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By DENISE GRADY As America faces its largest outbreak yet of illness caused by West Nile
virus — hundreds of cases so far, and as many as thousands more 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 35 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 serious 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 Nile. 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 unfamiliar symptoms. 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 million-a-year
industry. 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 called the bird flu 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 and other foul were slaughtered around the world, but
researchers did not discover the source of the outbreak or determine
what enabled the virus to infect humans. Bird flu continues to make the
rounds, but has been overshadowed by the Swine Flu Pandemic. Now we come up to 2009 and Recently another version of the flu known as
H1N1 or The Swine Flu is rapidly circling the World, It has infected
Millions and killed well over 6,000 people in the last 2 months and we
can expect more of the same in the near future! 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 meat. 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 tests. 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 as Realists I
think it would be a mistake to ignore it."
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