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Ehrlichia species are small, obligate intracellular bacteria, similar to rickettsia. Organisms are transmitted to humans through tick bites, most commonly Amblyomma americanum (Lone Star tick) or Ixodes, which is also associated with Lyme disease. Ehrlichia chaffeensis is the causative agent of human monocytic ehrlichiosis (HME), while Anaplasma phagocytophilum is responsible for human granulocytic ehrlichiosis (HGE). Incubation period between tick bite and disease is generally 7 to 10 days.
HGE frequently presents with fever, myalgia, and malaise, with abdominal pain, nausea, vomiting, diarrhea and arthralgia in less than half of patients, and rash in less than 10%. Especially during the first week of illness, thrombocytopenia, leukopenia, and elevation of hepatic transaminases are common. CSF pleocytosis and meningo-encephalitis are rare in HGE. Overall mortality rate is 0.5-1%.
HME infections are caused by E. chaffeensis. The most frequent presenting symptoms are fever, malaise, and headache however, secondary symptoms of anorexia, nausea, vomiting, diarrhea, and abdominal pain are more frequent than in HGE. The illness closely resembles Rocky Mountain spotted fever, except that rash is present in only 36% of cases. Serious complications include hypotension, respiratory failure, meningoencephalitis, acute renal failure, & coagulopathy. Laboratory findings include leukopenia, thrombocytopenia, and elevated hepatic transaminases. CSF often shows elevated protein and pleocytosis, usually lymphocytes. The mortality rate of HME is 2-7%.
Recommended testing for acute HME/HGE includes PCR and serology. HME/HGE serology includes IgG and IgM antibody for both organisms. Diagnostic titers usually appear by the third week after symptom onset. Cross-reactivity between HME & HGE antibodies is common. Since both IgM and IgG antibodies may be negative in early infection, PCR for HME/HGE is recommended for suspected acute disease. Specimen requirement is one red top tube for serology and one lavender top tube, refrigerated, for PCR. Ticks should not be submitted for identification or testing. Likewise, the examination of peripheral blood smears for ehrlichia morulae is very low yield (≤20%) and is unreliable as a diagnostic test.
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