Links between rodents' environment, climate and leptospirosis infection

Leptospirosis is a water-related bacterial disease with a high incidence in Southeast Asia. People usually become infected through exposure to water contaminated by the urine of infected animals, mainly rats and mice. In the framework of the CERoPath program, IRD researchers and their partners have revealed the relationship between rodents' environment and infection by leptospirosis bacteria. They showed that, whereas people mainly get infected in rice fields, the bacteria are present in a variety of environments, and particularly at the frontiers of fields and forests. The use of remote sensing and Geographic Information Systems (GIS) has helped to expand knowledge on rodents' habitats. These studies allow a better understanding of their behaviour, depending on land use changes, and infection risks caused by human activities, including leisure.

Rodents are major reservoirs of human pathogens such as Leptospira spp., the bacteria responsible for leptospirosis. This bacteria is transmitted to humans via the animals' urine, mainly through skin lesion in contact with contaminated water. The disease occurs in most parts of the world. The number of cases, which is estimated at 500,000 a year, is constantly growing. The rate is dramatically high in Southeast Asia, where it is considered as endemic.

In the framework of the CERoPath program, IRD researchers and their partners have demonstrated the links between rodents' environment, the climate, and leptospirosis infection.

Climate and the environment: determining factors

Taking a sample of nearly 3,000 rats and mice from about twenty different species in Lao P.D.R., Cambodia and Thailand, researchers have made a unique collection of tissues and parasites. This sampling allowed them to cover and compare different environments, regarding a gradient of anthropogenic disturbances, from forests to villages. For instance in the two study sites in Cambodia, most of the species carried Leptospira, with an average of 11% of the rodents identified as positive. This proved that the bacteria were present in various environments. Rice fields, secondary forests and their neighbouring fields had the highest prevalence among rodents, implying that such areas could present the highest risk of infection for people staying there. The data also demonstrated the impact of seasons on the risk of infection. As bacteria survive longer in a wet environment, they are spreading more efficiently during the rainy season.

Satellites to study rodents

The study used of Geographic Information Systems or GIS, remote sensing and geo-statistics. The GPS allowed to georeference each animal to their sampling location. Various kinds of information, such as the characteristics of its environment, were integrated into the GIS. Historical comparisons were made, based on archive and recent images from the SPOT observation satellites in the late 1980s. This study measured changes in the Thai, Cambodian and Laotian landscapes, and how it may have impacted the rodent and pathogens population dynamics.

New areas of disease transmission

These historical analyses of land use showed a progression in urban and agricultural areas at the expense of forests, with various patterns in the different countries. For instance on the Mondolkiri site in eastern Cambodia, massive deforestation has reduced wooded areas from 92% in 1988 to 81% in 1998, and down to 58% in 2008. In Thailand where deforestation started earlier, a fragmentation of wooded, farming and urban areas is clearly visible. Such abrupt changes in land use increase the contact between different populations of rodents, livestock and humans. As a result, diseases transmitted by animals, like leptospirosis, are at a much greater risk of spreading, emerging, and becoming endemic.

A multi-faceted disease

Most of the leptospirosis outbreaks in tropical countries start after flood episodes. The disease can take many clinical forms in humans. Drawn-out fever, headaches, muscle and joint pains are the first symptoms. Severe forms can cause acute renal failure, neurological damage and more or less severe haemorrhages, which can be fatal when there is no antibiotic treatment.

The study of rodents is a major public health concern, due to their role in transmitting numerous diseases to humans, particularly in Southeast Asia, which is considered as a "hotspot" of biodiversity. An in-depth knowledge of their behaviour following human activities will help to define more precisely the potential risks of infection and preventive actions that can be implemented.