Lyme Disease


Lyme disease is caused by the bacterium Borrelia burgdorferi.


  • An infection caused by Borrelia spirochetes, transmitted primarily by ixodid ticks
  • Ixodes scapularis (deer ticks) in the Northeast and Great Lakes areas
  • Ixodes pacificus in the West (black-legged ticks and Western black-legged ticks)


In 2019, a total of 34,945 confirmed and probable cases of Lyme disease where reported to CDC. Approximately 300,000 people may get Lyme disease each year in the United States.

High incidence U.S. states: Wisconsin, West Virginia, Virginia, Vermont, Rhode Island, Pennsylvania, New York, New Jersey, New Hampshire, Minnesota, Massachusetts, Maryland, Maine, District of Columbia, Delaware, Connecticut

Predominant age: most common in children ages 5 to 14 years and in adults aged 55 to 70 years of age

Etiology and Pathophysiology

  • Average incubation period 7 to 10 days after tick bite
  • Most transmissions occur in May to September when nymphal tick activity is highest.
  • If a tick is infected, the chance of transmission increases with time attached: 12% at 48 hours, 79% at 72 hours, and 94% at 96 hours of attachment.
  • Primary animal reservoir is the white-footed mouse.
  • Spirochetes multiply and spread within dermis, resulting in characteristic (erythema multiforme [EM]) rash. Hematogenous dissemination results in involvement of central nervous system (CNS), cardiovascular, or other organ stems.

Human leukocyte antigen haplotype DR4 or DR2 increases susceptibility to prolonged arthritis.

Risk Factors

Lyme endemic area. Ixodid ticks are common on deer; hunters at increased risk

General Prevention

  • Wear appropriate clothing when outdoors in endemic areas during times of high tick activity. Clothing should cover the ankles and pretreat clothes, shoes, and tents with 0.5% permethrin.
  • “Tick checks”: Examine skin after outdoor activities.
  • Remove ticks as soon as possible to limit transmission.
  • To prevent of tick bites: N,N-diethyl-meta-toluamide (DEET), picaridin ethyl-3-(N-n-butyl-N-acetyl) aminopropionate (IR3535), oil of eucalyptus (OLE), p-menthane-3,8-diol (PMD), 2-undecanone
  • Prophylactic treatment with 1 dose of 200 mg of doxycycline within 72 hours of a tick that has been attached for at least 36 hours is indicated in endemic areas. Number needed to treat = 50 to 53

Commonly Associated Conditions

  • Coinfection with other tick borne illness (e.g., babesiosis, ehrlichiosis, anaplasmosis)
  • Comorbid human granulocytic anaplasmosis and/or babesiosis in patients living in endemic regions

There's more to see -- the rest of this topic is available only to subscribers.