Persistent efficacy of doramectin against experimental nematode infections in calves

Pharmacokinetics of subcutaneously administered doramectin in alpacas

The objective of this research was to evaluate comparative pharmacokinetics of doramectin in alpacas, after subcutaneous administration of 0.2 mg/kg dose. Six healthy adult alpacas, mean age of 5 years ± 1, (three female and three gelded males) of mean bodyweight of 62 kg ± 16 kg with an average body condition scored 2.8 ± 1 out of five, were used in this study. Serial blood samples were collected from the jugular vein before the administration until day 21 afterwards to establish the pharmacokinetics of doramectin after its subcutaneous administration at 0.2 mg/kg dose. The blood samples were analysed using high-performance liquid chromatography (HPLC), fluorescence detection method with precolumn derivatisation, validated for alpacas. The pharmacokinetic parameters were calculated using a noncompartmental model, and results showed Cmax (6.05 ± 5.34 ng/ml), Tmax (3.83 ± 2.48 days), AUC (62.12 ± 18.86 ng/ml × d), terminal half-life (6.2 ± 4.9 days) and MRT (11.56 ± 4.43 days). The results of this study showed that the Cmax and AUC were much lower than in cattle and sheep at the same dosage. Tmax remained similar to cattle and sheep. This study presents valuable information about pharmacokinetics of doramectin in alpacas, which can be utilised in its future efficacy studies.

Anthelmintic treatment and the suppression of egg production in gastro-intestinal nematodes of sheep and cattle: fact or fallacy?

Evidence for a temporary suppression of egg production in gastro-intestinal nematodes of sheep and cattle following anthelmintic treatment is reviewed and the possible relevance of this phenomenon to the appropriate sampling time in the faecal egg count reduction test is discussed. The results of this review suggest that if such suppressive effects do occur in sheep nematodes, then they are unlikely to be of much practical significance and that little benefit would be derived from extending the post-treatment sampling interval beyond 10 days. For cattle nematodes, however, the issues are less readily obvious but indicate, at least when testing for resistance to persistent anthelmintics, that adoption of a longer post-treatment interval might be advisable.

Spirocerca lupi in dogs: prophylactic effect of doramectin

Spirocerca lupi is primarily a parasite of dogs, which typically causes oesophageal nodules, aortic aneurysms, and spondylitis. This study investigated the efficacy of doramectin as a prophylactic agent for canine spirocercosis. Five beagle dogs were injected subcutaneously with doramectin (400 microg/kg on 3 occasions 30 days apart q30d), while 5 other beagle dogs served as untreated controls. All dogs were inoculated with 40 infectious S. lupi larvae (L3) one month after the last doramectin treatment. All control dogs and 4/5 treated dogs became infected. Two control dogs died of ruptured aortic aneurysms, while no deaths occurred in treated dogs. Oesophageal nodules appeared 40-103 day later in treated as compared to control dogs, and eggs appeared in the faeces 49-106 day later in treated as compared to control dogs. The mean faecal egg count on day 223 in the treatment group was reduced by 99.77%. All control dogs had thoracic radiographic changes during the study, while only 2/5 study dogs showed radiographic changes. This study shows that although doramectin did not entirely prevent canine spirocercosis it reduced the clinical signs associated with infection and delayed and reduced egg output.

Evaluation of doramectin for the treatment of experimental canine spirocercosis

The nematode Spirocerca lupi is primarily a parasite of dogs, which causes typical lesions of esophageal nodular granulomas, aortic aneurysms and spondylitis. In order to evaluate the therapeutic effect of doramectin on experimental canine spirocercosis, seven beagle dogs experimentally infected with 40 infectious S. lupi larvae (L(3)) were treated with doramectin. Treatment was commenced following endoscopic visualization of esophageal granulomas, and typical S. lupi eggs were detected in the feces. The treatment protocol included six treatments of doramectin (400 microg/kg subcutaneously) at 2 weeks intervals, followed by monthly injections until the disappearance of the esophageal granulomas or the end of the study (768 days post-inoculation). Eggs could not be found on fecal examinations 3-10 days after the first or second doramectin treatment. In addition, a gradual decrease in size of granulomas was noticed in all seven dogs during the course of the study. Esophageal granulomas had completely resolved in six of the seven dogs between day 35 and day 544 post-initial doramectin treatment, by day 35 in one dog (after three treatments), by day 43 in two dogs (after four treatments), by day 98 in one dog (after seven treatments), by day 460 in one dog (after 18 treatments) and by day 544 in another dog (after 21 treatments). In one dog, remnants of S. lupi granulomas could still be seen 544 days post-initiation of treatment with doramectin. Multiple subcutaneous injections of doramectin (400 microg/kg) were shown to be effective and safe in the treatment of canine spirocercosis.

Effects of treatments with endectocide on the weight gain of grazing cattle in a warm temperate climate

Five groups of 20 weaned beef calves were injected subcutaneously with either an ivermectin, a doramectin, an abamectin long-acting formulation, an ivermectin long-acting formulation or a saline control, at turnout and 60 and 120 days later. The animals grazed the same pasture and were sampled and weighed at turnout and 30, 60, 90, 120, 150 and 180 days later. At turnout the mean bodyweights of all the groups were similar and faecal culture showed that they had a mixed strongyle infection of Cooperia, Haemonchus, Ostertagia, Oesophagostomum and Trichostrongylus species. After 180 days, the mean bodyweight gains of each group were respectively 62.1 kg, 102.2 kg, 106.4 kg, 107.3 kg and 110.1 kg for the control, ivermectin, doramectin, ivermectin long-acting and abamectin long-acting groups. All the products significantly improved the weight gains of the cattle, and significantly reduced their faecal egg counts.

