THE mystery of why cholera outbreaks tend to occur in seasonal cycles may have been solved.
In south Asia, outbreaks of the deadly disease tend to peak after the monsoons and spring rains. None of the proposed explanations, such as blooms of plankton that promote bacterial growth, are satisfactory. Now John Mekalanos of Harvard Medical School in Boston and his colleagues in India and Bangladesh have shown that the cycles of infection could be driven by bacteriophages 鈥� viruses that infect bacteria.
The investigators focused on two phages, which respectively infect O1 and O139, the two most dangerous strains of the Vibrio cholerae bacterium that causes cholera. In 221 samples taken over a period of three years from two major rivers and a lake in Dhaka, Bangladesh, they hardly ever found either phage present at the same time as the bacterial strain it infects.
Advertisement
鈥淧eople think contamination causes cholera. But it may be that water coming in washes out cholera-killing viruses鈥�
Medical records show that outbreaks of cholera coincided with times in the year when no phages were present, and that outbreaks fizzled out when blooms of the viruses wiped out the bacteria (Proceedings of the National Academy of Sciences, DOI: 10.1073/pnas.0408992102). The findings suggest that Vibrio strains thrive only when they are not under attack by their viral nemesis, when heavy rains or floods have diluted the phage.
The finding could also explain why cholera outbreaks are common after natural disasters such as tsunamis. 鈥淧eople think it is contamination from infected latrines,鈥� Mekalanos says. 鈥淏ut it is also conceivable that new water coming in washes phages out, so bacteria get more freedom to infect people.鈥�
While in theory phages could be added to water to kill cholera bacteria, growing the quantities needed would be impractical, Mekalanos says. Nor would the phages be useful medically, because treatment would come too late after infection. But it might be possible to predict when epidemics are likely by monitoring the phages. 鈥淚t could give you an early warning system,鈥� he says. 鈥淲hen phages are plentiful, there鈥檚 less of a risk.鈥�
This approach has the advantage that phages are easy to detect, says cholera specialist Bo Drasar of the London School of Hygiene and Tropical Medicine. You simply pour concentrated water samples onto lab colonies of the dangerous bacteria. 鈥淚f the phages are present, the bacteria die.鈥�
