Comparative efficacy of afoxolaner and ivermectin in dogs naturally infested with Rhipicephalus sanguineus sensu lato: A clinical field study conducted in Thailand

Afoxolaner is a novel insecticidal and acaricidal of the isoxazoline family, which is used in veterinary practice to control infestation of dogs by fleas, ticks, and mites (NexGard, Boehringer Ingelheim). Ivermectin is an avermectin administered at microdoses to prevent infection of dogs with Dirofilaria immitis and is used off-label to control Rhipicephalus sanguineus infestation of dogs in numerous countries, including Thailand. Here we conducted two trials to assess the efficacy of afoxolaner for treating natural R. sanguineus sensu lato (s.l.) infestations of dogs residing in the Chiang Mai area of Thailand. The first trial compared the efficacies of afoxolaner and ivermectin in dogs infested with < 500 ticks. A randomized, investigator-blinded, controlled study was conducted of 16 dogs, allocated into the groups as follows: afoxolaner (2.7-6.9 mg/kg, PO, group 1; n = 8), ivermectin (300 mu g/kg, SC, group 2; n = 5), untreated (group 3; n = 3). Tick counts and drug administration were performed on days 0, 28, and 56. Mean numbers of ticks on day 0 in groups 1, 2, and 3 were not significantly different (225, 169, and 123, respectively; p = .36). The mean number of ticks (%efficacy to control) in groups 1, 2 and 3 on day 28 were 7 (97.05%), 230 (2.95%), and 237, respectively; on day 56 were 4 (96.11%), 93 (9.71%), and 103, respectively; and on day 84 were 1 (98.65%), 44 (40.54%), and 74, respectively. The efficacy of afoxolaner was > 96%, whereas the efficacy of ivermectin was significantly lower compared with that of afoxolaner (p < .05) and never achieved the 90% efficacy threshold claimed by registration agencies. The second trial assessed the efficacy of afoxolaner for treating dogs with heavy tick infestations ( > 500 ticks/dog), including four dogs from two households. The dogs were treated monthly with Afoxolaner. The mean values of the numbers of ticks on dogs in the 2 households were not significantly different (913 and 800 on day 0, p = .18). The numbers of ticks significantly decreased thereafter, and the efficacy of afoxolaner was > 99% on days 28, 56, and 84. Adverse reactions were not observed in either trial. In conclusion, this study confirms the efficacy of afoxolaner against adult R. sanguineus s.l. that naturally infests dogs that inhabit Thailand.