Environmental Engineering Reference
In-Depth Information
become dangerous to us upon molting into nymphs. During the summer of 1992, follow-
ing the 1991 masting event, both populations of mice and larval ticks had exploded as a
result of two distinct responses by wildlife to acorn abundance. We predicted that abun-
dant larvae with access to abundant mice would result in high densities of infected
nymphal ticks in 1993, two summers following the masting event. And again, this is
exactly what we found (
Ostfeld et al. 1996; Ostfeld 1997
). Similarly, our prediction that
acorn failure in 1992 would lead to few mice and few larval ticks in 1993, and conse-
quently to few infected nymphs in 1994, was confirmed.
It's the Community Dynamics
Although mice were the best known of the many species that host immature ticks, and
a well-established Lyme-disease reservoir, continued study at the Cary Institute sites
revealed the existence of other equally important players. Extensive monitoring of acorns,
small mammals, ticks, and spirochetes from the early 1990s to the present demonstrated
that interannual variation in Lyme disease risk is not highly predictable based on the
dynamics of any one host species, but is predictable from knowledge of the dynamics of
the broader community of small mammals. Using maximum likelihood statistics and
model comparison approaches, my colleagues and I found that the abundances of chip-
munks and white-footed mice were equally important (and largely additive) in determin-
ing subsequent abundance of nymphal ticks. The infection prevalence of those nymphal
ticks was influenced most strongly by acorn production two years earlier.
Although abundance and space used by deer affected local abundance of larval ticks the
following summer, deer had essentially no influence on subsequent abundance of nymphs
at our study sites. It appears that the abundance of larval ticks has a much weaker effect on
subsequent nymphal abundance than does the availability of high-quality hosts for larval
ticks (
Ostfeld et al. 2006
). Mice and chipmunks tend to fluctuate synchronously because
both depend on acorn production for overwinter survival and reproduction. So, acorns are
connected to Lyme disease risk through multiple pathways.
Demonstrating that acorns affect risk of human exposure to Lyme disease is only part
of the puzzle. Whether variable risk, as measured by tick abundance and infection preva-
lence, results in parallel changes in numbers of reported cases of Lyme disease has seldom
been addressed. To determine whether variable abundance of rodents or acorns has a
direct public health impact, Eric Schauber, Andrew Evans, and I asked whether acorn den-
sity at the Cary Institute predicted per capita incidence of Lyme disease in human popula-
tions two summers later. Based on the expectation that acorn production is at least
moderately synchronized over large geographic areas, we asked this question of human
populations in Dutchess County, New York, where the Cary Institute is located, and also
in surrounding states. We found that acorn production on two 2.25-ha plots at the Cary
Institute predicts (detrended) incidence of Lyme disease in Dutchess County and the adja-
cent state of Connecticut (
Schauber et al. 2005
). Perhaps, not surprisingly, predictive power
declined with distance from the Cary Institute.
Although mice and chipmunks play important roles as favorable hosts for ticks and
competent reservoirs for the Lyme spirochete, many other mammals and birds also serve
