The 2001 outbreak of foot-and-mouth disease in British livestock forced travelers to walk through chemical disinfectants on their way through Customs. As inconvenient as that sounds, at least it helped stop the disease from spreading to other countries. Some other diseases, however, might be crossing international borders without ever stopping through the airport. In this Science Update, you’ll find out how.
Bugs that don't need wings to fly. I'm Bob Hirshon and this is Science Update.
While most of us need plane tickets and passports for intercontinental air travel, microbes can make the trip on a cloud of dust. Dale Griffin's a microbiologist at the US Geological Survey in St. Petersburg, Florida. He says in the summer, the Caribbean and Southeastern United States get lots of dust blown in from the Sahara Desert. He and his colleagues wanted to see whether this dust carried any hitchhikers.
If we take air samples on St. John in the Virgin Islands on clear days, versus days when there's African dust in the atmosphere, we easily recover about ten times more microorganisms than when there's no dust in the air.
That includes many different types of bacteria, fungi, and possibly viruses. Before, scientists thought that exposure to the sun's ultraviolet light killed the microbes in dust clouds. But Griffin says the clouds may be thick enough to filter out much of the light.
So microbes traveling at lower altitudes in those clouds don't receive the higher doses that the microbes at the higher altitudes do.
What the researchers still need to learn is whether these microbes are actually responsible for diseases seen in plants, animals, or humans. For the American Association for the Advancement of Science, I'm Bob Hirshon.
Making Sense of the Research
It may surprise you that dust can travel this far, but it can. Using some NASA satellites, such as the Total Ozone Mapping Spectrometer, scientists like Griffin can actually follow the path of dust clouds that form over the Sahara and cross the Atlantic Ocean.
How do these dust clouds make it across the sea? It seems that the same winds that push hurricanes across the Atlantic actually push the clouds of dust as well. It takes about 5-7 days for the dust clouds to move from the Sahara to the Caribbean and southeastern United States. But America doesn't get hit from just Africa. It also gets dust from the Asian deserts. Once a cloud rolls off the coast of China, it takes about 9 days for it to move across the Pacific and arrive in the United States.
The dust clouds themselves are actually very large. In fact, they're huge. They extend from the sea surface to as high as 10 kilometers. Griffin and his colleagues believe that the upper portions of the dust clouds serve to filter out UV light, which is lethal to microorganisms. As a result, the microbes at the lower levels are shielded from the light and survive the voyage. Scientists have found as many as 20-40 colonies of bacteria growing in some of these dust clouds. In addition to the bacteria, they also see virus-like particles that could infect plants and animals. According to the National Institutes of Health, airborne dust is a number one cause of respiratory stress worldwide, even without the microorganisms that are present in the dust clouds. So if it turns out that these microbes are able to cause diseases, that's all the more reason to keep an eye on the levels of dust in the air.
Griffin and his colleagues are hoping their research will lead to some type of monitoring or warning system, so when a cloud forms in one part of the world, public health authorities in a region that could eventually be affected could be alerted days ahead of time. This advance warning would enable them to prepare hospitals and other health care agencies for additional admissions. It could also help people with respiratory problems or weaker immune systems reduce their chances of infection by avoiding the outdoors.
Now try and answer these questions:
- From where does the dust that appears in the Caribbean and southeastern United States in the summer blow in?
- Why are scientists studying this dust?
- How is it that so many microbes in the dust can survive the trip across the Atlantic Ocean?
- What is the significance of this research in terms of public health?
- If dust can travel to America on the wind, it can travel from here as well. What kinds of health risks would you expect this country to impose on other parts of the world?
- Can you think of other instances in which an environmental problem in one area can affect the environment in someplace far away?
For more information about the research being conducted by the USGS scientists, visit the Earth Observatory site called When the Dust Settles. You can read about or watch a Scientific American Frontiers episode on this issue called Dust Busting.
The National Institute of Allergy and Infectious Disease also has good information on this topic.