A. A. Latif

An epidemiological study of tick-borne diseases and their effects on productivity of zebu cattle under traditional management on Rusinga Island, western Kenya.

Abstract Ten farmers on Rusinga Island, Lake Victoria, Kenya were selected to study the epidemiology of tick-borne diseases (TBDs) and their possible effects on calf performance. During the years 1986–1988 a total of 162 calves were ear-tagged, weighed monthly and monitored closely for TBDs and endoparasitism. The mean (± SD) body weights at 4 months, 8 months and 12 months were 35 ± 7 kg, 47 ± 10 kg and 61 ± 12 kg, respectively. The average growth rate from birth to 12 months of age was 120 g day −1 , but there were significant effects of year, season of birth and farm on calf liveweight gain. Calf mortality from birth to 12 months of age was high, ranging from 17 to 56% among farms, and averaging 33%. Twenty one percent of mortalities were diagnosed as due to East Coast fever (ECF) and 14% as due to endoparasites. The average annual ECF incidence rate was 22% and the ECF case fatality rate 21%, but these deaths were recorded on only two of the ten monitored farms. Forty-nine percent of dams had one or more damaged teats due to the tick species Amblyomma variegatum . Attempts were made using this small data set to analyse for effects of ECF and number of damaged teats on calf performance but no statistically significant effects could be demonstrated.

Histopathology of attachment sites of Amblyomma variegatum and Rhipicephalus appendiculatus on Zebu cattle of varying resistance to ticks

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.

Feeding Performance of Amblyomma variegatum (Acarina: Ixodidae) Fed Repeatedly on Rabbits

Rabbits infested with different stages ofAmblyomma variegatum Fabricius, 1794 became resistant to subsequent infestations by the same tick. Resistance was manifested by a reduction of 73.6% in the mean engorged weight of female ticks by the 3rd infestation. By the 5th infestation, only 70% of the nymphs engorged and their mean engorged weight was 57.7% of that of nymphs fed on tick-naive control rabbits. In the case of larvae, there was a 22.8% reduction in their mean engorged weight by the 6th infestation. Based on increases in body length while feeding on tick-naive rabbits, the nymphs could be divided into four feeding categories, i.e. Nl (unfed), N2, N3 and N4 (increasing states of engorgement). While 92–96% of ticks which dropped from susceptible rabbits fitted into feeding category N4, only 28% of those that dropped from tick-resistant rabbits fitted into that category. The remainder of the ticks fitted into categories N3 (60%) and N2 (12%). The majority of those ticks with reduced weight developed into adults which had an atypical scutal ornamentation pattern, in that an additional pair of lateral spots was regularly observed on the males. The females of such ticks deposited small numbers of eggs from which no larvae hatched.

Derivation of an index for assessing the resistance of zebu cattle to Rhipicephalus appendiculatus Neumann (Acarina: Ixodidae)

A quantitative measure for assessing the level of resistance of zebu cattle to Rhipicephalus appendiculatus is discussed. Basic data for the derivation of the mathematical index were from artificial infestation of cattle to adults, nymphs and larvae of the Rusinga Island stock of the tick species. Observed variables were the weights and numbers of the engorged/detached instars.The index derivation process was by the use of the stepwise discriminant function procedure and the Mahalanobis distance. These procedures led to die derivation of an index which was a function of the engorged weights of the female and nymphal instars, the numbers of detached females and the Mahalanobis distance. Variables associated with the larval instar were found to be insignificant and were therefore not included in the index function.The derived index is capable of discriminating between susceptible and resistant cattle, and can be used to monitor changes in the resistance levels of cattle over time.RésuméUn index mathématique base sur les résultats obtenus aprésune infestation des vaches avec le Rhipicephalus appendiculatus a été étudie. L’index est dérivé d’une fonction analytique discriminante qui a comme principle variables le nombre des femelles ayant pris du sang et les poids des femelles et chrysalides engorgées. Cet index est capable de discriminér entre les vaches sucepttbles et résistantes et peut être utilises pour observer les changements du niveau de resistance chez les vaches.

Non-tick stock-specific index for assessing resistance of zebu cattle to Rhipicephalus appendiculatus Neumann (Acarina: Ixodidae)

A quantitative index of resistance to ticks, which had been derived from results of artificial infestation of East African zebu cattle with a Rusinga Island (Lake Victoria) stock of Rhipicephalus appendiculatus, was tested for consistency using three additional tick stocks—from a laboratory (Muguga), Intona (Trans Mara) and Ukunda (Kenya coast). Results from all the stocks gave similar indices when compared with corresponding values for the Rusinga stock, confirming both the validity of the mathematical function and of the use of any local stock of the tick species for assessing the resistance of cattle to R. appendiculatus.RésuméUn index quantitative de la résistance aux tiques dérivant du résultat d’une infestation artificielle du zebu africain par le Rhipicephalus appendiculatus de l’ile de Rusinga (Lac Victoria) a été testé pour sa consistence lorsque 3 différentes espèces des tiques du laboratoire (Muguga), de Intona (Trans Mara) et Ukunda (Coast Kenyanne).Le résultat étaient similaire pour toutes les espèces. Ceci confirme la validité de la fonction mathématique et l’utilization de m’importe quelle espèce des tique pour évaluer la resistance.