• The O139 cholera variant was first detected in 1992
  • It sparked fears that it could cause a cholera pandemic
  • However, it "largely declined" by 2015

A newly detected cholera variant became so dominant in the 1990s that it was feared to cause a cholera pandemic, but it eventually disappeared instead. A team of researchers has now found what has caused the once-feared strain to go away.

The O139 and O1 variants of the Vibrio cholerae, the bacterium that causes cholera, are the only ones out of 200 serogroups that are known to cause epidemics and pandemics, the researchers wrote in their paper, published Tuesday in the journal Nature Communications.

The O139 variant was detected in India in 1992 and for a while, it "displaced" the O1 variant as the dominant one, the University of Cambridge noted in a news release. It quickly caused outbreaks in India and southern Bangladesh and continued to affect parts of Asia until the mid-2000s, sparking concerns among experts that it could cause a cholera pandemic.

However, it died out instead of causing a cholera pandemic. By 2015, the O1 became the dominant variant once again, while the feared O139 had "largely declined."

"No study has been able to answer why the potential eighth cholera pandemic (8CP) causing V. cholerae O139 emerged so successfully and then died out," the researchers wrote.

The researchers conducted a genomic study of the O139 variant, covering the period from its emergence in 1992 up until 2015, and found "two key genomic evolutionary changes" that might have led to its decline and "failure to seed the eighth cholera pandemic."

One of them resulted in a change in the toxins it produced, while the other caused it to gradually lose its antimicrobial resistance (AMR).

"We hypothesize that these two changes contributed to the eventual epidemiological decline of O139," the researchers wrote.

The O139's AMR was likely the key to why it was so successful initially, the university noted. And as it gradually lost this resistance, the O1 strain that has been around since the 1960s gained resistance, establishing itself yet again as the dominant strain, said study senior author Dr. Ankur Mutreja, of the University of Cambridge.

Cholera continues to affect people all over the world. The seven cholera pandemics, all caused by the O1 variant, has claimed the lives of "millions of people" in the last 200 years, noted the University of Cambridge. And today, there are still large outbreaks in Yemen and Somalia, while there are concerns about its emergence in the war-torn Ukrainian city of Mariupol. Iraq also reported cases amid a cholera outbreak in June.

"There's a real possibility that another cholera variant may emerge with the potential to cause large outbreaks, which could lead to the eighth cholera pandemic," Mutreja said. "Continuous surveillance of the variants in circulation is our best chance of preventing mass outbreaks."

Vibrio Cholerae/Cholera
Pictured: Transmission electron microsope image of Vibrio choleraethat has been negatively stained. Vibrio choleraeis the bacteria responsible for the gastroinestinal disease cholera. In order to get the disease cholera, the bacteria must be able to colonize in the small intestine and a critical factor necessary for this colonization is the toxin-co-regulated pilus(TCP). 0395 is a wild type strain, showing the normal bundling of toxin-co-regulated pilus(TCP). Wild-type pili are clearly visible as 7 nm fibres that form bundles @ 0.2Ð0.3 µm wide and 3Ð6 µm long. Tom Kirn, Ron Taylor, Louisa Howard-Dartmouth Electron Microscope Facility/Wikimedia Commons (Public Domain)