LYME DISEASE
Lyme disease is a tickborne infection caused by the spirochete bacteria, Borrelia burgdorferi. The deer tick, Ixodes scapularis, is the main tick responsible for transmitting the disease to humans. Discovered in 1975 in the Connecticut town of Lyme, this disease has become the most common tickborne disease in the United States. Most cases are reported in the spring, summer, and early fall, correlating with tick activity. The disease not only affects humans but has been reported to affect dogs, horses, and cattle.
Clinical
Findings: Erythema migrans is the characteristic cutaneous rash of Lyme disease.
Erythema migrans typically manifests as a solitary “bull’s-eye” macule at the
site of the tick bite. There is a central red macule surrounded by nonaffected
skin, which is then entirely surrounded by an expanding erythema that blends in
with the normal skin. The rash of erythema migrans is larger than 2 cm in diameter.
The rash manifests soon after the tick has transmitted the bacteria into the
skin. Occasionally, the central portion of the lesion forms a vesicle or bulla.
Solitary skin lesions are the most frequent skin manifestation, but one can
also encounter early disseminated Lyme disease. This results in multiple areas
of skin involvement. The numerous skin lesions are smaller than the original
lesion, lighter in color, and not as fully developed as bull’s-eye lesions.
This early dissemination of B. burgdorferi occurs in one quarter of
infected individuals. Most patients also exhibit constitutional symptoms at the
time of diagnosis, including headache, fever, and malaise.
Erythema
migrans occurs in approximately 75% of those infected with the spirochete.
Individuals who do not exhibit the rash and those who go without treatment are
likely to develop chronic disease, which manifests in many ways. Lyme arthritis
is one of the most frequent manifestations of chronic Lyme disease; it is
typically oligoarticular in presentation. Another of the more frequently seen
manifestations is Bell’s palsy, which is caused by involvement of the central
nervous system. The cardiovascular, nervous, musculoskeletal, and hematological
systems may all be involved in chronic Lyme disease.
Histology:
Skin
biopsies of erythema migrans show a lymphocytic superficial and deep dermal
infiltrate. Numerous plasma cells may be seen in conjunction with eosinophils.
Spirochetes are seen in fewer than half of specimens. The pathological findings
of erythema migrans are used to help confirm the clinical findings. However,
one should not wait for the pathology report to treat a patient with clinical
evidence of Lyme disease. Pathogenesis: B. burgdorferi is
a spirochete that is transmitted to humans via the bite of the deer tick (I.
scapularis). The white-tailed deer and the white-footed mouse are the two
reservoirs for B. burgdorferi. These two animals are typically
unaffected by the bacteria. The larval, nymph, or adult form of the I.
scapularis tick takes a blood meal from one of these reservoirs and
acquires the bacteria. The spirochete causes the tick no harm and can survive
in the gut of the tick for prolonged periods. The tick can then transmit the
bacteria to an incidental host such as a human. Transmission of the bacteria
is increased the longer the tick is attached to the host. It is generally
believed that a tick must be attached for 24 hours to transmit the bacteria.
Treatment:
Treatment
of erythema migrans consists of a 3-week course of doxycycline. The therapy is
highly effective and has an excellent safety profile. Amoxicillin can be used
for patients who cannot take doxycycline and for young children. Central
nervous system involvement requires intravenous therapy with ceftriaxone or
penicillin. Prevention is critically important. Permethrin-based insect
repellants are effective at repelling deer ticks. Clothing impregnated with permethrin can be
purchased for those who spend time outdoors in endemic areas.
After being
in a wooded region, people should check their skin for the presence of ticks
and remove them immediately, because the transmission of the spirochete
requires approximately 24 hours of attachment. This inspection method works for
adult ticks, but the larvae and nymphs are too small to see routinely and
are almost always
overlooked.
