly becoming significantly heavier than the vaccinates at the end of the trial. The mean group weight gains over the trial period were 89 . 92, 63 . 87, 88 . 67 and 98 . 70 kg in groups 1, 2, 3 and 4 respectively.     

The effect of simulated rainfall on the efficacy of doramectin pour-on against nematode parasites of cattle.

Two studies were conducted with doramectin topically administered at 500 microg/kg body weight to assess retention of therapeutic efficacy against nematode infections of cattle before, and after, simulated rainfall. In the first study, 50 heifers, with patent nematode infections, were allocated to one of five treatment groups. An untreated control group and one doramectin-treated group were not exposed to simulated rainfall. Simulated rainfall was applied at a rate of 25.4 mm of water in 35 min to three of the five groups: one group immediately before treatment, the second group 90 min after treatment, and the third group 24 h after treatment. Fecal samples were collected for determining egg counts 14 days after treatment. Percentage efficacy ranged from 97.3% to 100% in all treated calves, regardless of exposure to simulated rainfall. The second study involved 40 mixed-sex cattle that were allocated to one of four treatment groups (one saline control and three doramectin-treated groups). All cattle were confirmed to be free of nematode infections prior to oral gavage with infective larvae of Dictyocaulus viviparus, Cooperia oncophora, and Ostertagia ostertagi. Twenty-six days after infection, three groups were treated with doramectin pour-on and exposed to 20 mm of simulated rainfall over 40 min: one group 60 min before treatment, the second 20 min after treatment, and the third 40 min after treatment. Approximately two weeks after treatment, all cattle were necropsied for worm counts. In all treated groups, the percentage efficacy against O. ostertagi and D. viviparus was >99% to 100%. Percentage efficacy against Cooperia ranged from 97% to 98%. Results indicated that doramectin pour-on remains efficacious against nematodes of cattle when administered immediately before or after rainfall.     

Chemoprophylaxis and immunity to parasitic bronchitis in cattle — a field experiment comparing topical ivermectin and an oxfendazole intraruminal device

Seeder calves infected with Dictyocaulus viviparus were used to contaminate a field divided into three similar paddocks. Twenty-four autumn-born calves were allocated to three matched groups; one group was given topical ivermectin treatments (0.5 mg/kg) at 3, 8 and 13 weeks after turnout (Day 0); each member of a second group was given an oxfendazole pulse-release intraruminal device (OPRB) at turnout; while a third group was kept as untreated controls to monitor the natural epidemiological pattern of events. Severe pasteurella pneumonia exacerbated by lungworm infection occurred in the controls after Day 24. Two died and repeated doses of antibiotic and anthelmintic therapy were necessary to save the remainder. Clinical signs were much milder in the ivermectin and OPRB groups and resolved with only a single dose of antibiotic. The OPRB group excreted some lungworm larvae at this time, but none was detected in the faeces of the ivermectin group during the grazing season. At housing, five calves from each group and four lungworm-naive calves were challenged with D. viviparus larvae. The infection became patent in all challenge-control calves, but no larvae were passed by any of the trial animals. Post-mortem worm-counts revealed percentage takes for the challenge controls, trial controls, ivermectin and OPRB groups of 16.7, 0.01, 0.9 and 0.2, respectively. All trial groups had therefore developed a substantial immunity.     

Anthelmintic efficacy of albendazole against adult Dictyocaulus viviparus in experimentally infected calves.

Anthelmintic activity of albendazole against adult Dictyocaulus viviparus was evaluated in a controlled experiment. Calves were raised nematode-free to approximately 8 weeks of age and were each given 4,000 infective 3rd-stage larvae. Twenty calves with patent parasitisms were allotted to 2 groups of 10 calves each. Calves in group 1 were used as nonmedicated controls, and calves in group 2 were given albendazole in paste formulation at the dosage concentration of 7.5 mg/kg of body weight on the 30th day after administration of infective larvae. At necropsy, nonmedicated control calves had a total of 308 adult D viviparus, whereas the albendazole-treated calves had 11, for an average efficacy of 96.4%. These reductions were statistically highly significant (P less than 0.01). At necropsy, none of the treated calves was passing 1st-stage C viviparus larvae in their feces, whereas control calves were passing an average of 46 larvae/10 g of feces.     

