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| Fallow rice paddies in eastern Taiwan |
The pressures of
global trade may heighten disease incidence by dictating changes in land use. A
boom in disease-carrying ticks and chiggers has followed the abandonment of
rice cultivation in Taiwanese paddies, say ecologist Chi-Chien Kuo and
colleagues, demonstrating the potential for global commodities pricing to drive
the spread of infections.
Their work appears
in the September issue of ESA's journal Ecological Applications.
After Taiwan joined
the World Trade Organization in 2001, active cultivation of rice paddies fell
from 80 percent to 55 percent in just three years. The government of Taiwan
subsidized twice-yearly plowing of abandoned fields to reduce the spread of
agricultural pests into adjacent fields still in cultivation. Compliance has
been spotty. Kuo found that, while plowing did not suppress rodent populations,
it did inadvertently reduce the presence of the ticks and chiggers that use
rodents as their primary hosts.
"The
government considers only agricultural pests such as insects and rodents. They
don't think about the disease factors," said Kuo. But land use policy can
have complex and unexpected reverberations in the ecology of the landscape.
Chiggers, the
larval stage of trombiculid mites, spread scrub typhus (Orientia tsutsugamushi),
a bacterium that gets its name from the scrubby, dense vegetation that often
harbors its flesh-loving host. Scrub typhus is a common culprit underlying
visits to Southeast Asian hospitals for flu-like symptoms. Without antibiotics,
the infection is often fatal. Ticks (Ixodidae) transmit bacteria spotted fever
group rickettsiae, causing fever, aches and rash similar to Rocky Mountain
spotted fever. Neither pest prefers to live underwater.
Hualien, Kuo's
study area, is one of the least populous of Taiwan's counties, yet had nearly
the highest incidence of scrub typhus from 1998-2007. The county is a
smattering of small villages surrounded by a patchwork of flooded, plowed, and
abandoned rice paddies. Flooded paddies are poor habitat for ticks and
chiggers, and so cultivation of rice, which locally means carefully managed
flooding of fields to drown agricultural pests, likely suppresses ticks and
chiggers as well. Even the seemingly unkillable ticks die after a few weeks of
submersion, and chiggers are similarly terrestrial. Though studies are few,
limited data indicate that most chiggers die after a month under water.
This study did not
assess flooded paddies due to the difficulty of finding and collecting rodents,
ticks, and chiggers underwater. Instead, Kuo trapped rodents in fallow and
plowed fields and examined their tick and chigger passengers, testing the
arachnids for presence of disease-causing rickettsial bacteria. He found 6
times as many ticks on the rodents living in fallow fields -- and the
proportion of infectious ticks in fallow fields was three times higher,
compounding the risk. Chiggers rode rodents at a rate 3 times higher in fallow
fields than plowed fields.
"This study is
a great example of the kinds of indirect effects that trickle down from human
policies," said Bob Parmenter, an ecologist unaffiliated with the study.
"It tells a nice story about how changes in international trade barriers
can have unforeseen consequences." Parmenter is director of the USDA's Scientific
Services Division at Valles Caldera National Preserve near Los Alamos, New
Mexico, and an expert on the influence of ecology on deadly Hantavirus
outbreaks, like the current episode in Yosemite National Park (California, USA)
that has infected nine visitors and killed three.
The consequences of
economic pressures on land use are also present in the eastern United States,
where the small farms of the eighteen and nineteenth centuries have reverted,
to a large degree, to forest. With the return of deer and wildlands has come a
rise in ticks, and concurrent rise in Lyme disease. Conversely, opening new
land to farming or housing can bring its own disease risks.
Many studies have
investigated influence of global forces on disease, said Kuo. "Most are
focused on how climate change, global travel, or habitat destruction will
affect the emergence of vector-borne and zoonotic disease. We show that
economic organizations can actually affect human health, by influencing the
landscape."
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