Conservative estimates show that the bacterial infection, transmitted from mammals to humans, infects and kills more than other causes of haemorrhagic fever, the authors say.
The study, published yesterday in the journal PLOS Neglected Tropical Diseases, reviewed leptospirosis literature from 1970 to 2008 to generate a statistical model that estimates global leptospirosis cases and deaths. It found that South and South-East Asia were the most affected, with over 200,000 cases and 14,000 deaths a year in each region.
We are seeing only the tip of the iceberg,” says Albert Ko, an epidemiologist at the Yale School of Public Health in the United States and a senior author of the study, because “the model is based on severe cases of leptospirosis”, which account for less than a fifth of all infections.
Furthermore, leptospirosis data is sparse, with many suspected but unconfirmed cases. For example, information on morbidity and mortality rates in Africa was available from only two studies out of 80 reviewed.
This study will serve as an important reference in discussing the global burden of leptospirosis, says Martin Grobusch, who studies leptospirosis in Africa at the University of Amsterdam in the Netherlands.
Ko says policymakers should seek to improve leptospirosis surveillance and diagnosis, and clinicians should generate local data to inform local management of the disease. “Our study presents an investment case to address key challenges that make leptospirosis a neglected disease of neglected populations,” he says.
Leptospirosis is endemic in many tropical countries. It is caused by Leptospira bacteria that can infect many mammals. The bacteria spreads in the urine of infected animals and can survive in water and moist soil for months.
“Humans are accidental hosts of Leptospira when we contact the bacteria through infected animals or the environment,” says Ko. Rodents, cattle, pigs, cats and dogs are the Leptospira hosts most often associated with humans. Hence rural communities and urban slums — all vulnerable populations — bear the heaviest burden of leptospirosis.
Severe leptospirosis leads to acute bleeding in the lungs and can lead to kidney failure, but it is hard to diagnose and control. Patients develop symptoms such as fever, headaches and muscle pain that are easily confused with other infections such as dengue fever, says Paluru Vijayachari who heads the WHO Collaborating Centre for Diagnosis, Reference, Research and Training in Leptospirosis in India. “We need laboratory tests to confirm clinical diagnosis of leptospirosis,” says Vijayachari, but many tropical countries lack the required facilities.
According to the review, scientists expect the leptospirosis burden to worsen as climate change causes more extreme weather events and floods that promote leptospirosis outbreaks, and urban slums swell, causing more frequent contact between humans and rats in particular.