Although hepatitis B first came to the attention of the Western world as an iatrogenic disease transmitted accidentally by inoculation of contaminated blood ("serum hepatitis"), it is self-evident that this is not the natural mode of spread. In the populous areas of the developing world of high HBV endemi-city (Southeast Asia including China, Indonesia, Philippines, and the Pacific islands, the Middle East, Africa, and the Amazon basin), where the majority of people are seropositive and 8-15% are chronic carriers, most become infected at birth or in early childhood. Some 5-12% of parturient women are HBsAg positive, of whom nearly half are also HBV DNA positive, resulting in efficient perinatal transmission with a 70-90% probability of the infant itself becoming a carrier. As the infant generally becomes HBsAg positive only 1-3 months after birth, it is considered that most perinatal infections result from contamination of the baby with blood during parturition, rather than trans-placentally; breast milk or maternal saliva are probably responsible occasionally. However, up to half of all children in medium to high prevalence communities who become carriers acquire the infection from intrafamilial contact with chronically infected siblings or parents secreting virus in oozing skin sores, blood, or saliva; between 1 and 5 years of age the probability of becoming a chronic carrier following infection is of the order of 25%. In adolescents and adults transmission is principally by sexual intercourse; only 1-5% of primary infections acquired at this age progress to chronicity.

The picture is quite different in the developed world, where the carrier rate is generally less than 1% except in ethnic minorities (e.g., Asian immigrants) and in injecting drug users. Perinatal spread is correspondingly less common, and sexual (including homosexual) transmission among adolescents and adults is a significant risk. Percutaneous transmission by iatrogenic invasive procedures represents the most common identifiable mode of spread, with injecting drug users constituting the largest cohort of carriers. Posttransfusion hepatitis B and infection of hemophiliacs by contaminated factor VIII have now almost disappeared as a result of routine screening of blood and organ donors, but hepatitis B can represent a major occupational risk for laboratory workers who are vulnerable to accidental infection by blood spill or needle-stick injury. Less than 1 |xl of blood contaminating a syringe or needle can readily transmit hepatitis B from one individual to another. Professionals occupationally at risk include dentists, surgeons, pathologisfs, mortuary attendants, and technicians and scientists working in serology, hematology, biochemistry, and microbiology laboratories in hospitals or public health institutions, blood banks, or hemodialysis units. However, the availability of an effective vaccine has greatly reduced this occupational risk. Tattooing, acupuncture, and ear-piercing without rigorous sterilization of equipment constitute other potential routes of transmission, as do certain body-contact sports such as wrestling and rugby football.

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