Eprinomectin pour-on for control of Boophilus microplus (Canestrini) ticks (Acari: Ixodidae) on cattle

Macrocyclic lactones and ectoparasites control in livestock: Efficacy, drug resistance and therapeutic challenges

Macrocyclic lactones (MLs) are the cornerstone of parasite control in livestock due to their broad-spectrum activity against endo (nematodes) and ecto (lice, ticks, mites) parasites. These molecules, introduced into the veterinary pharmaceutical market 40 years ago, have substantially improved animal welfare and productivity by offering extended high efficacy, reducing treatment frequency, and displaying a favorable safety profile. However, their widespread and intensive use has led to a significant challenge nowadays: the development of parasite resistance. This review focuses on the critical link between drug pharmacokinetics (variation in concentration profiles and exposure over time) and pharmacodynamics (drug efficacy) and the ability of both avermectin and milbemycin MLs families to control livestock ectoparasites. This review discusses the integrated assessment of drug behavior in the host, its diffusion into target parasites, and the impact of different pharmaceutical formulations on enhancing drug delivery to infection sites. These are considered critical research/development areas to optimize the use of MLs, preventing treatment failures and finally extending the lifespan of these essential pharmaceutical ingredients. Finally, the importance of the rational use of MLs, guided by parasite epidemiology and pharmacological knowledge, is emphasized as a key strategy to preserve the antiparasitic efficacy of these still very useful molecules.

Pharmacokinetics and milk excretion pattern of eprinomectin at different dose rates in dairy cattle

This study aimed at determining the plasma disposition kinetics of eprinomectin (EPM) and EPM excretion pattern through milk after topical administration to dairy cattle at the recommended dose of 0.5 mg/kg and at 1 and 1.5 mg/kg. A high variability in the plasma concentration profiles was observed among animals, particularly in the Cmax values, with a coefficient of variation between 39 and 53%. The Cmax and AUC values were significantly affected by the dose administered at 1.5 mg/kg. However, such differences did not seem to follow a linear pattern among treatments. These parameters did not differ among dose rates after dose normalization; nevertheless, the simulation of a linear kinetic disposition showed a mean plasma AUC value of 254 ng.d/ml instead of the observed value of 165 ng.d/ml. EPM concentration profiles in milk were significantly lower than those measured in plasma. The Cmax and AUC milk-to-plasma ratios ranged from 0.14 to 0.26 and 0.16 to 0.21, respectively (p>0.05). The low milk-to-plasma ratio of EPM accounted for a low percentage of the fraction of the administered dose excreted through milk, being significantly higher at a dose rate of 0.5 mg/kg (0.07%) of EPM than at 1.5 mg/kg (0.04%) (p<0.05). The topical administration of EPM to lactating dairy cows at higher doses than that recommended for gastrointestinal nematodes showed a milk excretion pattern with a zero milk withdrawal period. In conclusion, the administration of topical EPM formulation at 1 or 1.5 mg/kg may be a valuable tool to be used in regional strategic deworming programs aimed to control ectoparasite infections in dairy production systems.

Resistance of the cattle tick Rhipicephalus (Boophilus) microplus to ivermectin in Argentina

Resistance to ivermectin in populations of the cattle tick Rhipicephalus microplus in Argentina was diagnosed in this work. The in vitro larval immersion test (LIT) was used to determine quantitatively the levels of resistance to ivermectin in different populations of R. microplus. Additionally, field trials to control natural infestations of R. microplus on cattle with a commercial formulation of ivermectin 3.15% were carried and jointly analyzed with the in vitro assays. The phenotypic response of the populations analyzed was not uniform. Five of them were classified as susceptible, four populations as resistant, and one in the category "incipient resistance". Regarding the field trials, the therapeutic efficacy in a population classified with LIT as susceptible achieved values higher than 94% two weeks after treatment, and no reproductively viable females were observed after the second day post-treatment. Conversely, the values of efficacy percentage in a population (named as "San Martín") classified with LIT in the category "incipient resistance" never exceeded the 70.8%, and engorged females were collected in practically all counts. The population "San Martín" was classified in the category "incipient resistant" with LIT analysis, but the field trial unambiguously shows that this tick population is resistant. The comparison of the results obtained with LIT in vitro assays and through field trials shows that biased estimations of resistance levels may occur when resistance ratios (RR) values are ≤2, and additional field efficacy trials could be needed to know with precision the status of the tick populations evaluated.

Injectable eprinomectin for cattle: Tick efficacy and pharmacokinetics

The aims of the present study were to evaluate the pharmacokinetic profile and efficacy of eprinomectin (EPM) against Rhipicephalus microplus in cattle of a new injectable form of EPM (Voss Performa®). The product was administered subcutaneously at a dose of 200 μg EPM/kg, in a single dose. The efficacy of EPM against R. microplus in cattle was evaluated through field and stall tests. Studies were performed to estimate the pharmacokinetic parameters of EPM with the purpose of better understanding the kinetics of the formulation. The formulation was effective in controlling R. microplus in both naturally and artificially infested cattle, providing efficacy greater than 95%. The results of pharmacokinetic study were C_max of 47.15 ± 22.20 ng/ml, T_max of 1.33 ± 0.492 days, T_1/2 of 2.96 ± 1.212 days, AUC_0-t of 228.08 ± 57.30 ng day ml^-1 , and AUC_0-∞ of 240.50 ± 58.44 ng day ml^-1 . Therefore, the new injectable EPM formulation becomes an important alternative for the control of cattle tick in Brazil.

