Submitted to: Preventive Veterinary Medicine
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: August 4, 1998
Publication Date: N/A
Interpretive Summary: The Fever Tick Eradication Program depends on the dipping of tick-infested cattle in vats containing the organophosphorus (OP) pesticide coumaphos to treat cattle imported into the U.S. from Mexico and to eradicate outbreaks of cattle fever ticks in south Texas where re-infestations with ticks are a chronic problem. The occurrence in Mexico of populations of southern cattle ticks resistant to Op and pyrethroid pesticides makes it imperative for effective alternative pesticides to be available. Amitraz, an amidine group pesticide, was used in a dipping vat to treat cattle infested with all three parasitic life stages of the southern cattle tick. Four concentrations ranging from 0.0088 to 0.0231% amitraz each provided >99% control of the ticks on the cattle. Each of the treatments except the lowest concentration protected the cattle against being re-infested for 14 days after the animals were treated. Because amitraz degrades rapidly if it is not kept at a high pH, hydrated lime is added to vat fluids to maintain a pH of about 12. The presence of lime in a dipping vat makes it difficult to re-suspend the amitraz after the vat has not been used for several weeks, but this problem does not prevent proper use and management of dipping vats containing amitraz.
Four groups of cattle infested with Boophilus microplus (Canestrini) were each dipped in a different concentration of amitraz diluted from a 12.5% EC formulation to determine the efficacy and performance of the product in a 11,400 l dipping vat. The amitraz in the vat was stabilized with hydrated lime to maintain a pH of ca 12. Analyses of vat samples showed that concentrations of amitraz in the vat were 7.6 to 13% lower than the targeted concentration of 0.010, 0.015, 0.020, and 0.025% active ingredient (AI) for dilutions prepared according to instructions on the manufacturer's label. The amount of hydrated lime added to the vat (10 kg/1000 l) may have interfered with analysis of vat samples. Therapeutic efficacy of each of the 4 concentrations (0.0088, 0.0131, 0.0174, and 0.0231% AI) of amitraz was excellent (>99% control). The rapid detachment of all ticks from an animal within a few hours after treatment with amitraz that often has been observed was not pronounced in the present study. Only 47% of the B. microplus detached in the first 4 hr post-treatment and 84% detached within the first 24 hr. All of the treatments except the lowest concentration provided protection of cattle against reinfestation by B. microplus larvae for 14 days post-treatment. Possibly as a result of the formation of a compact layer of lime and amitraz on the bottom after the vat was undisturbed for 6 wk, intense agitation was required to re-suspend the active ingredient.