Climate Change Is Making Europe’s Shellfish Dangerous

Climate change is putting Europeans at an increased risk of contracting Vibrio-related infections, according to an assessment recently released by the European Food Safety Authority (EFSA).

Warming sea temperatures are expecting to increase the prevalence of the bacteria in seafood, especially in low-salinity or brackish waters, with a rise in antimicrobial resistance among the species creating additional cause for concern.

Vibrio bacteria, traditionally a problem in tropical and subtropical regions, can cause a range of illnesses, from mild gastroenteritis and ear infections to life-threatening conditions like sepsis.

Experts in Europe are calling for proper surveillance, funded research, and public awareness campaigns.

What Is Vibrio?

Vibrio species are gram-negative bacteria typically found in warm marine waters. The most clinically significant strains include Vibrio cholerae, infamous for causing cholera; V parahaemolyticus, commonly associated with seafood consumption; and V vulnificus, the most dangerous, often linked to wound infections that can rapidly lead to necrotizing fasciitis or septicemia, especially in immunocompromised individuals.

Human infection with Vibrio usually occurs through the ingestion of contaminated seafood, particularly raw or undercooked shellfish such as oysters, or through open wounds exposed to seawater. Those with liver disease, diabetes, or weakened immune systems are particularly vulnerable to more severe infections, especially from V vulnificus.

“Vibriosis is uncommon but not rare,” explained Niall Conroy, MD, MPH, adjunct professor of public health at University College Cork, Cork, Ireland.

“Non-cholera Vibrio infection isn’t notifiable in most EU countries. But in the US, they usually see around 2000-4000 cases of human infection each year,” he told Medscape Medical News.

Vibrio infections peak during the summer months, when swimming and seafood consumption also peak. “The temperature of seawater is one of the most important factors in determining the concentration of Vibrio in seafood samples. As water temperatures rise, the likelihood of ingesting contaminated seafood increases,” Conroy explained.

Impact of Climate Change

“Europe has seen a rise in Vibrio infections over the past 20 years due to an increase in extreme weather events, like heatwaves. Warmer coastal waters have led to an expansion of areas where Vibrio bacteria can multiply, resulting in a higher risk of infections from the consumption of contaminated seafood,” Winy Messens, PhD, food microbiology expert at EFSA, told Medscape Medical News. “To mitigate risks, it’s crucial that seafood is handled and cooked properly, especially for vulnerable populations.”

The Baltic Sea, North Sea, and coastal areas of the Mediterranean have been identified as high-risk zones for Vibrio proliferation, with cases reported as far north as Scandinavia. The Baltic and North Seas are the fastest warming seas in Europe, and as the Baltic Sea is brackish, it is becoming increasingly suitable for Vibrio growth, the European Environment Agency has noted.

This year saw the hottest summer temperatures on record in Europe. According to the European Centre for Disease Prevention and Control (ECDC), such environmental shifts are likely to make Vibrio infections a recurring seasonal health risk across the continent.

What Do the Data Say?

The growing prevalence of Vibrio infections, especially in northern European countries unaccustomed to managing them, is placing new demands on healthcare systems, with urgent care settings and infectious disease specialists increasingly likely to encounter cases, the European Society of Clinical Microbiology and Infectious Diseases Annual Congress heard earlier this year. Clinicians, thus, need to be vigilant in recognizing symptoms of Vibrio infections, especially in patients presenting with sepsis or necrotizing fasciitis after marine exposure.

However, underreporting of vibriosis because it is non-notifiable in Europe and a lack of precise epidemiologic data, such as date of infection and place of exposure, mean the true scale of Vibrio infections across the region is unclear.

“We know that there is a hidden epidemiology of Vibrio infections in Europe,” said Jaime Martinez-Urtaza, PhD, professor at the Department of Genetics and Microbiology at Universitat Autònoma de Barcelona, Barcelona, Spain, a leading expert and published researcher on Vibrio. “Cases are not reported but occur everywhere when conditions are suitable for the bacteria. We have started different studies in areas where Vibrio infections had not been previously reported, and we identified Vibrio infections when cases are investigated retrospectively.”

Many of the large outbreaks of Vibrio illness have been associated with foreign strains and genetic variants that have been introduced to the area a long time ago. These strains remained hidden in coastal areas until environmental conditions changed enough to cause Vibrio blooms, followed by the detection of infections, Martinez-Urtaza told Medscape Medical News. “This has been a common pattern in Europe and in other regions of the world where large epidemics and outbreaks have been detected recently, mostly associated with conditions driven by climate change: Warming and drop in salinity caused by extreme weather events.”

Many of the countries along the Baltic Sea coast have their own national surveillance systems for vibriosis, including Denmark, Estonia, Finland, Germany, and Sweden. In those areas with a good reporting system, there is a clear trend showing an increase in the number of cases, said Martinez-Urtaza.

Various studies using whatever data are available confirmed that Europe has seen an alarming rise in Vibrio infections in recent years.

According to the recent EFSA analysis published in July, 32 outbreaks caused by Vibrio in seafood were reported in the European Union between 2010 and 2021, causing 221 cases, of which 57 included hospitalization.

A significant spike in Vibrio infections was recorded in Germany in 2018 and 2019 during major heatwaves as water temperatures in the Baltic Sea exceeded 20 °C, creating perfect conditions for the bacteria’s proliferation.

A study published this August in the journal Eurosurveillance found an increase in recorded vibriosis from 1994 to 2021, along with a widespread presence of Vibrio bacteria in Baltic Sea countries that collect data. More than 1500 cases were recorded in the study.

The study authors’ prediction model suggested that Vibrio infections may be underestimated in countries without official monitoring, leaving potential health risks to coastal zone visitors unknown.

However, the ECDC does monitor Vibrio growth in the Baltic Sea during the summer using a near real-time model. If and when the risk for Vibrio growth is determined as medium or above, notifications are reported in the weekly threats reports, a spokesperson told Medscape Medical News.

Antimicrobial Resistance and Risk for New Variants

Another area of concern is an emergence of Vibrio resistance to various antibiotics, including ampicillins, tetracyclines, and carbapenems, over the past two decades. This means it is becoming increasingly important to monitor the antibiotic profiles of clinical and environmental Vibrio spp.

“Data is somewhat limited on the preponderance of resistance genes within Vibrio isolates, but it tends to be fairly consistent,” said Conroy. “We are seeing resistance patterns that stretch across a range of important antibiotics, including those that would have traditionally been used to treat antibiotic-resistant infections. This is obviously concerning. This all goes to show the interdependency between things like rational antibiotic use, being climate conscious, and the safety of our food and water. A One Health approach to threats like vibriosis is needed.”

Martinez-Urtaza echoed these concerns. “Vibrio is very active in terms of biological innovation and evolution, and there is a risk of the emergence of new variants with pandemic potential, particularly with major pathogenic species like V cholerae and V parahaemolyticus.”

What Happens Next 

The rising incidence of Vibrio infections necessitates a greater public health response and increased awareness efforts, the experts told Medscape Medical News.Improved surveillance and alerts on Vibrio bacteria are needed, in addition to compulsory reporting of related infections and funded research. Public health campaigns warning vulnerable populations, such as those with chronic liver disease or diabetes, about the dangers of consuming raw shellfish and exposing wounds to seawater were also suggested.