Acaricidal efficacy of orally administered macrocyclic lactones against poultry red mites (Dermanyssus gallinae) on chicks and their impacts on mite reproduction and blood-meal digestion

Background

The poultry red mite (PRM), Dermanyssus gallinae, is one of the most economically deleterious threats to laying-hen industry worldwide. Macrocyclic lactones (MLs) have been widely used in control of mites in mammals, but the effects of MLs on PRMs are not well studied. The main objective of the present study was to systematically evaluate the effects of three MLs, i.e. eprinomectin (EPR), moxidectin (MOX) or ivermectin (IVM), on PRMs fed on chicks following oral administration.

Methods

Chicks in treatment groups were orally administrated with EPR, MOX or IVM at a dose of 5.0 mg/kg bodyweight. Chicks in the control group received the carrier solvent without drug. Chicks in each cage were then infested with 200 starved adult D. gallinae. After infestation and feeding for 12 h, engorged mites were collected to evaluate the acaricidal efficacy of the MLs, and its impacts on the reproduction and blood-meal digestion of D. gallinae.

Results

MOX, IVM and EPR demonstrated higher acaricidal efficacies post-treatment compared with the control, i.e. 45.60% for MOX, 71.32% for IVM and 100% for EPR on Day 10. MLs did not have significant effects on the blood-meal ingestion of PRMs, but significantly slowed down blood digestion (P < 0.0001). The oviposition rate, egg hatching rate and fecundity of PRMs in treatment groups were remarkably reduced. Among the three MLs, EPR exhibited the highest performance against PRMs, with an oviposition rate of 1.04%, fecundity of 0.33 eggs per mite and a zero egg hatching rate in EPR treated groups.

Conclusions

EPR, MOX or IVM administrated orally to chicks increased the mortality of D. gallinae, significantly slowed down their blood-meal digestion and significantly reduced their reproductive capability which included the oviposition rate, fecundity and egg hatching rate. The present study highlights the potential of MLs in the control of PRMs.

Relationship between pharmacokinetics of ivermectin (3.15%) and its efficacy to control the infestation with the tick Rhipicephalus (Boophilus) microplus in cattle

This study aimed to determine the relationship between the variation in plasma concentration of ivermectin 3.15% over time and its efficacy against the cattle tick Rhipicephalus (Boophilus) microplus. In addition, a trial was conducted to infer if the application of successive treatments with ivermectin 3.15% could affect its accumulation in cattle. A noticeable variation of ivermectin plasma concentration was observed among the treated heifers. However, these differences did not have a significant effect onthe therapeutic efficacy of the treatment at the end of the trial. No significant differences were observed in the levels of tick infestations between heifers of the treated group; moreover, no significant correlation was detected between the plasma AUC_0-21 of ivermectin 3.15% and the cumulative number of ticks of each heifer. Levels of therapeutic efficacy higher than 80% were observed only from day 7 post-treatment, when levels of ivermectin concentration were higher than 8 ng/ml. The lowest values of therapeutic efficacy were observed during the first and the second days post-treatment, when plasma concentrations of ivermectin 3.15% were lower than 8 ng/ml. Viable engorged females were collected from the heifers belonging to the treated group from days 1-5 post-treatment. There was a significant accumulation of the drug after the second dose of ivermectin 3.15%. Ivermectin concentrations in fat biopsies were 366 ng/g (51 days after the first treatment), 275 ng/g (51 days after the second treatment) and 15 ng/g (64 days after the second treatment). These results suggest that applications of successive treatments with ivermectin 3.15% might increase its accumulation in cattle tissues, extending the withdrawal period indicated for the commercial formulation.

Comparative efficacy of pour-on eprinomectin and ivermectin against Sarcoptes scabiei in buffaloes

The efficacy of eprinomectin and ivermectin pour-on was evaluated against Sarcoptes scabiei in naturally infested buffaloes. Eighteen animals were allocated into three groups (A, B and C) of six animals each. Animals in group A were treated with eprinomectin (pour-on) @ 0.5 mg/kg; group B with ivermectin (pour-on) @ 0.2 mg/kg while group C served as untreated controls. Skin scrapings were examined on days 0, 7, 14, 21, 28 and 42 post-treatment (PT). Skin scrapings were found free for mites on day 21 PT, with marked clinical improvement in the lesions after day 28 of treatment. Untreated control group continued to show severe progressive lesions throughout the study period. No clinical side-effects of the tested drugs were observed after treatment. This preliminary report on the efficacy of eprinomectin (pour-on) against S. scabiei in buffaloes is encouraging and results could be of considerable importance.

