Spectrum of activity of doramectin against cattle mites and lice

Effectiveness of a fixed-dose combination injectable (0.2 mg/kg doramectin + 6.0 mg/kg levamisole hydrochloride) against Rhipicephalus microplus and sucking lice infesting cattle

Unmanaged tick and sucking lice infestations negatively impact the health and production potential of cattle. Described herein are two non-interference dose confirmation studies evaluating the efficacy of a single administration of a new fixed-dose combination injectable (FDCI) endectocide consisting of 0.2 mg/kg doramectin + 6.0 mg/kg levamisole hydrochloride, against either laboratory-induced Rhipicephalus microplus infestations in Australia or naturally acquired sucking lice (Linognathus vituli) infestations in the US. This FDCI is available as Dectomax V® in Australia and New Zealand and as Valcor® in the United States. To evaluate therapeutic efficacy against R. microplus, 12 calves were each exposed to 10 infestations of ∼5000 larvae per infestation between Days -24 and -2. Calves were either treated on Day 0 with the FDCI or left untreated (control). Additional R. microplus infestations of ∼5000 larvae were conducted on Day 2 and then three times weekly to also evaluate persistent efficacy of the FDCI. Tick collections were conducted daily from Day -3. Group mean live tick counts, egg production, and egg viability were analyzed for significant differences between the two groups. To determine efficacy of the FDCI against lice, 24 cattle with active sucking lice infestations based on Day -7 counts were allocated to two groups and treated on Day 0 with either saline (control) or the FDCI. Lice counts were conducted weekly from Day 14 through 42 and again on Day 56. Mean group lice counts on each count day were compared between treatment groups. In the R. microplus study presented here, cattle in Queensland, Australia treated with the FDCI (Dectomax V®) showed > 90 % reduction in tick counts based on arithmetic means within 48 h of treatment when compared to untreated cattle, and counts were > 95 % reduced from post-treatment Day 5 through Day 30. In the sucking lice study conducted in the US, the FDCI (Valcor®) displayed 100 % efficacy against sucking lice infestations (L. vituli) from first count day (Day 14 post-treatment) through Day 35 and then 99.9 % efficacy through Day 56 post-treatment. No treatment-related adverse events were reported for cattle in either study. Using R. microplus and sucking lice as representative ectoparasites, these studies demonstrate the ectoparasite activity of doramectin is retained in the new FDCI.

Tests on the efficacy of amitraz, ivermectin and eprinomectin for control of lice in goats

Ectoparasitic infestations with Linognathus stenopsis and Bovicola caprae are a serious concern in goat farms, especially during the winter and spring. This study tested the efficacy of three modern insecticide drugs (ivermectin, eprinomectin and amitraz) applied via different routes for control of sucking and chewing lice in naturally co-infected goats. Each of drugs was tested on 30 goats. The animals from the first flock were treated with a single dose of ivermectin, applied subcutaneously at 0.2 mg/kg, the second one received a single spot on treatment with 1 mg/kg еprinomectin and the third flock was treated twice with amitraz, through spraying at a dose of 4 mL/1 L water. The results were evaluated before the treatments and at post treatment days 3, 14 and 60. On post treatment day 60, the application of ivermectin resulted in reduction of Bovicola caprae population by 73.05%, and for that of Linognathus stenopsis: by 92.70%. Eprinomectin demonstrated 100% reduction of both lice. The reduction of L. stenopsis and B. caprae caused by amitraz was by 92.20% and by 98.61% respectively. The macrocyclic lactone eprinomectin showed the highest efficacy (Р<0.05), was not toxic for goats and therefore, may be used as a treatment of choice for lice control.

Quantitative Monitoring of Selected Groups of Parasites in Domestic Ruminants: A Comparative Review

Parasites have had a significant impact on domestic ruminant health and production for a long time, but the emerging threat of drug resistance urgently requires an improved approach to parasite monitoring and control activities. The study reviewed the international literature to analyze the different proposals for the sampling approach and the quantitative estimation of parasite burdens in groups of animals. Moreover, the use of thresholds to decide when and which animal to treat was also investigated. The findings of the study highlighted the presence of a wide-ranging literature on quantitative monitoring for gastrointestinal nematodes (GIN), while more limited data were found for coccidia, and no specific indications were reported for tapeworms. Concerning liver flukes, bronchopulmonary nematodes (BPN) and permanent ectoparasites (lice and mange mites), the diagnostic process is usually aimed at the detection of the parasite rather than at the burden estimation. The main research gaps that need further investigation were also highlighted. For some groups of parasites (e.g., GIN and coccidia) the quantitative approach requires an improved standardization, while its usefulness needs to be confirmed for others (e.g., BPN and lice). The development of practical guidelines for monitoring is also encouraged.

Resistance against macrocyclic lactones in Psoroptes ovis in cattle

Background

Psoroptic mange is an important disease in Belgian Blue cattle. Treatment failure of macrocyclic lactones against Psoroptes ovis has been reported, but clear evidence of in vivo resistance is lacking. This study assessed the efficacy of macrocyclic lactone products on 16 beef farms in Belgium and the Netherlands in vivo and in vitro.

