Moxidectin pour-on for control of natural populations of the cattle tick Boophilus microplus (Acarina: Ixodidae)

Acaricides of Natural Origin. Part 2. Review of the Literature (2002–2006)

Acari are responsible for millions of dollars worth of damage each year as a result of infestations of animals, plants and man. They directly affect our health and prosperity as animal and plant parasites, vectors of disease, and producers of allergens. The indiscriminate use of pesticides has quickly induced resistance in many parasites. At present, the control of acarid parasitic diseases in agriculture, human and veterinary medicine is mainly based on the use of drugs; and for this reason the lack of effective drugs often prevents the control of some parasitic diseases, making them more serious and important. The use of commercial drugs involves many problems, besides the drug-resistance shown by the most important parasites. Environmental damage and the toxicity of many synthetic drugs, represent the main problems that strongly limit drug use. In addition, drug residues in plant and animal food products are important reasons for further economic losses for farmers and must be regarded as potentially hazardous to man and the environment. Plant-derived compounds are generally more easily degradable and could show a smaller negative environmental impact with respect to synthetic drugs. For these reasons, the evaluation of the antiacarid activity of plant extracts is increasingly being investigated in order to obtain new leads, as demonstrated by recent studies that have evaluated and confirmed the effectiveness of many plant compounds on bacteria, fungi, protozoa, helminths and arthropods. This review will be limited to the class Arachnida, sub-class Acaridi, particularly to their control in agriculture, veterinary and human medicine using natural methods.

Moxidectin and the avermectins: Consanguinity but not identity

The avermectins and milbemycins contain a common macrocyclic lactone (ML) ring, but are fermentation products of different organisms. The principal structural difference is that avermectins have sugar groups at C13 of the macrocyclic ring, whereas the milbemycins are protonated at C13. Moxidectin (MOX), belonging to the milbemycin family, has other differences, including a methoxime at C23. The avermectins and MOX have broad-spectrum activity against nematodes and arthropods. They have similar but not identical, spectral ranges of activity and some avermectins and MOX have diverse formulations for great user flexibility. The longer half-life of MOX and its safety profile, allow MOX to be used in long-acting formulations. Some important differences between MOX and avermectins in interaction with various invertebrate ligand-gated ion channels are known and could be the basis of different efficacy and safety profiles. Modelling of IVM interaction with glutamate-gated ion channels suggest different interactions will occur with MOX. Similarly, profound differences between MOX and the avermectins are seen in interactions with ABC transporters in mammals and nematodes. These differences are important for pharmacokinetics, toxicity in animals with defective transporter expression, and probable mechanisms of resistance. Resistance to the avermectins has become widespread in parasites of some hosts and MOX resistance also exists and is increasing. There is some degree of cross-resistance between the avermectins and MOX, but avermectin resistance and MOX resistance are not identical. In many cases when resistance to avermectins is noticed, MOX produces a higher efficacy and quite often is fully effective at recommended dose rates. These similarities and differences should be appreciated for optimal decisions about parasite control, delaying, managing or reversing resistances, and also for appropriate anthelmintic combination.

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

The efficacy of a commercial pour-on formulation of eprinomectin, a macrocyclic lactone, against experimental infestations of Boophilus microplus (Canestrini) ticks was evaluated in two trials involving 27 Bos taurus calves. The first trial was designed to evaluate the effects of a single treatment at a dose of 0.5 mg/kg of body weight against standard size B. microplus females (4.5-8.0 mm long). A significant reduction in tick numbers (P<0.05, Wilcoxon test) was observed between treated calves as compared to untreated ones from Day 3 (44% efficacy) after treatment to the end of the trial on Day 28 (96.9%), with a peak efficacy of 97.1% on Day 21. In the second trial the effect of eprinomectin on standard size tick numbers, engorgement weight and fertility of female ticks from calves with a single treatment dose of 1 mg/kg on Day 0 and calves treated twice at a dose of 0.5 mg/kg on Days 0 and 4 was evaluated. An efficacy >93% was obtained from Day 2 to Day 28 after treatment in calves treated twice at 0.5 mg/kg, and to the end of the trial (Day 35) in calves treated once with 1 mg/kg. The 1mg/kg treatment provided >98% residual efficacy for at least 7 days. During the first part of the second trial the efficacy of eprinomectin resulted from a dramatic adverse effect on engorgement weight and fertility of female ticks, with 100% control on Day 5 (dosage of 1 mg/kg) and on Days 6 and 7 (two doses of 0.5 mg/kg). Following Day 7, most of the effect was due to reduction in the number of standard size female ticks.

