HUMAN BABESIOSIS

Epidemiology

There appears to be two distinct epidemiological patterns of babesiosis in human populations in the U. S. The first pattern typically involves spleen-intact human cases who were frequently subclinical and reported mainly from coastal islands in the Northeast. The main etiologic agent was proven to be Babesia microti, transmitted by the blacklegged ticks. 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. Both the nymphal and adult stages of Ixodes scapularis are proven competent vectors of the infection, however, the role of adults in transmission of the infection to humans appears to be limited. On one hand, it is easier for people to notice the attachment of an adult tick, thus the tick is more likely removed before transmission occurs. On the other hand, people tend to have less outdoor activities during the cold months in early spring and late fall when adult ticks are more active. The risk of acquiring Babesia microti infection is greatest during June and July when the nymphal stage of Ixodes scapularis is most abundant. Transfusion of platelets, liquid red blood cells, and frozen-thawed red blood cells from asymptomatic donors may result in human babesiosis. However, such a risk is minimal.

HUMAN BABESIOSIS

TICK VECTORS AND RESERVOIR HOSTS

SYMPTOMS, DIAGNOSIS, AND TREATMENT