Anxiolytic and anticonvulsant properties of doramectin in rats: behavioral and neurochemistric evaluations

Doramecin is an antiparasitic drug that may interfere with gamma-aminobutyric acid (GABA) neurotransmission. Some behavioral manifestations are related with GABAergic neurotransmissions as anxiety and seizures. The objective of the present study was to examine the possible central nervous system (CNS) effects of doramectin (100, 300 and 1000 microg/kg, SC) in rats, using anxiety behavioral models, susceptibility to seizures and central neurotransmitter evaluations. The open-field results showed (i) few alterations in locomotion frequency; (ii) a biphasic effect on rearing frequency that may be the consequence of least habituation in open-field; (iii) the reduction of grooming durations might be attributed to a possible anxiolytic effect of doramectin since GABAergic agonists reduced this parameter in apparatus. Our data in the hole board showed no effects in locomotion and rearing frequencies but increased head dipping frequency of rats administered doramectin similarly to anxiolytic drugs. In plus-maze test, doramectin administration increased the number of entries and time into open arms, indicating also an anxiolytic effect. Doramectin protected animals from convulsant effects of picrotoxin, indicative of an anxiolytic pharmacological profile of a drug with GABAergic properties. The alterations observed in central dopaminergic, noradrenergic and serotoninergic neurotransmissions might be the consequence of reinforcement in central GABAergic neurotransmission induced by doramectin. The present results suggest that doramectin has the pharmacological profile of an anxiolytic/anticonvulsant drug with GABAergic properties.

The effectiveness of doramectin pour-on in the control of gastrointestinal nematode infections in cow-calf herds

Two field studies were conducted in the USA to determine the efficacy of a single strategically-timed dose of doramectin pour-on in the control of gastrointestinal nematodosis in beef cow-calf herds and the resultant effects on calf productivity. One study was carried out between May and October 1996 in a spring-calving herd at a site located in Idaho (ID) and the other between January and July 1997 in a fall-calving herd at a site located in Mississippi (MS). In each study, cow-calf pairs were randomly allotted by sex of calf to pastures and one of two treatment groups (doramectin pour-on at the recommended dose rate of 500 microg/kg body weight or untreated control). There were four pasture replicates per treatment at each site. Each pasture contained 12 cow-calf pairs at the ID site and 15 cow-calf pairs at the MS site. Treatment was administered to cows and calves on 21 May 1996 (ID) or 23 January 1997 (MS). Following treatment, cow-calf pairs were assigned to their designated pastures where they remained until the calves were weaned 140 (ID) or 168 (MS) days later. Cow and calf fecal egg counts and calf body weights were recorded on treatment day and then at monthly intervals until study termination. Doramectin treatment reduced nematode egg output in cows and calves over the entire grazing season compared to untreated controls and resulted in calf weight gain improvements of 9.8kg (p=0.295) at the ID site and 17.4kg (p=0.0002) at the MS site.

Evaluation of the persistent efficacy of doramectin and ivermectin injectable against Ostertagia ostertagi and Cooperia oncophora in cattle

The persistent efficacy of doramectin and ivermectin injectable against moderate and high infection levels of Ostertagia ostertagi and Cooperia oncophora were evaluated in cattle. Calves were allocated to six groups of six animals. On Day 0 animals of Groups I1/I2 and D1/D2 were treated with 0.2mg/kg ivermectin and doramectin injectable, respectively. Animals of the C1, I1 and D1 groups received a daily (moderate) infection of 1000 L3 of O. ostertagi and 1000 L3 of C. oncophora, and animals of the C2, I2 and D2 groups received a daily (high) infection of 10,000 L3 of each species. The animals were infected for 21 days with both species, the infections with C. oncophora and O. ostertagi started from Days 8 and 15 post treatment, respectively. Animals were necropsied on Day 40. The calculation of the persistent activity of ivermectin and doramectin was based on the efficacy against the different developmental and adult stages of both parasites. The present study confirmed that infection levels may influence the duration of persistent efficacy of an anthelmintic. Doramectin had at the moderate infection level a persistent efficacy of at least 35 days against O. ostertagi and at least 28 days against C. oncophora; at the high infection dose persistent efficacy was somewhat shorter i.e. up to 33 days and approximately 28 days, respectively. The duration of persistent efficacy of ivermectin against O. ostertagi at the moderate infection level was between 14 and 25 days, at the high dose level up to 25 days. Persistent efficacy of ivermectin against C. oncophora could, at both infection doses, not be measured, with the present experimental design.

Efficacy of doramectin against naturally acquired nematode infection in Iberian swine

Studies were carried out to determine the therapeutic efficacy of doramectin, administered intramuscularly at a dose of 300 microg/kg live weight, against naturally acquired helminths of extensively farmed Iberian pigs. The first study (slaughter study) evaluated, through necropsy of the study animals, the product's efficacy against gastrointestinal and pulmonary nematodes (Ascaris suum, Oesophagostomum dentatum and Metastrongylus sp.) whilst the second, faecal egg count reduction study, (FECR study) evaluated the drug's efficacy only against gastrointestinal helminths (A. suum, Trichuris suis and Oesophagostomum sp.). The first study used 20 animals divided into two equal groups of 10 on the basis of body weight and faecal egg count. One group constituted saline treated controls and the other was doramectin treated. On Day 14 post treatment half of the animals in each group were necropsied and the number of parasites present counted. On Day 15 the remaining half of each group underwent the same procedure. The second study was carried out with 40 animals divided equally into two groups of 20. This study determined the effect of doramectin treatment on faecal egg counts as an indicator of parasite burden. The first study demonstrated an efficacy of 100% against adult Metastrongylus sp. and A. suum, whilst the efficacy against O.dentatum was 96.3%. The second study indicated that at Day 21 post treatment there was a 100% reduction in egg counts in faeces in comparison to untreated controls.

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.

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.

Comparison of the persistent efficacy of the injectable and pour-on formulations of doramectin against artificially-induced infection with Dictyocaulus viviparus in cattle

The persistent efficacy of the injectable and topical formulations of doramectin was compared against experimental challenges with infective larvae of Dictyocaulus viviparus in two separate studies. Four groups of 10 randomly-assigned calves, negative for lungworm larvae by the Baermann technique, were used in each study. Calves were treated subcutaneously in the midline of the neck or poured down the midline of the back with saline (1 ml/50 kg. injection: 1 ml/10 kg. pour-on) on Day 0 or doramectin (200 microg/kg = 1 ml/50 kg. injection: 500 microg/kg = 1 ml/10 kg. pour-on) 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. Calves were inoculated daily with a gavage of approximately 100 larvae of D. viviparus from days 35 to 49 for the injectable study and days 28 to 42 for the pour-on study. The two larval viability monitor calves received approximately 3000 infective larvae in the same manner on Day 49 or 42 for the injectable and pour-on studies, respectively. Equal numbers of calves from each treatment group as well as the larval viability monitor calves were necropsied on days 14 and 15 after the last lungworm inoculation to enumerate the worm burden. The worms recovered were quantified and identified. For each study, geometric mean worm recoveries for each treatment group were back transformed from the natural log-transformed data (worm count +1) and were used to estimate percentage reduction. Doramectin injectable solution was 100.0% efficacious against lungworms for up to 49 days and the pour-on formulation was 100.0%, 93.1% and 81.5% effective in reducing lungworm infection resulting from challenge infection for up to 28, 35, and 42 days post-treatment, respectively.