Therapeutic and persistent efficacy of a single injection treatment of ivermectin and moxidectin against Boophilus microplus (Acari: Ixodidae) on infested cattle

The effectiveness of a single treatment with either ivermectin or moxidectin was determined by administering a single subcutaneous injection of each endectocide at 200 microg per kg body weight to cattle infested with all parasitic developmental stages (adults, nymphs, and larvae) of Boophilus microplus (Canestrini). The percentage reduction in the number of females that reached repletion following treatment (outright kill) was 94.8 and 91.1% for ivermectin and moxidectin, respectively. In addition, the reproductive capacity of the females that did survive to repletion was reduced by > 99%, regardless of the endectocide. Based on these two factors, the therapeutic level of control obtained against ticks on the cattle at the time of treatment was 99.0 and 99.1% for ivermectin and moxidectin, respectively. Engorged females recovered from either group of treated cattle weighed approximately 3-times less than untreated females, and the egg masses produced by treated females weighed approximately 5-8-times less than egg masses produced by untreated females. Partitioning of data into three separate 7-d post-treatment intervals allowed for an estimation of the efficacy of each endectocide against each individual parasitic development stage (adult, nymph, and larva). Results indicated that both endectocides were > or =99.7% effective against ticks that were in either the adult or nymphal stage at the time of treatment. However, the level of control against ticks in the larval stage of development at treatment was significantly lower at 97.9 and 98.4% for ivermectin and moxidectin, respectively. Analysis of the persistent (residual) activity of the two endectocides indicated that neither material provided total protection against larval re-infestation for even 1-wk following treatment. Against larvae infested 1-4 wk following treatment, the level of control with moxidectin ranged from 92.4% (1 wk) to 19.5% (4 wk). These control levels were higher at each weekly interval than for ivermectin, which ranged from 82.4% (1 wk) to 0.0% (4 wk). The potential for the use of these injectable endectocide formulations in the US Boophilus Eradication Program is discussed.

Therapeutic and persistent efficacy of a single application of doramectin applied either as a pour-on or injection to cattle infested with Boophilus microplus (Acari: Ixodidae)

The efficacy of a single treatment with a pour-on application or a subcutaneous injection of the macrocyclic lactone endectocide, doramectin, was evaluated in separate trials on Hereford heifers infested with Boophilus microplus (Canestrini). Significantly fewer ticks per calf were recovered from both groups of treated animals than from the complimentary untreated calves. The mean weights of engorged females and egg masses from both pour-on-treated and injectable-treated calves were also significantly smaller than the complimentary variables for the two groups of untreated calves. Among the treated groups, the mean weight of females from calves treated with the subcutaneous injection was 55% less than females from cattle that received the pour-on treatment and the weights of egg masses were 71% lighter than those from the pour-on-treated group. The estimates of percentage control for the two treatments were 88.6 for the pour-on formulation and a notably higher 99.8 for the injectable formulation. To obtain estimates of the effect of the treatments on the parasitic stages of the tick, cattle were infested with B. microplus larvae at three weekly intervals beginning 18 d pretreatment to ensure that, on the day of treatment, ticks in all three parasitic stages (adult, nymph, larva) would be on the cattle. The effect of the treatments on each parasitic stage was estimated by partitioning detached females into three groups by noting in which of the three 7-d intervals after detachment of engorged females began that detachment occurred. There was no difference for either the pour-on or injectable in the effect of the specific treatment on each parasitic stage. The persistent efficacy of the pour-on treatment against larvae placed on the hosts 1 wk after treatment was zero. The persistent efficacy of the injectable treatment ranged from 100 to 82.1% (mean, 93.7%) against the larvae placed on calves the first 3 wk after treatment and was still 44% against the fourth weekly posttreatment infestation. The injectable doramectin is a potential alternative to the coumaphos product now used as a precautionary treatment at USDA, Veterinary Services, Livestock Import Stations, for cattle exported from Mexico.