Efficacy of eprinomectin pour-on against Rhipicephalus (Boophilus) microplus on buffaloes

A trial was conducted on 12 buffaloes naturally infested with Rhipicephalus (Boophilus) microplus to evaluate the efficacy of eprinomectin pour-on at a dose of 0.5 mg/kg body weight. A reduction in live tick count by 45.94, 63.96, 81.53, 90.54, 98.19 and 100 % was observed on days 3, 5, 7, 10, 14 and 21 post-treatment, respectively. The reinfestation of ticks was not observed up to 42 days of trial period. On the basis of the present trial of eprinomectin pour-on, it can be recommended for use in dairy buffaloes against Rhipicephalus (Boophilus) microplus infestation.

Biological differences between two allopatric populations of Amblyomma cajennense (Acari: Ixodidae) in Argentina

The fertility of hybrids from two distinct populations of Amblyomma cajennense from different ecological regions of Northern Argentina was analyzed. Two colonies of A. cajennense from El Rey National Park (RNP), Salta Province (24º41'S, 64º36'W), and Copo National Park (CNP), Santiago del Estero Province (25º55'S, 61º43'W) were established infesting rabbits with adults collected from vegetation. Reproductive parameters of the first generation in laboratory of each colony and their crosses were evaluated considering engorged weight of females, engorged period of females, pre-oviposition period of females, minimum egg incubation period, reproductive efficiency index [REI = number of eggs laid/weight of the females in mg], and fertility efficiency index [FEI = number of hatched larvae/weight of the females in mg]. Infestations were made as follows: Group 1) RNP males and females; Group 2) CNP males and females; Group 3) males from CNP and females from RNP; Group 4) males from RNP and females from CNP. The engorgement weight of the females from CNP that mated with males from RNP was significantly lighter than those of the engorged females obtained in the other 3 crosses, and the engorged period of the females from CNP that mated with males from RNP was significantly longer that the engorged period of the females belonging to the remaining groups. The females from group 3 and 4 had a FEI extremely low in comparison with the FEI obtained from the engorged females originated from the groups 1 and 2. Biological implication of these findings is discussed.

Efficacy of eprinomectin pour-on treatment in sheep naturally infected with Dictyocaulus filaria and Cystocaulus ocreatus

The efficacy of eprinomectin on Dictyocaulus filaria and Cystocaulus ocreatus in naturally infected sheep was evaluated in the present study. In total, 30 infected sheep were randomly divided into two groups: treated (n = 15) and untreated (n = 15). A single pour-on dose of eprinomectin (0.5 mg/kg) was administered to the treated group. No medication was used in the untreated group. Faecal larval counts were performed on pre-treatment (day 0) and post-treatment (days 7, 14, 21 and 42) days. Eprinomectin was found to be 100% effective against D. filaria on day 7 post-treatment when compared with the untreated group and it maintained this effect on days 14, 21 and 42. However, the decrease in faecal larval count of C. ocreatus was found to be 86, 86 and 91%, on days 14, 21 and 42, respectively.

Difficulties to control natural infestation with Otobius megnini (Acari: Argasidae) nymphs in cattle with systemic biocides

A study was carried out to determine the efficacy of topical eprinomectin against nymphal infestation of Otobius megnini in cattle, where a group of 14 individuals were treated with a dose of 0.5mg/per kg of body weight applied on the dorsal midline, and a group of 14 individuals remained as control. Tick burdens between treated and control groups showed no statistically significant differences (P>0.05), and the mortality of the nymphs was similar in both groups (P: 0.828). All females obtained from nymphs of both groups were able to copulate with males from the corresponding group and laid eggs that produced visually normal larvae. The failure of eprinomectin and other biocides applied per os or by injection to control nymphs of O. megnini in cattle indicate that sanitary measures and applications of biocides into the ears would represent the most reasonable approach to control of this tick on cattle.

The acute effects of single-dose orally administered doramectin, eprinomectin and selamectin on natural infections of Syphacia muris in rats

This study was designed to determine the acute effects of a single-dose of orally administered doramectin, eprinomectin and selamectin on Syphacia muris infection in rats. Rats, naturally infected with S. muris, were divided into four groups: three different treatment groups (n=7) and one positive control (n=7). Cellophane tape preparations were obtained from the treated rats on day 0 pre-treatment and on days 2, 4 and 6 post-treatment. Syphacia sp. eggs were counted. Eprinomectin was found to be 100% effective in eliminating eggs on two post-treatment. However when egg counts on day 6 post-treatment were compared with pre-treatment egg counts, doramectin and selamectin were found to be 99.32 and 98.77% effective in eliminating eggs, respectively. On day 7 post-treatment, blood samples were obtained from all groups, and then the rats were necropsied. Doramectin, eprinomectin and selamectin were found to be 100% effective in eliminating adult S. muris, when compared with the positive control group.