Borrelia burgdorferi and Lyme Disease (page 3)
(This chapter has 6 pages)
© Kenneth Todar, PhD
Transmission of Lyme Disease
Lyme disease is spread by the bite of ticks of the genus Ixodes
that are infected with Borrelia burgdorferi. Ixodes,
known as the deer tick (or bear tick), normally feeds on the
mouse, the white-tailed deer, and certain other mammals. It is
for transmitting the spirochetes to humans in the northeastern and
United States. On the Pacific Coast, the bacteria are transmitted to
by the western black-legged tick, and in the southeastern
by the related black-legged tick.
Distribution of Ixodes
ticks that transmit Lyme disease in the U.S. CDC.
Ixodes ticks are much smaller than common dog and cattle
In their larval and nymphal stages, they are no bigger than a pinhead.
Adult ticks are slightly larger. The tick nymphs, which are most likely
to feed on a person and are rarely noticed because of their small size
(less than 2 mm), are usually involved in the transmission of the
Ixodes ticks: larva,
nymph, adult male, adult female. CDC.
Spirochete prevalence in adult Ixodes ticks is highly
variable depending on geographic location. It was shown
be present in approximately 35% of ticks in the Baraboo Hills northwest
Wisconsin, while in regions of California, 2% prevalence has been
reported, and in
regions of New York, 50% has been reported.
For Lyme disease to exist in an area, at least three closely
elements must be present in nature: the Lyme disease bacteria, Borrelia
can transmit them, and mammals
(such as mice and deer) to provide food
for the ticks in their various life stages.
The tick life cycle
of three distinctive stages: larvae, nymphs, and adults. A blood meal
required for ticks to molt from the larvae stage to the nymph stage and
from the nymph stage to the adult stage. The tick larvae and nymphs
become infected with Borrelia burgdorferi when they feed on
animals, particularly the white-footed mouse. The bacteria remain in
tick as it changes from larva to nymph or from nymph to adult. Infected
nymphs and adult ticks then bite and transmit the bacteria to
other small rodents, other animals, and humans, all in the course of
normal feeding behavior. Adult ticks preferentially feed on the
deer, which thereby becomes an important reservoir in regions of
The tick life cycle takes two years to complete (see diagram below).
Lyme disease occurs in domestic animals, as well. In dogs, the
usually presents as arthritis. Domestic animals can carry infected
into areas where humans live, but whether pet owners are more likely
others to get Lyme disease is not known.