Advances in DNA sequencing have enabled scientists to track the spread of some diseases by measuring mutations in the pathogen's DNA when the DNA replicates. Now a new research illustrates how DNA analysis in combination with geolocation can be used to map the source and spread of infection.
In the study, a team of scientists at the Wellcome Trust Major Overseas Programme created geographical and genetic maps of typhoid fever cases in Kathmandu, Nepal. The disease is caused by two bacteria, Salmonella typhi and Salmonella paratyphi, that are both found in the capital city. However, Kathmandu's lack of street names and difficulties detecting and measuring mutations in the pathogen's DNA have made it challenging for researchers to study how typhoid-causing bacteria evolve and spread at a local level.
To overcome these challenges, health workers took blood samples from hospitalized patients and then visited their homes and used GPS to capture the exact location. After isolating the organism from the blood samples, scientists analyzed the typhoid strain's genotype and mapped the data onto Google Earth. According to a Wellcome Trust release, researchers found:
...extensive clustering of typhoid infections in particular locations. Yet, perhaps counter-intuitively for a disease that spreads amongst humans, this clustering was unrelated to the density of the local population. In fact, the study showed that people living near to water spouts, for whom these provide their main source of water, and people living at a lower elevation are at substantially greatest risk of contracting the disease.
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The research has also shed light on the role of asymptomatic carriers of the disease in the spread of typhoid. As these carriers do not show symptoms, they are likely to be unaware of their infection and can unwittingly spread the disease. The most famous of such cases was a cook in New York in the early twentieth century, nicknamed 'Typhoid Mary', who is believed to have spread the disease to dozens of people.
Previously: HealthMap provides global surveillance of health outbreaks, New web-based application tracks evolution of pathogens and Following Google Flu Trends, researchers use queries to track MRSA
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