Spinose ear tick (Otobius megnini) infestation: Parasite burden and milk production response following permethrin aerosol in-ear treatment in a California dairy herd.

The spinose ear tick (Otobius megnini) is an ectoparasite of cattle whose impact on dairy production is poorly defined. This study evaluated the efficacy of a permethrin-based control protocol on tick burden and milk yield in Holstein cows under commercial California dairy conditions. Midlactation cows (n = 647) were systematically allocated to treatment groups by ear tag (odd or even) to receive either no treatment (CON; n = 300) or topical applications of permethrin aerosol spray (0.5%; ∼3 mL per ear; a 296-mL can treated approximately 50 cows) at wk 0 and 2 (TT; n = 347). Tick scores (TSc; 0 = none; 1 = 1-5; 2 = 6-10; 3 = >10 ticks/ear) were assessed at baseline and at wk 2 and 3. Weekly milk yield was derived from automated parlor records for -7 to 28 d relative to treatment. Cumulative logit models were used to evaluate changes of TSc over time, and linear mix models adjusted for milk yield at baseline and pen were used to evaluate weekly milk-yield data. A significant reduction in TSc was observed for TT compared with CON, with a significant treatment × week interaction. The predicted probability of TSc = 0 (no ticks/ear) at wk 3 postintervention was 0.1% (95% CI: 0.0-0.2) for CON and 83.4% (95% CI: 75.9-88.9) for TT versus 0.3% and 1.2%, respectively at wk 0. After adjusting for baseline milk yield and pen, no treatment effect on weekly milk yield was observed: 39.7 versus 39.6 kg/d for CON and TT, respectively. Results demonstrated that permethrin aerosol effectively reduced spinose ear tick infestation but did not improve short-term milk production. This study provides field-based evidence that permethrin can substantially reduce tick burden in US dairy cattle and calls for longer-term studies evaluating production, welfare, and reinfestation outcomes.