THE EPIDEMIOLOGY OF CANINE LEISHMANIASIS - TRANSMISSION RATES ESTIMATED FROM A COHORT STUDY IN AMAZONIAN BRAZIL

We estimate the incidence rate, serological conversion rate and basic case reproduction number (R-0) of Leishmania infantum from a cohort st udy of 126 domestic dogs exposed to natural infection rates over 2 yea rs on Marajo Island, Para State, Brazil. The analysis includes new met hods for (1) determining the number of seropositives in cross-sectiona l serological data, (2) identifying seroconversions in longitudinal st udies, based on both the number of antibody units and their rate of ch ange through time, (3) estimating incidence and serological pre-patent periods and (4) calculating R-0 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 calculat ion of prepatent periods, and hence the more accurate estimation of in cidence: an infection-conversion model fitted by maximum likelihood to serological data yielded seasonally varying per capita incidence rate s with a mean of 8.66 x 10(-3)/day (mean time to infection 115 days, 9 5 % 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, R-0, as 5.9 (95 % C.L. 4.4-7.4). R-0 is a determinant of the scale of the l eishmaniasis control problem, and we comment on the options for contro l.