Breakthrough in the understanding of the spread of cholera

Apr 3 2005

Cholera is an acute intestinal infection caused by the bacterium Vibrio cholera and is spread by contaminated water and food. Scientists from the University of Haifa and Technion, the Israel Institute of Technology in Haifa, Israel, have reported in Environmental Microbiology, an important breakthrough in our understanding of the continental and intercontinental distribution of cholera.

Prof Meir Broza and Dr Yechezkel Kashi have found a biotic association between V.cholerae and the non-biting midges (chironomids), one of the most common invertebrate groups in freshwater; and also the potential of these airborne adult midges to serve as vectors of the disease.

The paper includes evidence and small scale experiments suggesting that adult non biting midges, a highly mobile flying insect group (Chironomidae), are the vectors that may support continental and intercontinental distribution of cholera. Until 1967, researchers believed that humans may be the reservoir of cholera disease in the inter-epidemic period. In 2001, the University of Haifa team, led by Prof Meir Broza suggested that egg masses of non biting midges serve as a natural reservoir of cholera. It was discovered quite 'accidentally'.

Broza explains: "We were looking for a safe way to control nuisance midges in an Israeli drinking water system. One day hundreds of egg masses were brought to the lab and left overnight. The next morning they had apparently vanished. We discovered that V. cholerae serogroup O9 was the microbial agent responsible for digesting the egg masses and making them disappear."

A cooperative study by both Israeli groups showed that the most abundant protein secreted by V. cholerae is a protease used by bacteria to utilize the gelatinous like matrix of the midge egg masses as a nutrition source for their growth.

In the second phase of the research they asked; could these waterborne bacteria, which grow and multiply in egg masses within the water, be found and survive on airborne non-biting midge adults? To test this theory, first they used killed adults that had been caught in a huge swarm of non biting midges driven by a northeast wind onto the shore of Lake Victoria, Kenya. V.cholerae serogroup O2 was found flourishing on selective agar plates a few hours later."

The next step was to isolate viable V. cholerae from adult midges caught by light traps all over Israel, close to bodies of water as well as above sand dunes and date plantations near a desert oasis. In laboratory simulation Dr Yechezkel Kashi confirmed that fluorescent, tagged pathogenic O139 V. cholerae could be transferred by adult midges from one water container to another. In all parameters associated with insects, the pathogenic strains were distinguished from the environmental serogroups. It seemed that air currents, marine transportation and human travel and pilgrimage could be considered as combined and prevailing modes of dissemination of cholera disease.

Kashi, leader of the Technion team, added, "It is surprising that we could have a new basic finding on V. cholera, one of the most studied bacteria in the last century. This finding gives us the first hint of a new way to try and follow the bacteria in nature, a mission we are about to undertake. This will enable us to monitor the insects, combined with meteorological analysis to predict outbreaks, timing and direction."