Methods

On each farm a group of animals (n = 7–14) with psoroptic mange was treated with two subcutaneous injections of a macrocyclic lactone product with 7–10 days interval (15 farms) or a single injection with a long-acting macrocyclic lactone (1 farm). In vivo efficacy was assessed by the reduction in mite counts, clinical index (proportion of the body surface affected by lesions), the proportion of the animals with negative mite counts after the first treatment round and the number of treatment rounds needed to obtain zero mites counts in all animals. A mite population was categorized as sensitive when the mite count reduction after the first treatment round > 95% and the lower limit of the uncertainty interval > 90%. Resistance was detected when both parameters were below their threshold and suspected when one parameter was too low. In vitro knockdown and mortality were evaluated in a contact test.

Results

The proportion of the animals with negative mite counts after the first treatment round varied from 0 to 80%. All farms needed two or more treatments rounds to obtain zero mite counts on all animals. Clinical index only started to reduce after the second treatment round. Mite populations from three farms were categorized as sensitive, one as suspected resistant and 12 as resistant. No correlation was found between in vitro lethal dose 50 and knockdown dose 50 values and in vivo efficacy parameters.

Conclusions

Unambiguous treatment failure was detected on 12 out of 16 farms, confirming the presence of macrocyclic lactone resistance on Belgian Blue beef farms. In vitro parameters could not discriminate the farms based on their in vivo sensitivity. The mean reduction in mite counts and the lower limit of the confidence interval are proposed as parameters to identify acaricide resistance.

The efficacy of four avermectins on the synanthropic mite Lepidoglyphus destructor under laboratory conditions

The effect of four avermectins on the population growth of pest mite Lepidoglyphus destructor was tested in laboratory experiments. The avermectins (abamectin, doramectin, emamectin-benzoate and ivermectin) of analytical purity were incorporated into an experimental diet at the same molar concentrations, ranging from 0.16 to 8 nmol/3 g of diet. Using an initial population of 50 mites, the population growth was recorded after 21 days at 85 % relative humidity and 25 °C; 12 repeats were performed per avermectin concentration and control. The diets containing the avermectins successfully suppressed the population growth of L. destructor. The EC(50) recalculated to ng of substance per g of diet showed different suppressive effects of the avermectins: doramectin (181 ng/g diet), abamectin (299 ng/g diet), emamectin-benzoate (812 ng/g diet) and ivermectin (992 ng/g diet). Of the tested avermectins, abamectin is registered for the control of phytophagous mites and ivermectin against parasitic mites, i.e., Psoroptes ovis. Although emamectin-benzoate and ivermectin were less effective on L. destructor, all of the tested avermectins are highly suitable compounds for the control of synanthropic mites.

Efficacy of two avermectins, doramectin and ivermectin against Argulus siamensis infestation in Indian major carp, Labeo rohita

The efficacy of two avermectins, doramectin and ivermectin via oral and parenteral routes and their prophylactic effects against Argulus siamensis infestation in Indian major carp, Labeo rohita was studied. Doramectin and ivermectin were fed once to L. rohita sub-adults naturally infested with A. siamensis parasites at graded levels of 1000, 750 and 500 μg/kg body weight and 500, 300 and 200 μg/kg body weight, respectively. The mean time of clearance of parasites was 24 ± 0.149 h for 1000 and 24 ± 0.210 h for 750 μg of doramectin/kg body weight fish, however, the dose of 1,000 μg resulted in adverse reactions and mortality in fish. A single dose of 500μg/kg body weight of ivermectin also cleared the infestation in 24 ± 0.258 h. In second experiment, L. rohita naturally infested with A. siamensis were administered with doramectin and ivermectin intramuscularly as single dose of 200, 150 and 100 μg/kg body weight. Both doramectin and ivermectin at 200 μg/kg body weight cleared the infestation in 4 ± 0.149 and 4 ± 0.258 h respectively, however, those resulted in adverse reactions viz., black pigmentation, lethargy and poor appetite in fish. The mean time of clearance of parasites from fish was 6 ± 0.210 and 12 ± 0.314 h for doramectin and ivermectin, respectively, when given at a lower dose of 150 μg/kg body weight. The prophylactic effects of both drugs were also evaluated by challenging naïve rohu with metanaupliar larvae of A. siamensis after feeding the fish single dose of the doramectin and ivermectin at 750 and 500 μg/kg body weight, respectively. The effects of both the drugs were found to last for 17-18 days. The effects of bath treatment with doramectin and ivermectin at different concentrations on adult A. siamensis and metanaupliar larvae of A. siamensis were also evaluated. The results suggested that ivermectin could act more rapidly compared to doramectin. However, detailed studies on the toxicity and pharmacokinetics of doramectin and ivermectin upon their administration to aquatic organisms and the environmental impacts of their usage are required to be undertaken before finally recommending the safe use of these drugs for control of parasitic infections.

