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The international shipping industry could have unintentional cargo; new research says bacteria and viruses that cause human diseases such as cholera are hitchhiking between continents in the ballast water of ships.

Ships pump water into their ballast tanks to add stability during their voyages, but often empty that water in overseas ports when they load up on heavy cargo.

In North America, the millions of tonnes of ballast water dumped by ships every year have been blamed for successive invasions of non-native marine species, including pipe-clogging zebra mussels, predatory goby fish and the phenomenally reproductive fishhook waterflea.

But today's research, published in the journal Nature, suggests the microbial passengers in ballast water are much more numerous, and shouldn't be ignored.

"It's the same problem we encounter with aircraft; more planes and more people travelling means a disease can be carried from one destination to the next with a blink of an eye," Robert Fournier, a professor of oceanography at Dalhousie University, said.

"Now, we have more shipping traffic and we have more ballast water too; . . . the potential for disease transfer is there."

Scientists from the Smithsonian Environmental Research Center in Edgewater, Md., analyzed the ballast water in ships entering the busy port of Chesapeake Bay after an overseas journey.

On average, that ballast water had concentrations of 830 million bacteria, and 7.4 billion virus-like particles per litre.

All the water tested contained vibrio cholerae, the bacterium responsible for human cholera epidemics. More than 90 per cent of the ships were carrying a newly emerging cholera strain, too.

(Peru's first cholera outbreak in 1991 was believed to be the result of ballast water emptied in Lima's harbour by a Chinese freighter.)

According to Gregory Ruiz, a marine ecologist and the lead author of the research, cholera is known to exist in marine life and is of little threat to North American communities which lack two of the necessary conditions for an epidemic: warm temperatures and unsanitary conditions.

But, he said, if we know that different strains of cholera are being transported by ships around the world, the big question is what other pathogens are also on board. (Viruses such as hepatitis and the Norwalk virus can survive in water, too.)

"It probably includes a huge diversity of micro-organisms," Mr. Ruiz said. "And some of them are probably colonizing here based on what we see with larger organisms."

Microscopic plant life transported in ballast water is yet another potential threat to people. Toxins excreted by certain algal blooms get absorbed by shellfish and that can lead to dangerous illnesses, such as paralytic shellfish poisoning, for seafood lovers, Mr. Ruiz said.