LYME DISEASE
DESCRIPTION |
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Lyme disease is a vector-borne disease,
which means it is delivered from one host to another. In this case,
a tick bearing the Bb organism literally inserts it into a host's
bloodstream when it bites the host to feed on its blood. It is
important to note that neither Bb nor Lyme disease can be
transmitted from one person to another.
In the United States, Lyme disease accounts for more than 90% of all
reported vector-borne illnesses. It is a significant public health
problem and continues to be diagnosed in increasing numbers. More
than 99,000 cases were reported between 1982 and 1996. When the
numbers for 1996 Lyme disease cases reported were tallied, there
were 16,455 new cases, a record high following a drop in reported
cases from 1994 (13,043 cases) to 1995 (11,700 cases). Controversy
clouds the true incidence of Lyme disease because no test is
definitively diagnostic for the disease, and the broad spectrum of
Lyme disease's symptoms mimic those of so many other diseases.
Originally, public health specialists thought Lyme disease was
limited geographically in the United States to the East Coast. We
now know it occurs in most states, with the highest number of cases
in the eastern third of the country.
The risk for acquiring Lyme disease varies, depending on what stage
in its life cycle a tick has reached. A tick passes through three
stages of development--larva, nymph, and adult--each of which is
dependent on a live host for food. In the United States, Bb is borne
by ticks of several species in the genus Ixodes, which usually feed
on the white-footed mouse and deer (and are often called deer
ticks). In the summer, the larval ticks hatch from eggs laid in the
ground and feed by attaching themselves to small animals and birds.
At this stage they are not a problem for humans. It is the next
stage--the nymph--that causes most cases of Lyme disease. Nymphs are
very active from spring through early summer, at the height of
outdoor activity for most people. Because they are still quite small
(less than 2 mm), they are difficult to spot, giving them ample
opportunity to transmit Bb while feeding. Although far more adult
ticks than nymphs carry Bb, the adult ticks are much larger, more
easily noticed, and more likely to be removed before the 24 hours or
more of continuous feeding needed to transmit Bb. |
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