Sarcoptic mange in three alpacas treated successfully with amitraz

Sarcoptic mange is a serious skin disease in alpacas that can result in high morbidity and even mortality. Three alpacas were presented with sarcoptic mange that had previously failed to respond to repeated topical applications of eprinomectin, and an injection of doramectin. They were moderately to severely pruritic, had extensive lesions of alopecia, erythema, scaling and crusting, and had lost weight. As no drug is currently licensed for the treatment of sarcoptic mange in alpacas in the UK, they were treated with a topical solution of amitraz (50 mL in 10 L) after initial bathing with antibacterial or keratolytic shampoos. The clinical signs completely resolved with no relapse over a 10-month follow-up period. In this small group of alpacas, amitraz was an effective and well-tolerated treatment for sarcoptic mange.

Clinical and therapeutic studies on mange in horses

At Kafr El-Sheikh province, Egypt, out of 117 examined drafting horses, mites were detected in 20 (17.09%) horses. The recovered mites were 14 Chorioptes, four Psoroptes and two Sarcoptes whereas mites were not detected in four cases clinically showed typical mange lesions. Interestingly, neither the age nor the sex of the examined horses had a clear influence on the prevalence of the infection. Clinical signs observed in mange infested horses were in the form of irregular skin lesions, severe itching and sometimes biting of affected skin areas and decrease feed consumption. The skin lesions mainly start as erythematous area followed by developing of papules and crust formation. Skin scratches as a result of traumatized lesions usually occurred. Hair was lost on the affected parts developing irregular alopecic areas. Distribution of the lesions was varied according to the type of mite. Chorioptic mite was detected in para-anal fold, distal portion of legs and tail lesions, Psoroptic mite was detected in withers, mane, shoulder and flank lesions whereas Sarcoptic mite was isolated mainly from lesions on the head and neck. Complete clinical and parasitological cure for mite infestation were obtained within 2 weeks in both moxidectin and ivermectin treated groups with 100% recovery rate. Our results indicated that moxidectin oral gel is effective and good alternative for the treatment of chorioptic mange in horse to avoid drug resistance that may develop as a result of the intensive use of ivermectin alone for long periods.

Use of doramectin for treatment of sarcoptic mange in five Angora rabbits

The efficacy of administering doramectin after moxidectin treatment, which has previously proved only partially effective, was evaluated in five Angora rabbits naturally infested with Sarcoptes scabiei mange. Evaluations included physical examination for clinical signs of sarcoptic mange and collection of skin scrapings for determination of mites. The rabbits first received two subcutaneous injections, 10 days apart, of moxidectin 1% injectable solution at a dosage of 0.2 mg kg(-1) of bodyweight. Although moxidectin treatment resulted in clinical improvement within 10 days post initial injection, on days 10 and 35 post initial treatment live mites were present in skin scrapings. Administration of doramectin 1% injectable solution using the same route and dosage and at similar intervals to moxidectin led to complete disappearance of signs of scabies and parasitological cure in all rabbits.

Efficacy of a long-acting formulation of ivermectin against Psoroptes ovis (Hering, 1838) on cattle

A study was conducted in cattle experimentally infested with Psoroptes ovis to compare the prophylactic control against P. ovis provided by a long-acting injectable formulation of ivermectin to that of a commercially available injectable formulation of doramectin. Thirty Holstein steers were used. Animals were allocated by restricted randomization based on Day 0 body weight, forming six replicates of five animals each. Within each replicate, one animal was randomly allocated to one of the following treatment groups, with ivermectin and doramectin administered subcutaneously where indicated: (1) untreated controls; (2) ivermectin long-acting injectable (LAI) 630 mcg/kg, 56 days before challenge; (3) ivermectin LAI 630 mcg/kg, 42 days before challenge; (4) ivermectin LAI 630 mcg/kg, 35 days before challenge; or (5) doramectin 200 mcg/kg, 35 days before challenge. Animals were housed in individual pens 1 week prior to treatment. All animals were experimentally infested with P. ovis mites in the area between the shoulders, on the same day. Live mites were counted in scrapings from mange lesions at 2 sites on each animal 14, 21 and 28 days after challenge. Live mites were found in 33, 67 and 83% of the untreated controls on each respective evaluation. No P. ovis mites were found in steers treated with ivermectin LAI. Those animals showed lower (P < 0.05) mite counts than untreated controls on evaluations conducted 21 and 28 days after challenge. These results indicate that the ivermectin long-acting injectable formulation prevents induced infestations by P. ovis for at least 56 days after treatment. Doramectin injectable formulation, used at 200 mcg/kg, did not have a prophylactic effect 35 days after treatment.

Impact of doramectin treatment at the time of feedlot entry on the productivity of yearling steers with natural nematode infections

OBJECTIVE

To measure the reduction in fecal nematode egg counts and productivity impact of treatment of yearling steers with doramectin at entry into the feedlot, compared with control steers treated only with fenthion.

