Google Earth typhoid maps traces disease outbreaks
their main source of water, and people living at a lower elevation are at substantially greater risk of contracting the disease. These two variables, elevation and water spout proximity, are likely to be interconnected because the water spouts are more common in low-lying areas.
Typhoid incidence is likely to be associated with fecal contamination of ground water during the monsoon. As S. paratyphi A (a strain of S. paratyphi found in Nepal) appeared to spread downstream from the main focal point, this would put people living in areas with low elevation at higher risk.
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 such case 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 household due to direct transmission (either from an asymptomatic carrier or someone with symptomatic, acute disease) the researchers should have been able to isolate the same genotype from several individuals in the same household. In fact, the variation of genotypes was more or less random, suggesting that the disease infrequently spreads from asymptomatic carriers. Rather, infections are transmitted predominantly through the environment - for example, through the water source.
Dr. Baker adds: “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.”
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 nineteenth 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.”
— Read more in Stephen Baker et al., “Combined high-resolution genotyping and geospatial analysis reveals modes of endemic urban typhoid fever transmission,” Open Biology (October 2011) (doi: 10.1098/rsob.110008)
