In rabbits with cheyletiellosis is topical selamectin or injectable ivermectin a more effective treatment?

[...]

Appraisal, application and reflection

Although only one article was found to directly answer the PICO question (Mellgren & Bergvall, 2008), two other articles were also found to be relevant to this Knowledge Summary (Kim et al., 2008; and Coşkunserçe et al., 2012), as they allowed the comparison to be drawn regarding the effectiveness of either selamectin or ivermectin in infested rabbits. Whilst the heterogeneous nature of the study designs (one being a prospective case report and the other a clinical design study), rabbit populations and recorded data precludes direct comparison of their results with each other; these articles present clear evidence for the benefit of using either topical selamectin and injectable ivermectin to treat cheyletiellosis in rabbits.

Mellgren & Bergvall’s (2008) retrospective case study, in addition to being the most relevant article available to treat our PICO question, brings strong evidence in support of the efficacy of both drugs in treating cheyletiellosis in rabbits by comparing the effect of three different treatment programmes on a sample base from across two practices. The effectiveness of both topical selamectin (Stronghold^®/Revolution, 60 mg/ml, Pfizer Inc., New York) and injectable ivermectin (Ivomec^®vet. injectable, 10 mg/ml, Merial SAS, Lyon) was confirmed, whilst doubts about the success rate of oral ivermectin in treating cheyletiellosis were raised. Despite variable treatment success rates between groups, this was not found to be statistically significant. The short length of follow-up time, and variables of the study (environment, client bias) were its biggest limitations. The variability of thoroughness of the post-treatment assessment also raised some risks of re-infection through shedding of mites by ‘in remission’ rabbits, re-infecting themselves, causing a relapse. A replication of this study with a larger cohort, more control over external parameters, a longer follow-up period with clinical examinations and strict microscopic examinations, would allow for a more thorough interpretation of the comparative effectiveness of both drugs.

The prospective clinical design study led by Kim et al., (2008) evaluated the effectiveness of a single application of spot-on selamectin (Revolution®, Pfizer; Kalamazoo, MI, USA) in treating cheyletiellosis across a cohort of 23 pet rabbits. Signs of improvement were evaluated at 3 and 5 weeks post-treatment through clinical and microscopic examination of epidermal debris for mites and eggs. Complete cure was obtained after 5 weeks post-treatment for the whole group and no relapse was noted 12 weeks post-treatment, suggesting the effectiveness of a single spot-on application to treat the infestation. Although successful, the lack of statistical analysis and control over external factors were the main limitations of the study. However, if the study was to be repeated with a large cohort size and addressed some of its limitations it would be a great advocate for the use via the cascade of spot-on selamectin. With no side effects resulting from its application it would be a more client and pet friendly (as no injection is required) alternative to injectable ivermectin when treating cheyletiellosis.

Finally, in the case study led by Coşkunserçe et al. (2012), a single rabbit was treated for C.parasitivorax with subcuntaneous (SC) ivermectin (Iverkol, Etkin) injection to assess the effectiveness of high dose (1.2 mg/kg ) treatment in treating cheyletiellosis. The rabbit was then examined every 10 days for clinical sign improvements and every 7 days for skin scrapes to assess the presence of live mites. After day 14, no clinical signs nor microscopic signs of infestation were noted. However, as positive as the outcome of this case report is, and it describes the effectiveness of high doses of ivermectin in treating cheyletiellosis, it has many limitations: only one animal enrolled, descriptive approach but no statistical analysis of the relevance of the results, and the bias of interpretation as no grading system was used to assess the improvement of clinical signs. This study is relevant, as it demonstrates the effectiveness of ivermectin (Iverkol, Etkin) injections in the treatment of cheyletiellosis, however, it would need to address certain limitations if it was to be replicated to give more weight to its findings.

Despite numerous limitations within each study, evidence supports that both injectable ivermectin and topical selamectin are efficacious in treating cheyletiellosis in rabbits. Effectiveness of avermectins in controlling rabbit mange has been reported in previous studies for moxidectin, selamectin, doramectin, and ivermectin (Niaz & Shoaib, 2015). Altogether the effectiveness of the treatment, low rate of re-infestation and improvement of clinical signs, indicate that the use of both drugs can be warranted in treating cheyletiellosis. Both drugs have their individual benefits, topical selamectin being client friendly and non-painful as it is delivered as a spot on (Farmaki et al., 2009) and injectable ivermectin having proven efficacy and relying less on client compliance (Mellgren & Bergvall, 2008). Currently, injectable ivermectin is not licensed for use in rabbits and therefore, unless prescriptible under the cascade, topical selamectin should be used for the treatment of cheyletiellosis in rabbits (Robinson & Brennan, 2016). Although it has been seven years since their publication, there is no further evidence on this topic. However, with the risk of development of resistance, ivermectin should be used judiciously in treatment of rabbit mange (Coşkunserçe et al., 2012).

Additional well-designed (prospective, controlled, randomised) studies comparing the effectiveness of topical selamectin and injectable ivermectin in larger groups are needed to further evaluate the benefit of both drugs. Further research into the pharmacodynamics and bioavailability of each drug in rabbits post-treatment would also inform comparative results, assuming the literature is available, in helping a clinician choose between topical selamectin or injectable ivermectin (under the cascade).

[…]