Ehrlichiosis

An acute (abrupt onset) disease, first reported in humans in 1986, due to infection by the rickettsial agent, Ehrlichia canis. The brown dog tick, is the common vector (carrier). Ehrlichiosis is clinically similar to Rocky Mountain spotted fever with high fever, headache, malaise, and muscle pain but without a rash. Laboratory features include leukopenia (low white blood cell count), thrombocytopenia (low platelet count), mild anemia, and elevation in the levels of hepatic aminotransferase enzymes. Clinical symptoms and laboratory abnormalities respond promptly to therapy with the antibiotics tetracycline or doxycycline, and the majority of patients become afebrile within 24 to 48 hours after the start of such treatment. The diagnosis of ehrlichiosis rests on the detection of ehrlichia either by direct means or by the indirect means of serologic studies. Three methods are available for detection: inspection of peripheral-blood smears, PCR testing, and tissue culture. The disease ehrlichiosis is named for the great German Nobel Prize winning physician and bacteriologist Paul Ehrlich (1854-1915).

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Infection with leukocytic rickettsiae of the genus Ehrlichia; in humans, especially by E. sennetsu that produces manifestations similar to those of Rocky Mountain spotted fever.Species of Ehrlichia have long been recognized as causes of febrile hemorrhagic disease of variable severity in animals, including dogs and horses. Human infection with E. sennetsu, limited to the Far East, is a mononucleosislike illness. The first human case of e. in the western hemisphere was reported in 1986. Since then 2 distinct forms of the disease, each associated with a different species of Ehrlichia and a different group of tick vectors, have been recognized in human beings. More than 90% of patients give a history of having been bitten by several ticks. An animal reservoir has not yet been identified, but rodents and deer are suspected. After an incubation period of 1–4 weeks, infection begins as a nonspecific febrile illness with chills, sweating, headache, and joint and muscle pains. One-fourth of patients have a transitory nonspecific rash not related to the site of tick bite. Systemic complications may involve the respiratory tract (sore throat, cough, pulmonary infiltrates, acute respiratory distress syndrome), the digestive system (nausea, vomiting, abdominal pain, gastrointestinal bleeding), or the liver (80% have hepatitis). Other possible complications include meningitis, pericarditis, renal failure, and disseminated intravascular coagulation. Early studies of these infections, based on populations with more conspicuous and readily identifiable disease, overestimated case fatality rates. With treatment, the case fatality rate in the 2 forms of e. probably does not exceed 1%. Human granulocytic e. is clinically indistinguishable from human monocytic e., but in the former, morulae are found in neutrophils rather than in monocytes, and the disease is somewhat more severe. For both forms of e., polymerase chain reaction technology has yielded the most sensitive and specific serologic testing. Tetracycline and doxycycline are highly effective in arresting progression of either form of human e.. Because other agents, including chloramphenicol, have not been found to be effective, tetracyclines are used even in children, in whom they are generally contraindicated because of the risk of dental mottling and suppression of bone growth. Treatment is often begun on suspicion, pending confirmation of the clinical diagnosis by serologic test. Drug therapy is continued for 14 days.

- human e. a form of e. that presents clinically as a undifferentiated acute febrile illness characterized by fever, chills, diarrhea, and headache, following tick bite(s), probably by the Lone Star tick, Amblyomma americanum. Usually caused by Ehrlichia chaffeensis. First described in 1987. (Thought to be predominantly a monocytic form of e..)

- human granulocytic e. (HGE) an acute infectious disease characterized by fever, chills, headache, joint and muscle pains, and sometimes respiratory, gastrointestinal, hepatic, or other systemic involvement; first described in 1994 in northeastern and northern midwestern states and California; the causative agent can be distinguished only by molecular studies from Ehrlichia equi, the cause of equine e.. The deer tick, Ixodes scapularis, is the principal vector, and the peak incidence is in July. Hematologic studies show depression of RBC, WBC, and platelets. Clusters of developing organisms called morulae may be seen in neutrophils on stained blood smears, but serologic testing is more sensitive.

- human monocytic e. (HME) an acute infectious disease characterized by fever, chills, headache, muscle and joint pain, and variable respiratory, gastrointestinal, and systemic involvement; hematologic studies show depression of RBC, WBC, and platelets. The finding of clumps of developing organisms, called morulae, in the cytoplasm of monocytes in a stained smear of peripheral blood establishes the diagnosis, but their detection is often difficult. Serologic testing shows antibody to Ehrlichia chaffeensis, an organism closely resembling the agent of canine e., E. canis. This disease has been largely confined to the southeastern and south central United States. The Lone Star tick (Amblyomma americanum) and the American dog tick (Dermacentor variabilis) are the principal vectors, and the incidence is highest from April to September, during the peak activity of these ticks.

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ehr·lich·i·o·sis er-.lik-ē-'ō-sis n, pl -o·ses -.sēz infection with or a disease caused by rickettsial bacteria of the genus Ehrlichia

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ehr·lich·i·osis (ār-lik″e-oґsis) a type of tick-borne fever caused by infection with bacteria of the genus Ehrlichia. It is usually seen in domestic animals, but certain species can affect humans.