A highly contagious strain of cholera mysteriously disappeared at the time. But now researchers believe they have unraveled the mystery.

In the past 200 years, seven cholera pandemics have killed millions of people worldwide. In fact, the seventh is still ongoing today, with major outbreaks in Yemen and Somalia. In 1992, a new, highly contagious strain of the bacterium that causes cholera was discovered in India. Scientists feared an eighth widespread pandemic. But surprisingly, that didn’t happen. Strange, but now, thanks to a new study, explainable.

More about cholera
Cholera is a life-threatening infectious disease caused by the bacterium Vibrio cholerae. The disease is mainly transmitted through contaminated drinking water or food. The main feature of the disease is severe diarrhea and dehydration. Major outbreaks of cholera only occur in places where hygiene and sanitation are poor. And so the disease is mainly confined to the developing world. However, cholera can also form when water and sewage systems are disrupted by war or natural disasters. For example, in the Ukrainian city of Mariupol (which had been under fire for weeks from Russian shelling), the risk of a major cholera outbreak has now increased significantly.

The dominant variant of Vibrio cholerae is called O1 and originated in the sixties. But as mentioned, a new variant appeared in 1992: 0139. This new variant soon became dominant and caused major outbreaks in India and southern Bangladesh.

Fast spread

The rapid spread of 0139 across Asia surprised scientists, who feared an eighth cholera pandemic. But for some reason, that pandemic never happened. In 2015, the 0139 variant had unexpectedly disappeared and the 01 variant once again established itself as the dominant species. How could that happen? Scientists were in the dark. Until now.

Study

In a new study researchers analyzed as many as 330 samples of the 0139 variant, taken between 1992 and 2015. And that lifts a corner of the veil. The research team discovered two important changes in the genome of this cholera variant.

Declaration

Before the 0139 variant appeared, cholera was sensitive to many antibiotics. But 0139 proved resistant, which is probably why it became dominant very quickly. The researchers discovered several genes in the 0139 variant that made it resistant to antibiotics. “When this new variant first appeared, the 0139 variant was therefore antibiotic resistant,” says researcher Ankur Mutreja. “Over time, however, this resistance was lost. At the same time, the 01 variant became resistant to antibiotics.”

In addition, the researchers discovered a change in the types of toxin produced by the cholera bacteria. And these two significant changes in the genome may well be the reason for the unexpected decline of the dreaded 0139 variant.

Check

At present, the cholera bacterium is not routinely monitored. And that has to change, the researchers argue. “There is a real chance that another strain of cholera will emerge and cause major outbreaks,” Mutreja warns. “This could still lead to the eighth cholera pandemic. We should therefore constantly keep a close eye on the existing variants. Only then can we prevent massive outbreaks.”

The World Health Organization (WHO) estimates that 1.3 to 4.0 million people worldwide are diagnosed with cholera each year, resulting in 21,000 to 143,000 deaths. According to Mutreja, it is therefore necessary that the genes underlying antibiotic resistance and toxin production are continuously monitored. “This is the key to staying ahead of the spread of the cholera bacteria,” the researchers said. “In particular, this will help to make changes to vaccines and to take appropriate measures to avert future cholera outbreaks in a timely manner.”