EVALUATION OF THE POTENTIAL OF SYSTEMIC SLOW RELEASE CHEMICAL TREATMENTS FOR CONTROL OF THE CATTLE TICK (BOOPHILUS MICROPLUS) USING IVERMECTIN

SUMMARY Stall and field trials with cattle infested with various acaricide-resistant strains of cattle tick, have demonstrated the potential of ivermectin as a systemic tickicide. A dosage of 200 μg/kg, administered subcutaneously to animals naturally infested in the field, gave satisfactory tick control for 21 days, after an initial lag period of 2 days immediately following treatment, during which significant numbers of ticks survived. Daily subcutaneous treatments, administered so as to simulate slow release, indicated that a dosage of 15 μg/skg/day should give complete tick control if the chemical could be released continuously from a subcutaneous implant. The potency of ivermectin, assessed by in vitro tests against engorged adults, suggests that the tickicidal activity could be associated with the parent compound per se rather than a metabolite.     

THE EFFECT OF CATTLE TICK (BOOPHILUS MICROPLUS) ON PREGNANT AND LACTATING BRAHMAN-CROSS AND BRITISH-BREED COWS IN NORTHERN AUSTRALIA

Natural infestations of cattle tick (Boaphilus microplus) on breeding Hereford, Shorthorn and Brahman-cross cows were observed in a dry tropical environment. Herefords were dipped three times and Shorthorns four times during a 12 month period, while the Brahman-cross cattle were dipped twice during a 3-year period. No differences in tick infestations could be shown between pregnant and non-pregnant or lactating and non-lactating cows even though they were under severe nutritional stress. No consistent negative correlations were shown between body- weight changes and tick counts. This was probably due to the effect of dipping in the Herefords and Shorthorns. There was a trend towards significant negative correlations in July and August over each of three years for undipped Brahman-cross cows. Some form of tick control on breeding Brahman-cross cows, aimed at reducing tick burdens during July to September may be beneficial in the environment under study.     

THE TRANSMISSION OF BABESIA BOVIS IN HERDS OF EUROPEAN AND ZEBU X EUROPEAN CATTLE INFESTED WITH THE TICK, BOOPHILUS MICROPLUS

SUMMARY Transmission of Babesia bovis by the tick Boophilus microplus was studied in 4 breeding herds of European and Zebu x European cattle under different levels of tick infestation. The observations consisted of weekly counts of female ticks on the cattle, monthly serological tests for antibodies to B. bovis, examination of tick larvae from the pasture to determine B. bovis infection rates and comparison of the suitability of paddocks for tick reproduction. The rate of transmission (inoculation rate) was estimated in terms of the daily probability of infection and consisted of the product of the mean daily tick infestation head^-1 and the babesial infection rate in tick larvae. The theoretical value of the minimum inoculation rate required to produce stability of babesiosis is 0.01. This value was exceeded only in a herd of European cattle with minimal tick control and grazing on a pasture favourable for tick reproduction. Instability of babesiosis occurred in the other herds of European cattle after tick numbers had been reduced by pasture spelling and strategic dipping and after reduction in the babesial infection rate in ticks apparently caused by unfavourable environmental conditions. Over a period of years, the tick infection rates also declined as a consequence of the reduction in numbers of ticks. The Zebu x European cattle failed to generate inoculation rates greater than the minimum level, even though no tick control measures were applied. This was attributed to lower babesial infection rates in the ticks than those observed in a comparable herd of European cattle and to the high tick resistance of the Zebu crosses which maintained the tick populations at low levels. Both factors combined to produce a low inoculation rate.     

INFECTIVITY FOR CATTLE OF ANAPLASMA MARGINALE EXTRACTED FROM BOOPHILUS MICROPLUS TICKS EXPOSED TO CERTAIN TEMPERATURES

SUMMARY: Boophilus microplus ticks collected from calves with patent Anaplasma marginale infections were incubated at either 4 to 5°C, 14°C, 22°C, 27°C or 37°C for up to 14 days. Extracts prepared either from larvae, nymphs, immature females, adult males or mixtures of both sexes were infective for 14 of the 16 splenectomised calves inoculated. Extracts either from nymphs or from adult ticks deriving from nymphs moulted in vitro were infective for 11 of 12 nonsplenectomised calves. Possible application of the findings to producing a vaccine strain of A. marginale is discussed.