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.

A comparison of persistent anthelmintic efficacy of topical formulations of doramectin, ivermectin, eprinomectin and moxidectin against naturally acquired nematode infections of beef calves

Persistent anthelmintic efficacy of topical formulations (all at a dosage of 500 microg/kg) of doramectin (DOR), ivermectin (IVM), eprinomectin (EPR) and moxidectin (MOX), in comparison with untreated control cattle (CONT), was observed in stocker beef calves during a 112-day winter-spring grazing trial. Five groups of 15 calves per group were grazed on 15 separate 2 ha pastures following random assignment of animals to specific pastures and then to treatment groups. All of the 5 treatments were represented in each of the 15 pastures. All cattle were weighed on study Days 1, 0, 28, 56, 84, 111 and 112. Fecal samples for nematode egg counts were collected on Days 7, 0, at 7 day intervals through Day 56 and at 14 day intervals to Day 1 12. Pooled group fecal cultures for determining generic composition of nematode infections were prepared at 14 day intervals throughout the study. As based on fecal egg counts, anthelmintic activity of EPR and MOX was greater (p < 0.05) than values for IVM or CONT through Day 28. Activity of DOR was greater (p < 0.05) than that of IVM on Days 7 and 14 only. Although significance levels varied little among treated groups from Day 42 to the end of the study, egg counts and percent reduction values of EPR and MOX remained consistently lower numerically than egg counts and higher than reduction values respectively, of DOR and IVM through Day 70. From Day 70 on, IVM counts were numerically, but not significantly higher than values of CONT. Based on larval culture, Cooperia predominated from Day 0 through 28 and again from Days 70 to 98; Ostertagia was second in prevalence with highest percentages, which exceeded those of Cooperia, between Days 42 and 70. Bodyweights of all treated groups, with exception of IVM, were always significantly greater (p < 0.05) than weights of CONT. Weights of IVM were numerically greater, but not significantly greater than CONT only on Days 84 and 112. From Day 56 on, there were no significant differences between weights of DOR, EPR and MOX, however, numerical values for MOX were consistently higher than values for the other two. Final average total bodyweight gains were: 153.7 kg for MOX, 148.5 kg for EPR, 146.9 kg for DOR, 139.7 kg for IVM and 127.7 kg for CONT.

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 and ivermectin against Ascaris suum in experimentally infected pigs

A study was conducted to investigate the persistent nematocidal activity of two avermectins against experimentally-induced infections of Ascaris suum in swine. Seventy-two nematode-free cross-bred pigs of similar bodyweight were randomly allotted to nine treatment groups of eight pigs each. Eight of the groups were treated with injectable solutions containing 300 microg of doramectin/kg (IM) or 300 microg of ivermectin/kg (SC) either 0 (same day), 7, 14, or 21 days prior to an oral challenge of 50000 embryonated A. suum eggs. The ninth group (control) was challenged in parallel without any avermectin treatment. At 41 or 42 days after challenge, pigs were euthanatized and adult and larval stages of A. suum were collected from the gastrointestinal tract of each pig and counted. Both avermectins significantly (P < 0.0002) reduced nematode counts when given on the day of challenge (0 days prior), and the efficacy was 100% and 97.5% for doramectin and ivermectin, respectively. Doramectin given 7 days prior to challenge significantly (P < 0.0001) reduced nematode counts, and the efficacy was 98.4%. For all other avermectin-treatment groups, nematode counts were not significantly reduced compared to those in control pigs. These data indicated that anthelmintic activity of ivermectin against A. suum persisted for less than 7 days and the activity of doramectin persisted for more than 7, but less than 14 days.

Persistence of the efficacy of doramectin against Ostertagia ostertagi and Cooperia oncophora in cattle

The persistence of the efficacy of doramectin injectable against Ostertagia ostertagi and Cooperia oncophora was evaluated in two studies in calves. In both, the calves were allocated to six groups of six. Calves in the first control group (C1) and first treated group (T1) received a daily infection of 200 L3 of O ostertagi and 200 L3 of C oncophora; the calves in groups C2 and T2 received a daily infection of 1000 L3 of each species, and groups C3 and T3 received 10,000 L3 of each species per day. The calves in the three treated groups each received 0.2 mg/kg doramectin injectable on day 0. In the first study, the calves were infected for 21 days with Cooperia and for 28 days with Ostertagia, and they were slaughtered on day 33. In the second study, the calves were infected for 21 days with both species, the infections with Cooperia and Ostertagia starting from eight and 15 days, respectively, after the treatment, and the animals were slaughtered on day 40. The calculation of the persistence of the activity of doramectin was based on its efficacy against the different developmental and adult stages of the two parasites. The data from both studies indicated that the efficacy of doramectin against Ostertagia persisted for at least five weeks, but no conclusions could be drawn about the effect of the size of the infective doses on the persistence of the activity. In contrast, the Cooperia worm counts from the second study suggested that the efficacy of doramectin against Cooperia persisted for at least four weeks when the calves were exposed to a low or moderate infection level, whereas at the highest infection level it persisted for between three and four weeks.

Field efficacy of doramectin pour-on against naturally-acquired, gastrointestinal nematodes of cattle in North America

Seven studies were conducted under field conditions in North America to evaluate the therapeutic efficacy of doramectin in a pour-on formulation at a dosage of 500 microg/kg (1 ml/10 kg) for cattle harboring naturally-acquired infections of gastrointestinal nematodes, including species of Haemonchus, Ostertagia, Trichostrongylus, Bunostomum, Cooperia, and Oesophagostomum. In each study, 40 to 100 cattle were randomly allocated to a saline- or doramectin-treated group in a tiered manner based on Day -7 bodyweight. On Day 0, the cattle received either saline or doramectin topically, according to their treatment group. Weather and safety observations were made following treatment. No adverse reaction to treatment was observed at any time during these studies. Fecal egg count (FEC) determinations were carried out on each animal on Days -7, 0, 7, 14, and 21. Reductions in FEC for the doramectin-treated animals compared to saline-treated cattle were > or = 96.0% by Day -7 and > or = 99.0% on Days 14 and 21 for each study. Across all studies regardless of weather conditions, the reduction by Day 21 for the doramectin-treated animals compared to saline controls was 99.7% (p < or = 0.0001) and compared to pretreatment levels in doramectin-treated cattle was 99.9% (p < or = 0.0001). Doramectin pour-on should provide a useful new treatment for controlling nematode parasites of cattle.

Gastrointestinal nematode infections of first-grazing season calves in Western Europe: associations between parasitological, physiological and physical factors

Analysis of 85 studies on gastrointestinal nematode infections in first-grazing season (FGS) calves is presented. The studies cover a 26-year period and were carried out in 13 countries in Western Europe. Both control and chemoprophylactic-treated (early in the season) FGS calf groups were included. In 53 of the 85 studies, clinical outbreaks of parasitic gastroenteritis (PGE) were observed during the FGS in the control groups. The mean initial age (and weight) of the control calf group was significantly associated with PGE outbreaks: 82% of control calf groups < or = 6 months of age had outbreaks of PGE, compared to only 33% of control calf groups > 6 months of age. In 92% of trials where the geometric mean faecal egg count (MFEC) was > or = 200 EPG on Day 56, PGE outbreaks were observed, but where it was < 200 EPG, only 29% had PGE. The use of these two factors in assessing the likelihood of PGE outbreaks in untreated calf groups in the future FGS is therefore, proposed. No chemoprophylactic-treated groups had PGE, but there was a highly significant negative relationship between maximum faecal egg counts in the chemoprophylactic-treated calves and the proportion of the trial covered by the different chemoprophylactic systems. Higher stocking densities were significantly associated with higher pasture contamination in both control and chemoprophylactic-treated calves. A highly significant positive relationship between the weight gained in the chemoprophylactic-treated groups and the estimated duration of the various chemoprophylactic systems was found, but there were large variations in weight gains (60-160 kg) between groups even with the same chemoprophylactic. Despite this and other highly significant associations, it was not possible to indicate what weight gains were obtained by the end of the FGS, from factors measured early in the FGS.

The effect of different infection levels on acquired resistance to gastrointestinal nematodes in artificially infected cattle

The objective of this study was to evaluate the effect of different infection levels of Ostertagia ostertagi and Cooperia oncophora in a simulated 'first grazing season' on the resistance of calves to an artificial challenge infection. The infection levels were determined by the infection schedules and the chemoprophylaxis used. Thirty six 7-11-month old Holstein-Friesian bull calves were randomly divided into four groups. The animals of group B received an ivermectin sustained release bolus (ISRB) on day 0. The calves of group D were treated on days 0 and 56 with a subcutaneous injection of doramectin (0.2 mg kg(-1) BW). Group C was the untreated control group. The calves of group N were used as helminth-naive controls, while the animals of groups B, C and D were trickle infected for 24 weeks. The infection schedules were designed to simulate the expected infection pattern for each treatment group under set-stocked conditions in temperate climate areas. After the last infection, all animals were treated with oxfendazole. One week later, all animals received a challenge infection of 50,000 O. ostertagi L3 and 100,000 C. oncophora L3, spread over 10 consecutive days. During the primary infection period the faecal egg output and the serum pepsinogen and antibody levels reflected the different levels of host-parasite contact between the groups (group C > group D > group B > group N). After the challenge infection, faecal egg counts, total Ostertagia burden, size of the adult worms and abomasal globule leucocyte counts all indicated a positive relationship between the level of Ostertagia infection during the primary infection period and the level of acquired resistance. A reduction of host-parasite contact during the primary infection period, as a consequence of the infection schedule and the chemoprophylaxis used, resulted in a diminished level of resistance to the artificial challenge infection with O. ostertagi. Faecal cultures and small intestine worm counts indicated that all previously infected groups had acquired a high degree of resistance to the Cooperia challenge infection.

Ecotoxicology and residues of anthelmintic compounds

Anthelmintics and endectocides used for the treatment and prophylaxis of Ostertagia sp. in ruminants include benzimidazoles, levamisole, morantel and the avermectins and milbemycins. Most of these agents are excreted to some extent in the faeces of treated animals and it has been demonstrated that members of the avermectin/milbemycin group may have deleterious effects on non-target organisms utilising the faeces. The environmental impact of antiparasitic chemotherapy depends on the deleterious effect which the agent or its metabolites have on organisms in the locus of the excreta, the amount of active agent excreted, the temporal nature of the excretion and the stability of the ecotoxic residues. These have to be considered in the context of the overall proportion of excreted faeces from a herd which is contaminated and thus the availability of non-contaminated faeces which may act as refugia for dung utilising organisms. The contribution which weathering, faunal inhabitants, trampling by cattle and disturbance by birds have on the rate of dung degradation must also be considered. The greatest ecotoxicological risk is associated with sustained release delivery devices, delivering endectocides with potent activity against dipteran flies and coleopteran beetles. The relatively large proportion of most cattle herds excreting faeces with no endectocidal contamination is likely to reduce the impact that such treatment or prophylactic strategies have on non-target organisms.

A comparison of the efficacy of two treatments of doramectin injectable, ivermectin injectable and ivermectin pour-on against naturally acquired gastrointestinal nematode infections of cattle during a winter-spring grazing season

Four groups of 18 crossbred beef steer calves (three replicates of six per group) were used to compare persistent efficacy of doramectin injectable, ivermectin injectable and ivermectin pour-on against naturally acquired infections of gastrointestinal nematodes during winter-spring grazing in Louisiana. The experiment was initiated on January 11. Treatments administered on Day 0 and again on April 5 (Day 84, 12-week interval) were: Group 1, untreated controls (CONT); Group 2, doramectin (DOR) at 200 micrograms/kg, s.c. injection; Group 3, ivermectin (IVM-INJ) at 200 micrograms/kg, s.c. injection; Group 4, ivermectin pour-on (IVM-PO) at 500 micrograms/kg, back midline. The cattle were weighed and fecal samples (for egg counts and for culture-larval identification) were collected at regular intervals throughout the 161 day experiment. In the interval between Day 0 and 84, arithmetic mean egg counts of the CONT group averaged about 890 eggs per gram, but then decreased markedly between Days 119 and 126, and remained at a lower plane for the remainder of the experiment. From Day 28 to 56, egg counts of the DOR group were consistently lower (P < 0.05) than those of controls and both IVM-treated groups. Egg counts of the DOR group were always lowest after the second treatment, but differed (P < 0.05) only from IVM-PO counts between Days 119 and 140 (35 and 56 days after the second treatment). Ostertagia was the predominant genus, followed by Cooperia in all four groups. Oesophagostomum, Trichostrongylus, Haemonchus, and Bunostomum were other genera identified. Bodyweights of the DOR group remained significantly greater (P < 0.05) than those of all other groups from Day 112 through the end of the experiment. Total gains for the CONT, DOR, IVM-INJ, and IVM-PO groups were 96, 159, 147, and 150 kg, respectively; treated groups were significantly (P < 0.05) greater than CONT, but differences among treated groups was not significant (P > 0.05).

