Leptospirosis is one of the world’s leading zoonotic diseases – a bacterial disease that affects and passes between humans and animals – and currently causes more than 1 million new infections and nearly 59,000 deaths annually, with a fatality rate of 10%. A new study, published in PLOS Neglected Tropical Diseases, shows that it is probably more widespread than previously thought, and growing.

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The burden of leptospirosis is higher than has been previously recognized.

Leptospirosis is a “social-ecological problem,” prevalent in the context of social inequity, mostly affecting the poorest segments of society, and a “severely neglected disease,” according to Prof. Albert Ko, chair of the Department of Epidemiology of Microbial Disease at Yale School of Public Health in New Haven, CT.

Prof. Ko and his colleagues have conducted a systematic review of published morbidity and mortality studies and databases and developed a disease model to generate a worldwide estimate of leptospirosis’ human toll.

The report – co-authored by researchers from the Oswaldo Cruz Foundation and the Institute of Collective Health, both in Brazil, the University of Zurich in Switzerland and the World Health Organization (WHO) – claims that the incidence of the disease has the potential to grow in the coming decades due to global climate change and rapid urbanization. As the world’s slum population is predicted to double to 2 billion people by 2030, epidemics can be expected.

The bacteria that causes the disease is shed in the urine of rats, dogs, cattle and other mammals. The pathogen can survive in water and soil for weeks or months. From the water, it passes to humans.

Tropical countries carry the burden for 73% of cases, particularly the urban slums and poor rural farmland of Latin America, Africa, Asia and island nations. There, through inadequate sewage and sanitation, combined with extreme climatic events and heavy seasonal rainfall, people come into contact with contaminated environments.

Adult males aged 20-29 are statistically more likely to contract and die of leptospirosis, with implications for families, communities and the economy. The risk of fatality increases with age.

The US is not immune to the disease. According to the Centers for Disease Control and Prevention (CDC), in 1998, 775 people were exposed to it, of which 110 became infected, in the country’s largest recorded outbreak.

Cases have originated at lakes in Wisconsin and swamps in Florida. The number of cases among children in urban areas in the US is also reported to be increasing, and leptospirosis was reinstated as a nationally notifiable disease in January 2013.

The CDC mention farmers and mine workers, water sports and adventure race participants among those at risk. Approximately 100-200 cases occur annually in the US, 50% of them in Hawaii. In the US, the fatality rate is about 1-5%.

Leptospirosis can occur when spirochetal bacteria known as leptospires – a genus of Leptospira – that have been deposited in water or soil, enter the human body through abrasions in the skin, or by swallowing affected water or rubbing the eyes. It can cause a wide range of symptoms, some of which may be mistaken for other diseases, or there may be no symptoms at all.

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“The study identified an important health burden caused by this life-threatening disease, which has been long neglected because it occurs in the poorest segments of the world’s population.

At present, there are no effective control measures for leptospirosis. The study provides national and international decision-makers with the evidence to invest in initiatives aimed at preventing the disease, such as development of new vaccines, as well as targeting the underlying environmental and social conditions, rooted in social inequity, that lead to its transmission.”

The authors claim that until now, underestimation and lack of data on leptospirosis have led to its neglected status, hampering efforts to develop prevention and control measures. Their new report, they hope, will change this.

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