Correlating cholera outbreaks with environmental factors using remote sensing

Cholera outbreaks are caused by the presence of Vibrio cholerae in drinking water. It needs to be present in high enough levels in order to be at an infection-causing dose in when ingested by humans. Cholera outbreaks linked to drinking water from natural sources such as rivers, may well be heavily dependant on environmental conditions. These may include temperature, water levels, nutrient and plankton production that can be both favourable and unfavourable for the growth and reproduction of V.cholerae (Lobitz et al., 2000).

Cholera levels can’t be directly measured however can be remotely sensed to infer its presence. Lobitz et al., (2000) use satellite data to monitor cholera spread and timing and take specific environmental parameters into account to assess any effect they may have. They used cholera cases from 1980-1995 from the ‘International Centre for Diarrheal Disease Research, B Hospital surveillance program’, giving a sample of all the in- or out-patient treatments each week. Data included the number of tested patients, and the number that tested positive for cholera. The environmental parameters measured by remote sensing included sea surface temperatures (SST) and sea surface height (SSH). These were taken for each time date from one point off the coast of Bangladesh, though it is worth noting that similar patterns were also found for other locations. The SST data was taken from 1989-1995, whereas the SSH data was taken from 1992-1995.

In the years 1992, 1994 and 1995, cholera outbreaks showed a significant association with SST. This is to be expected as warmer oceans stimulate plankton blooms, blooms in this case containing Vibrio cholerae. Lobitz et al., (2000) suggest that these plankton blooms may also be closely linked to other environmental factors such as nutrient levels, upwelling etc. Plankton can serve as a reservoir to V.cholerae. They hypothesise that a delay in cholera cases to increased SST is due to the delay of the phytoplankton to the increase and so a delay in the subsequent zooplankton bloom.

In 1993 there was no association in cholera cases and SST even though SST was normal that year. However, 1993 saw the lowest SSH observed in all of the years. The SSH was very low until October, in which it was higher than expected. This led to a cholera outbreak. This was down to increased extent of tidal intrusion of plankton inland and so increased human contact to the plankton containing the V.cholerae. 1995 also saw higher than normal SSH, this also subsequently led to a cholera outbreak, showing both SST and SSH play a role in cholera outbreaks (Lobitz et al., 2000)

This paper shows a clear link between cholera outbreaks and environmental factors, showing clearly that periods where the SST is increased causes an increase in cholera cases and that periods of low SSH followed by high SSH also cause outbreaks. However I think it would have been useful if the SSH was recorded and correlated with outbreaks over the same period of time as the SST, this would further consolidate evidence that changing SSH causes outbreaks of cholera. The use of remote sensing data in this study provides a potentially important tool in future work into cholera, as mentioned in the study. Using RS data can give estimates of chlorophyll concentrations in phytoplankton. Using this alongside zooplankton measurements may make predictions of cholera outbreaks possible. The importance of this is obvious, being able to predict outbreaks may lead to huge reductions in the number of cases as a result. Looking into this method further will be hugely important.

Reference:

Lobitz, B., Beck, L., Huq, A., Wood, B., Fuchs, G., Farque, A., Colwell, R. . (2000). Climate and infectious disease: Use of remote sensing for detection of Vibrio cholerae by indirect measurement . Proceedings of the National Academy of Sciences. 97 (1), 1438-1443.