ANIMALS

6,096 crossbred yearling steers with a mean (+/- SD) body weight of 377.0 (+/- 37) kg.

PROCEDURE

Steers were implanted with zeranol and alternately separated to fill each of 24 pens. Groups of steers within 12 matched pairs of pens were randomly allocated to treatment with doramectin or no treatment with doramectin for internal nematodes. Fecal samples were collected from approximately every twentieth steer from each pen at day 0 and at reimplant (approx day 60). Each steer was weighed on day 0 and at reimplant and then mean body weights of steers per pen were determined at 120 to 140 days after trial initiation.

RESULTS

Treatment steers had a significantly lower fecal egg count at reimplant than control steers. Treatment steers had a significantly greater mean daily gain during the study, significantly greater feed consumption, significantly lower feed-to-gain ratio, and significantly better quality carcass grades at slaughter.

CONCLUSIONS AND CLINICAL RELEVANCE

Under the conditions of our trial, there was a significant fecal egg count reduction response to doramectin treatment, which resulted in significantly improved productivity. Results of economic analysis of return on investment indicated that even with low egg counts in heavy body weight cattle, nematode egg count reduction with doramectin significantly improved returns.

Comparative distribution of ivermectin and doramectin to parasite location tissues in cattle

Pharmacokinetic studies have been used traditionally to characterize drug concentration profiles achieved in the bloodstream. However, endectocide molecules exert their persistent and broad spectrum activity against parasites localized in many different tissues. The aim of this study was to compare the distribution of ivermectin (IVM) and doramectin (DRM) to different tissues in which parasites are found following subcutaneous administration to calves. Holstein calves weighing 120-140 kg were injected in the shoulder area with commercially available formulations of IVM (Ivomec 1% MSD AGVET, NJ, USA) (Group A) or DRM (Dectomax 1%, Pfizer, NY, USA) (Group B). Two treated calves were sacrificed at 1, 4, 8, 18, 28, 38, 48 or 58 days post-treatment. Plasma, abomasal and small intestinal fluids and mucosal tissues, bile, faeces, lung and skin samples were collected, extracted, derivatized and analyzed by high performance liquid chromatography (HPLC) with fluorescence detection to determine IVM and DRM concentrations. IVM and DRM were distributed to all the tissues and fluids analyzed. Concentrations >0.1 ng/ml (ng/g) were detected between 1 and 48 days post-treatment in all the tissues and fluids investigated. At 58 days post-treatment, IVM and DRM were detected only in bile and faeces, where large concentrations were excreted. Delayed Tmax values for DRM (4 days post-administration) compared to those for IVM (1 day) were observed in the different tissues and fluids. High IVM and DRM concentrations were measured in the most important target tissues, including skin. The highest IVM and DRM concentrations were measured in abomasal mucosa and lung tissue. Enhanced availabilities of both IVM (between 45 and 244%) and DRM (20-147%) were obtained in tissues compared to plasma. There was good correlation between concentration profiles of both compounds in plasma and target tissues (mucosal tissue, skin, and lung). Drug concentrations in target tissues remained above 1 ng/g for either 18 (IVM) or 38 (DRM) days post-treatment. The characterization of tissue distribution patterns contributes to our understanding of the basis for the broad-spectrum endectocide activity of avermectin-type compounds.

Persistence of the activity of topical ivermectin against biting lice (Bovicola bovis)

To assess the persistence of the activity of topical ivermectin against a natural challenge with biting lice (Bovicola bovis), 90 mixed-breed cattle that had been treated to remove lice, were blocked by bodyweight within sex and randomly allocated to three treatments: untreated control, doramectin at 200 micrograms/kg by subcutaneous injection, and ivermectin at 500 micrograms/kg by topical application. Forty-five pens were blocked into three groups of 15, and the blocks of pens were randomly allocated to three 14-day challenge periods starting 21, 28 and 35 days after treatment. There were five pens per treatment for each challenge period, and one B bovis-infested donor calf was introduced into each pen containing two principal calves at the start of the challenge period for that block of pens. The calves were examined thoroughly for B bovis seven, 14 and 21 days after the introduction of the donors. There were no significant differences between the control and doramectin groups for the numbers of animals infested, or the geometric mean louse counts at the final examination for any of the challenge periods. At the final examination for each challenge period, the louse counts of the cattle treated with topical ivermectin were all zero, and significantly (P < 0.05) fewer cattle treated with topical ivermectin were infested than either the controls or cattle treated with doramectin.

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.

Field efficacy of injectable doramectin against Chorioptes bovis in naturally infected cattle

A single subcutaneous injection of doramectin at a dose rate of 200 micrograms/kg bodyweight was effective in controlling an infection of Chorioptes bovis mites in naturally infected cattle. From 14 days after treatment, the geometric mean number of live mites was significantly lower (P < 0.001) in the doramectin-treated cattle than in the control group at each sampling until day 35. The percentage efficacy (treated versus controls) of doramectin against C bovis at day 35 was 99.9 per cent and the percentage reduction (day 35 versus day 0) in the treated animals was 99.3 per cent. At day 35, all seven controls were still positive for C bovis whereas five of the eight doramectin-treated animals were free of live mites.

