Bangalore: Hidden cholera in India may be much more prevalent than reckoned by experts changing the dynamics of the disease, which, despite a decline in infection, has seen an increase in outbreaks in recent years, says a new study in Thursday’s Nature magazine.
Using records of cholera mortality in 26 districts of West Bengal from 1891 to 1940 during the British rule, the statistical models of researchers from the University of Michigan, Ann Arbor, show the ratio of asymptomatic to disease-causing infections may be much higher than thought and that immunity to the disease may wane much more quickly.
“This is in stark contrast to previous estimates, which suggested that more infections resulted in disease, and that immunity waned more slowly,” says the lead researchers Aaron King.
The new discovery, therefore, means that many people may have been spreading cholera without realizing it, he adds.
For many important diseases, including HIV, inapparent or symptomless infections exist, so these findings are “interesting and important”, but not unusual, says G. Balakrish Nair, director of the National Institute of Cholera and Enteric Diseases, or Niced, in Kolkata. “However, the high number inapparent cases suggested by this model is significant; we’ll likely pursue these inferences.”
Cholera is a water-borne diarrhoeal disease caused by a bacterial infection. Six of the seven of the cholera epidemics that have swept across the world since 1817 have originated in the low-lying, densely populated regions north of the Bay of Bengal, where the disease is endemic. This is primarily because most cholera cases here are mild or asymptomatic but the true extent of this infection has been difficult to assess.
According to the mathematical analysis, patterns of cholera prevalence are best explained by assuming that only 0.4% of infections are fatal and that immunity wanes a few weeks or months after exposure to the pathogen. This is in contrast to earlier estimates which suggested that more infections resulted in disease and that the immunity disappeared over a period of three to seven years.
King and his colleagues believe epidemiologists and public health officials have traditionally underestimated inapparent infections as, understandably, most attention has been given to the severe cases. “Our results suggest quite strongly that if we want to forestall epidemics, public health officials must broaden the focus of their attention,” King said in an email.
Nair says it is wrongly perceived that cholera is on its way out. “We do see epidemics and 2007 was particularly bad when India and many other countries saw a huge burden of cholera,” he notes. While there’s been a decline in infections, the number of outbreaks has increased due to breakdown in infrastructure that causes drinking water to mix with the sewage.
King suggests cheap, “relatively ineffective vaccines, if delivered at the right time of the year, might be effective at forestalling cholera epidemics”, even if the vaccine coverage is not total.
But India has no immunization programme for cholera because the existing vaccines, mostly used today by western travellers, are prohibitively expensive and not licensed for use in the developing world. However, Nair is hopeful as Niced has a promising research under way in collaboration with the International Vaccine Institute, Seoul, South Korea.
“I anticipate a cheap, safe and efficacious cholera vaccine from an Indian company in two years.”