Efficacy of a single treatment of a fluralaner pour-on solution for cattle (Exzolt) against experimental infestations of Ixodes holocyclus on calves

Ixodes holocyclus (Australian paralysis tick) is an important ectoparasite of cattle in eastern Australia. Feeding adult female ticks secrete holocyclotoxin that can cause paralysis in young calves and naïve animals. Current control options have limitations in persistence and practicality, and no pour-on product is registered specifically for the control of paralysis ticks in cattle. Fluralaner, an isoxazoline acaricide with systemic and prolonged persistence, has demonstrated efficacy against I. holocyclus in companion animals. The present studies evaluated the therapeutic and persistent efficacy of a fluralaner pour-on solution against experimental I. holocyclus infestation in calves. Two randomised, controlled studies were conducted using a total of 40 healthy male dairy calves (20 calves per study). Within each study, calves were allocated to either an untreated control ( n = 10) or a fluralaner-treated group ( n = 10). Treated calves received a single topical application of a fluralaner pour-on formulation at a minimum dose of 2.5 mg/kg body weight on Day 0. Calves were experimentally infested with 10 (Study 1) or 15 (Study 2) unfed adult female I. holocyclus ticks on Day −1 and subsequently re-infested weekly for up to 6 weeks post-treatment. Therapeutic efficacy was evaluated 48 h after treatment against pre-existing infestation while persistent efficacy was assessed at 72 h following each subsequent infestation by comparing live tick counts between treated and untreated control calves. Ticks were assessed in situ at 24 h post-treatment and 48 h post-reinfestation, then assessed and removed at 48 h post-treatment and 72 h post-reinfestation. Therapeutic efficacy of fluralaner pour-on against existing infestation was 95.5% in Study 1 and 69.1% in Study 2. The persistent efficacy at 72 h following re-infestations remained 100% in Study 1 and ranging from 98.6% to 100% in Study 2. Efficacy declined below 95% after Week 4 in both studies. Fluralaner-treated calves had significantly fewer live ticks than the untreated controls at all 72 h assessments through to Week 4 ( P ≤ 0.002). Treatment was well tolerated with only one mild, site reaction observed. A single topical administration of fluralaner pour-on solution for cattle provided control of existing I. holocyclus infestations and sustained protection against reinfestation for 28 days in calves under controlled experimental conditions. This finding supports fluralaner pour-on as an effective and practical option for the management of the Australian paralysis tick in cattle. At the time of publication, paralysis tick control is not an approved indication on the Australian APVMA registration of fluralaner pour-on solution for cattle (Exzolt).