Evaluation of the therapeutic and protective efficacy of doramectin against psoroptic scabies in cattle

Three studies were conducted to evaluate the therapeutic and protective efficacy of doramectin when given by injection at a dose of 200 micrograms/kg against induced Psoroptes otis infestations of cattle. The first study investigated therapeutic efficacy. Mite infestations were established on 15 test animals held in stanchions by transfer of material from infested donor calves. Test animals were then allotted on the basis of mite counts to a treatment group (10 animals) which received doramectin and a control group (5 animals) which received saline. Skin scrapings were collected for mite counts on the day before treatment and on days 7, 14, 21 and 28 after treatment. Efficacy assessed on the basis of the proportion of animals cured by day 28 was 100%. The second study was designed to determine the duration of protective efficacy. Forty-eight scabies-free heifers were allotted to a treated group of 32 which received doramectin, and a control group of 16 which remained untreated. These treatment groups were each divided into eight subgroups. Commencing on treatment day and continuing at weekly intervals for 7 weeks, a subgroup of animals from each treatment was placed in stanchions and challenged by transfer of material from infested donor calves. Skin scrapings for mite counts were collected 7 and 14 days later. Infestations were successfully established on all untreated control calves. Doramectin prevented the establishment of infestation for three weeks and significantly (P < 0.05) reduced infestation levels for an additional two weeks. The third study established the duration of residual protection conferred by doramectin and ivermectin under contact transmission. Ninety-six scabies-free heifers were divided into two equal treatment groups. Animals in one group received doramectin and animals in the other group received ivermectin at its recommended dose of 200 micrograms/kg by subcutaneous injection. Each treatment group was then divided into eight subgroups of six animals. Commencing on treatment day and continuing at weekly intervals for 7 weeks a subgroup of animals from each treatment was exposed to purposely infested seeder animals for one week. Three animals from each treatment subgroup were then placed in individual stanchions in which grooming was prevented and the other three were placed together in a pen where normal grooming behavior was permitted. Skin scrapings for mite counts were collected at weekly intervals for up to 4 weeks. Doramectin provided complete protection against infestation for five weeks compared to four weeks for ivermectin. These periods were not influenced by grooming behavior.

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.

Evaluation of the protective efficacy of doramectin against sucking lice of cattle

The protective efficacy of doramectin against sucking lice was evaluated under natural challenge conditions in Canada. Two studies with a similar experimental design were conducted in sequence. In each study, two groups of louse-free cattle received either doramectin at a dose of 200 micrograms/kg by subcutaneous injection or no treatment and were then mixed with louse-infested animals for the winter housing period. All animals were examined for lice on treatment day and thereafter at weekly intervals for 13 weeks. In the preliminary study, acquisition of infestation in the face of a very mild challenge of Linognathus vituli was delayed by a mean period of 49 days in doramectin-treated animals compared with untreated controls. The difference in the delay between the groups was significant (P < or = 0.04). In the second study, test animals experienced a moderate challenge of a mixed infestation of L. vituli and Solenopotes capillatus. Acquisition of infestation was delayed by a mean period of 25.6 days in doramectin-treated animals compared with controls, the difference between the two groups being highly significant (P < or = 0.0001). These studies confirmed the protective efficacy of doramectin against sucking lice under natural challenge conditions showing that infestation is prevented for a period of about 4 weeks following administration of the drug at its recommended dose.

Efficacy of pour-on and injectable formulations of moxidectin and ivermectin in cattle naturally infected with Psoroptes ovis: parasitological, clinical and serological data

On the basis of Psoroptes ovis counts performed on day -7, 32 animals were randomly allocated to a control group of five animals or to four groups comprising six or seven animals which were treated, respectively, with pour-on ivermectin (IPO), injectable ivermectin (II), pour-on moxidectin (MPO) and injectable moxidectin (IM). Living mites were counted in skin scrapings on days 0, 7, 14, 28, 42 and 56 post-treatment (PT). Lesions were recorded on a standardized map on days 0 and 56 PT. Antibody kinetics were studied using ELISA on serially diluted sera. The antibody titres were expressed as the dilution giving the positive/negative cut-off. Until their treatment on day 28, the control animals remained parasitologically positive and their antibody titres increased. In treated groups, all living mite counts were negative on days 28 and 42 PT but some animals were still infected on days 7 and 14 PT. On day 56, living P. ovis were found in one animal of the IPO group. An equation of regression describing the antibody decrease was calculated with each individual data set. In most of the treated animals, the coefficient of determination R2, which describes the closeness of fit to the linear model, was above 0.9. The linear model could not be applied (low R2) to the antibody kinetics of four animals: the day 56 positive animal and its two neighbours in the IPO group and one animal from the MPO group. In the treated groups, the differences between the numbers of infected animals, the mean daily weight gains or the mean antibody titres were not statistically significant. Mean daily weight gains of the treated groups were higher than in control animals.

