Environmental Engineering Reference
In-Depth Information
Meanwhile, on the north shore of Long Island, across Long Island Sound from Lyme,
Connecticut, researchers surveying for the rickettsial agent of Rocky Mountain Spotted
Fever, typically found in Dermacentor ticks, accidentally discovered spirochete bacteria in I.
dammini ticks ( Burgdorfer et al. 1982 ). Suspecting that they might have found the causative
agent of Lyme disease, these researchers determined that Lyme disease patients indeed
were infected with these spirochetes and were producing antibodies ( Burgdorfer et al.
1982; Benach et al. 1983 ).
This newly discovered spirochete was named Borrelia burgdorferi ( Johnson et al. 1984 ).
Biomedical scientists now knew what caused Lyme disease, how the pathogen was trans-
mitted, and where cases were clustered geographically. Spurred by a strong public health
concern, biomedical researchers had discovered (or thought they discovered) a “new” tick
species and a “new” spirochete species, both of which were shockingly widespread and
abundant. But as new epidemiological and entomological information on this emerging
zoonosis arose, it became apparent that tremendous spatial and temporal variation in
Lyme disease existed. If the causes of this variation could be determined, mitigation strate-
gies might be developed to benefit human health.
IT'S THE DEER
The discipline of epidemiology is concerned with identifying risk factors for human dis-
ease. When an arthropod vector is known to be important, or even necessary, for a disease
agent to infect humans, epidemiologists often collaborate with medical entomologists to
study the life cycle, abundance, and distribution of the vector. A flurry of research on I.
dammini ensued in the late 1970s and 1980s to advance an understanding of risk factors for
Lyme disease and lay a foundation for preventative measures. Like other ixodid (hard)
ticks, I. dammini was found to undergo two immature stages (larva and nymph) in addi-
tion to the adult stage. At each stage the tick takes a single blood meal from a vertebrate
host to fuel transition to the next stage or, in the case of adults, for reproduction. Newly
hatched larvae seek a host in midsummer, and following their blood meal, they molt into
nymphs, which then overwinter on the forest floor before host-seeking the following late
spring or early summer. After taking their blood meal, nymphs molt into adults, which
seek a host in mid- to late fall. Adult females engorged with host blood overwinter before
depositing an egg mass in spring, from which the next generation of larvae emerge in
mid-summer. Thus, the tick life cycle lasts for two years and can involve three distinct
vertebrate host species, with considerable time spent in a resting state on the forest floor.
The abundance and distribution of organisms with such complex life cycles are often
assumed to be determined by a complex suite of biotic and abiotic factors, including avail-
ability of multiple hosts. However, even when first principles suggest that multiple factors
are important in determining species abundances and temporal dynamics, such multifac-
torial approaches are rarely a part of initial research strategies ( Lidicker 1978 ).
Medical entomologists working on islands off the coast of Massachusetts were so
impressed with the large numbers of I. dammini ticks found on hunter-killed deer
( Piesman et al. 1979 ) that the species was given the common name “deer tick.” Early stud-
ies of the relationship between deer and tick abundances, again carried out on islands,
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