Chronic progressive lymphedema (CPL), Chorioptes, and treatment with moxidectin

CPL remains a frustrating problem for owners of feathered draft horses, and the cause remains uncertain. This study found in 18 horses a 100% correlation between horses with CPL and Chorioptes bovis infection. The role of the mite in causing CPL is unknown but, logically, it would worsen the condition. A novel treatment of pour-on moxidectin was instituted and was very effective in treating what is normally a hard to treat condition. Dosage and results of treatment are below.

Alleviating lesions of chronic progressive lymphedema in Belgian draft horses by successfully treating Chorioptes bovis infestation with moxidectin 0.5% pour-on

Vet Parasitol. 2023 Nov 10:324:110074. doi: 10.1016/j.vetpar.2023.110074. Online ahead of print.

Authors
Marieke Brys 1 , Edwin Claerebout 2 , Koen Chiers 3
Affiliations

1 Laboratory of Veterinary Pathology, Department of Pathobiology, Pharmacology and Zoological Medicine, Faculty of Veterinary Medicine, Ghent University, Salisburylaan 133, 9820 Merelbeke, Belgium. Electronic address: Marieke.Brys@UGent.be.
2 Laboratory of Parasitology, Faculty of Veterinary Medicine, Ghent University, Salisburylaan 133, 9820 Merelbeke, Belgium.
3 Laboratory of Veterinary Pathology, Department of Pathobiology, Pharmacology and Zoological Medicine, Faculty of Veterinary Medicine, Ghent University, Salisburylaan 133, 9820 Merelbeke, Belgium.

Abstract

The aim of this prospective, randomized, single-blinded, and placebo-controlled clinical trial was to investigate the efficacy of a moxidectin pour-on solution for the treatment of Chorioptes bovis infestation in Belgian draft horses, and in addition, to evaluate the effect of this treatment on the clinical signs and lesions associated with chronic progressive lymphedema (CPL). Nineteen privately owned Belgian draft horses were randomly assigned to either a treatment group (moxidectin pour-on formulation, n = 10) or a placebo group (phosphate-buffered saline (PBS), n = 9). On Day 0, all 19 horses tested positive for the presence of C. bovis in superficial skin scrapings. Prior to treatment, all feathering on the distal limbs of the horses was clipped. Treatment was applied twice (Day 0 and 7). Pour-on moxidectin (Cydectin 0.5% Pour-On; Zoetis) was evenly distributed over the distal legs of the horses at a dose of 1.5 mg moxidectin/kg body weight. Animals in the placebo group were treated with PBS. Pretreatment and follow-up examinations consisted of counting living mites in superficial skin scrapings, scoring pruritus, and scoring mange-associated and CPL-associated lesions (skinfold score and skin lesion score). Horses in the placebo group and moxidectin group were followed up to 8 weeks and 24 weeks after the first treatment, respectively. On Day 14, no living mites were found in any of the horses in the moxidectin group (p = 0.013). These horses continued to remain free of mites, until the final sampling conducted at 24 weeks following the initial application of moxidectin, when three horses again showed living mites in skin scrapings. Treatment with moxidectin resulted in a significant reduction of both CPL-associated skin lesion scores (p = 0.003) and pruritus scores (p = 0.001) after only seven days. By Day 56, still no signs of pruritus (p < 0.0001) were detected, with significant improvement of mange-associated lesions (p < 0.0001). Although the skinfold score did not show a significant reduction by Day 56, the score for skin lesions associated with CPL had significantly improved (p < 0.0001). In conclusion, the results of this study demonstrate that pour-on moxidectin, at a high dose and applied directly to the mite predilection site, was an effective treatment for C. bovis infestation in feathered draft horses, providing positive effects on CPL lesions, pruritus and mange-associated lesions. Furthermore, these findings emphasize the therapeutic significance of addressing mange in the management of CPL-affected draft horses. Keywords: Belgian draft horse; Chorioptes bovis; Chronic progressive lymphedema; Moxidectin. Quoted from the discussion of the article and excerpted:
Discussion
In this study, a high dose of pour-on moxidectin was administered, considering the established oral dose recommendation of 0.4 mg/kg body weight for moxidectin in horses, and the recommended dose of 0.5 mg/kg body weight for moxidectin pour-on in cattle. As pour-on administration moxidectin in horses is off-label use and no recommendations for pour-on dosage are available, we determined the dosage for this study considering existing literature and several potential complicating factors for bioavailability, such as the presence of lesions including hyperkeratosis on the site of application in draft horses with CPL. Moreover, in horses, it has been demonstrated that a high dosage of pour-on macrocyclic lactones is necessary due to poor plasma availability after pour-on administration. This results in subtherapeutic plasma concentrations, which may promote the development of drug resistance in (gastrointestinal) parasites (Francisco et al., 2009, Gokbulut et al., 2010). Since, to the author’s knowledge, there is currently no research available on the pharmacodynamics, pharmacokinetics, and persistent efficacy of pour-on moxidectin in horses, the decision was made to administer the treatment twice with a one-week interval, taking into account the life cycle of C. bovis.

Based on clinical examinations, pour-on moxidectin at a dose of 1.5 mg/kg body weight did not result in any adverse effects in the present study. However, additional safety studies are required to confirm safety, with particular emphasis on neonates, foals, lactating mares, and debilitated horses.