Rotation of treatments between spinosad and amitraz for the control of Rhipicephalus (Boophilus) microplus populations with amitraz resistance.

A farmlet study was conducted over 4 years in which three treatments were applied to six groups of Holstein dairy calves. Calves in each group were infested with equal numbers of N-strain (susceptible) and Ultimo strain (amitraz and synthetic pyrethroid resistant) tick larvae to establish self-sustaining populations with an initial, measurable level of resistance to amitraz. Standard counts of all ticks between 4.5 and 8.0mm diameter on one side of each animal were made each week and treatment was applied when tick numbers exceeded a threshold of 25 engorged adults per side. The three treatments were: 1, spinosad spray whenever tick numbers exceeded the threshold; 2, amitraz spray whenever tick numbers exceeded the threshold; 3, spinosad whenever tick numbers exceeded the threshold for the first 2 months, then amitraz for 2 months, with alternation every subsequent 2 months. Engorged adult female ticks were collected from each treatment group on 10 or 11 occasions during the study and tested using the larval packet test bioassay (LPT) for acaricide resistance. Spinosad 250ppm provided effective control of amitraz-resistant tick populations in the field, using a similar number of treatments as in the amitraz and rotation groups. The initial infestations of all of the groups resulted in the establishment of populations with in vitro evidence of resistance to amitraz using the LPT. Treatment with spinosad or with a rotation between spinosad and amitraz every 2 months resulted in reduced levels of resistance to amitraz according to the LPT. The animals treated with amitraz alone showed increasing resistance to amitraz according to the LPT each summer and autumn with a return to full or almost full susceptibility to amitraz in early spring in all years. This pattern suggests a relative lack of fitness of amitraz-resistant ticks that might be exploited by using an acaricide rotation strategy.

Responses of calves to injections of ACTH and their relationship with growth rate.

A study was made of the effects of exogenous adrenocorticotropin (ACTH) on the levels of blood components in 109 dairy replacement calves and the statistical correlations between these effects and the growth rates of the calves from birth to six months. Blood samples were taken from a jugular vein before ACTH was injected and then at two, four, six and eight hours afterwards, and analysed for plasma cortisol concentration, total white cell counts, packed cell volume, haemoglobin, plasma glucose, sodium, potassium, magnesium and inorganic phosphorus, erythrocyte sodium, potassium and magnesium, serum ionised calcium and total protein and total plasma calcium concentration. The injection of 1.1 +/- 0.02 iu/kg of ACTH intramuscularly resulted in a peak plasma cortisol concentration after two hours which had not returned to normal after eight hours. It also resulted in leucocytosis, lymphopenia, neutrophilia, eosinopenia and hypophosphataemia; the mean changes were repeatable (P < 0.05) in 49 of the calves tested two months later. The weight gains to six months of age could be predicted from the degree of the changes in several blood constituents. Significant partial regression coefficients were found for the change in glucose concentration (0 to four hours), absolute neutrophil count (0 to two hours), absolute lymphocyte count (0 to four hours) and loge absolute eosinophil count (0 to two hours). The multiple regression sum of squares was highly significant (P < 0.0001), and the multiple coefficient of determination was 0.305. It was concluded that the changes in these blood components after an injection of ACTH might be used to predict the weight gains of dairy replacement calves.