Abstract:
Background: We implemented a longitudinal study to determine the incidence of Brucella infection in cattle, camels,
sheep and goats that were being raised in a pastoral area in Isiolo County, Kenya. An initial cross-sectional survey was
implemented to identify unexposed animals for follow up; that survey used 141 camels, 216 cattle, 208 sheep and
161 goats. Sera from these animals were screened for Brucella spp. using the Rose Bengal Plate test (RBPT), a modiied
RBPT,
and an indirect
multispecies Enzyme
Linked
Immunosorbent
Assay
(iELISA). Results
of RBPT and iELISA
were
interpreted
in parallel to
determine
seroprevalence.
A total
of 30 camels,
31 cattle,
22 sheep and 32 goats that
were
seronegative
by
all the above
tests
were
recruited
in a subsequent longitudinal
study for
follow
up.
These
animals
were
followed
for
12
months
and tested
for
anti-Brucella
antibodies using iELISA. Seroconversion
among these
animals
was deined by
a positive
iELISA test
following
a negative
iELISA result
in the previous
sampling period.
All
seropositive
samples were
further
tested
using real-time
PCR-based assays
to
identify Brucella
species.
These
analyses
targeted
the alkB
and BMEI1162
genes for
B.
abortus,
and B.
melitensis,
respectively.
Data from
the longitudinal
study
were
analysed using Cox
proportional
hazards
model that accounted
for
within-herds
clustering
of Brucella
infections.
Results: The overall incidence rate of Brucella infection was 0.024 (95% conidence interval [CI]: 0.014–0.037) cases
per animal-months at risk. Brucella infection incidence in camels, cattle, goats and sheep were 0.053 (0.022–0.104),
0.028 (0.010–0.061), 0.013 (0.003–0.036) and 0.006 (0.0002–0.034) cases per animal-month at risk, respectively. The
incidence rate of Brucella infection among females and males were 0.020 (0.009–0.036) and 0.016 (0.004–0.091),
respectively. Real-time PCR analyses showed that B. abortus was more prevalent than B. melitensis in the area. Results
of multivariable Cox regression analysis identiied species (camels and cattle) as an important predictor of Brucella
spp. exposure in animals.
Conclusions: This study estimated an overall brucellosis incidence of 0.024 cases per animal-months at risk with
camels and cattle having higher incidence than sheep and goats. These results will inform surveillance studies in the
area.
