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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. Human monocytic ehrlichiosis (HME) and human granulocytic ehrlichiosis (HGE) were first reported in the United States in 1987 and 1994, respectively. Ehrlichia chaffeensis is the causative agent of human monocytic ehrlichiosis (HME), while Anaplasma phagocytophilum is responsible for human granulocytic ehrlichiosis (HGE). Most HGE patients have a history of tick exposure 7 to 11 days before onset of illness. Peak incidence is in June & July, with another peak in November, secondary to the emergence of adult ticks. HGE frequently presents with fever, myalgia, 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. Peripheral blood smears may show bacterial inclusions, known as morulae, in the neutrophils. Up to 16% of infections are complicated by sepsis, myocarditis, or polyneuropathy. CSF pleocytosis and meningoencephalitis are rare in HGE. Overall mortality rate is 0.5-1%. HME infections are also seasonal (April to September) and are caused by E. chaffeensis, primarily in the south-central, southeast, mid-Atlantic states, and California. Incubation period between tick bite and disease is generally 7 to 10 days. 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. The mortality rate of HME is 2-7%. 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. Morulae are only rarely seen in infected monocytes. Recommended testing for acute HME/HGE includes PCR and serology. HME/HGE serology includes IgG antibody for both organisms. Diagnostic titers usually appear by the third week after symptom onset. Cross-reactivity between HME & HGE antibodies is common. Since IgG antibody may be negative in early infection, PCR for HME/HGE is recommended for suspected acute disease. Early treatment of suspected TBRD is strongly advised due to significant morbidity and potential mortality from these infections. Doxycycline is the drug of choice. 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. Pediatric Ehrlichiosis Under-Diagnosed According to a recently published study (Arch Pediatr Adolesc Med 2002; 156:166-170), cases of pediatric ehrlichiosis may be under-diagnosed in the "tick-belt region" which includes Kansas City, Oklahoma City, Nashville, Memphis, Little Rock & Louisville. Blood samples from children between the ages of 1 and 17 were analyzed for antibodies to E. chaffeensis. Altogether, 13% of the 2000 children tested had antibodies that indicated a previous infection. Symptoms of ehrlichiosis include many that are non-specific, such as fever, headache, myalgias, anorexia & malaise. Rash is more common in children than adults. Important laboratory findings include leukopenia, thrombocytopenia and elevations of hepatic transaminases. Rarely, inclusions referred to as 'morulae' or 'mulberry lesions' may be observed in leukocytes, but the diagnostic test of choice is serology. The illness typically lasts about 3 weeks and carries a mortality rate of 3%. |
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