We estimate the incidence rate, serological conversion rate and
basic
case reproduction number (R0) of Leishmania infantum
from a cohort study of 126 domestic dogs exposed to natural infection rates
over 2 years on Marajó Island, Pará State,
Brazil. The analysis includes new methods for (1) determining the number
of
seropositives in cross-sectional serological
data, (2) identifying seroconversions in longitudinal studies, based on
both
the number of antibody units and their rate
of change through time, (3) estimating incidence and serological pre-patent
periods and (4) calculating R0 for a potentially
fatal, vector-borne disease under seasonal transmission. Longitudinal and
cross-sectional serological (ELISA) analyses
gave similar estimates of the proportion of dogs positive. However,
longitudinal analysis allowed the calculation of pre-patent periods, and
hence
the more accurate estimation of incidence: an infection–conversion
model
fitted by maximum likelihood to serological data yielded seasonally varying
per
capita incidence rates with a mean of
8·66×10−3/day (mean
time to infection 115 days, 95% C.L. 107–126 days), and a
median
pre-patent period of 94 (95% C.L. 82–111) days. These
results were used in conjunction with theory and dog demographic data to
estimate the basic reproduction number, R0,
as 5·9 (95% C.L. 4·4–7·4). R0
is a determinant of the scale of the leishmaniasis control problem, and
we
comment on the options for control.