Efficacy of doramectin against intestinal nematodes and sarcoptic manage mites in naturally infected swine

Endectocidal efficacy of doramectin administered intramuscularly at a dosage of 300 micrograms/kg was evaluated in 464 pigs naturally infected with intestinal nematodes or mange mites on 14 commercial farms in Japan. By doramectin treatment, fecal egg counts were reduced > 99% for Ascaris suum, Strongyloides ransomi, Oesophagostomum dentatum, and Trichuris suis; worm counts of T. suis and mite counts of Sarcoptes scabiei reduced 90.1% on Day 21 and 99.5% on Day 28 following treatment, respectively.

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.

Activity of doramectin against nematode and arthropod parasites of swine

The efficacy of doramectin, a novel avermectin, was assessed against both naturally-acquired and experimentally-induced infections of gastrointestinal roundworms, lungworms, kidneyworms, lice and mites in studies conducted across North America and Europe. Twenty-two studies evaluated efficacy against fourth larval and adult stages of the following nematode species: Hyostrongylus rubidus, Ascaris suum, Strongyloides ransomi, Oesophagostomum dentatum, Oesophagostomum quadrispinulatum, Trichuris suis, Metastrongylus spp. and Stephanurus dentatus. Efficacy was evaluated against the louse Haematopinus suis in six studies and against the mite Sarcoptes scabiei in four studies. A common study design was employed for each study type. In all studies, animals were allotted at random to a doramectin-treated or a saline-treated group. The doramectin-treated group received the drug at 300 micrograms kg-1 by intramuscular injection while the saline-treated group received saline by the same route. In the nematode studies, worm burdens were determined for each animal at slaughter 14-16 days after treatment. Efficacy against each nematode species/stage was assessed on the basis of percentage reduction in geometric mean worm burden in doramectin-treated animals compared with saline-treated controls. In louse and mite studies, counts were made immediately before treatment and then at weekly intervals for four weeks. Efficacy was based on a comparison of the level of infestation on the day of treatment with that on the last day of test. Data from individual studies were combined to derive a single estimate of efficacy against each of the parasite species represented in the study program. Efficacy of doramectin was 98% or greater against all nematode species except T. suis for which the efficacy was 87% and 79% against adult and fourth larval stage, respectively. Efficacy was 100% against both Haematopinus suis and Sarcoptes scabiei.

Doramectin systemic activity against cattle grubs, Hypoderma lineatum and H. bovis (Diptera: Oestridae), and cattle lice, bovicola bovis (Mallophaga: Trichodectidae), Linognathus vituli and Solenopotes capillatus (Anoplura: Linognathidae), and Haematopinus eurysternus (Anoplura: Haematopinidae), in Wyoming

Seven individual trials were conducted in Wyoming to evaluate the therapeutic efficacy of doramectin administered subcutaneously at a dosage of 200 micrograms kg-1 against multiple, natural infestations of cattle grubs or cattle lice. Insect species present and the number of trials that included each species were: Hypoderma lineatum, 2; Hypoderma bovis. 1; Bovicola bovis, 5; Haematopinus eurysternus, 1; Linognathus vituli, 5; and Solenopotes capillatus, 3. Examinations for lice were performed prior to treatment and either weekly or bi-weekly thereafter for 28 days. Examinations for cattle warbles were performed either weekly or every 4 to 5 weeks from time of first appearance through last appearance in the backs of the cattle. No H. lineatum, H. bovis, H. eurysternus, L. vituli, or S. capillatus were found on doramectin-treated animals at any time following treatment. By 28 days following treatment, the number of B. bovis was reduced between 58 and 98%. Treatments applied later in the season, i.e. in March, were more efficacious against B. bovis than those applied in January or February.

Structure and activity of avermectins and milbemycins in animal health

The avermectins and, to a lesser extent, the milbemycins, have revolutionized antiparasitic and antipest control over the last decade. Both avermectins and milbemycins have macrocyclic lactone structures that are superimposable, they are produced by the same genus of soil dwelling organisms, they have the same mode of action, they exert this action against the same nematode/acarine/insect spectrum of targets, and they show the same mechanism-based toxicity in mammals. Reports suggesting that milbemycins have a different mode of action from avermectins with implications that there will be no mutual resistance to the groups have been shown to be false. Contributing to the belief that there were differences in mode of action between the two groups are the vague definitions of resistance presently in use which rely on the ability of the parasite to survive treatment at the manufacturer's recommended use level. More appropriately, drug resistance should be defined as 'a change in gene frequency of a population, produced by drug selection, which renders the minimal, effective dosage previously used to kill a defined portion (e.g. 95%) of the population no longer equally effective'. This type of definition would allow us to detect changes in susceptibility of a population earlier and is essential when comparing different chemicals to determine if there is mutual resistance to them. It is concluded that much effort has been expended by pharmaceutical, government, and academic scientists searching for broad-spectrum second generation avermectin and milbemycin products, but none has exceeded the original avermectin in any fundamental way. The newer avermectin and milbemycin compounds that have appeared claim niches in the market place based on emphasis of certain narrow parts of the overall spectrum. Consequently, there are no second generation avermectins and milbemycins at present and all newer compounds from this mode of action class are viewed as siblings of the first generation.

