Abstract:
The impact of Rhipicephalus appendiculatus tick
presence (exposure) on Theileria parva infection
seroprevalence (outcome) on a farm was assessed in a
group of cattle using population attributable fractions
(PAF). The analysis was based on a representative
sample of 80 traditional smallholder mixed farms from
Mbeere District, Kenya. The PAFs were estimated using
sequentially partitioned PAF approach that estimated a
PAF associated with the exposure after adjusting for any
effect that the confounder (agro-ecological zone [AEZ])
may have had by influencing the prevalence of the
exposure. The resultant PAF was compared with Bruzzi
approach PAF that estimated the proportion of T. parva
infection cases directly attributable to the exposure after
controlling for confounding by AEZ. The estimated
PAF on the Bruzzi approach was 26.4% [95% CI:
9.6%, 43.2%]) whereas the partitioned PAF was 15.5%
[95% CI: 1.5%, 29.6%]) implying that about 11% of
the estimated impacts was driven by AEZ effects. Both
approaches were consistent in estimating a relatively
low impact of farm vector tick presence with a relatively
high level of uncertainty. Overall, the results suggested
that under endemic instability in Mbeere District, (1)
presence of R. appendiculatus was not a good indicator
of T. parva infection occurrence on a farm, and (2)
ecological variation could play a role in determining
infection impacts. This study provides a preliminary
basis for evaluating the potential value of estimating
PAFs for variables amenable to control in tick-borne
diseases epidemiological studies.
