The second pattern included those cases reported from California, Georgia, and Washington. The etiologic agents involved have not be morphologically identified. Improved serodiagnostic and molecular techniques are needed for characterizing Babesia species and elucidating the epidemiology of babesiosis in these areas. Clinical presentations of human Babesia microti infection appear to vary among patients. The severity of the disease can be affected by several risk-increasing factors.
Splenectomy appears to correlate with life-threatening and fatal cases.
Simultaneous occurrence of Lyme disease and babesiosis in humans has been commonly observed in areas endemic for both infections. In addition, patients with HIV infection and AIDS have presented severe manifestations of babesiosis. Age appears to be another important risk factor. The intensity of clinical babesiosis caused by Babesia microti has been greater in adults over 40 years of age than in younger adults or children, although debilitating illness appears to occur among all age groups. Sufficient transmission of Babesia microti through the bite of an infective tick requires approximately 36-48 hours of attachment.