Nematode reinfection following treatment of cattle with doramectin and ivermectin

A study was conducted to investigate reinfection with nematodes in calves following treatment with doramectin or ivermectin administered subcutaneously at a dose rate of 200 micrograms kg-1 of body weight under conditions of natural challenge. Thirty calves were allocated to three groups of 10 calves each based on body weight, sex, breed and pre-treatment faecal egg counts (FEC) after grazing together on a common pasture for three months. Treatments were doramectin, ivermectin and no treatment. Calves were returned to the same pasture for 56 days, placed on dry lot from days 56 to 63 and then necropsied over days 64-66. Faecal egg counts were done at days -1 and 0, then bi-weekly from day 14 to 56 and day 63. Mean FEC in control calves continued to rise throughout the grazing period. Trichostrongyle FEC were significantly (P < 0.05) greater in the control group compared to either treated group at each sample time following treatment. At necropsy, a total geometric mean of 19,847 nematodes per calf was recovered from untreated controls, of which eight genera were identified. The predominant nematode genera were Ostertagia (8749), Nematodirus (3702) and Cooperia (1927). In the ivermectin-treated calves, geometric mean worm burden was similar to that of the untreated controls: A total of 20,349 nematodes per calf was present including the genera Nematodirus (8633), Ostertagia (4700) and Cooperia (1740). In contrast, the geometric mean worm burden in doramectin-treated calves was 12,173, including Ostertagia (4310), Cooperia (1141) and Nematodirus (1667). Doramectin was more effective than ivermectin in protecting calves from reinfection over a 56-day post-treatment grazing period under conditions of natural challenge as measured by accumulated mean worm burdens.

Therapeutic and protective efficacy of doramectin injectable against gastrointestinal nematodes in cattle in New Zealand: a comparison with moxidectin and ivermectin pour-on formulations

Two similar studies were conducted in New Zealand to compare the therapeutic and persistent activity of doramectin injectable, moxidectin pour-on, ivermectin pour-on and oxfendazole oral drench when administered to nematode-infected cattle which were then grazed on common pastures. On day 0 (treatment day), 40 cattle were weighed, faecal sampled and allocated on the basis of day--3 faecal egg counts (FEC) to four treatment groups. Cattle were then treated with either doramectin by subcutaneous (s.c.) injection, moxidectin and ivermectin by topical application, or oxfendazole orally using label-recommended dosages. Oxfendazole treatment served primarily as a control to monitor reinfection without persistent activity. Faecal samples for nematode egg counts and coprocultures for larval differentiation were collected six times between day 0 and day 56 and all cattle were reweighed on day 56. Doramectin reduced pretreatment FEC by 99.1% in the first study and by 100% in the second study when assessed at 14 days posttreatment. Corresponding reductions for moxidectin were 80.8% and 85.2%, for ivermectin 86.0% and 80% and oxfendazole 78.3% and 100%, respectively. Posttreatment rise in FEC indicated that reinfection of moxidectin-treated animals occurred at the same time as oxfendazole controls in both trials. Posttreatment rise in FEC with ivermectin pour-on was similar to moxidectin and oxfendazole in one study, but in the other study ivermectin pour-on delayed the rise by 14-21 days. The rise in FEC for doramectin was delayed for 14-21 days in one study and at least 21 days in the other. The better parasite control provided by doramectin resulted in greater weight gains for cattle over the 56-day period as compared to moxidectin pour-on, ivermectin pour-on and oxfendazole in both trials. Gains of doramectin treated cattle were significantly (p < 0.05) greater than those of ivermectin and moxidectin groups in one trial and the oxfendazole group only in the other.

The effectiveness of a single treatment with doramectin or ivermectin in the control of gastrointestinal nematodes in grazing yearling stocker cattle

Four studies were conducted to a similar experimental design in the U.S. to evaluate the effectiveness of doramectin injectable administered to yearling stocker cattle in the control of gastrointestinal nematodiasis over the subsequent grazing period. Studies were conducted in Wisconsin (WI) and Arkansas (AR) during the summer season. The other two studies were conducted in Georgia (GA) and Mississippi (MS) during the winter/spring season. Doramectin was compared with both ivermectin injectable and ivermectin pour-on in the WI study, with ivermectin injectable alone in the GA study and with ivermectin pour-on alone in the other two studies. At each study site, an area of permanent pasture previously grazed by parasitized animals was subdivided by fencing into equal pasture units each with its own water supply. A treatment designation (non-medicated control, doramectin injectable, ivermectin injectable or ivermectin pour-on) was randomly assigned to each pasture unit. Weaned beef calves with confirmed gastrointestinal nematode infections were randomly allotted to a pasture unit and corresponding treatment group. Each treatment group consisted of three replicates of seven animals per pasture unit (total 21 animals) in the WI study, three replicates of four or six animals per pasture unit (total 16 animals) in the AR study, five replicates of six animals per pasture unit (total 30 animals) in the GA study and three replicates of 12 animals per pasture unit (total 36 animals) in the MS study. Treatments were 1% doramectin injectable solution, 1% ivermectin injectable solution, 0.5% ivermectin pour-on solution or non-medicated controls. The injectables were administered at a dose of 1 ml/50 kg body weight (200 micrograms doramectin or ivermectin/kg) by subcutaneous injection in the neck. Ivermectin pour-on solution was administered topically at a dose of 1 ml/10 kg body weight (500 micrograms ivermectin/kg). After receiving their prescribed treatment, animals were placed on their designated pasture unit where they remained for the entire grazing period (84-140 days). Fecal nematode egg counts and body weights were monitored at predetermined intervals throughout each study. Doramectin treatment reduced pretreatment egg counts by between 95 and 100% by 21 days post-treatment. Subsequent rises in egg output from exposure to infective pastures were delayed by two to four weeks resulting in substantial reductions in total egg deposition over the grazing period and, therefore, potential pasture recontamination. Doramectin treatment resulted in substantial average daily weight gain advantages (0.152-0.272 kg) over the grazing season compared to non-medicated controls. Advantages were statistically significant (P < 0.05) in three of the four studies. There were no significant differences (P > 0.05) in average daily gain between the doramectin and ivermectin injectable or ivermectin pour-on treated groups.

