Organisms of the genus Ehrlichia and Anaplasma are pleiomorphic, obligate intracellular bacteria of the family Rickettsiaceae that parasitize the phagosomes of mononuclear or polymorphonuclear leukocytes (Eng et al., 1990; Anderson et al., 1991). Human ehrlichioses are recently recognized tick-borne infections (McQuiston et al., 1999). In 1986, a clinically novel form of human monocytic ehrlichiosis (HME) was described and shown to induce an E. canis-reactive humoral reaction (Maeda et al., 1987). The advent of taxonomic determination based on 16S rRNA gene sequencing led to the discovery that E. canis-like human ehrlichiosis was actually caused by a previously undescribed species, subsequently named E. chaffeensis (Anderson et al., 1991). Evidence of human infection with E. canis, the cause of canine monocytic ehrlichiosis, has also been reported (Perez et al., 1996). Finally, another agent causing granulocytic ehrlichiosis in dogs, E. ewingii, was reported to cause human illness in four patients (Buller et al., 1999).
The main vector of E. chaffeensis and E. ewingii is the Lone Star tick, Amblyomma americanum, and the dog tick, Dermacentor variabilis (Walker and Dumler, 1996). From 1986 to 2006 more than 2300 human cases of HME have been reported to the CDC, mainly from the south-eastern and south-central United States (Ismael et al., 2010). In 1990, canine and human infections with granulocytic ehrlichiae were discovered in Minnesota and Wisconsin (Bakken et al., 1994). Human granulocytic anaplasmosis (HGA), formerly human granulocytic ehrlichiosis (HGE) is caused by an organism closely related to E. equi and E. phagocytophila (Chen et al., 1994), now known as Anaplasma phagocytophilum (Nicholson et al., 2010).
The main vector of HGA agent is I. scapularis in the mid-western and north-eastern United States (Walker and Dumler, 1996). Since first identified, more than 3000 human cases of HGA have been reported to the CDC, mainly from the northeastern and upper mid-western United States (Ismael et al., 2010). The zoonotic nature of human ehrlichioses and anaplasmosis is supported by reports of natural infections with the same species in dogs, deer, horses, and rodents. Dogs are likely to contribute to the enzootic cycle and human infection. Dogs can also become infected with E. chaffeensis in experimental (Dawson and Ewing, 1992) and natural conditions (Breitschwerdt et al., 1998). In dogs, disease caused by infection with A. phagocytophilum is typically characterized by acute onset of fever, depression, myalgia, anorexia, lameness, and reduced platelets. In people, A. phagocytophilum infection is characterized by fever, headache, lethargy, myalgia, elevated liver function enzymes, and reduced platelets. Human fatalities can occur, although reportedly in anorexia, rash in 20% of the patients (for HME, less common for HGA), leucopaenia, thrombocytopaenia, anaemia, hypertension, coagulopathy, renal failure, pancytopaenia, hepatocellular injury, and elevated serum hepatic aminotransferase levels (Walker and Dumler, 1996; McQuiston et al., 1999; Ismael et al., 2010).
The severity of the disease ranges from asymptomatic seroconversion to fatal infection. Case-fatality rates that were as high as 5% for HME and 10% for HGA (Dumler and Bakken, 1995) have now declined to less than 2% (HME) and 1% (HGA). Human and animal ehrlichioses are mainly diagnosed by indirect immunofluorescence assay, although PCR assays are increasingly used (McQuiston et al., 1999; Ismael et al., 2010). Treatment of infections caused by Ehrlichia spp. in dogs is mainly based on the use of doxycycline at 100mg twice a day for 5–7 days (Tan, 1997). Prevention is based on the same measures as reported for Lyme disease.