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

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 micrograms/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 micrograms/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.

Prophylactic and persistent efficacy of doramectin against Cochliomyia hominivorax in cattle

Two prophylactic studies and one persistent efficacy study were conducted in Brazil to evaluate one injection of doramectin at a dose rate of 200 micrograms kg-1 against induced infestations of the New World screwworm, Cochliomyia hominivorax. In each prophylactic study, 12 calves were allocated to two groups of six animals. Six calves were treated with doramectin and six with saline. Two hours after treatment, animals were anesthetized and received two incisions, one in the shoulder and one in the rump. Each incision was then infested with 50 first instar larvae. In the persistent efficacy study, 24 calves were allocated to six groups (T1-T6) of four animals each. On the day of treatment, three groups (T1, T3 and T5) were treated with saline and three groups (T2, T4 and T6) with doramectin. All animals were anesthetized and received four incisions, one in the shoulder and one in the rump of each side. Each incision was then infested with 30 first instar larvae of C. hominivorax with the following schedule: T1 and T2 at Day 3, T3 and T4 at Day 7, and T5 and T6 at Day 14 post-treatment. Calves were observed daily and incisions evaluated for 8 days post-infection. Larvae that completed development and exited the wounds were collected and incubated to evaluate viability. In all experiments, doramectin was 100% efficacious in preventing screwworm infestation caused by field isolates of C. hominivorax. First instar larvae were eliminated from the incisions of doramectin-treated calves between 24 and 48 h post-treatment, and no third instar were recovered at any time. As a consequence, healing started at 24 h and was completed at 96-120 h post-infection. Screwworm infestations and viable third-instar larvae developed in all the incisions in non-treated calves, and healing did not begin until larvae had completed their normal life cycle and exited the wounds. The persistent efficacy of a single injection of doramectin extended beyond 14 days.

Persistent efficacy of doramectin against experimental nematode infections in calves

Three studies were conducted involving cattle exposed to experimental nematode infections. These studies were designed to investigate the prophylactic activity of a single subcutaneous treatment of doramectin at 200 micrograms kg-1 body weight against infections of Ostertagia ostertagi, Cooperia oncophora and Dictyocaulus viviparus. For each study, parasite-naive calves were randomly allocated to either a treated or a matched control group. One group received doramectin and the other received doramectin and the other received either no treatment or an injection of saline at 1 ml per 50 kg body weight by the subcutaneous route. Thereafter, all calves received a daily oral challenge of infective larvae of the particular parasite species on test in each study. Challenge of each pair of treatment/control groups continued for periods of 14, 21 or 28 days. An interval of 14-21 days was then allowed to permit the parasites which had established to mature, after which all animals were slaughtered and their worm burdens determined using standard techniques. Geometric mean worm burdens were calculated from the log worm counts and used to estimate percentage efficacy. Accumulated burdens of C. oncophora in doramectin-treated cattle resulting from a daily challenge infection for 14 or 21 days were reduced by 99.2% and 90.7% respectively, in comparison with those of non-treated control animals. For D. viviparus, burdens were reduced by 100% and 99.9% after a 21 or 28 day challenge, respectively. The corresponding figures for O. ostertagi were 99.9% after a 21 day challenge and 93.7% after a 28 day challenge.

Therapeutic and persistent efficacy of doramectin against Dermatobia hominis in cattle

One therapeutic and one persistent efficacy study were conducted in Brazil to evaluate doramectin at a dose rate of 200 micrograms/kg-1 against induced infestations of the tropical warble-fly, Dermatobia hominis. Doramectin was very effective in both the treatment of established infestations and also in the prevention of damage caused by the parasite. In the therapeutic trial, 12 calves were infested along the dorsal line with 25 first instar larvae of recent field isolates of D. hominis but in one calf nodules did not develop. Twenty-four days later animals were allocated to two groups on the basis of the number of parasite nodules present. Six calves were treated with doramectin, and five received saline solution. Animals were examined daily for 11 days post-treatment and the number of nodules mapped and recorded. Larvae that completed development were collected and incubated to evaluate viability. In the persistent efficacy study, 24 calves were allocated to six groups (T1-T6) of four animals each. On the day of treatment, three groups (T1, T3 and T5) were treated with saline and three groups (T2, T4 and T6) with doramectin. At 21 days, 28 days and 35 days post-treatment, 25 first instar D. hominis larvae were seeded along the dorsal line of each calf of T1 and T2, T3 and T4, and T5 and T6, respectively. Animals were examined daily for 18 days and the number of nodules mapped and recorded 6, 12 and 18 days post-infestation.(ABSTRACT TRUNCATED AT 250 WORDS)

