Selected US Ticks and Tick-borne Pathogens - tabpi

searchers and epidemiologists have the technical means to
find pathogens they couldn’t find before. “Ticks are not changing
their ability to transmit disease,” explains Dryden, “but we
are recognizing more diseases because we are looking for
them.”
Adds Kocan: “Molecular technologies are fabulous. We
can do things that we could never do before. For instance,
you can collect ticks and, using polymerase chain reaction
[PCR], you can test for half a dozen parasites at one time. So
ticks may have carried these pathogens all along, but we
just haven’t recognized them.”
Because of tick spread, increased tick density and recognition
of more diseases from tick vectors, veterinarians
might find it useful to include vector-borne diseases that are
not common in their area as part of the diagnostic differentials
if the animal’s clinical signs and presentation match the
disease definition. Dryden adds: “What I tell practitioners
when I get in front of an audience is very simple: ‘Whatever
you knew about ticks 5 years ago, it is different today and
whatever you know about ticks today will be different 5
years from now.’
“And that is because these ticks continue to move and
expand and increase in importance. Just because you did
not diagnose this disease in your practice yesterday doesn’t
mean you won’t diagnose it tomorrow. The situation is not
static. This is a dramatic, dynamic situation we have today in
the eastern half of the United States.”
Hard ticks begin their life cycle as larvae, which hatch
from eggs and immediately begin seeking hosts, often rodents.
After the larvae successfully feed, they fall off the
host and live in the soil and decaying vegetation where they
then molt into nymphs, which seek their blood meal from a
small vertebrate. If a nymph fails to find a blood meal, it
dies. If it succeeds, it falls off the host and lives in the soil to
molt into an adult. The adult seeks a larger host for its blood
meal, and the preferred host for I. scapularis and A. americanum
is the white-tailed deer, where the adult feeds and mates
over the winter. The adult female tick is capable of laying
several thousand eggs before she dies, according to Kocan.
“The thing that drives the tick populations is deer. If we
didn’t have deer, we wouldn’t have half the tick populations
that we have,” Kocan says.
When the United States was young, white-tailed deer
flourished in this country, but they were over-hunted, explains
Dryden. Through the 1800s, they were a major source
of food in this country — even Army units supplied themselves
by hunting deer. “The estimates indicate there were
only between 200,000 and 300,000 white-tailed deer left in
the early 1900s. Then it became illegal to hunt deer in many
states,” Dryden says, as many state gaming departments
tried to repopulate the species.
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May 2007 | Veterinary Forum 49