Contrasting spatial distribution and risk factors for past infection with scrub typhus and murine typhus in Vientiane City, Lao PDR

Abstract

Background: The aetiological diagnostic of fevers in Laos remains difficult due to limited laboratory diagnostic facilities. However, it has recently become apparent that both scrub and murine typhus are common causes of previous undiagnosed fever. Epidemiological data suggests that scrub typhus would be more common in rural areas and murine typhus in urban areas, but there is very little recent information on factors involved in scrub and murine typhus transmission, especially where they are sympatric - as is the case in Vientiane, the capital of the Lao PDR.

Methodology and principal findings: We therefore determined the frequency of IgG seropositivity against scrub typhus (Orientia tsutsugamushi) and murine typhus (Rickettsia typhi), as indices of prior exposure to these pathogens, in randomly selected adults in urban and peri-urban Vientiane City (n = 2,002, ≥35 years). Anti-scrub and murine typhus IgG were detected by ELISA assays using filter paper elutes. We validated the accuracy of ELISA of these elutes against ELISA using serum samples. The overall prevalence of scrub and murine typhus IgG antibodies was 20.3% and 20.6%, respectively. Scrub typhus seropositivity was significantly higher among adults living in the periphery (28.4%) than in the central zone (13.1%) of Vientiane. In contrast, seroprevalence of murine typhus IgG antibodies was significantly higher in the central zone (30.8%) as compared to the periphery (14.4%). In multivariate analysis, adults with a longer residence in Vientiane were at significant greater risk of past infection with murine typhus and at lower risk for scrub typhus. Those with no education, living on low incomes, living on plots of land with poor sanitary conditions, living in large households, and farmers were at higher risk of scrub typhus and those living in neighborhoods with high building density and close to markets were at greater risk for murine typhus and at lower risk of scrub typhus past infection.

Conclusions: This study underscores the intense circulation of both scrub and murine typhus in Vientiane city and underlines difference in spatial distribution and risk factors involved in the transmission of these diseases.

Figure 1. Spatial distribution of households with a member IgG positive against scrub typhus in Vientiane city in 2006.

* The « neighborhood » is the primary administrative unit in Laos and constituted the primary sampling unit of the seroprevalence survey within Vientiane. ** Equal Interval Classification method divides a set of attribute values into groups that contain an equal range of values. Note: Cartographic files from “Atlas Infographique de Vientiane” .

Figure 2. Spatial distribution of households with a member IgG positive against murine typhus in Vientiane city in 2006.

* The « neighborhood » is the primary administrative unit in Laos and constituted the primary sampling unit of the seroprevalence survey within Vientiane. ** Equal Interval Classification method divides a set of attribute values into groups that contain an equal range of values. Note: Cartographic files from “Atlas Infographique de Vientiane”.

Figure 3. Percentage IgG positivity against scrub typhus and murine typhus among adults (aged >35 years) of different age (in 5 years age classes), Vientiane city, 2006.