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Contrary to patterns described elsewhere, tick numbers on Rusinga Island cattle drop markedly after the onset of the rainy season. One possible explanation is that this is related to seasonal differences in host behaviour, since cattle grazing on the island are restricted during the crop-growing season (the rainy season) and free grazing afterwards. In a field experiment, cattle were divided into two groups, free grazing and restricted, for 2 months each during the rainy season and during the dry season, and the number of ticks infesting each group was compared. In both seasons, the free-grazing group carried more ticks than the restricted group. Not only did the restricted group come into less contact with ticks, they were also exposed to tick-predating chickens for longer periods each day than the free-grazing group. Management practices that affect host behaviour should, therefore, be taken into consideration in tick population studies.

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A study was conducted on the population dynamics of four tick species infesting livestock on Rusinga Island. Infestations of Rhipicephalus appendiculatus Neumann, Rhipicephalus evertsi evertsi Neumann, Amblyomma variegatum (F.)., and Boophilus decoloratus (Koch) were monitored on East African shorthorn Zebu cattle at monthly intervals for 2 yr. These cattle were of unimproved stock under traditional husbandry without any tick control practices. Most ticks were on cattle between September and March, with a peak in December-February. Following the onset of the rainy season in April, tick numbers on the cattle declined markedly; this decline continued until July or August, when numbers started to increase again. No significant correlation could be made between these population changes and any of the four climatic factors recorded. Instead, there was a notable relationship between tick population changes and local farming practice. The area and duration of cattle grazing activity were severely curtailed during the cropping season from April to September, which reduced tick-host contact and thus the tick burdens of the cattle at that time.

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Mature, unimproved East African Zebu cattle (Bos indicus) naturally exposed to ticks from birth were ranked for resistance to ticks by repeated whole-body counts of infesting ticks. Four cattle of high resistance, four of low resistance and two of intermediate resistance were artificially infested with nymphs of Amblyomma variegatum and Rhipicephalus appendiculatus. Two Friesian cattle (Bos taurus) with less exposure to ticks were similarly infested. Biopsies of tick attachment sites were examined histologically. All attachment sites showed acute inflammatory lesions, and sites of both tick species on high resistance cattle showed delayed hypersensitive reactions with intra-epidermal pustulation and significant increases in the numbers of granulocytes. The predominant cells infiltrating attachment sites on high resistance cattle were eosinophils with A. variegatum and neutrophils with R. appendiculatus. Such differences need to be taken into account in developing immunological tests for selecting cattle for resistance but there are sufficient common features of reactions to the two infesting tick species to justify dermal hypersensitivity tests.

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Monthly collections were made of all ticks from 25 mature East African Zebu cattle and 5 yearlings for 13 mo on Rusinga Island, Lake Victoria, Kenya, from September 1986 to September 1987. The most common species were Rhipicephalus appendiculatus Neumann and Amblyomma variegatum F.; Boophilus decoloratus Koch and Rhipicephalus evertsi evertsi Neumann also were present in fewer numbers. Cattle were ranked according to their degree of resistance to each of the four species and to each stage in the life cycle of the three-host ticks. Ranking according to infestations of females and nymphs of R. appendiculatus, nymphs and larvae of A. variegatum, or all three stages of these two species when compared with the total burden gave statistically significant correlations. Sampling difficulties prevented the ranking of cattle for infestations of larvae of R. appendiculatus, although the numbers on cattle were high. On the other hand, failure of attempts to rank the cattle on females of A. variegatum, B. decoloratus, or R. e. evertsi counts was attributed to the small differences between tick numbers on cattle. Highly resistant cattle showed little or no seasonal fluctuations in tick numbers for most of the period compared with animals of low resistance, which showed an up to sevenfold increase in the magnitude of the tick burden when the tick challenge was high. Cattle with low resistance were responsible for much of the tick multiplication in the field, whereas highly resistant animals tended to limit the populations. There were no distinct differences between the numbers of each tick species on yearlings and mature cattle.

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Groups of previously tick-exposed East African Zebu cattle together with tick-susceptible Friesian steers as controls were artificially infested with adults, nymphs, and larvae of Rhipicephalus appendiculatus Neumann on two occasions. Comparison of the feeding performance data from the two different breeds of cattle showed significant differences in the number and weight of engorged females and the weight of engorged nymphs. Data from the first challenge were used to derive a function for discriminating between resistant and susceptible categories. The use of the function gave repeatable results on resistance rankings when compared with original ranking of cattle based on natural tick counts following natural exposure. The function was converted into an index that could then be used to monitor the seasonal and annual fluctuations in tick resistance rankings of individual animals in a herd.

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Tick-susceptible Friesian steers were used as bait animals to assess the availability of adult Rhipicephalus appendiculatus Neumann on the pasture. Assuming that initial gross pickup rates of ticks by cattle were similar, the numbers of female ticks that attached and the numbers that subsequently fed to maturity on individual cattle were compared based on data obtained from bait animals. The proportion of successful attachments on high-resistance (HR) cattle (0.30) was significantly less than that on the low-resistance (LR) animals (0.51). Similarly, the proportion of attached ticks that fed to maturity also was significantly lower on HR animals (0.15) than on LR animals (0.33). Female survival was calculated as the product of these two parameters. On the HR cattle, female survival (0.05) was significantly less than on LR (0.18). About 12% of the cattle had low resistance, and up to 73% of the females survived on them. These animals were responsible for much of the tick multiplication in the field population.

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In the course of a study of ticks of livestock, five Nile monitor lizard specimens were caught and examined for ticks. Although none of the tick species infesting livestock were found on the reptiles, a large number of adults and presumed nymphs and larvae of Aponomma flavomaculatum (Lucus) and adults of Amblyomma sparsum (Neumann) were collected. The ticks demonstrated a distinct predilection for their attachment sites. This is the second report of A. flavomaculatum from this part of Kenya.

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During a study of an experimental population of Rhipicephalus appendiculatus and its transmission of bovine theileriosis to cattle, some supplementary information was obtained on the role of rodents and the small carnivores in the development and maintenance of R. appendiculatus populations. Observations were also made on other common tick species found on these hosts. A total of 530 individual hosts were examined during the 4-yr period, of which the majority included, among the carnivores, the white-tailed mongoose (Ichneumia albicauda), the large spotted genet (Genetta tigrina) and the zorilla (Ictonyx striatus). Among the 482 rodents 95% included Otomys irroratus, Rhabdomys pumilio and Lophuromys flavopunctatus. While all stages of R. appendiculatus thrived on cattle the role of rodents and carnivores was negligible. Similarly, while the other tick species like H. leachi group, immatures of R. hurti/jeanneli group and I. thomasae avoided cattle, they thrived on the carnivores and rodents. However, the adults of R. hurti and R. jeanneli preferred cattle to carnivores and rodents.

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