causes Lyme disease (named after a town in Connecticut). It is a spirochete that can be visualized by darkfield microscopy and by Giemsa's and silver stains.
Transmission and Epidemiology
is transmitted by tick bite, called Ixodes dammini on the East Coast and Midwest, and Ixodes pacificus on the West Coast. The life cycle of the Ixodes tick lasts two years. The main reservoir of the tick consists of small mammals (white-footed mouse, woodrat), with large mammals such as deer being an obligatory host (though not reservoir) in the tick's life cycle. Eggs deposited in spring on deer Þ larvae Þ feed on small mammals during summer Þ nymphs during spring Þ adults in fall Þ mate on deer. The nymphal stage transmits the disease more than any other stage. Nymphs feed primarily in the summer, which accounts for the high incidence of disease at that time. The spirochete enters into the blood by tick saliva. After a latent period, it appears to disseminate to surrounding skin, regional lymph nodes, and to distant organs via the bloodstream.
Lyme disease is the most common vector-borne disease in the United Stages
. Other bacterial diseases transmitted by ticks in the U.S. include Rocky Mountain spotted fever, ehrlichiosis, relapsing fever, and tularemia.
Pathogenesis is associated with spread of spirochete to various organs, especially the heart, joints, and CNS
No exotoxins, enzymes, or other important virulence factors have been identified.
Stage I: Erythema chronicum migrans (ECM), a spreading, nonpruritic, painless, circular red rash with a clear center at the bite site is most common finding. Average diameter is 16 cm, and lasts up to 30 days. Nonspecific "flu-like" symptoms including fever, chills, fatigue, and headache.
Stage II: Weeks to months later. Cardiac and neurologic involvement. Myocarditis or pericarditis, and heart block, occurs. Acute (aseptic) meningitis and cranial neuropathies are prominent. Peripheral neuropathies also occur. ECM lesions recur in 50% of patients. A latent phase lasting weeks to months ensues.
Stage III: Arthritis or large joints (knees) is characteristic. Synovial fluid WBC's range from 500-98,000 cells/mm3. Chronic progressive CNS disease also occurs.
History and physical, culture (this almost always fails), serologic diagnosis done by detecting either IgM antibody or a rising titer of IgG antibody with ELISA. Unfortunately there are cross-reacting antibodies against spirochetes in the normal flora, so this test is no perfect.
Treatment and Prevention
Doxycycline or amoxicillin for early Lyme.
Ceftriaxone or penicillin G for late Lyme.
Prevention includes wearing protective clothing and using insect repellents when out in the woods, and careful skin examination for ticks.