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To determine the relationship of human herpesvirus-6 (HHV-6) infection to febrile convulsions, cerebrospinal fluid (CSF) from patients with a history of febrile convulsion were tested by polymerase chain reaction (PCR) amplification for HHV-6 DNA. HHV-6 DNA was detected in 9 of 10 samples from patients with exanthem subitum who showed neurologic symptoms. Also, 8 of 10 CSF samples from 8 patients who had three or more febrile convulsions and 1 of 7 CSF samples from patients who had a single febrile convulsion contained HHV-6 DNA. These data suggest that HHV-6 may invade the brain during the acute phase of exanthem subitum and that recurrence of febrile convulsions may be associated with reactivation of HHV-6.

An encephalitic illness with a fatal outcome occurred in a 9 month old girl with virologically confirmed exanthem subitum. Human herpes-virus-6 (HHV-6) DNA was found in the cerebrospinal fluid at the acute stage of the disease by the polymerase chain reaction, but the virus antigen was not detected in her brain tissue. This suggests that HHV-6-induced encephalitis/encephalopathy may be due to a non-infectious process.

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.

Eight cases of acute human herpesvirus type 6 (HHV6) infection in infants were diagnosed serologically by the demonstration of IgM anti-HHV6 (8/8) and a significant change in total anti-HHV6 antibody titre (6/8). Four infants were sufficiently ill to require admission to hospital and further investigations: one with encephalitis and three with gross hepatosplenomegaly, two of whom had evidence of simultaneous infection with another herpes-virus. The remaining four infants had an illness compatible with roseola infantum, although this diagnosis had not been made clinically. Sera from two of those infants with rash had been sent for analysis to exclude rubella because the infants' mothers were pregnant. The other two had received antibiotics when febrile, and the subsequent appearance of the roseola rash had raised the possibility of antibiotic allergy. The data suggest that there are clinical syndromes in addition to roseola infantum associated with the presence of IgM anti-HHV6, in which serological screening for evidence of acute HHV6 infection may be useful.

The polymerase chain reaction was used to detect human herpesvirus 6 (HHV-6) DNA in peripheral blood mononuclear cells from patients with exanthem subitum. Amplified products were detected by agarose gel electrophoresis and dot blot hybridization with a cloned DNA probe. No cross-hybridization with DNAs of five other human herpesviruses was observed in this system, and all HHV-6 strains gave positive reactions when the primer pairs were used. Immunoglobulin M antibody appeared about 5 days after the onset of disease, reaching a maximum after about 2 to 3 weeks and then decreasing to less than 1:10 1 month after the onset of disease in almost all patients. Mononuclear cells from seven patients with exanthem subitum all gave positive reactions in this system, and viral DNA was found in samples even during the convalescent phase of the disease. We conclude that this test, using polymerase chain reaction amplification, which takes only 1 to 2 days, is useful for the diagnosis of HHV-6 infection.

The susceptibilities of seven T-cell lines to human herpesvirus 6 (HHV-6) infection were examined. MT-4 cells were the most susceptible of these lines to infection with this virus. Therefore, chemically adhered MT-4 cell monolayers were used for infectious HHV-6 assay by indirect immunofluorescent-antibody (IFA) staining. When cell monolayers were fixed 30 to 45 h postinfection, the foci stained with IFA were easy to count and a linear relationship was observed between the number of foci and the virus concentration. MT-4 cell monolayers were also used for a focus reduction neutralizing-antibody test. In this test, sera from patients in the convalescent stage of exanthem subitum all showed significant neutralizing activity (1:80 to 1:320), whereas sera from patients in the acute stage of disease showed no detectable neutralizing activity. The titers of neutralizing antibody correlated well with the levels of anti-HHV-6 antibodies detected by IFA.

Human lymphocytes and MT4 cells infected with a virus isolated from a patient with exanthem subitum were examined by transmission and scanning electron microscopy. The most striking characteristic of the ultrastructure of this herpes-type virus was that nucleocapsids located outside the nucleus were each coated distinctly with a tegument of moderate electron density. Tubular structures formed due to some mistakes in the viral assembly were also detected in the nucleus. Morphological differentiation of this virus from the other human herpesvirus was discussed. From these observations it was concluded that this virus has the same ultrastructural characteristics as HBLV (HHV-6).