The bacterium Vibrio vulnificus is a rare threat in Spain, but one that is potentially serious for vulnerable people

Bacteria of the genus Vibrio occur naturally in saltwater environments, particularly in temperate climates. Salinity and temperature are factors that affect the growth of this bacterium, so it stands to reason that global warming and the warming of coastal waters will increase the prevalence of this microorganism and the number of infections it causes—as has, in fact, been observed in recent years.

Vibrio vulnificus is acquired through a skin wound that comes into contact with contaminated water, or by eating undercooked, contaminated seafood. Wound infection is the most common route of transmission, and although most cases are mild, there have been reports of very serious complications such as sepsis, myositis, or necrotizing fasciitis, particularly in susceptible individuals. Necrotizing fasciitis is a severe infection that rapidly destroys the skin and soft tissues, which has earned this bacterium the nickname “flesh-eating bacteria.” However, it is important to remember that this term is a journalistic, not a scientific, term and that it describes a rare complication. Even so, it is important to inform the public that the risk exists, to identify vulnerable individuals, to warn that if a wound worsens rapidly upon contact with seawater, it is necessary to consult a doctor, and to raise awareness that climate change has real consequences, such as an increase in these types of infections.

Vibrio vulnificus is a salt-tolerant bacterium that thrives in seawater. It causes sepsis when raw shellfish are ingested (be careful with oysters) and can enter the body through open wounds. When doctors advise against swimming in the ocean (because of Vibrio) or in rivers (because of Leptospira) if you have open wounds, they’re not just saying that for no reason.

What are Vibrio bacteria?

“Vibrio bacteria are naturally found in aquatic ecosystems. This genus of bacteria includes important human pathogens such as V. cholerae and V. vulnificus. Of these two species, V. vulnificus is known as the ‘flesh-eating marine bacterium.’ Furthermore, this species is pathogenic not only to humans but also to various species of aquatic animals, some of which are of interest in aquaculture.

V. vulnificus inhabits brackish waters found in temperate, tropical, and subtropical regions. Its optimal growth temperature ranges from 28 to 37 °C, which is why its geographic distribution is expanding toward higher latitudes as a result of global warming. This aquatic pathogen can be filtered and concentrated by shellfish, such as oysters or cockles, which act as reservoirs.

V. vulnificus infects humans primarily through two routes: contact and ingestion. In the first case, infection usually occurs when an open wound comes into contact with water containing the bacterium. In the second, the bacterium is ingested with raw or undercooked shellfish and infects the intestine.”

What is the actual risk to the general population?

“In healthy people, the infection is usually mild, self-limiting, or may even go unnoticed. However, if the person belongs to a high-risk group, both wound infections and intestinal infections can progress to severe forms.”

What is the greatest risk of infection with the bacterium Vibrio vulnificus, and what are its common consequences or symptoms?

“First, it should be clarified that infections with V. vulnificus are mostly mild and go unnoticed, but they can be severe in susceptible individuals with preexisting conditions that compromise the immune response to infection in some way. These include diabetes; various diseases affecting the liver, such as hepatitis, cirrhosis, or liver cancer; iron metabolism disorders, such as hemochromatosis; and diseases or treatments that affect the normal functioning of the immune system, such as AIDS or immunosuppressive therapies. Furthermore, although it is not a disease in itself, very advanced age also increases the risk of severe infections.

If a person has an open wound, it is advisable to avoid swimming in the ocean, estuaries, or brackish waters, especially during periods of high temperatures and, above all, if they belong to a high-risk group. Under no circumstances should recent surgical wounds, ulcers, deep cuts, injuries that have not healed properly, or even fresh tattoos be exposed to seawater. However, if contact occurs or a person sustains an injury while swimming, the area must be thoroughly disinfected with clean water, soap, and an antiseptic, and the wound should be monitored closely. If you experience severe pain or pain that worsens rapidly, redness, swelling, bloody blisters, a change in skin color, fever, chills, or general malaise, you should go to the emergency room immediately.

Severe forms of wound infection begin with the spread of the bacteria into deeper tissues, accompanied by tissue destruction (necrosis)—both directly, through the action of the bacteria’s enzymes and toxins, and indirectly, due to the local inflammatory response. If the infection is not treated promptly, it may require surgery or even amputation. Occasionally, the bacteria can enter the bloodstream, survive, and cause secondary sepsis.

In cases of infection resulting from the consumption of raw or undercooked shellfish, you should go to the emergency room if you experience fever, chills, vomiting, diarrhea, dizziness, confusion, severe weakness, or any signs of systemic illness. In severe cases, the bacteria can cross the intestinal barrier, enter the bloodstream, and directly cause primary sepsis.

In at-risk patients, the rapid progression of symptoms is a critical warning sign. If symptoms progress within a few hours, it is especially important to go immediately to the emergency room and clearly explain that there has been recent exposure to seawater or brackish water, or consumption of raw or undercooked shellfish. Fortunately, V. vulnificus is a bacterium sensitive to most antibiotics used in clinical practice, so early diagnosis and rapid treatment are essential for recovery.”

What is or has been its presence along the Spanish coast? And in Europe? What environmental conditions favor its presence?

“In Spain, and if we focus on the Mediterranean coast, V. vulnificus is not widely present on all beaches. Its presence depends heavily on local environmental conditions. The Mediterranean offers one favorable condition—high temperatures during the warm months—but it also presents a significant limiting factor: the salinity of the open sea is usually too high for V. vulnificus to multiply optimally.

Therefore, in the Spanish Mediterranean, I would expect to find it primarily in specific coastal environments where seawater mixes with freshwater inflows, creating conditions of lower salinity—such as river mouths, coastal lagoons, marshes, canals, or estuarine areas. In other words, I wouldn’t speak of a uniform presence along the entire coastline, but rather of a possible presence at specific locations where high temperatures, moderate or low salinity, and nutrient availability coincide.

In Europe, the clearest example is the Baltic Sea. There, salinity is much lower than in the Mediterranean, and during hot summers, two factors highly favorable to vibrios combine: high temperature and low salinity. That is why the Baltic Sea is under special surveillance by European authorities during the summer. Favorable conditions can also occur in other European areas with low salinity, such as estuaries or coastal areas with limited water exchange.”

It can cause sepsis and amputations—how common is this, really?

“V. vulnificus can cause sepsis, necrotizing wound infections, amputations, and even death, but it’s very important to put this into context. For the general population, V. vulnificus infections are rare.

The key is not to cause alarm, but to clearly identify at-risk groups, avoid unnecessary exposure, and seek emergency care promptly if compatible symptoms appear.

As for the actual frequency of sepsis and amputations, in Spain we do not have sufficiently detailed official data to estimate it accurately, but my impression is that the frequency must be very low in absolute terms.

As a point of reference, in the United States—where surveillance is more developed and where health alerts are issued every summer in high-risk areas such as the Gulf Coast—approximately 150–200 V. vulnificus infections are reported annually (0.06 cases per 100,000 inhabitants per year). These are diagnosed and reported cases—that is, those that reach the healthcare system; mild infections are likely underreported.

The key point is that, among reported cases, V. vulnificus can be very serious: approximately one in five people infected with this bacterium dies, and some patients may require intensive care, surgery, or even amputation. Therefore, while the overall incidence in the population is very low, when the infection affects a vulnerable person, it can progress rapidly and have very serious consequences.”