Persistent efficacy of doramectin against experimental challenge with Ostertagia ostertagi in cattle

Two studies were conducted in North America to evaluate the persistent efficacy of doramectin injectable solution against experimental challenge with infective larvae of Ostertagia ostertagi. In both studies, four groups of 10 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-1) on Day 0 or doramectin (200 micrograms kg-1 = 1 ml 50 kg-1) 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. Beginning on Day 14 and continuing through Day 28, the 40 treated calves each were given approximately 1000 infective larvae of O. ostertagi by gavage daily; the two larval-viability monitors were inoculated in a similar manner with approximately 30,000 larvae as a single dose on Day 28. Animals were slaughtered on Day 42 in one study and on Days 42, 43, or 46 in the second. The abomasum from each calf was harvested and processed for worm recovery. A 2% aliquot of abomasal contents plus wash was examined for worm quantification and identification. Geometric mean O. ostertagi burdens were calculated from the log (O. ostertagi count + 1) and were used to estimate percentage reduction. In both studies, doramectin injectable solution was > or = 99.6% efficacious in reducing infection resulting from challenge with infective larvae of O. ostertagi for at least 21 days posttreatment; by 28 days posttreatment, efficacy was 87.3% in one study and 99.7% in the other.

Comparative persistent efficacy of doramectin, ivermectin and fenbendazole against natural nematode infections in cattle

A study was conducted in Argentina, to investigate the period of protection of a single injection of doramectin administered subcutaneously (s.c.) at 200 micrograms kg-1 (1 ml/50 kg) compared with single treatments of ivermectin (200 micrograms kg-1 s.c.) and fenbendazole (5 mg kg-1 p.o.), against field infections of gastrointestinal parasites of cattle. Eighty-three animals were selected and ranked on the basis of serial fecal egg counts (e.p.g.'s). From this group, three animals were slaughtered before treatment and their lungs, abomasum, small and large intestines, were processed for parasite counts and identification. The remaining 80 animals were allocated in ranked groups of four to a control or one of three treated groups. Animals of the four groups were grazed together in the same pasture for the duration of the study. Treatments were administered on Day 0. Individual fecal samples were collected at weekly intervals for the first 49 days post-treatment and twice a week from Day 52 to Day 84 (end of study). At each collection day fecal samples were pooled for coprocultures. On Day 28 and 56, two animals from each group, previously identified on Day 0, were killed and their parasite burdens determined. The duration of protection of a single injection of doramectin was longer than ivermectin or fenbendazole treatment. On Day 56, the total number of parasites found in doramectin-treated animals was significantly (P < 0.05) lower than parasite burdens found in either ivermectin- or fenbendazole-treated animals. The longer persistent activity of doramectin was expressed by the lower number of adults and L4 stages of Ostertagia ostertagi. Data from this experiment demonstrated the limitations of using fecal egg counts to evaluate the persistent efficacy of anthelmintics. The duration of activity of doramectin was demonstrated more accurately by parasite counts in cattle from each group since decreasing e.p.g.'s were seen in non-medicated animals without changes in total parasite burdens.

Duration of anthelmintic efficacy of doramectin and ivermectin injectable solutions against naturally acquired nematode infections of cattle

A comparison of persistent efficacy of doramectin injectable (D) and ivermectin injectable (I) was investigated under field conditions with treated permanent principal (PP) and interval-grazed principal (IGP) calves. The experiment was initiated on October 13, 1992 (day 0). Cattle used were crossbred beef heifers of 185 kg average weight and 8 to 10 months old. By random allotment, 66 calves were divided into two groups of 15 PP-D and PP-I calves for each treatment and two groups of 15 IGP-D and IGP-I calves for each treatment. Three extra or replacement calves were allotted for each group. Permanent principal calves in three replicates of five cattle per treatment grazed continuously on nematode-contaminated replicate pastures from day 0 to day 70. At 2-week intervals, i.e., days 0 to 14, 14 to 28, 28 to 42, 42 to 56 and 56 to 70, one IGP-D and one IGP-I calf was grazed with each of the respective PP-D and PP-I calf replicates and necropsied 21 days after removal from pasture. All respective PP calves and IGP calves were treated with doramectin at 200 micrograms kg-1 or ivermectin at 200 micrograms kg-1 by s.c. injection on day 0. After the day 0-14 interval, all IGP-D calves had zero egg counts. From the day 14-28 interval through the next three grazing intervals, the arithmetic mean egg counts of IGP-D calves were 18, 90, 281 and 31; those of IGP-I calves were 30, 226, 74 and 185. This suggested a persistence effect of approximately 2 to 4 weeks. In PP-D calves, egg counts reached a mean maximum at day 56 of only five eggs per gram, while counts of PP-I calves reached a peak of 40 on day 42. From the day 14-28 interval and through all subsequent intervals, arithmetic mean total worm counts from IGP-I calves were 58 to 73% greater than those in IGP-D tracers. A maximal total worm count of 4159 was observed in IGP-D calves of the day 42-56 interval; total worm counts in IGP-I calves from the day 14-28 interval through the day 42-56 interval were: 5420, 6739 and 9979, respectively. Haemonchus and Cooperia were higher in prevalence than Ostertagia in both treatments. Results of PP-D egg counts and total worm burdens in IGP-I calves indicated a high level of doramectin persistent activity for approximately 4 to 5 weeks and an advantage over persistence activity of ivermectin.

