Date Published: May 29, 2019
Publisher: Public Library of Science
Author(s): Cho Naing, Simon A. Reid, Saint Nway Aye, Norah Htet Htet, Stephen Ambu, Senaka Rajapakse.
Leptospirosis is probably the most widespread zoonotic disease in the world especially in tropical countries. There has been an increase in individual studies, which assessed the frequency of leptospirosis in flood conditions. Some studies showed contact with floods was significantly associated with the occurrence of leptospirosis while other studies reported differently. The objective of this meta-analysis was to synthesize the evidence on the risk factors which are associated with human leptospirosis following flooding. We set up the inclusion criteria and searched for the original studies, addressing leptospirosis in human with related to flood in health-related electronic databases including PubMed, Embase, Ovid Medline, google scholar and Scopus sources. We used the terms ‘leptospirosis’, ‘flood’, ‘risk factor’ and terms from the categories were connected with “OR” within each category and by “AND” between categories. The initial search yielded 557 citations. After the title and abstract screening, 49 full-text papers were reviewed and a final of 18 observational studies met the pre-specified inclusion criteria. Overall, the pooled estimates of 14 studies showed that the contact with flooding was a significant factor for the occurrence of leptospirosis (pooled OR: 2.19, 95%CI: 1.48–3.24, I2:86%). On stratification, the strength of association was greater in the case-control studies (pooled OR: 4.01, 95%CI: 1.26–12.72, I2:82%) than other designs (pooled OR:1.77,95%CI:1.18–2.65, I2:87%). Three factors such as ‘being male’(pooled OR:2.06, 95%CI:1.29–2.83), the exposure to livestock animals (pooled OR: 1.95, 95%CI:1.26–2.64), the lacerated wound (pooled OR:4.35, 95%CI:3.07–5.64) were the risk factors significantly associated with the incidence of leptospirosis following flooding in the absence of within-study heterogeneity (I2: 0%). We acknowledge study limitations such as publication bias and type 2 statistical errors. We recommended flood control and other environmental modifications that are expected to reduce the risk of leptospiral infection, and a multi-sectoral effort to this aspect would have long-term benefits.
Leptospirosis is probably the most widespread zoonotic disease in the world  caused by corkscrew-shaped pathogenic bacteria Leptospira interrogans of the genus leptospira . Human infection occurs through direct contact with the urine of animal reservoirs or through contact with contaminated soil or water . The global burden of disease study estimated that approximately 2.9 million (1-25-4-54 million) Disability Adjusted Life Years were lost to leptospirosis. This is more than 70% of the global burden of cholera in that year. On stratification, males were approximately 80% and young adults aged 20–49 years were 52% of the total burden . Although a widespread zoonotic in the world, it is particularly common in the tropics . Clinical manifestations range from a mild form of illness including fever, chills and headache to life-threatening conditions (e.g., Weil disease), characterized by jaundice, renal impairment and hemorrhage . The microagglutination titre (MAT) remains the definitive serological investigation technique in both humans and animals .
The current study adhered to the preferred reporting items for systematic reviews and meta- analyses (PRISMA)  (S1 Table).
A four-phase flow chart of the study selection process is illustrated in (Fig 1). The initial search yielded 557 citations. After the title and abstract screening, 49 full-text papers were reviewed and a final of eighteen studies met the pre-specified inclusion criteria [11,12, 14,15,19, 22–34]. Table 2 provides characteristics of these included studies. A summary of the thirty-one studies [5,7,13,35–62] are provided in S3 Table.
The current study containing eighteen observational studies from twelve endemic countries provides evidence on the effect of flooding and the associated risk factors for the occurrence of leptospirosis following flooding. The major observations of the current analysis are as follows;
There are some limitations in the present meta-analysis. Leptospirosis is considered to be widespread in many tropical countries. The limited number of studies to the limited countries made a limited geographic representativeness. Studies in non-English language were not included. This concern was supported by the presence of publication bias observed in the current analysis. Flooding results from the interaction of many factors including rainfall, surface run-off, sea level, catchment size and local topography . Besides flooding as a factor, several other factors can affect the isolation of leptospira from water and soil, such as pH, temperature, characteristics of water and soil and the presence of animals that are considered as reservoirs of leptospira. The lack of significance in the remaining risk factors (e.g. barefooted walking) might partly be due to the relatively small number of studies with small sample sizes, which was an issue of Type II statistical error. Hence, the association between flooding and the occurrence of human leptospirosis observed in the present analysis could be interpreted in the light of these issues. Nevertheless, our analysis used the adjusted ORs controlling common factors for the strength of associations, and this made a confidence in our estimates.
Leptospiral infections cluster in the flooded areas was documented in the present analysis. Flooding is related to peri-domiciliary infrastructure deficiencies (inadequate rainwater drainage with silt and refuse, blockage of drainage system). Considering that leptospira can survive for weeks to months in fresh water, and are efficiently carried and disseminated by water  (e.g. flooding in this case). The identified risk factors are conditions that are potentially correctable not by individuals, but at the municipal and community level . Hence, better control strategies and immediate response system involving multisectoral approaches including health and non-health sectors (e.g. land development, rural/urban settlement, animal husbandry, municipalities) would have a significant impact on the reduction of leptospirosis transmission amongst the targeted populations. For example, improving sanitation in slum communities and equitable access to improved sanitation  would reduce rat infestations and further exposure to an environment contaminated by the urine of rodent reservoirs.
Findings suggest that flooding was associated with the risk of occurrence of leptospirosis in endemic countries. Future public health interventions such as implementation of disease prevention measures including rodent control programs and health education should be valuable for preventing exposure to animal urine and subsequent leptospirosis. These findings may serve as evidence-based information for the national health planners for better formulation of a package of emergency disease control strategy containing leptospirosis. To substantiate the findings, future well designed prospective studies complement to proper health care services during flooding are required.