HEALTH: Monitoring Environmental Change Could Help Predict Epidemics

By IPS Correspondents Reprint | | Print |

Judith Perera

LONDON, Dec 21 1996 (IPS) - A combination of data from oceanography, ecology, microbiology, epidemiology, medicine, marine biology and satellite imagery would enable scientists to better understand cholera, and ultimately prevent global epidemics.

So argues Rita R. Colwell, president of the American Association for the Advancement of Science in her Presidential Lecture, the text of which is published in this week’s issue of the U.S. magazine Science, published on Dec. 20.

She uses the history of cholera epidemics to illustrate the effect of environmental changes on infectious disease, tracing the history of seven cholera pandemics, which began in 1817 and continue today on six continents.

“Before 1817”, she says, “cholera was most probably a sporadic, summertime illness, perhaps emerging in it violent epidemic form in the early 19th century.”

Colwell, from the University of Maryland Biotechnology Institute, explains how cholera, which is a waterborne disease, is affected by environmental and social changes.

For example, cholera thrives in salty waters, but can grow in water with low salinity if the water temperature is relatively high and there is a high concentration of organic nutrients. It can survive in unfavourable environmental conditions in a dormant state to be revived by heat shock. It attaches itself to certain types of plankton, in particular a zooplankton known as copepods.

“Ocean currents sweeping along coastal areas thereby translocate plankton and their bacterial passengers,” she explains. “The great pandemics followed coastlines of the world’s oceans.”

The second pandemic lasted from 1829 to 1851, the third from 1852 to 1859, the fourth from 1863 to 1879, the fifth from 1881 to 1896 and the sixth from 1899 to 1923. Then there was a long gap.

“From 1926 to 1960 many believed that cholera would not recur in pandemic form because water supplies had been improved world- wide, ” she says.

“But nature prevailed and the seventh pandemic began in 1961 and continues to the present in six continents.” This was caused by a new biotype of cholera which emerged in Indonesia and spread around the world.

Water systems with agricultural and industrial waste provide the nutrients cholera needs to flourish. There is a potential for epidemics and pandemics, “especially in developing countries having poor sanitation, lack of hygiene and crowded living conditions.”

Also, the El Nino climatic event, an unusual warming of the central Pacific region, brings an influx of warmer surface water and nutrients which trigger plankton blooms that carry the disease.

That plankton may then be ingested in a glass of water, in areas where there are no proper water treatment facilities. Colwell notes that between 1991 and 1995 the El Nino lasted for over three years – the longest period since monitoring began in the 1870s.

Colwell suggests that models to predict cholera outbreaks can be developed by using remote sensing to measure plankton blooms and computer processing to integrate ecological and epidemiological data.

“Because phytoplankton blooms can be measured by satellite imagery, and zooplankton blooms quickly follow phytoplankton blooms, conditions associated with a cholera outbreak or epidemic can be measured by satellite.”

Changes in global climate are likely to affect cholera epidemiology. Colwell notes: “Recent interannual changes in the strength and seasonal evolution of the surface level south west monsoon winds have been related to variations in summer phytoplankton blooms of the north-western Arabian Sea and also the Bay of Bengal.”

In the Bay of Bengal synthesis of satellite remote sensing with analysis of hydrographic and meteorological data has been related to cholera case data for Bangladesh. This has “provided evidence that cholera cases occur following a rise in ocean surface temperature.”

Predicting conditions conducive to pandemics of cholera “should allow public health measures to be taken prospectively, rather than retrospectively,” she concludes. For example, the fact that cholera attaches to plankton suggests there may be simple ways of protecting against or minimising the risk of infection.

Cholera frequently coincides with flooding in Bangladesh and during severe flooding conditions deteriorate to the point where building a fire to boil water is just not possible.

However, Colwell suggests that a filter could be made using a net constructed from nylon or material from a sari, which would be readily available. This would remove the plankton from the water along with the attached cholera.

“From all this evidence”, says Colwell, ” it is now possible to utilise remote sensing and computer processing to integrate ecological, epidemiological and remotely sensed spatial data for the purpose of developing predictive models of cholera outbreaks. ”

She notes: “Cholera offers an excellent example of how information concerning environmental factors permit better understanding of disease — not only virulence, but equally important, transmission and epidemiology.”