Comparison of the persistent activity of ivermectin, abamectin, doramectin and moxidectin in cattle in Zambia

The persistent efficacy of four commercially available macrocyclic lactones (ML) in maintaining reduced faecal egg counts in cattle grazing naturally infested pastures was evaluated in 44 zebu animals aged 1-2 years in Zambia. The study started in February (rainy season) when the strongyle egg output was increasing. Four days before the start of the trial, all animals were treated with a double dose of oxfendazole. They were then divided into five groups which were again treated on day 0. Groups A, D, I and M received 0.2 mg kg-1 of abamectin, doramectin, ivermectin and moxidectin, respectively. Animals of group C received albendazole (7.5 mg kg-1). Faecal samples were collected twice a week for egg counts and larval differentiation. Faecal egg counts in the C group increased from day 21 onwards and plateaued from day 42 between 180 and 380 eggs per gram. The main genera found in cultures were Cooperia (90%) and Haemonchus (7%). Faecal egg excretion in groups M, A, D and I started on day 35, 42, 42 and 45, respectively. Subsequently and until day 84, average counts in these four groups were always significantly lower than in group C. Compared with albendazole, all four ML gave over 95% reduction in cumulative faecal egg counts for 42 days after treatment. The percentage efficacy was still over 84% by day 84 when an average cumulative egg count of 11320 eggs per gram faeces was calculated in group C. In addition, there was no significant difference in efficacy between the four ML groups at any of the sampling dates. During the trial no significant difference in weight gain between any of the groups was observed.

Persistence of the efficacy of pour-on and injectable moxidectin against Ostertagia ostertagi and Dictyocaulus viviparus in experimentally infected cattle

The persistence of the efficacy of moxidectin 0.5 per cent pour-on and moxidectin 1 per cent injectable against Ostertagia ostertagi and Dictyocaulus viviparus in calves was studied in two experimental trials. In the first trial two groups of seven calves were treated with either the pour-on or the injectable formulation, while a third group remained untreated. All the animals were infected daily from Monday to Friday with infective stages of O ostertagi and D viviparus between the day of treatment (day 0) and day 33, and were necropsied for worm counts three days later. The experimental design of the second trial was similar to that of the first but the period of infection was from 28 to 45 days after the treatment, and the necropsy was five days after the last infection. In both trials both moxidectin formulations had very high efficacies (99.6 per cent) against adult and developing stages of O ostertagi and D viviparus. The higher efficacy of the moxidectin pour-on preparation against early fourth stage larvae in both trials suggested that its effect was more persistent. It was calculated that the efficacy of moxidectin against O ostertagi persisted for at least five weeks for the injectable formulation and six weeks for the pour-on. The efficacy of moxidectin against D viviparus lasted for at least six weeks for both formulations.

Persistent efficacy of abamectin and doramectin against gastrointestinal nematodes of cattle

OBJECTIVE

To assess the persistent activity of injectable formulations of abamectin and doramectin against gastrointestinal nematodes of cattle.

DESIGN

Controlled slaughter study assessing residual efficacy.

PROCEDURE

Nematode-free calves were treated with abamectin or doramectin (each at a dose of 200 micrograms/kg) and infections then induced with repeated doses of infective larvae of Trichostrongylus axei, Haemonchus placei, Ostertagia ostertagi and Cooperia species. The duration of challenge ranged from 14 to 28 days. The calves were slaughtered at either 38/39 or 45/46 days after the treatments and nematodes recovered from the gastro-intestinal tract.

RESULTS

Significant reductions in numbers of O ostertagi occurred for both abamectin and doramectin treatments (> 93%) relative to counts in untreated calves, when challenge was administered up to 21 days after treatment. For T axei and Cooperia spp significant reductions occurred when the challenge occurred for 14 days after treatment (99%). Although differences from untreated animals were not significant, the results for H placei suggested high efficacy (> 85%) for up to 21 days for doramectin and up to 28 days for abamectin.

CONCLUSIONS

There was no significant difference between abamectin and doramectin for any parasite at any challenge point, indicating that there is equivalent persistent activity of doramectin and abamectin against important gastrointestinal nematodes of cattle.

Inefficacy of moxidectin and doramectin against ivermectin-resistantCooperiaspp. of cattle in New Zealand

Two cases are described in which strains of Cooperia spp. of cattle were found to be resistant to either the oral or the injectable formulations of ivermectin. Injectable moxidectin was not effective against the latter strain and doramectin was not effective against either of the ivermectin-resistant Cooperia isolates. In both cases, the anthelmintic efficacy of oral levamisole was 100%.

The combined effect of fenbendazole treatment and a move to aftermath 7 or 9 weeks after turnout on Dictyocaulus viviparus infections in calves

A grazing study was performed with the main objective of examining the effect of fenbendazole (FBZ) in a 'dose and move' system on nematode infections in calves with special emphasis on Dictyocaulus viviparus. Three groups of six calves were grazed from May to October 1993. One group (DM7) was treated with FBZ and moved to aftermath (pasture which had only been mown) 7 weeks after turnout. The second group (DM9) was similarly treated and moved 9 weeks after turnout and the third group served as untreated pasture control group (PC) and was moved to aftermath 9 weeks after turnout. FBZ treatment removed adult lungworms from DM7 and DM9. Tracer calves grazed during the first 7 or the first 9 weeks after turnout acquired mean burdens of 18 and 125 lungworms, respectively. In PC faecal larval counts increased until the end of August. Most of the animals in this group were then suffering from lungworm disease and emergency treatment with ivermectin was given. In both FBZ-treated groups, larvae reappeared in the faeces of some of the calves 4-5 weeks after treatment. Subsequent reinfection resulted in higher mean faecal larval counts in both groups 2 months after treatment, although variation in faecal larval counts was high. In DM7 values tended to be higher than in DM9. These higher larval counts were associated with mild signs of parasitic bronchitis in some calves of DM7, whereas no signs were seen in DM9. At the end of the experiment, all calves, and also a group of six permanently housed non-infected control calves (HC), were infected experimentally with 5000 D. viviparus larvae to evaluate development of immunity. The worm counts at necropsy showed that all calves on pasture had developed immunity.

