NSF PR 99-70 - November 19, 1999
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Human Plague Cases Increasing In Southwest
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Male Xenopsylla cheopsis (oriental rat
flea) engorged with blood. This flea is
the primary vector of plague in most large
plague epidemics in Asia, Africa, and
South America. Both male and female fleas
can transmit the infection.
Photo Credit: courtesy of the Centers
for Disease Control and Prevention
Flourescence antibody positivity is seen
as bright, intense green staining around
the bacterial cell.
Photo Credit: courtesy of the Centers
for Disease Control and Prevention
Wayson stain of Yersina pestis, displaying
characteristic "safety pin" appearance
of the bacteria.
Photo Credit: courtesy of the Centers
for Disease Control and Prevention
Note
About Images |
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Human plague, that dreaded disease of centuries past
caused by the bacterium Yersinia pestis, is
on the rise, at least in the southwestern United States.
Biologists at the National Science Foundation (NSF)-funded
LongTerm Ecological Research (LTER) site in Sevilleta,
New Mexico, have found that human plague cases in
New Mexico occurred more frequently after wetter than
average winter-spring time periods (October to May).
In fact, during years much wetter than normal, a 60%
rise in the number of cases of human plague resulted.
The scientists report these results in a paper published
in the November issue of the American Journal of Tropical
Medicine and Hygiene.
Plague occurs episodically in many parts of the world,
explains Robert Parmenter, director of the Sevilleta
LTER field research station, and lead author of the
paper. "Some outbreaks appear to be related to increased
abundance of rodents and other mammals that serve
as hosts for the fleas that then transmit the disease
to humans." Increased winter-spring precipitation,
say Parmenter and his colleague Ken Gage of the Center
for Disease Control office in Fort Collins, Colorado,
enhances small mammal food resources (plants and insects),
leading to an increase in the abundance of plague
hosts. In addition, moister climate conditions may
act to promote flea survival and reproduction, also
enhancing plague transmission. "Our results can be
used by physicians and public health personnel to
identify and predict periods of increased risk of
plague transmission to humans," Parmenter hopes.
Global climate dynamics are proposed by some scientists
as the culprit in recent outbreaks of infectious diseases;
others warn that long-term global warming could increase
the risk of contracting such diseases. "The recent
El Nino events of the 1990s have been associated with
large increases in rodent populations in both North
and South America," explains Scott Collins, LTER program
director at NSF. "These increases in rodent populations
have been shown to correlate with numerous diseases,
including Lyme disease and hantavirus pulmonary syndrome.
Climate studies may provide a forecasting tool for
identifying periods of increased risk of disease transmission."
Plague was introduced into North America via San Francisco,
California, in 1899-1900 by shipboard transport of
plague-infected rats from Asia. These rats quickly
infected native mammal populations, especially ground
squirrels, and plague spread throughout western North
America. Plague is now most commonly found in the
southwestern United States -- in New Mexico, Colorado,
Arizona and California. Major risk factors for humans
include contact with diseased wild mammals or their
infected fleas, and exposure to infected fleas carried
by pets like dogs and cats.
"If climate conditions become favorable for reproduction
and survival of either the wild mammal populations
or their flea populations, then the probability of
human infection via animal-fleahuman contacts will
also increase," write Parmenter and his colleagues.
With known risk of plague transmission following unusually
moist winter-spring seasons, localized public health
warnings and heightened medical surveillance can be
arranged prior to the peak in human cases during the
summer, the scientists suggest. "Local physicians
and health care professionals can determine if their
cities and towns have received higher-than-normal
precipitation, and increase their cognizance for plague
patients," the researchers write.
Editors: Images of the plague bacterium are
available at a resolution of 200 dpi. Please contact
Peter West at: 703-306-1070/pwest@nsf.gov
Broadcasters: B-roll is available on Betacam
SP. Please contact Dena Headlee at: 703-306-1070/dheadlee@nsf.gov
or pager: 1-888-937-5249
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