Around the World, Warmer Temperatures Mean More Infections

April 25, 2006 — -- At first glance, an outbreak of diarrhea among passengers on board a cruise ship in Alaskan waters in the summer of 2004 seemed to be relatively harmless.

Health officials theorized it might be the Norwalk virus, a bug that often affects people living in close quarters, such as in nursing homes, hospitals and cruise ships. While certainly annoying, Norwalk usually doesn't cause serious illness.

But then the lab reports started trickling in, and it showed that indeed a more serious problem was at hand -- many of the afflicted the passengers had eaten raw oysters raised in Alaska that were infected with a type of cholera-like bacteria, Vibrio parahaemolyticus, that normally grows on shellfish harvested in much warmer waters.

The finding not only signaled a dangerous new risk to the Alaskan seafood industry, it also highlighted how surprisingly and directly global warming can affect human health, particularly in terms of infectious diseases, experts say.

"Depending on the warming trend that unfolds in the years ahead, we have to accept that habitats will change ... new bugs can be expected to settle in. Every organism will find a niche," said epidemiology professor Colin Soskolne, of the University of Alberta in Canada. "With the tampering of the environment, we really can't predict with much certainty exactly what those changes will be."

Not Safe for Consumption

Global warming is caused by an increase in carbon dioxide and other industrial and auto emissions, which trap heat in the atmosphere and increase air and water temperatures.

While he has personally noticed Alaska's shrinking glaciers and ice floes, global warming wasn't on the mind of Dr. Joseph McLaughlin as he investigated the cruise ship disease outbreak.

He simply wanted to know why the oysters were suddenly at risk -- before this outbreak, no seafood in Alaska had ever tested positive for Vibrio because the ocean water was simply too cold for the bacteria to multiply.

But that was no longer true: An analysis showed that Alaskan water was no longer as chilly as it once was, giving Vibrio a new home up north.

"There's a sort of threshold level, above which concentrations of Vibrio in oysters become (accumulated) enough to cause illness in humans," said McLaughlin, a medical epidemiologist with the Alaska Department of Health and Social Services in Anchorage. "That temperature is about 15 degrees Celsius. What we found was that 2004 was the first summer on record during which the temperature exceeded that."

Or as McLaughlin said in a report published in the New England Journal of Medicine last October: "Rising temperatures of ocean waters seem to have contributed to one of the largest known outbreaks of V. parahaemolyticus in the United States."

The warmer water is unlikely to go away. Buoys placed by the National Oceanic and Atmospheric Association in 1976 have detected a steady annual increase of .4 degrees in the Gulf of Alaska, he said.

"We're not talking about a little bay in Prince William Sound," McLaughlin said.

Thankfully, the state health department got the word out about the outbreak and advised oyster farm owners to keep oysters deep enough where they would not be exposed to water any warmer than about 15 degrees C.

It is just one of the many ways that Alaskans have had to adapt to documented changes in the climate and environment, McLaughlin said. But it serves as a strong warning signal.

"We thought this was probably the best example of the potential of global warming impacting human health, as far as available evidence goes," McLaughlin said.

Mosquitoes and Ticks Spreading

Across the world, another microscopic bug, malaria, has caused havoc in the lowlands of Africa. It is transmitted by mosquitoes, which thrive in hot, damp areas near stagnant bodies of water.

Until recently, the mile-high city of Nairobi, Kenya, was relatively free of the disease.

But now Nairobi, despite its elevation and climate, is in the midst of a malaria outbreak.

And like the Alaskan bacterial outbreak, warmer temperatures are to blame, say scientists with the University of Michigan, the University of Hawaii, the University of Barcelona in Spain and the London School of Hygiene and Tropical Medicine.

In their report, published in last month's Proceedings of the National Academies of Science, they determined that the mosquitoes were thriving in part because it was steadily getting warmer in East Africa's higher altitudes.

This comes as no to surprise to public health researcher Dr. Paul Epstein, the associate director of the Center for Health and the Global Environment at Harvard Medical School and a medical doctor trained in tropical public health.

Along with mosquito-borne diseases like malaria, yellow fever and West Nile virus, tick-borne diseases also are an increasing problem, he said.

When winters are milder, more deer ticks reproduce year-round, transmitting infections such as Lyme disease and Rocky Mountain Spotted Fever.

According to the Centers for Disease Control and Prevention, Lyme disease cases increased from 11,700 cases in 1995 to 21,304 cases in 2005. This year could be a bad one for diseases spread by warm weather-loving insects -- 2005 was the warmest year in the last 100 years, said NASA scientists.

This not only causes more biting bugs to flourish, it also indirectly impacts humans' health in other ways. For example, when trees are weakened by drought, such as what's happening in pine forests from Alaska to Arizona, beetles eat the bark, weakening the trees even more and making them prone to wildfires, Epstein said.

And those wildfires not only release more harmful carbon dioxide and soot into the atmosphere, they trigger another major health effect of global warming: asthma.

Ragweed Loves Carbon Dioxide

For years, doctors have noted an increase in asthma and allergies.

While no one can link the increase directly to global warming, Epstein has strong evidence: When ragweed -- one of the most allergenic plants on the planet -- is grown in a carbon dioxide-rich environment, it grows 10 percent faster than normal, but produces 60 percent more pollen.

While that's good for the plants, it's bad for people. Most people are allergic to something, and often it is plant pollen. In children especially, being exposed to an allergen can trigger a dangerous asthmatic response, or inflammation of the lungs.

"As we see the seasons change and warmer weather has an earlier arrival in the spring, we're beginning to see shifts in asthma and allergies," Epstein said.

What's worse, he said, is that the tiny dust particles emitted from diesel fuel attach to plant pollens, and these diesel particles further irritate the lining of the lung.

Though there haven't been published studies showing that the rise in asthma cases is tied to global warming, Epstein said it is.

"We're seeing it and we're all experiencing it," he said.