

Ultrastructure of A.phagocytophilum by transmission electron microscopy. Photo by V.Popov, reprinted from Dumler JS et al. Human granulocytic anaplasmosis and Anaplasma phagocytophilum. Emerg Infect Dis;11:1828-34.


Like Ehrlichia species, Anaplasma organisms are small, gram-negative, and intracellular. A. phagocytophilum targets neutrophils, alters their function in the host, and forms morulae within vacuoles. Most of the damage it causes appears to be related to host inflammatory processes, as there is little evidence of a correlation between the number of organisms and host disease severity.

Anaplasmosis is a global infection, occurring in North America, most of Europe, and eastern Asia. Ticks from the Ixodes persulcatus-complex are the vectors: I. scapularis in the northeastern and upper Midwestern regions of the United States; I. pacificus in the Pacific Northwest; I. ricinus in Europe; and I. persulcatus in Asia. A. phagocytophilum is maintained in nature by cycling between these ticks and various small mammals, primarily mice and other small rodents. Because Ixodes ticks are also the vectors for Lyme disease, babesiosis, and tick-borne encephalitis, Anaplasma coinfection with these other diseases can and does occur in humans.

In the United States, cases of HGA have outnumbered those of Human Monocytic Ehrlichiosis (HME) Similar to HME and human ewingii ehrlichiosis, the median age of patients with HGA is around 50 years old.

Signs and Symptoms

The clinical course of HGA ranges from asymptomatic infection to fatal disease. Initial symptoms, occuring about five to ten days after tick bite, are largely non-specific: fever, chills, severe headache, and myalgias;j nausea, cough, and arthralgias may also occur. Rash is uncommon but has been reported.

Compared with HME, HGA appears less likely to involve the central nervous system, but peripheral neuropathies (e.g., numbness and tingling) are more common and can last weeks to months. Among the neurologic findings reported in the medical literature are facial palsy, demyelinating polyneuropathy, and brachial plexopathy. Respiratory distress syndrome and a septic or toxic shock-like syndrome have been reported, but appear to be less common than in HME. The overall fatality rate from HGA is very low (less than 1%), with most of the deaths resulting from opportunistic infections (for example, herpes simplex esophagitis, Candida pneumonitis, and pulmonary aspergillosis) in immunocompromised patients.



