Therapeutic effect of GP543 strain of Bacillus thuringiensis against Amblyomma cajennense ticks in vitro and in naturally infested cattle.

The current study was focused on evaluating the in vitro effect of the spore-crystal complex of GP543 Bacillus thuringiensis strain on the viability of Amblyomma cajennense, and the in vivo reduction of the parasite load of this ectoparasite on naturally infested cattle. In vitro, 30 adult ticks were treated with 1.2 mg/ml of B. thuringiensis GP543 strain spore-crystal complex in 10 µl/ml of AGREX F® surfactant and 20% glycerin, subjected to the immersion test to assess their viability every 1.5 h up to 6 h and finally at 12 h post-immersion. GP543 was compared to 125 µg/ml of the commercial drug BOMBARD® (amitraz). In vivo, the same concentrations were tested on naturally infested Swiss-Zebu heifers in grazing conditions. Treatments were applied on the perineal region in an area of 30 × 15 cm on days 0 and 7, and the parasite load was determined on days 0, 3, 7, 9, 14, and 21. In vitro, GP543 decreased the viability rate a 96.6% at 12 h post-immersion, while the commercial drug induced a 100% decrease of viability. On heifers, the parasite load decreased after 21 days in the group treated with GP543 in a similar way to the group treated with amitraz. Considering the above, GP543 strain of B. thuringiensis is proposed as a candidate for the control of Amblyomma cajennense.