Modelling the field efficacy of a genetically engineered vaccine against the cattle tick, Boophilus microplus.

The cattle tick Boophilus microplus Canestrini (Ixodidae) is an obligate parasite of bovines in the tropics. Vaccination against the tick, using concealed antigens, has previously been demonstrated to give partial protection which can vary under field conditions. In this paper computer modelling is used to evaluate the potential impact of the vaccine on populations of ticks on grazing cattle. The degree and duration of protection required to maintain tick numbers at acceptable levels is investigated. Results of the computer analyses have been used to guide the further development of the vaccine and to support an application for registration of the vaccine for control of the cattle tick in Australia. They demonstrate the contribution that modelling can make in evaluating the novel impacts that new technologies may have on the host-parasite relationship in different ecological environments.

Ecological aspects of cattle tick control in central Zambia.

In ecological studies in central Zambia, both climate and ecotype affected population dynamics of tick species. Below average rainfall for several years caused a suppression in numbers of Rhipicephalus appendiculatus Neumann adults. Reduction in rainfall leading to changes in grazing patterns is thought to have been responsible for an increase in numbers of Amblyomma variegatum Fabricius adults in a grassland habitat. There were reasonable correlations between numbers of each tick species on individual hosts over 1 year old. However, there were no relationships between numbers of ticks and bovine lymphocyte antigens (BoLA).

Effect of tick control on liveweight gain of cattle in central Zambia.

Field trials were carried out during 1982-86 in two different ecotypes in central Zambia to determine the impact of tick control on the liveweight gain (LWG) of cattle. During the first 2 years of the trial a diamidide acaricide (Amitraz) sprayed at weekly intervals caused periodic depression in LWG in young animals. Thereafter a pyrethroid acaricide (cypermethrin) was used. During the 1984-85 tick season infestations of 50-120 adult Amblyomma variegatum Fabricius in untreated animals caused significant reductions in LWG. From the differences in LWG between treated and untreated cattle, the induced loss in LWG was estimated at 46-61 g per engorged female A.variegatum.

Development rates, fecundity and survival of developmental stages of the ticks Rhipicephalus appendiculatus, Boophilus decoloratus and B. microplus under field conditions in Zimbabwe.

To determine development rates, fecundity and survival of Rhipicephalus appendiculatus, Boophilus decoloratus and B. microplus, a study was carried out in long and short grass in the highveld of Zimbabwe. Engorged adult females of the three species and engorged larvae and nymphs of R. appendiculatus were buried beneath the soil in small cages in the rainy, cool and hot seasons in 1980 and 1981. Half the number of cages were examined regularly to determine development rates and half were left undisturbed to determine survival rates and the fecundity of engorged females. Development was most rapid during warm conditions and slowest during cool conditions, but high temperatures appeared to prolong the preoviposition periods of all species. The relationship between fluctuating temperatures and rate of development in the field was defined using a least-squares procedure. Survival of engorged females was usually high, but was reduced by predation when they were not protected. Fecundity was reduced in long grass during the cool season and in short grass during the hot season. A higher percentage of eggs hatched in the rainy season than in the cool or dry seasons. The survival of engorged larvae and nymphs was usually high in all seasons. Engorged nymphs were the hardiest stage and eggs the most suceptible stage to adverse microclimatic conditions.

Ability of adults of Amblyomma hebraeum (Acarina: Ixodidae) to feed repeatedly on sheep and cattle.

Adult Amblyomma hebraeum ticks were exposed at 14-day intervals on five sheep on 46 occasions and on three cattle on 15 occasions to determine the effects of repeated feeding. The engorged weight and percentage of ticks which fed successfully on sheep declined significantly at first but subsequently they increased. On cattle there was no progressive decline in engorged weight, although the percentage which fed successfully declined significantly and was lower than that for sheep. This decline was attributed to increased grooming by cattle and was the only apparent mechanism by which resistance was expressed.