POSTED 1 NOV 2000
Not a new problem
Why are we fretting about emerging diseases now, when disease has been
part of life and death since the beginning?
Biological warfare and bio-terrorism. With smallpox eliminated from the wild, nobody gets vaccinated anymore. That's perfect if you're considering making an attack with the virus. The bacterial disease anthrax is another favorite of biological warfarers, since it's easy to grow and spread.
A disease of the environment?
As emerging disease emerges as a distinct field of study, scientists are
examining the role environment plays in infection. On the planetary scale,
ecological changes are being driven by the ongoing global warming. We don't
have space to explain the entire interaction between warming and infectious
disease (for the gory details, see "Climate Change ..." in the bibliography).
However, here's how global warming could affect those diseases carried by
vectors:
The deer tick (Ixodes scapularis) adult female, adult male, nymph, and larva on a centimeter scale. Courtesy CDC Division of Vector-Borne Infectious Diseases
Adaptable
Smaller-scale environmental changes may also play a role in vector-borne
diseases, since pathogens and vectors may adapt easily to environments altered
by human population, food scarcity, and changes in land use.
"Vectors (insects) adapt rapidly to changed forest habitats," and may prefer living near cities, David Molyneux of the Liverpool School of Tropical Medicine wrote us. "There is strong evidence of historic changing patterns of disease associated with deforestation -- of all categories of forest."
As people travel deeper into forests in South America and Africa to cut wood or hunt, they have brought back an assortment of new diseases, likely including big-ticket bugs like AIDS and Ebola.
But regrowth of forests can also cause problems, Molyneux wrote. "In leishmaniasis [caused by parasites] the attainment of climax vegetation in Sudan resulted in resurgence of disease."
Lyme, juiced
Lyme disease, first recognized in the United States in the 1970s, demonstrates
how forest regrowth can improve habitat for disease vectors. The fever,
malaise, fatigue, headache, and muscle and joint aches of Lyme are caused by a
bacterium that is transmitted by the bite of deer ticks and black-legged
ticks.
Deer ticks live on -- you guessed it -- white-tail deer.
Yo! My ticks carry Lyme disease! Now take down that garden
fence and we'll talk... © Texas Parks & Wildlife
Lyme now accounts for more than 95 percent of reported vector-borne illnesses in the U.S., totaling more than 128,000 cases since 1982.
To understand Lyme, you have to appreciate the degree of ecological change over the past century or so. Harvard's Andrew Spielman says that period in the United States and Europe has seen "an extraordinary amount of environmental change through reforestation" which has affected the distribution of a whole assortment of animals.
After farmers moved west in the 1800s, the East Coast was dominated by second-growth and patchy fields that are perfect white-tail deer habitat. "Deer were virtually absent from the East in 1900," Spielman says. "By 1950, that had changed enormously, and there's been an explosion of the deer herd since then." Lending gravity to the situation, Lyme may be accompanied by two other emerging diseases: human babesiosis and human granulocytic ehrlichiosis. "It's the same vector, the some reservoir, and a person can be infected by all three from the same tick," Spielman says.
Take-home lesson?
The fact that both deforestation and reforestation can cause epidemics points
to the danger of generalizing about the relationship of environmental change
to disease. Molyneux stresses that the natural history of diseases like Lyme
and Ebola, which live in animal hosts, "is dependent on human behavior
patterns, the capacity of the insect vector to adapt [to different hosts], and
on the behavior of disturbed animal populations, to name a few."
Cause
of the aches and pains of Lyme disease, this corkscrew-shaped Borrelia burgdorferi
bacterium emerged after white-tail deer occupied second-growth forest in
the Eastern United States.Courtesy CDC Division
of Vector-Borne Infectious Diseases
One generalization that does hold, however, is that disease surveillance -- the key step of simply knowing the enemy -- is often rotten near the forests where many diseases emerge. "Health services at the forest interface are usually poor even if they exist," says Spielman.
Global warming, on top of population growth, soaring international travel, and the breakdown in public health services makes for a miserable combination. Is there any cause for optimism? Spielman paused, uncharacteristically, then responded. "I can't think of anything that qualifies as good news."
Pathogens often jump from animals to people. How?
