Vibrio vulnificus will proliferate due to the ideal conditions resulting from warmer oceans and high rainfall
“Chances are that Vibrio vulnificus cases are underreported in India,” says Lekshmi N, a researcher at Translational Health Science and Technology Institute, Faridabad. While pursuing a PhD at Rajiv Gandhi Centre for Biotechnology in Kerala in 2019, Lekshmi and her colleagues heard of a 55-year-old man in Thiruvananthapuram district who had developed a painful swelling in his right leg.
The swelling lead to gangrene, then septicaemia, and the patient died within 24 hours. Curious about the case, the researchers managed to get his blood sample from the hospital where he was treated and identified Vibrio vulnificus as the microorganism responsible for the death.
People can get V vulnificus by eating infected raw shellfish (which results in diarrhoea, vomiting, fever) or by exposing wounds to waters where the bacteria live (which can cause life-threatening flesh-eating disease that kills about 20 per cent of the infected in one or two days).
The Kerala case was only India’s sixth case of V vulnificus since 2007, and seventh till date, as per data collected by Down To Earth from case reports in peer-reviewed journals. Experts, however, believe that the number could increase significantly in coming years.
These pathogens thrive in the tropics or subtropics, where sea or brackish water temperatures reach 20°C or higher. They also prefer waters with low salinity.
As climate change increases the frequency and intensity of cyclones, rain and flooding, coastal communities across the world could face a higher risk of exposure to V vulnificus. High rainfall also reduces salt levels in the sea, which suits the bacteria.
In India, the sea surface temperatures average 28°C, making the waters conducive to growth and proliferation of V vulnificus. The Arabian Sea and the Bay of Bengal are growing warmer by 0.013°C and 0.016°C per year, as per 2021 studies in Nature and Scientific Reports, respectively. The salinity in the Bay of Bengal is one of the lowest in the world since it gets high precipitation and freshwater influx from rivers.
Despite conducive marine conditions and a long coastline, reports of human infections of V vulnificus from India are sparse.
"This could be probably due to a lack of awareness among both clinicians and laboratory personnel," experts from the Puducherry-based Jawaharlal Institute of Postgraduate Medical Education and Research write in a 2019 paper in BMJ Case Reports.
Lekshmi and colleagues recommend that doctors should consider testing for this pathogen when patients exhibiting signs of flesh-eating disease live close to a marine environment.
Researchers have also routinely found V vulnificus in seafood and marine environments in India. The incidence of V vulnificus in marine fish ranges from 13 per cent to 16 per cent; in molluscan shellfish, it is 38.5 per cent; and in oysters, it is 43 per cent to 75 per cent, as per a study in Microorganisms in 2020.
The microbes can enter the digestive tract when people eat raw or undercooked seafood. “As Indians mostly consume properly cooked seafood, they are less at risk of contracting the infection through this route,” says Iddya Karunasagar, senior director, Nitte University, Mangaluru.
V vulnificus belongs to the same family as Vibrio cholerae, which causes cholera, a highly contagious diarrhoeal disease. Though Vibrio cholerae is responsible for an estimated 3-5 million cases and 100,000-120,000 deaths every year, the fatality rate of cholera can be 1 per cent if properly treated.
For V vulnificus, the mortality rate is 15-50 per cent despite prompt diagnosis and treatment. “V vulnificus is not a pandemic like cholera, where you have millions of cases, but if the bacteria infect lesions on a limb or thumb, doctors will be forced to perform an amputation. So it is sufficiently concerning,” says Rita Colwell, Distinguished University Professor at University of Maryland, US.
“Treatment becomes difficult when the bacteria enter the bloodstream. And when people are already immunocompromised,” says Karunasagar. The risk is even higher among people with comorbidities such as chronic liver disease, cancer, chronic kidney disease and diabetes.
The first case of V vulnificus in humans was recorded in 1976 in the US. A 2023 study in Nature Scientific Reports documents that between 1988 and 2018, wound infections due to V vulnificus increased eightfold in the eastern US. The country now records at least 100 cases every year.
The species has also been moving northwards at 48 km per year, the study says. In the late 1980s, infections were rare above the country’s Georgia state, but by 2018, they were regularly reported further north in Philadelphia.
By 2041-2060, the bacteria may reach New York. And by 2081-2100, the infections could emerge in every eastern US state under medium-to-high future emissions and warming, where temperatures are likely to rise to 2.8-4.6°C above the preindustrial levels the study states.
Northern Europe has also seen a spike in the number of human infections, which were associated with swimming or bathing in coastal waters. The Baltic Sea has been identified as a high-risk environment since it has low salinity and high warming rates.
Many Vibrio cases, including V vulnificus, were reported in the Baltic Sea region in 2006, 2010 and 2014, which had an especially warm summer, says a 2022 paper in Eurosurveillance.
Most of the Vibrio cases were documented during heatwave years, such as 1994, 1997, 2003, 2006 and 2010, states a 2016 analysis in PNAS. The researchers stress that such events are likely to increase in frequency and intensity with global warming. V vulnificus infections are also reported in South Korea, Taiwan, Japan, and Mexico.
The northward movement of V vulnificus is through marine organisms. “Like humans have Escherichia coli in our gut, fish have Vibrios in their gut. But only a few harm them,” Colwell explains.
The first case of pathogenic V vulnificus in marine organisms was documented in Japanese eel in 1975. The pathogen arrived in Spain through imported eels in 1985.
Since then, it has spread to Europe and Canada, where it has caused outbreaks, and several farms have been shut down. It produces a toxin that is thought to interfere with the eel’s immune system. It also infects other organisms such as derbio (Trachinotus ovatus), tilapia (Oreochromis sp), trout (Oncorhynchus mykiss) and shrimp (Penaeus vannamei).
In 2018, India documented an outbreak of V vulnificus in a tilapia farm in Kerala. Originally from Africa and West Asia, tilapia is one of the most traded food fish globally. “The significance of vulnificus as a pathogen in fish health is not studied enough in Indian waters,” says Sumitra. There is no database on fish farm outbreaks.
Researchers try to predict the abundance of V vulnificus at a given time to manage human health risks. Colwell and her team are developing one such risk-warning tool.
They use satellite-based sensors to measure sea surface temperature and chlorophyll levels from phytoplankton—microscopic organisms that form the base of the marine food chain. High phytoplankton blooms are associated with increased V vulnificus infections and deaths, Colwell explains.
The European Centre for Disease Prevention and Control has developed Vibrio Map Viewer—a tracker for Vibrio species to calculate the infection risk index based on sea surface temperature and sea surface salinity.
In Japan, people eat bivalves, which include oysters and mussels, only in the winter. They avoid eating them in the summer when the bacteria are in high numbers. “This has helped manage the infection to a large extent,” says Karunasagar.
This was first published in the 16-31 August, 2023 print edition of Down To Earth
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