Evidence of Ivermectin Resistance in Sarcoptes scabiei var. canis Infesting Dogs: A Clinical and in vitro Study

Abstract

Sarcoptic mange caused by Sarcoptes scabiei var. canis is a contagious canine dermatosis, and apparent ivermectin treatment failure raises concern about reduced acaricide susceptibility. This study evaluated the clinical response to a three-dose ivermectin protocol and compared in vitro ivermectin susceptibility between mites recovered from clinically refractory dogs and a reference susceptible isolate. Thirty naturally infested dogs received subcutaneous ivermectin at 300 µg/kg on days 0, 14, and 28, with serial skin scrapings and pruritus assessment through day 42. Mites from refractory dogs were pooled as a field-resistant isolate and tested against a susceptible isolate using ivermectin contact bioassays. Twenty-four dogs (80%) became mite-negative with more than 75% pruritus resolution, whereas six dogs (20%) remained mite-positive with less than 25% pruritus reduction and were classified as clinically refractory. At 1% ivermectin, 24-hour mortality was 34% in the field-resistant isolate compared with 91% in the susceptible isolate. The 24-hour LC₅₀ was 0.71% for the field-resistant isolate and 0.09% for the susceptible isolate, corresponding to a resistance ratio of approximately 7.9. LT₅₀ at 1% increased from 2.1 to 9.4 hours. These findings are consistent with a marked phenotypic reduction in ivermectin susceptibility associated with clinical treatment failure and support confirmatory follow-up testing, careful acaricide selection, and continued surveillance.