A detailed assessment of the pattern of moxidectin tissue distribution after pour-on treatment in calves

The use of topical (pour-on) administration of endectocide drugs in cattle has reached world-wide acceptance. However, only limited information is available on the kinetic behaviour for topically administered moxidectin (MXD). To improve our understanding of the relationship between pharmacokinetics and efficacy for pour-on preparations, MXD concentration profiles were measured in tissues of endo- and ectoparasites location over 35 days postadministration. MXD distribution to the fluid content and mucosal tissue of the abomasum and different intestinal sections (duodenum, ileum, caecum and colon) was assessed. The comparative patterns of MXD distribution to skin and hypodermic tissue from different anatomical sites (backline, rib cage, thigh and face) were also investigated following the pour-on administration. Wide tissue distribution and long residence time characterized the kinetics of topically administered MXD. MXD was recovered between 1 and 35 days post-treatment in all the tissues investigated. The highest MXD availabilities were observed in the skin layers at the site of administration (backline) and in the fat tissue. The fluid contents of different intestinal sections showed MXD concentrations higher than those measured in their respective mucosal tissues, particularly at day 1 post-treatment. MXD concentrations in the skin (epidermis + dermis) were higher than those measured in the hypodermic tissue. Large differences in the availability of MXD in skin from different anatomical regions (backline > rib cage > thigh > face) were observed. The low plasma and the high skin availability indicate the formation of a skin depot of the drug, being released slowly to the plasma and reaching concentrations in systemic tissues (abomasal mucosa, lungs, etc.) similar to those measured after subcutaneous administration. These findings demonstrate that target parasites may be exposed to markedly different drug concentrations according to their location sites, which is particularly relevant for ectoparasites located in different anatomical regions. Knowledge of the tissue distribution of topically administered endectocides contributes to understand the differences observed in efficacy and/or persistence of activity and to optimize their use in cattle.

Effect of moxidectin against natural infestation of the cattle tick Boophilus microplus (Acarina: Ixodidae) in the Mexican tropics

The study was divided in to two trials and carried out in a ranch in eastern Yucatan state, Mexico. In the first trial, two groups of 15 BostaurusxBosindicus heifers, 6-12 month of age and naturally infested with Boophilus microplus ticks were used. Heifers in Group 1 were treated with a 1% injectable formulation of moxidectin at the dose of 0.20mg/kg body weight by subcutaneous injection. The other group remained as untreated controls. Number of immature and engorging female ticks were assessed on days 0, 7, 14, 21, 28 and 35 post-treatment (PT). The efficacy of moxidectin on adult ticks from day 7 to 28 PT was greater than 95%. The efficacy decreased to 74.9% by day 35. In the second trial, animals in Group 1 were treated with the moxidectin product as before, while cattle in Group 2 were treated according to the routine procedure for the control of ticks on that property (125 g/l amitraz as a dip). Treatment of all cattle was repeated four times at intervals of 28 days. The efficacy of the experimental moxidectin treatment was similar to that of the routine amitraz treatment, i.e., greater than 99%.

Efficacy of moxidectin 1% injectable against natural infection of Sarcoptes scabiei in sheep

Thirty ewes naturally infected with Sarcoptes scabiei var. ovis, were allocated into three groups of 10 animals each. Animals in groups B and C were treated on day 0 and on days 0 and +10, respectively, with moxidectin 1% injectable at a dose of 0.2mg moxidectin/kg body weight (BW). Group A remained untreated. Seven days before treatment, the geometric mean of Sarcoptes scabiei var. ovis per square centimeter of skin in groups A, B and C were not significantly different. From the day of treatment to the end of the trial, the average number of mites/cm(2) increased in untreated animals and decreased in groups B and C, but these values were higher for group C. Active lesions produced by S. scabiei var. ovis consistently increased during the trial in the untreated animals; in group B the minimum count occurred on day +56 this reduction being more evident in group C (no lesions on days +49 and +56). Also in this group, the number of cured animals was 100%, therefore, the application of two treatments with moxidectin (group C) showed higher efficacy than a single treatment (group B). Body condition score decreased in the three experimental groups along the trial. All animals were individually weighed on days -1, +28 and at the end of the trial. No adverse reactions were observed in the animals treated with 0.2mg moxidectin/kg BW.