Confronting cholera in Peru

The year-long cholera epidemic in Peru refocussed scientific attention worldwide on the causes of the disease and the various strains of cholera bacteria.

Published: Monday 31 August 1992

GLOBAL attention was drawn to the cholera bacteria (Vibrio cholerae) following the epidemic raging in Peru since January 1991, which claimed as many as 4,000 lives and affected another three lakh lives in the country. In all, till early July 1992, 5.3 lakh cases of cholera and 4,700 deaths were reported from 19 countries in South and Central America.

The epidemic, perhaps the worst of the century, is believed to have started when a Chinese freighter, moored in Lima harbour, discharged contaminated bilge water containing a cholera strain found in Asia -- V cholerae O1 biotype El Tor. Paul Epstein, a tropical disease specialist at the Harvard Medical School, said in a letter to The Lancet (Vol. 339 No. 8802) that DNA studies by scientists showed "genetic" identity" between strains found in Peru and in Bangladesh.

The El Tor strain of cholera had never before been spotted in Peru, where the V cholerae non-O1 strain, found in the country's sewage lagoons, is responsible for Peru's seasonal cases of cholera. However, the seasonal cycles of V cholerae O1 in countries where it is endemic coincide with the seasonal cycles of V cholerae non-O1 in Peru.

Scientists have long been puzzled by the seasonal fluctuations of cholera, malaria, yellow fever and other blood-based parasitic infections. The first indications that cholera bacteria may not always be waterborne came in 1976 and thereafter began the search for environmental reservoirs that could explain these fluctuations, particularly in Asia and Africa. Researchers discovered that cholera bacteria "hibernate" under unfavourable conditions and attach themselves to algae and plankton. In 1990, scientists from the Johns Hopkins University and the Maryland Biotechnology Institute found such cholera "reservoirs" off the coast of Bangladesh.

The cholera reservoirs respond to changes in temperature, pH (a measure of acidity and alkalinity), salinity and nutrition raising the fear that global warming and the resulting oceanic changes will encourage the growth of plankton and algae, thereby increasing habitat for the "hibernating" cholera bacteria. As a result, scientists fear that cholera, at present seasonal, may become a round-the-year phenomenon.

While the cholera reservoirs explain the seasonal nature of the disease, there are other reasons as well for the Peruvian epidemic. According to a report in The Lancet (Vol. 340, No. 8810), based on the findings of a team of 16 health professionals who probed the South American epidemic, the high mortality rate in some Peruvian cities "was largely associated with drinking contaminated water from the municipal water system". Many cities in Peru discontinued chlorination of water supplies after the US Environment Protection agency published studies to show chlorine enhances cancer risk and is responsible for around 700 cases of cancer each year in USA.

The Lancet condemned this decision in an editorial, saying the cancer risk figures could be put "in perspective" by the 4,000 deaths in one year in Latin America. The threat of cholera is obviously more dangerous than the threat of cancer.

Use of sewage water for irrigation of vegetables which are eaten raw, such as cabbages, has also been identified as a source of cholera. Even if waste water were to be treated in waste stabilisation ponds, as recommended by the World Health Organisation, V cholerae would still spread because blue-green algae (Anabaena variabilis) and various types of plankton, commonly found in such ponds, would act as a reservoir for the cholera bacteria. These algae and plankton would then carry the cholera germs into rivers and streams. The need therefore is to study the extent to which sewage treatment techniques remove cholera germs.

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