Environmental Engineering Reference
In-Depth Information
found that the two were positively correlated (
Wilson et al. 1985, Anderson et al. 1987
).
On the basis of these early studies and the common name “deer tick,” it quickly became
axiomatic that deer are indispensable for tick populations. Later studies by ecologists and
medical entomologists have shown that the situation is much more complicated. Islands
are known to have reduced faunal diversity, and on mainland sites where a number of
other medium-sized to large mammals co-occur with deer, adult ticks commonly parasit-
ize these nondeer hosts. Probably as a result of the effects of nondeer hosts, several studies
of Lyme-disease ecology at mainland sites have found no relationship, either spatially or
through time, between deer abundance and that of ticks (reviewed in
Ostfeld et al. 2006
).
Several studies have even found that, when deer are excluded from areas less than a few
hectares, the number of ticks
increases
, apparently owing to the use of other hosts that ben-
efit from local exclusion of deer (
Perkins et al. 2006
). Studies in which the role of deer,
compared to that of other hosts, is determined remain rare but are much needed for
understanding the complex ecology of ticks.
Even the scientific name
Ixodes dammini
and associated moniker “deer tick” had to be
discarded. Studies in the early and mid-1990s revealed that what had been considered a
“new” species in fact was a northern population of the widespread species
Ixodes scapu-
laris
, or blacklegged tick. The bases for this conclusion were (1)
I. dammini
from
Massachusetts and
I. scapularis
from Georgia interbred readily in the lab and produced fer-
tile offspring through several generations; (2) no discrete differences between the two
groups existed in morphology, chromosomes, or isozymes; and (3) no discrete differences
between the two groups existed in phylogenetically informative nucleotide sequences of
either ribosomal or mitrochondrial loci (
Oliver et al. 1993; Wesson et al. 1993; Norris et al.
1996
). As a result of these studies, it is generally accepted that what was called
I. dammini
actually consists of northern populations of
I. scapularis
, which was described in 1821, and
the correct common name for members of these populations is “blacklegged tick.”
It's the Mice
Even though deer were assumed to be essential for the occurrence of blacklegged tick
populations, another key element is necessary for these ticks to pose a threat to human
health—the ticks must be infected with
B. burgdorferi
. Because these spirochetes are not
passed from mother ticks to eggs, each generation of ticks must acquire
B. burgdorferi
infec-
tions from hosts to be capable of transmitting infection to people. Deer were known to be
highly inefficient at transmitting spirochetes to ticks, so this role must be served by other
hosts. Early ecological studies (e.g.,
Donahue et al. 1987
) demonstrated that larval ticks
acquired
B. burgdorferi
more efficiently from white-footed mice (
Peromyscus leucopus
)than
from any other host, and that this high “reservoir competence” in mice led to high prevalence
of infection in the next generation of nymphs (
Mather et al. 1989
). Again, studies of the sim-
plified ecology of island communities, where few nonmouse hosts for immature ticks
occurred, led investigators to the paradigm that deer are required for the existence of a black-
legged tick population and mice are required for the ticks to be infected with Lyme disease
spirochetes (Spielman et al. 1985). This paradigm has been highly resilient even in the face of
considerable contradictory evidence and strong support for the role of nondeer, nonmouse
hosts in determining numbers and infection prevalence of blacklegged ticks (see later).
