Systemic activity of closantel for control of lone star ticks, Amblyomma americanum (L.), on cattle

Cattle were treated once at 5 mg/kg orally or subcutaneously or daily at 0.1-5 mg/kg orally or 0.1-1 mg/kg subcutaneously with closantel, N-[5-chloro-4-[(4-chlorophenyl)cyanomethyl]-2-methylphenyl]-2-hydroxy-3, 5-diiodobenzamide, and numbers and weights of engorged females, weights of egg masses and hatch of eggs of lone star ticks, Amblyomma americanum, were recorded. Effectiveness of treatments on reproduction was determined by comparing total estimated larvae (EL) (EL = wt. egg mass X est. % hatch X 20000) of ticks from treated cattle with that of ticks from untreated cattle. With certain treatments, we also determined the effect of manure of treated cattle on survival of larvae of the horn fly, Haematobia irritans, or effect on survival and of fecundity of adult horn flies or stable flies, Stomoxys calcitrans, fed on blood from treated animals. The single oral treatment afforded essentially complete control of total EL only of ticks placed on the animal on the day of treatment, while the single subcutaneous treatment afforded greater than 92% control of total EL of ticks placed on animal on treatment day and for 6 weeks posttreatment. Daily treatments of 0.5 mg/kg or greater orally and 0.1 mg/kg or greater subcutaneously afforded essentially complete control of total EL of ticks throughout the treatment period (3-12 weeks) and for 1-7 weeks after treatment was discontinued. An estimated concentration of greater than 9 micrograms/ml of blood was calculated by probit analysis to be necessary to provide greater than 90% control of total EL of lone star ticks; that same concentration also provided greater than 90% control of hatch of eggs laid by treated females. A higher concentration (40 micrograms/ml) was necessary to prevent engorging of the females. No treatments tested were effective against larvae of the horn fly or adult horn flies or stable flies.