First discovered in 1994, human ehrlichiosis (HE) is a bacterial infection caused by several types of rickettsiae that spread by tick vectors from dogs and other animals to humans. Like Lyme disease, ehrlichiosis is occurring in a context of widening suburban sprawl in North America, Europe, and Japan. Once in the bloodstream, the bacteria invade and kill white blood cells. There are two types of ehrlichiosis: human granulocytic (HGE) and human monocytic ehrlichiosis (HME). The threat to the blood supply appears to be primarily from HGE. From 1986 to 1997, there were only 449 cases of HGE reported in the United States. Symptoms include relatively mild and self-limited fever, headache, and malaise. However, 10-20 percent of people with HGE go on to suffer encephalitis, acute respiratory distress syndrome, and opportunistic infections, and up to 5 percent of infections may be fatal. Treatment with antibiotics such as tetracylines is effective. The incidence of HGE is not well understood among blood donors, with perhaps only one presumptive case in a blood recipient. Preliminary studies indicate that HGE can survive in refrigerated blood for up to 18 days (Walker and Dumler, 1997; Leiby etal., 2002).

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