Cholera Hitches A Ride On The Backs Of Soft-Shell Turtles
You can catch cholera from drinking contaminated water.
You can catch it from raw or undercooked shellfish.
And you can catch it from soft-shell turtles.
That's the finding of a study published this month by scientists at the Chinese Center for Disease Control and Prevention. And it's a particular concern in China and many other countries in East Asia, where turtle meat is often used in stews and soups.
The researchers found that the bacterium that causes cholera, Vibrio cholerae, can colonize many of the outer surfaces of a soft-shell turtle, including its shell, legs, neck and calipash — a gelatinous material just underneath the shell and highly prized as a delicacy. The bacteria can also live in turtles' intestines. The study was published in the scientific journal, Applied and Environmental Microbiology.
Although China has relatively few cholera cases compared to other countries, several small outbreaks of cholera are linked to soft-shell turtles every year, often at rural banquets — a troubling sign, considering that turtle consumption in the country has grown to somewhere between 220 million and 330 million pounds per year.
"We found that soft-shell turtles really can carry Vibrio cholerae and cause cholera outbreaks," said Meiying Yan, one of the study's authors. "The surface of the turtle was the most important source of Vibrio cholerae O139." O139 is a strain of cholera circulating in Asia that was discovered in 1992.
Soft-shell turtles live in ponds, slow-moving rivers or brackish estuaries, and their preferred homes overlap with rivers, estuaries and coastal waters where cholera can be found. Cholera can also live on the bodies of many aquatic animals, like shrimp and other shellfish. Cholera is commonly spread by contaminated water but can also be contracted by eating contaminated food.
In outbreaks where soft-shell turtles have been suspected of transmitting cholera, it's usually because the meat wasn't cooked thoroughly, was improperly stored at room temperature too long or because kitchen utensils used with raw turtle meat were also used with other food without being cleaned.
Prior to this study, evidence of cholera infections linked to soft-shell turtles has been limited. Even though there have been clusters of cholera among attendees of banquets where turtle meat was served for many years, few of those cases have thorough enough laboratory work-ups to pinpoint the source of infection. Chinese authorities have sometimes found genetic evidence that the same strain of cholera bacteria were found on turtles and in infected people. But this study makes it clear that soft-shell turtles can carry strains of cholera that cause human disease.
China has only had 200 cases per year in the past decade and only reported 13 cases in 2015. But other countries have seen devastating outbreaks. According to the World Health Organization, anywhere from 1.3 million to 4 million people get cholera and the disease causes 21,000 to 143,000 deaths globally. Haiti has an ongoing outbreak that started after the 2010 earthquake and Yemen is fighting an outbreak with more than 200,000 suspected cases.
For the current study, researchers studied how cholera colonizes soft-shell turtles by inserting bioluminescent genes into cholera bacteria, soaking the turtles in a solution of the mutant glowing bacteria and then watching for signs of colonization by cholera bacteria. Within a few days, there were obvious signs. The researchers also injected cholera solution into the turtles' stomachs and found that the bacteria can colonize turtle intestines.
"It proves that as a transmission agent, [the turtle] is another species, along with copepods [tiny aquatic animals], and invertebrates like oysters and shrimp" that can spread cholera, said Rita Colwell, a professor at the University of Maryland at College Park and at the Johns Hopkins Bloomberg School of Public Health. Colwell was among the earliest scientists to describe how cholera survives in aquatic environments and is a former director of the National Science Foundation.
Soft-shell turtles may be uniquely suited as an animal to study cholera. They can survive outside of water, unlike zebrafish or shrimp. Because soft-shell turtles don't appear to be affected by V. cholerae and because the bacteria can live on turtles' skin and shell, it can be easier to use turtles to grow and study cholera than animals that can only host cholera in their internal organs, like mice or rabbits. Turtles are unlikely to completely replace other animals used to study cholera, but they can be used to explore the mechanics of how the bacteria attach to different surfaces.
Rina Shaikh-Lesko is a science journalist who writes about medicine, global health and the life sciences. She can be reached @rinawrites
Copyright 2020 NPR. To see more, visit https://www.npr.org.