New Diseases Emerge As Old Ones Re-Emerge
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ON AN ALTERED PLANET, NEW DISEASES EMERGE AS OLD ONES RE-EMERGE
By Denise Grady
New York Times
August 20, 2002
As America faces its largest outbreak yet of illness caused by West Nile
virus -- 251 cases so far this year, and as many as 1,000 expected --
inevitable questions arise. Why here? Why now? Until 1999, the disease had
never even been detected in North America.
No one knows how the virus came to the United States. But it made itself at
home, and by 2001 had infected 29 species of mosquitoes, 100 species of
birds and many mammals, including humans. It has now reached 36 states and
the District of Columbia.
Researchers say West Nile may be just one example of an infectious disease
whose incidence and geographic range have expanded because of human
activities affecting the mosquitoes, birds, rodents and other animals that
help spread the infection.
Since the mid-1970's -- a time when it was widely assumed that most
infectious diseases had been conquered or at least controlled -- a troubling
array of previously unknown diseases has emerged, including Lyme disease,
AIDS, mad cow disease, the Ebola virus, Legionnaires' disease and a host of
others. In addition, old diseases like yellow fever, malaria and dengue
fever have reappeared in their former haunts and spread to new areas. Some
microbes, like the ones that cause tuberculosis, malaria and food poisoning,
have become dangerously drug resistant.
In a 2000 report, the World Health Organization identified a half-dozen
factors that could affect the distribution and emergence of infectious
diseases. The factors include ecological changes like those from global
warming and changes in land use; human factors like population growth,
migration, war, sexual behavior, intravenous drug use and overcrowding;
international travel and commerce; technological and industrial factors like
food processing, livestock handling and organ transplants; microbial changes
like the development of antibiotic resistance; and breakdowns in public
health measures like sanitation, vaccination and insect control.
In the case of West Nile virus, researchers say global warming caused by
rising levels of greenhouse gases may be contributing to the warm winters
and summer droughts that seem to favor the spread of the virus.
Dr. Paul R. Epstein, associate director of the Center for Health and the
Global Environment at the Harvard Medical School, said an important
consequence of warming was an increase in "extreme weather events" --
droughts punctuated by torrential rains. Drought, he said, helps the
mosquito species Culex pipiens, which plays a major role in spreading West
He added that drought might also wipe out darning needles, dragonflies and
amphibians, which destroy mosquitoes. Drought may also aid the spread of
infection by drawing thirsty birds to the pools and puddles where mosquitoes
breed. "Hot weather plays a role, too," Dr. Epstein said. "Warmth increases
the rate at which pathogens mature inside mosquitoes."
Climate is not the only factor. As wilderness is developed and animals'
specialized habitats are destroyed, opportunistic creatures like rats and
crows often take over. Known as generalists or opportunists, animals that
thrive near developed areas tend to be hardy species that can eat almost
anything and live almost anywhere. If, like crows, they also happen to be
capable of carrying a disease and spreading it through mosquitoes to people,
they become important factors in outbreaks.
Dr. Epstein described a similar sequence of events for Lyme disease, which
is spread to people by ticks that feed on deer and white-footed mice. The
factors that helped Lyme disease emerge, he said, include "the social and
human activities that bring us in touch with fragments of forest, like
sprawl and suburban life, and the fact that there are lots of deer but few
predators of deer."
In addition, he said, "warming has contributed to the northern movement of
ticks, and warm winters allow for overwintering."
Globally, warming is widely thought to be contributing to the spread of
malaria and dengue, each carried by mosquitoes, to high altitudes in Africa
and Central and South America where the diseases had not occurred before.
Slowing emissions of greenhouse gases could help blunt the warming trend in
the long run, but the climate system will take decades to respond. And so,
many scientists say, it is important to use antibiotics more judiciously, to
slow the endless buildup of resistant bacteria. It is also essential, they
say, to monitor weather patterns and populations of insects, birds and
rodents to anticipate outbreaks and try to head them off. Nations, the
scientists add, also have to be on the alert for outbreaks of illnesses with
Among recently recognized diseases, one of the most alarming was a brain
infection, encephalitis, caused by the Nipah virus, which suddenly appeared
at pig farms in Malaysia in 1998 and killed more than 100 people. The
outbreak is described in a book, "Conservation Medicine: Ecological Health
in Practice," to be published this month by Oxford University Press.
To stop the epidemic, Malaysian authorities killed about a million pigs in
1999. The slaughter dealt a severe blow to pig farming in Malaysia, a $400
Initially, researchers had no idea where the virus was coming from. But in
2000, scientists in Malaysia found that fruit bats were carriers, and
suggested that the bats -- perhaps driven out of the rain forest by logging
and forest fires that left them hungry -- were attracted to pig farms, where
they spread the virus to pigs, which then infected people.
The outbreak is still being studied, and researchers fear that the bats,
which migrate, could spread the disease to other countries. Moreover, Nipah
can infect dogs, cats and horses, which may also be able to infect people.
In 1997 in Hong Kong, a strain of flu virus jumped from chickens to people,
the first time such a virus had gone directly from birds to people without
first passing through pigs. More than a million chickens were slaughtered,
but researchers did not discover the source of the outbreak or determine
what enabled the virus to infect humans.
Human activity is believed to have played an essential role in the birth of
the AIDS epidemic. Research has shown that the virus was originally a
chimpanzee virus -- simian immunodeficiency virus, or S.I.V. -- and
scientists think it jumped species into humans who were exposed to infected
blood while hunting and butchering chimpanzees for food or for sale as bush
Now, some researchers say, they fear that a related virus may also make the
jump into people. Such a jump could be disastrous, leading to a contagious
infection in people -- one that could not be detected by current blood
The scientists' concern stems from an enormous expansion in Africa of the
bush-meat trade. It has in part grown because logging roads have opened up
remote regions for hunting and shipping of the animal carcasses. Researchers
have found surprisingly high rates of S.I.V. infections in meat taken from
primates in bush-meat markets in Cameroon. In May, a team from the United
States, Cameroon, France and Belgium reported that members had screened more
than 700 primate carcasses and found S.I.V. infection in 20 percent of them.
More than 30 primate species are now known to carry strains of H.I.V.
Dr. Beatrice Hahn, a professor of medicine at the University of Alabama at
Birmingham and a member of the team, said: "Three things were surprising:
the rate of infection, the diversity of viruses and the amount of bush-meat
hunting that was going on. It shows for the first time that there is no
doubt that humans are routinely exposed to a wide variety of viruses from
this activity. We suspect some may have already jumped. And you do not want
to transfuse the blood of a person who might have gotten such an infection.
You don't have to be freaked out or a doomsday monger, but I think it would
be a mistake to ignore it."
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