In August 2009, Thomas Shurley accidentally flipped his boat during a fishing trip in Galveston Bay, Texas. He scraped his left knee while righting it and continued fishing. After returning home the next day, he felt sick but no one worried. On day two his leg was too swollen to move, and his friends rushed him to Baytown Methodist Hospital. The doctors told him a type of bacteria coursed through his blood from that wound on his leg, and it needed to be amputated. He agreed, and the doctors took the blackened dead mass that was once his left leg. The surgery came too late, and Shurley died of organ failure five hours later.
Shurley was one of an increasing number of people who die each year from marine-borne bacteria called vibrio. The vibrio bacteria has many different “breeds”—some responsible for diseases like the open-wound infection that killed Shurley and others that cause cholera. Between 1999 and 2003, 27 people were infected with septicemia, the same open-wound infection that killed Shurley, along the coasts of North Carolina. Now, vibrio populations and their subsequent diseases are on the rise, and new research suggests increasing plankton populations in warmer oceans may be the reason.
“We know that the world is warming very rapidly, and it’s affecting the oceans and seas,” says Phillip Reid, senior research fellow at the Sir Alister Hardy Foundation for Ocean Science. Rising sea temperature drives up the plankton population and therefore the presence of vibrio in the North Sea, according to research led by Luigi Vezzulli, a marine biologist at the University of Genoa, where Reid is a researcher. The study was published in the International Society of Microbial Ecology Journal in October.
The plankton and the vibrio bacteria responsible for diseases like cholera and septicemia thrive in warmer waters, and as the tropical tides push northward, they drive potential growth areas for the bacteria farther north as well. That means areas like the North Sea, which are normally too cold for the cholera-type vibrio to grow well, are warming up to temperatures now suitable for the pathogens to develop and multiply.
According to Reid, vibrio bacteria attach themselves to the exoskeleton of the plankton, which the shellfish eat. If not cooked properly, those shellfish retain the bacteria, and the bacteria enter people’s bodies causing nausea, vomiting and can result in hospitalization, even death.
The vibrio bacteria also enter the body through open wounds and sores, an infection called septicemia. When people with cuts swim at beaches with high concentrations of the vibrio or vibrio-carrying plankton, the bacteria enter the wounds, infecting the surrounding skin within one to two days. Left untreated, these infections can create ulcers and lesions on the skin and into the muscle tissue, eventually killing both and, if not treated, the person as well. Most of the North Carolina cases and Shurley had some immunodeficiency, which strengthened the vibrio diseases’ lethal potential.
Vezzulli’s team used data collected from the Continuous Plankton Recorder survey, a system in the United Kingdom holding the longest and most widespread collection of marine organism data in the world. The survey system catalogues data from as far back as 1946 and records information monthly in 22 different ship routes around the world.
The researchers studied plankton collected from the top 7 meters of the sea surface at two source areas—off the Rhine and Humber estuaries of the North Sea—by dragging enormous, fine-mesh nets behind ships. They used 55 samples collected every August from 1961 to 2005 because August is the month of peak vibrio population count in seawater.
“What’s groundbreaking about this research is that we were actually able to extract DNA of vibrio bacteria from the preserved plankton, which has never been done before,” says Reid. The SAHFOS organization that Reid works with runs the Continuing Plankton Recorder survey. Using the vibrio DNA from preserved plankton, the team plotted a timeline of vibrio populations in the two estuaries.
The data showed that the plankton carrying the vibrio bacteria living along the coastal waters increased throughout the North Sea. According to Reid, data from other sources around the world show the same trend as the North Sea data. Reid says, “Sometimes the plankton blooms are so thick, they change the color of the entire sea.”
Not all the different types of vibrio create illness; however, many are responsible for infectious diseases, most notably cholera. Endemic to India and surrounding countries, cholera spread throughout the world, causing widespread illnesses and deaths in areas like New Orleans after Hurricane Katrina and Haiti following the earthquake.
Other reasons outside of warmer sea temperatures could attribute to the rise in cholera and septicemia outbreaks, according to Janelle Thompson, assistant professor in the department of civil and environmental engineering at MIT. For example, as ships from a cholera vibrio-infested area travel to another port, they increase the chances of dumping those bacteria into the water. However, Thompson says, “Regardless of other factors, we know tropical waters are moving north, and now we’re seeing more disease carrying vibrio in northern areas as well.”
Vezulli and the rest of the team plan on conducting further investigation into areas that have already been hit with cholera to see if a direct link exists between the elevated sea temperature and the vibrio bacteria.