A case of child-to-father transmission of hepatitis B virus in Japan: a rare infection route

This report describes a rare horizontal transmission of hepatitis B virus (HBV) from an unvaccinated 6-year-old boy to his father. The father had been diagnosed with acute hepatitis B 1 month earlier; therefore, when the child visited the clinic with fever, he was screened for HBV markers and diagnosed as an asymptomatic carrier. Neither the child nor his father was vaccinated against HBV, whereas the child’s mother and sister, having received the HBV vaccination as they were medical staff and a nursing student, respectively, tested negative for the hepatitis B surface antigen (HBsAg) and positive for anti-HBs. We performed a phylogenetic analysis of HBV in the child and his father, and identified 100% homologous strains of identical genotype C. At diagnosis, the father tested positive for IgM anti-hepatitis B core with a high titer, whereas the child tested negative for this marker. These data strongly indicated a child-to-father transmission. In this case, the HBV infection route was speculated as close contact including saliva-based transmission between the child and father, mainly attributed to their daily food habits. When clinicians diagnose patients with acute or chronic HBV infection, the household members should have been examined for HBV markers immediately. If some household members are susceptible to HBV infection, all members should be vaccinated against HBV.