Research Article: Contrasting Spatial Distribution and Risk Factors for Past Infection with Scrub Typhus and Murine Typhus in Vientiane City, Lao PDR

Date Published: December 7, 2010

Publisher: Public Library of Science

Author(s): Julie Vallée, Thaksinaporn Thaojaikong, Catrin E. Moore, Rattanaphone Phetsouvanh, Allen L. Richards, Marc Souris, Florence Fournet, Gérard Salem, Jean-Paul J. Gonzalez, Paul N. Newton, David H. Walker

Abstract: BackgroundThe 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 FindingsWe 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.ConclusionsThis 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.

Partial Text: Scrub typhus and murine typhus are important but under-recognized treatable causes of fever, morbidity and mortality in South-East Asia [1], [2], [3], [4]. There has been a recent resurgence of interest in these diseases, which both cause undifferentiated fever, headache and myalgia progressing, in a minority, to jaundice, pneumonitis and meningo-encephalitis [5], [6], [7], [8], [9]. Scrub typhus, caused by Orientia tsutsugamushi, occurs in Asia and northern Australia and is transmitted by the bites of infected trombiculid mites [10]. Murine typhus, caused by Rickettsia typhi, occurs globally and is transmitted through the infected flea bite site or by scratching infected faeces into the skin [11], [12]. Scrub typhus and murine typhus were first differentiated, in Malaysia, in 1936 [13]. Chiggers and rodents are thought to be most important reservoirs of scrub typhus and murine typhus infection, respectively. Studies suggest that scrub typhus is more common in rural areas and murine typhus in urban areas [11], [13], [14], [15], [16], [17], [18], [19] but there is very little recent information on the epidemiology of scrub and murine typhus in places where both diseases occur.

The research program entitled “Urbanization, Governance and Spatial Disparities of Health in Vientiane” aimed at describing and analysing the organization of urban areas (resulting from geographical, social, cultural, political, environmental, and behavioural combinations) as sources of intra-urban health inequalities. To provide health data on population of Vientiane, the Institut de Recherche pour le Développement and University Paris Ouest La Defense carried out a health survey in which indicators of health status and health seeking behaviour were studied [23], [24].

This study examined the presence of IgG antibodies as surrogate markers for past infection with agents known to cause two common rickettsial diseases in and around an Asian city. Patients with both diseases present at Vientiane health facilities and are sympatric at the district level [20]. However, adults living in central, more urbanised area of Vientiane had a higher seropositivity against murine typhus and, adults living in peripheral less urbanised Vientiane had a higher seropositivity against scrub typhus. This confirms that those living in more rural areas are at higher risk of scrub typhus infection and, those living in urban areas are at more risk of murine typhus infection, which is consistent with what has been observed elsewhere [11], [12], [15], [16], [17], [18], [19]. The absence of previous serological surveys does not allow examination of temporal changes in transmission of these diseases, which were first described in Laos in 2006 [20]. In a paper resulting from the same survey, variation according to the level of urbanization was also noticed for the spatial distribution of flavivirus exposure in Vientiane city with anti-flavirus IgG prevalence significantly higher among individuals living in the central city (60.1%) than those living in the periphery (44.3%) [29].