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Leptospirosis comes with the rain

People that contract the disease show mild symptoms such as fever, headache, conjunctivitis irritation and muscle, joint, and bone pain that are commonly mistaken for other diseases. It can also show serious clinical conditions that may lead the death, such as liver and kidney failure, bleeding, and meninges issues.

The World Health Organization claims that every year there are more than 500,000 cases of leptospirosis in the world, and although it occurs around the world, it is especially prevalent in countries with subtropical and moist climates; therefore the climate is essential to understand the dynamics of the disease.

The El Niño–Southern Oscillation (ENSO) is the main climate modulator in Colombia, impacting with frequencies that range from three to seven years. The ENSO has two phases, a cold phase, known as La Niña, where the rains increase and temperatures tend to be lower; and the warm phase known as El Niño, characterized by dry seasons and little or no rain.

Faculty of Agrarian Sciences, Ecology Ph.D. Clara Susana Arias, and Universidad Nacional de Colombia (UNal) in Medellín, Faculty of Mines Hydraulic Resources Engineering Ph.D. Alejandro Builes, carried out a research project that showed that Colombia has an intimate relationship with leptospirosis and the excess or lack of rain-related to the ENSO.

Read more: The more it rains, the more palm trees in Chocó (in Spanish).

Cases and incidence

The purpose of the researchers was to analyze the change in case numbers and the rate of incidence of leptospirosis during La Niña and El Niño episodes, in three spatial scales: country, provinces, and municipalities.

In epidemiology, the incidence is related to the number of new cases in a determined population and time. To calculate it, the number of cases is divided among the susceptible population and multiplied by a factor which normally is 100,000 inhabitants.

For the research project, they recorded different situations according to the analyzed scale. In Colombia, the national scale shows that the number of monthly leptospirosis cases increases by 25% during La Niña, therefore the rain increase is an indication to produce early alerts that could lead to prevention and case reduction.

The statistical significance showed that the correlation between the increase in cases and La Niña periods is also presented with a lag of up to 7 months, in other words, once the La Niña is present, more cases can be expected for up to seven months later.

At the province level, there were more cases in both phases of the ENSO, according to the location of the provinces in the country. The provinces of Valle del Cauca, Antioquia, and Atlántico reported a greater number of cases. The main impacts are in the Provinces of Guaviare, Risaralda, San Andrés, Atlántico, and Valle del Cauca.

The analysis reveals that the provinces of Atlántico, La Guajira, Bolívar, Santander, and Norte de Santander (in northern Colombia), along with Valle del Cauca, Cauca, Meta, Guaviare, and Guainía, increased their monthly leptospirosis cases during the La Niña phenomenon, which brings more rain.

Read more: The Saharan air layer is not dangerous but does require follow-up.

However, they also discovered that in some provinces, cases increased during the El Niño season. “This result was surprising because leptospirosis is linked to the presence of water and increased rains, as it is caused by bacteria that survive very well in water,” said Arias.

Showing an increase in leptospirosis cases in other regions during the El Niño months opened two new hypotheses, one that possibly some regions get temperatures that help bacteria to survive and thrive, such as the case of the Province of Nariño, where mean temperatures are low, therefore the optimal range for the bacteria would increase. The other hypothesis indicates that high temperatures also promote their proliferation of rodents, a common source of infection for human beings; and in warm weather when the temperature increases people go swimming in rivers more.

Concerning the municipal scale, they analyzed those municipalities with a greater incidence of cases and discovered that in 17 municipalities leptospirosis increased during the La Niña months; of these 7 also showed cases during the months of El Niño. The municipalities with more reported cases were Cali, Barranquilla, San José del Guaviare, Cartagena, and Turbo.

In general, in municipalities with long drought periods and rain linked to La Niña showed more leptospirosis cases; but in municipalities that are moist the whole year, such as Urabá, for instance, the increase in the rain brought by La Niña doesn’t seem to have any repercussion.

Between 2007 and 2015, the cases of leptospirosis in Colombia increased by 25% during the rainy season of the La Niña phenomenon and decreased by 17% during the warm period of El Niño.

Opportune precautionary measures

Considering that leptospirosis is present around the country and that according to the research project, retroactive data helps as a reference to prevent an increase in cases toward the future, when the La Niña phenomenon approaches there should be precautionary measures in place.

Read more: With the rain, mosquito-transmitted diseases return (in Spanish).

Especially, the authorities need to reinforce risk management prevention in northern Colombia, where the cases for La Niña increase, and to the proneness of floods in certain areas, people need to avoid contact with water that could be tainted and wash their food well.

The contrasting results of each spatial scale reinforce the fact that leptospirosis is a disease where several factors have an influence (multidimensional), and with complex interactions between its determinants, therefore more research is necessary.

Arias and other UNal researchers will continue to explore the relationship between elevation and the disease to become cognizant if with time it is escalating in height or if it is staying in areas susceptible to flooding, to try to link it to future climate change scenarios. While Dr. Builes will explore more extended time frames, the consequence of extreme rain with the disease at the municipal level and a more precise timeframe, analyzing not only months but weeks or days.



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