Hepatitis A epidemiology and demographics
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Editor-In-Chief: C. Michael Gibson, M.S., M.D. [1]
Epidemiology and Demographics
Person-to-person transmission through the fecal-oral route is the primary means of HAV transmission in the United States. Transmission occurs most frequently among close contacts, especially in households and extended family settings. Because the majority of children have asymptomatic or unrecognized infections, they play a key role in HAV transmission and serve as a source of infection for others. In one study of adults without an identified source, 52% of their households included a child aged <6 years, and the presence of a young child was associated with HAV transmission in the household. In studies in which serologic testing of the household contacts of adults without an identified source of infection was performed, 25%-40% of contacts aged <6 years had serologic evidence of acute HAV infection (IgM anti-HAV).
Common-source outbreaks and sporadic cases also can occur from exposure to fecally contaminated food or water. Uncooked foods have been recognized frequently as a source of outbreaks. Cooked foods also can transmit HAV if cooking is inadequate to kill the virus or if food is contaminated after cooking, as occurs commonly in outbreaks associated with infected food handlers. Waterborne outbreaks of hepatitis A are infrequent in developed countries with well-maintained sanitation and water supplies. The majority of waterborne outbreaks are associated with sewage-contaminated or inadequately treated water. Outbreaks in the context of floods or other natural disasters (e.g., hurricanes) have not been reported in the United States.
Depending on conditions, HAV can be stable in the environment for months. Heating foods at temperatures >185°F (>85°C) for 1 minute or disinfecting surfaces with a 1:100 dilution of sodium hypochlorite (i.e., household bleach) in tap water is necessary to inactivate HAV.
On rare occasions, HAV infection has been transmitted by transfusion of blood or blood products collected from donors during the viremic phase of their infection. Since 2002, nucleic acid amplification tests such as polymerase chain reaction (PCR) have been applied to the screening of source plasma used for the manufacture of plasma-derived products.
In experimentally infected nonhuman primates, HAV has been detected in saliva during the incubation period. However, transmission by saliva has not been demonstrated.
Can I get viral hepatitis from an animal?
No. Hepatitis viruses are not zoonotic meaning that they cannot be transmitted between, or be shared by, animals and humans. No natural animal or insect hosts or vectors are known to exist. While humans are the only natural hosts, some non-human primates can be experimentally infected for research purposes.
Prevalence
In the U.S. population, the overall age-adjusted prevalence of HBV infection (including persons with chronic infection and those with previous infection) was 4.9% in the third National Health and Nutrition Examination Survey (NHANES III, 1988--1994). Foreign-born persons (particularly Asian/Pacific Islanders) who have emigrated from countries in which HBV is endemic contribute disproportionately to the burden of chronic HBV infection in the United States. The prevalence of chronic HBV infection among foreign-born persons immigrating to the United States from Central and Southeast Asia, the Middle East, and Africa varies (range: 5%--15%) and reflects the patterns of HBV infection in the countries and regions of origin for these persons. During 1994--2003, approximately 40,000 immigrants with chronic HBV infection were admitted annually to the United States for permanent residence (78; CDC, unpublished data, 2005).
During 1990-2004, overall incidence of reported acute hepatitis B declined 75%, from 8.5 to 2.1 per 100,000 population. The most dramatic declines occurred in the cohort of children to whom recommendations for routine infant and adolescent vaccination have applied. Incidence among children aged <12 years and adolescents aged 12--19 years declined 94%, from 1.1 to 0.36 and 6.1 to 2.8 per 100,000 population, respectively (Figure 2). Since implementation of routine childhood immunization, an estimated 6,800 perinatal infections and an additional 18,700 infections during the first 10 years of life have been prevented annually in the United States.
Although infections in infants and children aged <10 years represented <10% of all HBV infections before implementation of childhood immunization programs, childhood infections resulted in an estimated 30%-40% of the chronic HBV infections among persons who acquired their infections in the United States. In two population-based studies conducted among Asian/Pacific Islander children who were born in the United States before perinatal hepatitis B prevention programs were widely implemented, 61%-66% of the chronic HBV infections occurred in children born to HBsAg-negative mothers. A substantial proportion of these chronic infections would not have been prevented by a selective program of identification and immunization of only infants born to HBsAg-positive mothers.
In addition to declines in incidence among all age groups, racial disparities in hepatitis B incidence among children have been substantially reduced. The reduction of the disparity between Asian/Pacific Islander and other children is consistent with recent observations noting a decline in seroprevalence of HBV infection after successful implementation of routine hepatitis B vaccination among Asians who have recently immigrated to the United States. However, as hepatitis B incidence has declined among U.S.-born children, unvaccinated foreign-born children account for a high proportion of infections. During 2001--2002, of 19 children born after 1991 in whom acute hepatitis B had been verified, eight (42%) were foreign born.