The approach, which uses advanced molecular methods to measure risk for infection, may also be applicable to other water-borne bacterial diseases. The findings will be published on line August 21 in advance of the September issue of the journal Public Library of Science (PLoS) Medicine.
"What we found supported our hypothesis that severe leptospirosis in the Peruvian Amazon is associated with higher concentrations of more virulent forms of the bacteria at sites of exposure and transmission," said Joseph Vinetz, M.D., Associate Professor of Medicine in UCSD's Division of Infectious Diseases..
This approach to risk assessment of environmental surface waters is globally applicable, and can connect the presence of water-borne pathogens to the risk of mild versus severe human disease, according to the researchers. Scientists will be able to determine if a sample of water contains Leptospira, as well as quantify how many of the bacteria are present. More densely contaminated water sources would be more likely to cause human disease than water with a lower concentration of Leptospira.
"This can have direct policy implications for health departments in monitoring the safety of water for bathing, cleaning and swimming – all ways that diseases are spread, not just by drinking contaminated water," Vinetz said, adding that the next step is to intervene and clean up the water sources, and assess the impact of cleanup on the incidence rate of human leptospirosis.
The researchers successfully used a powerful molecular technology called polymerase chain reaction (PCR) to measure and compare levels of Leptospira in environmental surface waters at urban and rural sites in the Peruvian Amazon region of Iquitos. Leptospirosis, caused by bacteria of the genus Leptospira, is the most common disease in the world transmitted from animals to humans.
Annually, tens of millions of human cases of leptospirosis occur worldwide, and fatality rates can range as high as twenty to twenty-five percent in some regions. It occurs in both industrialized and developing countries, but is particularly prevalent in tropical countries where poor people live under highly crowded conditions, or in rural areas where people are exposed to water contaminated by the urine of Leptospira-infected mammals such as livestock or rats. Transmission also appears to coincide with warm weather and the occurrence of flooding, which washes soil contaminated with animal urine into water sources such as wells and streams.
Recent outbreaks worldwide among adventure athletes, military personnel and travelers highlight the risk for acquiring leptospirosis in tropical environments. Noted risk factors include the use of well or stream water, minding livestock, walking barefoot and the presence of rats and cats in the home.
Previous data from the Peruvian Amazon region indicated that the overall incidence of human leptospirosis was similar in urban and rural sites. However, the severe form of the disease had only been observed in urban areas. Vinetz and his research team tested two hypotheses: first, that concentration of more virulent Leptospira would be higher in urban than in rural environmental surface waters; and that the concentration and species of disease-causing Leptospira in the water would be associated with both the risk of acquiring the disease and its severity.
Standard lab culture-based methods of identifying Leptospira in water and soil sources are time-consuming, laborious, and usually unable to identify pathogenic Leptospira at all. To overcome these limitations, the scientists analyzed relatively small quantities of surface water – for instance water from gutters, wells, puddles and streams – using quantitative real-time PCR assays that amplify small amounts of DNA. By measuring DNA, the researchers were able to effectively describe and quantify the amount of pathogenic bacteria present in the water samples.
The scientists then were able to connect the molecular identification of the bacterial pathogens in the water samples to those actually infecting people and causing acute leptospirosis in rural and urban areas. This approach allowed for a precise mechanistic connection between source of infection and human disease, allowing the researchers to measure the environmental risk for bacterial infection – an approach never used before.
"This study is important because we have connected clinical and basic science to provide a quantitative risk assessment for water-borne diseases," said Vinetz. "This method of risk assessment for infection may also be applicable to other water-borne diseases such as those caused by Shigella, Salmonella, Cryptosporidium, and E. coli."
Debra Kain | EurekAlert!
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