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Three species of Ehrlichia in the United States are known to cause disease in humans. Ehrlichia chaffeensis, the cause of human monocytic ehrlichiosis, occurs primarily in southeastern and south-central regions and is primarily transmitted by the lone star tick, Amblyomma americanum (Figure 16-8). Human granulocytic ehrlichiosis is caused by Anaplasma phagocytophila or Anaplasma equi and is transmitted by Ixodes ticks. Ehrlichia ewingii is the most recently recognized human pathogen, with cases reported in immunocompro-mised patients in Missouri, Oklahoma, and Tennessee.

After an incubation period of about 5 to 10 days following the tick bite, initial symptoms generally include fever,

Table 16-16 Features of Common Tick-Borne Diseases in the United States*




Primary Vector(s)

Approx. Distribution




Common Initial Signs and Symptoms





Fatality Rate (%)

Rocky Mountain Spotted fever (Rickettsia rickettsii)

American dog tick (Dermacentor variabilis), Rocky Mountain wood tick (D, andersoni); brown dog tick (Rhipicephalus sanguineus) in Arizona

Widespread in United States, especially in south Atlantic and south-central states

2 to 14

Fever, nausea, vomiting, myalgia, anorexia, headache

Thrombocytopenia, mild hyponatremia, mildly elevated hepatic transaminase levels

Maculopapular rash ; appears about 2 to 4 days after fever onset in 50% to 80% of adults and more than 90% of children; may involve palms and soles

5 to 10

Human monocytic ehrlichiosis

(Ehrlichia chaffeensis)

Lone star tick (Amblyomma americanum)

Southern, south-central, mid-Atlantic, and northern states; isolated areas of New England

5 to 14

Fever, headache, malaise, myalgia

Leukopenia, thrombocytopenia, elevated serum transaminase levels

Rash appears in more than 30% of adults and in about 60% of children

2 to 3

Human granulocytic anaplasmosis

(Anaplasma phagocytophilum)

Black-legged tick (Ixodes scapularis and I, pacificus) in United States

North central and Pacific states; New England

5 to 21

Fever, headache, malaise, nausea, vomiting

Leukopenia, thrombocytopenia, elevated serum transaminase levels



Ehrlichia ewingii infection

Lone star tick

South Atlantic and south-central states; isolated areas of New England

5 to 14

Fever, headache, myalgia, nausea, vomiting

Leukopenia, thrombocytopenia, elevated serum transaminase levels


None documented

Modified from Chapman AS, Bakken JS, Folk SM, et al. Diagnosis and management of tick-borne rickettsial diseases: Rocky Mountain spotted fever, ehrlichioses, and anaplasmo-sis—United States. A practical guide for physicians and other health-care and public health professionals. MMWR 2006;55(RR-4):3.

"Treatment for these diseases is the same: adults should receive 100 mg of doxycycline (Vibramycin) orally or intravenously twice a day, and children who weigh less than 100 lb (45.4 kg) should receive 2.2 mg/kg of doxycycline orally or intravenously twice a day.

Figure 16-9 Target rash of erythema migrans in Lyme disease.


Figure 16-9 Target rash of erythema migrans in Lyme disease.


Figure 16-8 Lone star tick, Amblyommaamericanum.


headache, malaise, and muscle aches. Other signs and symptoms can include nausea, vomiting, diarrhea, cough, joint pains, confusion, and occasionally rash. Laboratory findings indicating ehrlichiosis include leukopenia, thrombocytope-nia, and elevated liver enzymes. Ehrlichiosis can be a severe illness, especially if untreated, and as many as half of all patients require hospitalization. Laboratory confirmation of ehrlichiosis requires serologic, molecular (PCR), or culture-based methods.

Appropriate antibiotic treatment should be initiated immediately when there is a strong suspicion of ehrlichiosis on the basis of clinical and epidemiologic findings. The treatment recommendations are the same as for Rocky Mountain spotted fever. Rifampin has been used successfully in a limited number of pregnant women with documented ehrlichiosis.

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