18 october 2011

Google Earth maps reveal secrets of typhoid outbreaks

Source: Wellcome Trust

Health

Oxford University scientists have used Google Earth to track the spread of typhoid in Kathmandu, Nepal.

The researchers based in Nepal and Vietnam combined DNA sequencing technology and GPS signalling to map typhoid cases in Google Earth and trace the source of outbreaks.

The Wellcome Trust-funded study is a modern equivalent of John Snow’s work in the mid-nineteenth century to map cases of cholera in Soho, London, which allowed him to trace the source of the outbreak to a contaminated water pump.

‘Until now, it has been extremely difficult to study how organisms such as the typhoid-causing bacteria evolve and spread at a local level,’ explains 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.’

A Google Earth map reveals a cluster of confirmed typhoid infections occurring within a square kilometre.

The researchers found extensive clustering of typhoid infections in particular locations. 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.

Typhoid incidence is likely to be associated with contamination of ground water during the monsoon with faeces. As one typhoid strain appeared to spread downstream from the main focal point, this would put people living in areas with low elevation at higher risk. Elevation and water spout proximity are likely to be interconnected factors, as the water spouts are more common in low lying areas.

The scientists, from the Wellcome Trust Major Overseas Programme in Vietnam and the Oxford University Clinical Research Units in Kathmandu, Nepal, and Ho Chi Minh City, Vietnam, have published their findings in the journal Open Biology.

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

Recent advances in DNA sequencing have allowed scientists to accurately track the spread of some diseases by measuring mutations in the DNA of the disease-causing microbe. However, tracing the spread of typhoid has proved challenging as these mutations are small in number and not detectable by most techniques in use.

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.

Dr Stephen Baker

Tracing outbreaks of typhoid in Kathmandu also carries its own problem: street names are not used in Nepal, so accurately mapping the outbreaks using the addresses of typhoid cases has proved challenging.

Instead, health workers visited patients’ homes and used GPS to capture the exact location. They also took a blood sample from the hospitalised patient to allow analysis of the typhoid strain’s genetic make-up.

The research also shed light on the role of carriers who show no signs of 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 ‘Typhoid Mary’, a cook in New York in the early 20th century who is believed to have spread the disease to dozens of people.

The researchers found that typhoid does not often spread from asymptomatic carriers. Rather, infections are transmitted predominantly through the environment – for example, through the water source.

Dr Baker says: ‘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,’ he adds.

Commenting on the research, Dr Jimmy Whitworth, Head of International Activities at the Wellcome Trust, said: ‘Just as John Snow’s pioneering cholera maps of the 19th century showed that poor sanitation leading to contaminated water was spreading disease, 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.’