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The assay for detecting IgM neutralizing (NT) antibody activity to human herpesvirus-6 (HHV-6) was developed by using pretreatment of blood sample with staphylococcal protein A. The activity was mostly present in IgM fractions of serum but not in IgA fractions separated by ultracentrifugation. The assay was used for seroepidemiological studies for HHV-6 infection. In primary HHV-6 infection, IgM NT antibodies appeared 5 to 7 days after onset of exanthem subitum, reached maximum titers at 2 to 3 weeks, and tended to decline to undetectable levels after 2 months. In contrast, reactivation of HHV-6 observed in organ transplants showed somewhat greater degree of IgM NT antibody responses that persisted for 2 to 3 months and became undetectable 5 to 6 months after transplantation. The level and persistence of NT antibody titers measured by the conventional method was generally greater than those of the IgM titers. The prevalence of the IgM NT antibodies was examined in healthy individuals. The antibody was first detected at 4 to 7 months of age (5%), reached maximum level at 8 to 11 months (40%), and was detectable by 4 to 6 years (17%). A few (4 to 5%) of adolescents and adults were positive for the antibody.

We determined IgG antibodies against Human Herpesvirus-6 (strain Uganda 1102, M. D. Griffin, London) in the indirect immunofluorescence test in sera from 1105 persons of various age groups. Of these sera 570 were retested using HHV-6 strain St. W. (Prof. Schneweis, Bonn). We could confirm that maternal antibodies decrease between birth and six months of age and the seropositive rate rises rapidly between seven months and five years of age up to 79.5%. Between six and ten years and up to 40 years, the antibody-positive rate lies around 81.3% and 66%, respectively. To confirm the causal nature of human herpes virus type 6 (HHV-6) for exanthema subitum we could demonstrate eight seroconversions testing sera from 14 patients with roseola infantum. In addition, the virus was isolated from peripheral blood lymphocytes of children during the acute fever phase in four cases in tissue culture and in six cases the virus was detected by positive hybridization. In single and some paired sera from patients with acute exanthematous diseases, rubella (n = 28), parvovirus B 19 (n = 24), measles (n = 17), mumps (n = 27), adenovirus (n = 27) and parinfluenza virus type 3 (n = 28) and in sera from patients with Kawasaki syndrome (n = 20), acute varicella-zoster- (n = 27), acute herpes simplex- (n = 18) and HIV-1 infection (n = 50), we found no HHV-6 IgM antibodies and no HHV-6 IgG antibody rises. We could only demonstrate an HHV-6 seropositive rate according to our age-prevalence study.(ABSTRACT TRUNCATED AT 250 WORDS)