Therapeutic and persistent efficacy of doramectin against Boophilus microplus in cattle

One therapeutic and one persistent efficacy study were conducted in Brazil to evaluate doramectin at a dose rate of 200 micrograms kg-1 against induced infestations of the single host tick, Boophilus microplus. Doramectin was highly effective in eliminating established tick populations from cattle and also in preventing infestation by the parasite. In the therapeutic study, 12 calves were infested three times a week along the dorsal line with 2500 recently hatched larvae, for a total of 11 times before treatment. Animals were allocated to two groups on the basis of uniformity of established engorged tick burdens. Six calves were treated with doramectin and six received saline solution. From Day -3 to Day 21 post-treatment, individual collections of detached engorged female ticks were made from each calf. In the persistent efficacy study, 12 calves were allocated to two groups of six animals. Six calves were treated with doramectin and six received saline solution. From Day 1 to Day 17 post-treatment, each animal was infested three times a week along the dorsal line with 2500 recently hatched Boophilus microplus larvae, for a total of nine times. From Day 18 to Day 42 post-treatment, daily collections of detached engorged female ticks were made from individual animals. In the therapeutic study, efficacy (reduction of collected engorged female ticks) progressed from 51% at 24 h post-treatment (p.t.) to at least 99% at 4 days p.t., and reached 100% at 8 days p.t. With the exception of one tick that did not lay eggs, recovered from one animal at 11 days p.t., no more ticks were recovered from doramectin-treated calves for the duration of the experiment. For the first 6 days after treatment, only a few detached engorged ticks were collected from treated animals, and their oviposition and hatchability declined rapidly. In the persistent efficacy study, doramectin treatment was highly efficacious in preventing the establishment of Boophilus microplus populations for 20 days after the first ticks completed their cycle in the non-treated group. The oviposition and hatchability of the few ticks that completed their life cycle in the doramectin group were severely reduced.

Efficacy of doramectin against warble fly larvae (Hypoderma bovis)

The efficacy of doramectin in the treatment of cattle harbouring naturally acquired infestations of first, second and third instar larvae of Hypoderma bovis was determined in two studies carried out in the Burgundy region of France. In the first study, cattle on six farms with a history of H. bovis infestations were treated during October 1989 with either doramectin at a dose of 200 micrograms kg-1 liveweight (186 animals) or with an equivalent volume of saline (157 animals). During the following March and April, all animals were examined for the presence of warbles. In the second study, cattle on four farms with warbles present in their backs were treated during March 1990, with either doramectin or saline (as before). The viability of larvae within each warble on all the animals was then assessed every 2 days for 14 days. In Study 1, no warbles were present in any of the doramectin-treated cattle at any time, whereas warbles were found in 135 saline-treated animals. In Study 2, all larvae in warbles on the backs of the doramectin-treated cattle were dead by Day 14 after treatment, whereas viable larvae were still present in warbles in the backs of all saline-treated cattle. No adverse reaction to doramectin treatment was observed in any animal at any time. It was concluded that doramectin is both safe and 100% efficacious in the treatment of first, second and third instar H. bovis infections of cattle.

Doramectin, a new avermectin highly efficacious against gastrointestinal nematodes and lungworms of cattle and pigs: two studies carried out under field conditions in Germany

Two field studies were carried out to confirm the efficacy of doramectin in cattle and pigs in Europe. A total of 40 cattle harbouring naturally acquired infections of gastrointestinal nematodes, including Ostertagia spp., Oesophagostomum spp., Cooperia spp., Haemonchus spp., Nematodirus spp. and Trichuris spp., were allocated to a treated or a control group (20 animals/group), and 100 harbouring naturally acquired infections of gastrointestinal nematodes and lungworms, including Oesophagostomum spp., Metastrongylus spp., Ascaris suum and Trichuris suis, were allocated to either a treated (60 pigs) or a control (40 pigs) group. Animals in the treated groups received an injection of doramectin at 200 micrograms/kg (1 ml/50 kg) for cattle or 300 micrograms/kg (1 ml/33 kg) for swine. Animals in the control groups received saline at 1 ml/50 kg for cattle or 1 ml/33 kg for swine. The cattle were injected by the subcutaneous route in the lateral mid-line of the neck and the pigs were injected intramuscularly in the neck. Faecal egg counts were determined for all animals on days--7, 0, 7, 14 and 21 of the studies, where day 0 was the day of treatment. The mean daily weight gain of doramectin-treated animals was significantly higher than that of the control animals over the 21 days of the studies (P < 0.0007 for cattle and P = 0.0001 for swine). At 21 days after treatment, the mean faecal egg counts of the doramectin-treated animals were significantly lower than those of the control animals and were reduced by 100% as compared with pretreatment levels. No adverse reaction to treatment was observed in any animal during either study.(ABSTRACT TRUNCATED AT 250 WORDS)