Alien invaders hitch a ride on ships

Microbes can travel the world in the ballast of ships

Ships may be spreading human diseases around the world in their ballast tanks, scientists have warned.

Billions of microbes, including the cholera bug, have been detected in the ballast water of ships.

Scientists checked commercial vessels arriving at Chesapeake Bay on the United States' east coast from foreign ports.

Each litre of ballast water contained about a billion bacteria and seven billion virus-like particles, said the team. Cholera-causing bacteria were detected in all of 15 ships examined.

Cholera can kill half the people it infects

Ships have used water for stability since the 19th century, discharging the contents of their ballast tanks at ports of call and during voyages.

In the US alone, more than 79 million tonnes of ballast water is dumped into the sea every year.

Alien invaders

The researchers, led by Gregory Ruiz of the Smithsonian Environmental Research Center, Maryland, USA, predict that microbes from ballast water frequently invade coastal areas.

"Ballast tanks carry a diverse community of organisms, resulting in many biological invasions," the team wrote in the scientific journal Nature.

"Pathogens (harmful microbes), including those affecting humans, are common in coastal waters and can also be transferred in ballast water."

The team warns that despite growing concern about the global spread of infectious diseases, the potential for ships to carry micro-organisms around the world has been "virtually unexplored".

They have called for immediate research into the health implications of the spread of microbes around the world in ballast water.