Typhoid Spread Mapped Using Google Earth

Scientists working in Kathmandu (Nepal) have combined sophisticated gene sequencing technology and global-positioning system (GPS) case localization to map the spread of typhoid and track its source.

Recent developments in DNA sequencing have allowed scientists to precisely trace the spread of some diseases by measuring mutations in the pathogen’s DNA when the DNA replicates. However, tracing the spread of typhoid has proved challenging as these mutations are small in number and not detectable by most techniques in use.

Typhoid fever is caused by two bacteria--Salmonella enterica serovars typhi and paratyphi. Both of these bacteria are found in Kathmandu and they typically spread through water or food contaminated with feces. Symptoms of the disease include fever, abdominal pain, and vomiting.

Tracing outbreaks of typhoid in Kathmandu also carries its own difficulty: street names are not used in Nepal, so capturing the addresses of typhoid cases, and therefore accurately mapping the outbreaks, has shown to be difficult for healthcare workers. In research published October 17, 2011, in the journal Open Biology, scientists from the Wellcome Trust (London, UK) Major Overseas Program in Vietnam and the Oxford University (UK) Clinical Research Units in Kathmandu and Ho Chi Minh City (Vietnam) have found a way to accurately map typhoid outbreaks in the city. Their study integrated DNA sequencing technology and GPS signaling, and mapped the data onto Google Earth.

“Until now, it has been extremely difficult to study how organisms such as the typhoid-causing bacteria evolve and spread at a local level,” explained Dr. Stephen Baker from the Oxford University Clinical Research Unit in Vietnam. “Without this information, our ability to understand the transmission of these diseases has been significantly hampered. Now, advances in technology have allowed us for the first time to create accurate geographical and genetic maps of the spread of typhoid and trace it back to its sources.”

To gather the data, healthcare workers would visit a patient’s home and use GPS to capture the exact location. They would also take a blood sample from the hospitalized patient to isolate the organism and to allow analysis of the typhoid strain's genotype.

The scientists discovered widespread clustering of typhoid infections in specific locations. However, conceivably counter-intuitively for a disease that spreads among humans, this clustering was unrelated to the density of the local population. In fact, the study revealed that people living near to water spouts, for whom these provide their chief source of water, and people living at a lower elevation are at considerably greatest risk of contracting the disease.

Typhoid incidence is likely to be associated with fecal contamination of ground water during the monsoon. As S. enterica paratyphi A (a strain of the serovar paratyphi found in Nepal) appeared to spread downstream from the key focal point, this would put people living in areas with low elevation at higher risk. These two variables, elevation and water spout proximity, are probably interconnected, as the water spouts are more common in low lying areas.

The research has also provided clues into the role of asymptomatic carriers of the disease in the spread of typhoid. Because these carriers do not show symptoms, they are probably 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.
If the disease was spreading within a family caused by direct transmission--either from an asymptomatic carrier or someone with symptomatic, acute disease, the researchers should be able to isolate the same genotype from a number of individuals in the same household. In fact, the variation of genotypes was comparatively random, suggesting that the disease seldom spreads from asymptomatic carriers. Instead, infections are transmitted predominantly through the environment--for example, through the water source.

Dr. Baker added, “Improvements in infrastructure are fundamental to the control and elimination of typhoid. Poor water quality, sanitary conditions and the presence of carriers mean that the organisms will persist in the community long after the limited window of immunity given by the current vaccine. Without integrating improvements in infrastructure alongside other control measures such as diagnosis, treatment, and vaccination, it is unlikely that typhoid can be adequately controlled in places like Kathmandu in the long-term.”

Fighting infectious diseases is one the strategic priorities of the Wellcome Trust. Much of this work is conducted at a local level in regions where disease is endemic. This includes several major overseas programs, including the Wellcome Trust Major Overseas Program in Vietnam.

Commenting on the research, Dr. Jimmy Whitworth, head of International Activities at the Wellcome Trust, said, “This study, which combines accurate mapping with the latest in genotyping technology, further reinforces the importance of improving the quality of water supplies and infrastructure for sanitation if we are to seriously tackle diseases such as typhoid.”

Related Links:
Wellcome Trust
Oxford University