Scientists in Calcutta have detected cholera bacteria genes in people’s stool samples that were negative for the microbe, evidence for silent or sub-clinical cholera in humans that they say could spread the infection into the environment.

Researchers at the National Institute of Cholera and Enteric Diseases (NICED) have said their findings suggest humans may serve as unwitting reservoirs of cholera, an infection believed to be confined to brackish and marine water.

They found cholera genes in three of 22 stool samples from patients at Calcutta’s Infectious Disease Hospital who had diarrhoea. The samples, however, were negative for cholera bacteria, implying that the patients carried bacterial counts too low to be detected through standard diagnostic tests.

“Such people who carry small numbers of cholera germs in their gut will not develop symptoms but could unknowingly become a source of cholera by shedding bacteria into the environment,” said Asish Mukhopadhyay, a NICED scientist involved in the study.

Earlier studies from Calcutta, Dhaka and the Philippines in the late-1960s had documented human carriers of cholera. Scientists had found cholera germs in stool samples of healthy people who had no symptoms but were shedding enough cholera to be detected.

“But cholera carriers were considered rare as most people developed symptoms,” Mukhopadhyay said. “Perhaps one in a thousand stool samples would be from a symptom-free person. In the NICED study, three (13 per cent) of the 22 stool samples contained cholera genes, a significant fraction of the sample."

When people have a few numbers of cholera bacteria in their intestines, they do not develop diarrhoea. But, the scientists said, the bacteria that such human carriers shed in their stools could slip into the local water networks, replicate and give rise to fresh infections.

“The new findings suggest that asymptomatic carriers of cholera could be more numerous than assumed until now,” said Thandavarayan Ramamurthy, a senior microbiologist who specialises in gastrointestinal infections but was not associated with the NICED study.

“We know asymptomatic carriers can spread many other infections such as typhoid, TB or Covid-19,” Ramamurthy said. “It is difficult to identify asymptomatic carriers because they don’t need to come for diagnosis or treatment.”

Bengal’s Gangetic plains and the Bay of Bengal have long been recognised as reservoirs of cholera bacteria. Cholera patients emerge throughout the year in and around Calcutta, although patients’ numbers are low during December and January. The standard treatment for cholera involves antibiotics.

The NICED scientists and collaborators in Japan conducted a so-called metagenomics study — an analysis of all genetic segments from all organisms in samples — of stool samples from patients with diarrhoea. Their study, published in the journal Scientific Reports, has indicated that people with diarrhoea may be simultaneously infected with multiple diarrhoea-causing organisms.

The 22 patients whose stool samples were negative for cholera had developed diarrhoea from other microbes — other bacteria, viruses or parasites.