Pharmacokinetics and bioequivalence of parenterally administered doramectin in cattle

Plasma concentrations of doramectin in 40 cattle dosed by subcutaneous (sc) or intramuscular (i.m.) injection (200 micrograms/kg) were compared to assess the bioequivalence of the two routes of administration. Peak concentration (Cmax), and areas under the concentration curve (AUC0-infinity) were determined from plasma concentrations. Animals treated by the sc route showed a mean AUC0-infinity of 457 +/- 66 ng.day/mL (+/- SD) and a mean Cmax of 27.8 +/- 7.9 ng/mL. Results from the i.m. treatment group showed a mean AUC0-infinity of 475 +/- 82 ng.day/mL and a mean Cmax of 33.1 +/- 9.0 ng/mL. Absorption constants (ka) determined by modelling were 0.542 +/- 0.336 day-1 after sc administration and 0.710 +/- 0.357 day-1 after i.m. administration. The 90% confidence limits on the difference between mean AUC0-infinity values for the sc and i.m. groups fell within 20% of the mean value for the subcutaneous group. Cmax was somewhat greater for the i.m. route. The 90% confidence limits on the difference in mean In(Tmax +1) also fell within 20% of the mean sc value. Based on this analysis, bioequivalence of the sc and i.m. formulation has been established.

Effect of chemoprophylaxis with avermectins on the immune response to gastrointestinal nematodes in first-season grazing calves

The influence of chemoprophylaxis on the immune response to gastrointestinal nematodes was evaluated in three groups of ten first-season grazing calves. Group C functioned as untreated controls, whereas Group L received two treatments, one at turn-out and one 8 weeks later, with doramectin (200 micrograms kg-1) and Group H three treatments, 3, 8, and 13 weeks post turn-out, with ivermectin (200 micrograms kg-1). At housing, five calves of each group were randomly selected for necropsied for worm counts. The remaining calves were challenged with 50,000 Ostertagi and 100,000 Cooperia oncophora and necropsied 4 weeks later. During the grazing season, Group C suffered from an outbreak of parasitic gastroenteritis which necessitated two anthelmintic treatments. In both treated groups no clinical symptoms were observed and growth rates were normal. Pepsinogen levels and antibody concentrations indicated a moderate and low gastrointestinal nematode infection in Group L and Group H, respectively. The Ostertagia and Cooperia worm populations present in the calves necropsied at housing were reduced in both treated groups in comparison with Group C. In the challenged calves, egg output was significantly higher in Group H than in Group L and Group C. Ostertagia and Cooperia worm burdens after challenge were high in Group H, moderate to low in Group L and low in Group C. This study indicates that highly suppressive prophylactic programmes, such as the ivermectin 3, 8 and 13 weeks schedule, can impair immune development against gastrointestinal nematodes during the first grazing season.

Efficacy of Michel's 'dose and move' system against Dictyocaulus viviparus infections in cattle using moxidectin as anthelmintic

Two grazing experiments were performed to study the effect of moxidectin in a 'dose and move' system on Dictyocaulus viviparus infections in calves. In the first experiment, three groups of four calves were experimentally infected with 20 larvae of D. viviparus 7 weeks before moxidectin treatment of two of these groups. A sufficient suppression of Dictyocaulus infections was observed in a 'dose and move' group, but also in a group which stayed on contaminated pasture after treatment. In contrast, high faecal larval counts and lungworm disease were observed in July-August in a non-treated pasture control group. Development of immunity against lungworm was sufficient in all three groups, on pasture. In the second experiment, four out of 26 calves, including two groups of six calves and four tracer calves, were experimentally infected with 20 larvae of D. viviparus 7 weeks before moxidectin treatment combined with a move of one of the groups. No lungworm disease was observed in this 'dose and move' group. In a pasture control group high faecal larval counts and severe clinical disease were observed in August-September and one calf had to be euthanized. Although a mean burden of 129 immature lungworms was present at the time of treatment, development of immunity appeared to be low in the 'dose and move group'. In contrast, high levels of immunity had developed in the pasture control group.

Control of gastrointestinal nematodes in first-season grazing calves by two strategic treatments with doramectin

Twenty first-season Holstein crossbred calves were used in a study conducted to investigate the efficacy of two strategic treatments with doramectin (s.c. 200 micrograms kg-1 bodyweight). They were allocated in two equal groups: Group D was treated with doramectin at turn-out and 8 weeks later and Group C served as control group. Tracers were used at turn-out, in August and before housing. At monthly intervals all principal animals were weighed, and their faeces were sampled for faecal egg counts and cultured for larval identification. Serum samples were collected for determination of pepsinogen. Animals were slaughtered for worm counts 3 weeks after housing. Clinical signs of parasitic gastroenteritis (PGE) in Group C necessitated a salvage treatment with pyrantel citrate. No signs of PGE were seen in Group D. The faecal egg output remained very low (less than 50 eggs g-1) in Group D, whereas in Group C a mean geometric maximum of 352 eggs g-1 was observed. Pepsinogen levels were below 2000 mU tyrosine in Group D, compared with maximum levels of 4190 mU tyrosine in Group C. The worm counts of the tracers at housing showed an overall reduction of 94% in the total number of gastrointestinal nematodes present on the D pasture, compared with the C pasture. Worm counts of the principal animals showed an 87.8% reduction in the total worm burden. The present results show clearly that doramectin given at turn-out and at Week 8 controls PGE in calves during the first grazing season.

Efficacy of doramectin against naturally acquired adult and inhibited larval infections by some nematode parasites in cattle in New Zealand

A study was conducted to determine the therapeutic efficacy of 1% doramectin injected subcutaneously at 200 microg/kg into cattle harbouring naturally acquired infections of inhibited Ostertagia ostertagi larvae. Sixteen yearling Friesian bulls, grazed without anthelmintic treatment throughout the autumn-winter, were selected on the basis of similar body weights and serum pepsinogen activities. After removal from pasture on day -23 they were weighed and randomly assigned to two treatment groups on the basis of this weight. On day 0, one group was given saline (1 ml/50 kg) while the second was treated with doramectin (200 microg/kg). Both treatments were given by subcutaneous injection. All stock were slaughtered 14-15 days after treatment. Moderate to high levels of adult O. ostertagi and Trichostrongylus axei and early and late 4th larval stages of O. ostertagi were recovered from saline-treated calves at necropsy. Doramectin was highly effective in eliminating all stages of O. ostertagi (99.9%; p<0.0001) and T. axei (100%; p<0.0001). No evidence of lesions were detected at the injection sites at necropsy. These results confirm that doramectin is an extremely effective broad-spectrum avermectin anthelmintic with efficacy against inhibited as well as maturing larval and adult forms of O. ostertagi.