Efficacy of ivermectin and levamisole against immature Dictyocaulus viviparus in cattle

Eighteen calves aged approximately three months were each infected with Dictyocaulus viviparus larvae at a rate of 30/kg bodyweight. Seven days later they were randomly allocated to three groups of six animals. Calves of group 1 were controls. Calves of group 2 were given levamisole at a dose rate of 10 mg/kg and calves of group 3 were given ivermectin at a dose rate of 200 micrograms/kg. The anthelmintic activity of these two drugs was compared using clinical, functional, parasitological and pathological parameters. The results showed that the efficacy of ivermectin, given at a therapeutic dose, against immature D viviparus was higher than that of levamisole, given at double the recommended dose.     

Efficacy of Cydectin moxidectin 1% injectable against experimental infections of Dictyocaulus viviparus and Bunostomum phlebotomum superimposed on natural gastrointestinal infections in calves.

Twenty male Holstein calves averaging 105 kg in weight and naturally infected with gastrointestinal nematodes and small numbers of lungworm and hookworm, were given experimental infections with the two latter species to provide adult and larval stages for anthelmintic evaluation. Following random allotment, one group of 10 calves was injected subcutaneously with moxidectin at a dosage of 0.2 mg kg-1 of body weight. A second group of 10 was injected subcutaneously with unmedicated blank vehicle at a dosage of 1 ml per 50 kg of body weight. Fecal samples were examined before treatment and at 7 and 13 days after treatment. The 20 calves were necropsied for worm recovery at 13 and 14 days after treatment. All calves were positive for lungworm and hookworm on the treatment date. Treatment was 100% effective in elimination of hookworm eggs and lungworm larvae and 99.9% in reducing total egg counts at both 7 and 13 days after treatment. Moxidectin was 100% effective (P less than 0.01) in eliminating the following 11 species of nematodes. Dictyocaulus viviparus mature and immature adults (E5), Bunostomum phlebotomum adults and L4, Ostertagia ostertagi adults and early L4, Ostertagia lyrata adult males, Haemonchus placei adults. Trichostrongylus axei adults, Cooperia spp., including Cooperia punctata, Cooperia spatulata, and Cooperia pectinata adults, Oesophagostomum radiatum adults and Trichuris discolor adults. No adverse reactions to moxidectin treatment were observed.     

Efficacy of levamisole pour-on compared with levamisole subcutaneous injection against Dictyocaulus viviparus infection in calves.

The efficacy of levamisole pour-on against Dictyocaulus viviparus was compared to that of subcutaneous levamisole injection. Eighteen calves were raised individually and artifically infected with D. viviparus larvae. Faecal samples were collected 27 and 28 days later and larvae per gram (l.p.g.) determined. The animals were then divided into three comparable groups. Group 1 animals remained untreated as controls. Group 2 animals received levamisole 10% w/v subcutaneous injection at a dose of 5 mg kg-1 and Group 3 received levamisole pour-on 20% w/v at a rate of 10 mg kg-1 applied transdermally. Results of l.p.g. measurements from faecal samples taken 7 and 8 days post-treatment indicated a dramatic reduction in the worm burden of animals in both treatment groups. Necropsies at 14 days post-treatment revealed few adult worms in these groups, indicating a 99 and 98% kill rate for pouron and subcutaneous injection, respectively.     

The therapeutic efficacy and prophylactic activity of doramectin against Dictyocaulus viviparus in cattle

SUMMARY Three groups of 8, 4-month-old male Jersey or Jersey-cross calves were infected with 2400 Dictyocaulus viviparus L₃ larvae and either left untreated or injected subcutaneously with 200 μg/kg doramectin 5 or 25 days after infection (DAI). Lungworms were found in all untreated cattle (geometric mean = 49) at necropsy 39 or 40 DAI. None was found in any of the treated cattle. In a second experiment, groups of 6, 8-month-old calves were untreated or injected with 200 μg/kg doramectin 28, 21 or 14 days before each calf was challenged with 2700 D viviparus larvae. Lungworms were recovered at necropsy 32 to 34 DAI. The geometric mean worm burden in the untreated cattle was 550. This was reduced by 100%, 99.5% and 94.1% in calves treated with doramectin 14, 21 or 28 days, respectively, before infection. It was concluded that doramectin is a highly effective anthelmintic against D viviparus adult or L₄ infections of cattle, and that reinfection of treated cattle will be significantly reduced for at least 28 days after treatment.