Research Article: Influence of Household Rat Infestation on Leptospira Transmission in the Urban Slum Environment

Date Published: December 4, 2014

Publisher: Public Library of Science

Author(s): Federico Costa, Guilherme S. Ribeiro, Ridalva D. M. Felzemburgh, Norlan Santos, Renato Barbosa Reis, Andreia C. Santos, Deborah Bittencourt Mothe Fraga, Wildo N. Araujo, Carlos Santana, James E. Childs, Mitermayer G. Reis, Albert I. Ko, Pamela L. C. Small.

Abstract: BackgroundThe Norway rat (Rattus norvegicus) is the principal reservoir for leptospirosis in many urban settings. Few studies have identified markers for rat infestation in slum environments while none have evaluated the association between household rat infestation and Leptospira infection in humans or the use of infestation markers as a predictive model to stratify risk for leptospirosis.Methodology/Principal FindingsWe enrolled a cohort of 2,003 urban slum residents from Salvador, Brazil in 2004, and followed the cohort during four annual serosurveys to identify serologic evidence for Leptospira infection. In 2007, we performed rodent infestation and environmental surveys of 80 case households, in which resided at least one individual with Leptospira infection, and 109 control households. In the case-control study, signs of rodent infestation were identified in 78% and 42% of the households, respectively. Regression modeling identified the presence of R. norvegicus feces (OR, 4.95; 95% CI, 2.13–11.47), rodent burrows (2.80; 1.06–7.36), access to water (2.79; 1.28–6.09), and un-plastered walls (2.71; 1.21–6.04) as independent risk factors associated with Leptospira infection in a household. We developed a predictive model for infection, based on assigning scores to each of the rodent infestation risk factors. Receiver operating characteristic curve analysis found that the prediction score produced a good/excellent fit based on an area under the curve of 0.78 (0.71–0.84).Conclusions/SignificanceOur study found that a high proportion of slum households were infested with R. norvegicus and that rat infestation was significantly associated with the risk of Leptospira infection, indicating that high level transmission occurs among slum households. We developed an easily applicable prediction score based on rat infestation markers, which identified households with highest infection risk. The use of the prediction score in community-based screening may therefore be an effective risk stratification strategy for targeting control measures in slum settings of high leptospirosis transmission.

Partial Text: In developing countries, leptospirosis is an emerging health problem affecting urban slum communities [1]–[4]. Annual epidemics of the disease typically occur during periods of seasonal rainfall [1], [5]–[8]. Lack of sanitation infrastructure such as open sewage systems and poor refuse collection services provide conditions for proliferation of rats, which are the main reservoir for leptospirosis in urban settings [1], [9]–[11]

As previously described, we enrolled 2,003 participants from 684 households in a community cohort study designed to measure risk factors and infection rates for leptospirosis [11]. Of these, 1,585 (79%), 1,324 (66%) and 1,394 (70%) participants completed the first, second and third year of follow-up, respectively. We identified 104 Leptospira infections in 97 participants residing in 80 households during the three year study period. We identified fifty-one infections (49%) in the first year, 26 (25%) in the second and 27 (26%) in the third year. In all but four cases (96%), L. interrogans serogroup Icterohaemorrhagiae was identified as the presumptive infectious serogroup based on agglutination titers. A majority of the case households (63, 79%) had a single participant with evidence of infection, while 17 (21%) case households had two participants. Seven participants had serologic evidence of two exposures. In addition to the 80 households defined as case-households we also included 115 households, out of a possible 186, meeting inclusion criteria as controls.

High levels of rodent infestation and the predominance of Rattus norvegicus are frequent features within urban slum areas, in Brazil and around the world. Efforts to implement and improve rodent management interventions to reduce urban leptospirosis have been hampered by the lack of readily available information and epidemiologically-based markers that allow identification and monitoring of households at increased risk for infection. Our study demonstrates not only the large proportion of houses in a typical Brazilian slum at risk of acquiring Leptospira transmission, but also that the risk is significantly associated with four markers of rodent infestation and environmental factors (R. norvegicus feces, rodent burrows, access to water, un-plastered walls). The risk score system we developed, by weighting and combining values for each of these features, accurately classified households into risk groups for Leptospira infection with high precision. Of note, similar household-based markers of rodent infestation and infection risk have been described in association with Lassa fever [31] and hantavirus pulmonary syndrome [32]. We propose that targeting households at higher predicted risk for leptospirosis for augmented chemical, environmental and educational interventions would result in the greatest reduction of Leptospira transmission.



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