Cholera

It Came From The Deep

CAROL EZZELL / Scientific American 19jun99

Scientists warn of outbreaks stemming from the ocean abyss

In December 1992 thousands of people on the southern coast of Bangladesh began vomiting and experiencing profuse, watery diarrhea that caused their tissues to lose so much fluid that their eyes appeared to sink within their sockets and the skin on their fingertips began to pucker. Within days, many had died of severe dehydration.

The scourge was not new: it was cholera, a waterborne infectious disease that had reached epidemic proportions there many times before. But scientists have noted that the outbreak was accompanied by an upwelling that brought deep-sea water to the surface near the Bangladeshi coast. They are now wondering whether it is a harbinger of what both the developed and the developing world can expect if humans continue to pump sewage--treated or not--into the oceans of the planet.

Increasingly, scientists are finding evidence of pathogenic microbes, many usually found only in human feces, at startling oceanic depths. "The deep ocean acts as a kind of refrigerator," says Rita R. Colwell, the director of the National Science Foundation. "It has been assumed that a human is not going to be exposed to these microbes if they're several thousands of meters below the surface, but currents carry waters such that they may appear on another shore," she says. "So it is always a possibility that these microbes--which are essentially lying there dormant--could at some point provide the entrèe to an epidemic."

Marine scientist D. Jay Grimes of the University of Southern Mississippi says that a variety of viruses that infect the human gastrointestinal tract--including poliovirus and rotavirus--have been identified in ocean water samples taken below 1,000 meters (3,300 feet). And the microbes can last: in the late 1980s Sagar M. Goyal of the University of Minnesota isolated gut bacteria from samples obtained at sewage sludge-dumping sites more than 170 kilometers offshore from New York City--30 months after the sites had been closed to further dumping. The bacteria were resistant to several antibiotics, a clear sign that they originated from humans taking the drugs.

According to Grimes, researchers are just beginning to realize the implications of disease-causing microbes in the deep ocean. "But the studies proving a link between an upwelling and a human outbreak haven't been done yet," he says.

Public health expert Paul R. Epstein of Harvard University suggests a different scenario, based on prior studies by Colwell, for the 1992 outbreak in Bangladesh. An upwelling could have brought nitrogen and phosphorus, nutrients abundant in deep-sea waters, closer to the surface, where they could have prompted a plankton "bloom." That would have caused populations of small, plankton-eating sea creatures called copepods to flourish. And cholera bacteria thrive in the guts of copepods, so their numbers would in turn have increased. "It's certainly possible that cholera bacteria from the deep could get washed up and cause disease, but that would be hard to prove," he states.

Benjamin H. Sherman of the University of New Hampshire agrees that the presence of pathogenic microbes at great depths is a general warning sign of the degree to which humans can affect the earth's ecosystems. "The prospect that we have decades-old or hundreds-of-years-old pathogens in the deep blue is interesting," he comments. But he points out that a more immediate problem is sewage released in coastal waters.

Nevertheless, Epstein says he is "very much" concerned about the presence of pathogens in the deep ocean, especially considering the proliferation of projects such as one in Boston, where a 17-kilometer-long pipeline is being built to take sewage from the city out to sea. Although that sewage will be treated, he cautions that some microbes are insensitive to chlorine

"We don't know the consequences" of adding sewage to the sea, Epstein warns. "We're just beginning to look at how climate change can affect ocean circulation and bring these bugs back to haunt us."

Carol Ezzell is a staff editor and writer.