Epidemiology of Viral Hepatitis A and E: A Global View

There are many similarities between the two human enterically transmitted hepatitis viruses, hepatitis A virus (HAV) and hepatitis E virus (HEV), in their epidemiology and the diseases they cause. However, important differences exist as well (Table 2.1). Both viruses are small, positive sense, single-stranded RNA viruses that replicate in the liver, causing similar symptoms and resulting in similar histopathology. Both viruses are non-enveloped when discharged from the liver into the biliary tree but also can be secreted into the bloodstream containing capsular material from the hepatocyte as a pseudomembrane. Both of these hepatitis viruses have been responsible for large epidemics and can be acquired from contaminated food. However, subclinical infection with HAV among infants and young children is very common especially in developing countries, but HEV infections typically occur as large waterborne epidemics in adolescents and adults in developing countries. Infants and young children have much lower rates of infection during these epidemics. A characteristic feature of HEV epidemics is a mortality of 1% or less in the general population but a mortality of 20% or higher among pregnant women. Sporadic infections from HEV occur among adults in industrialized countries from foodborne transmission. In contrast, clinically apparent hepatitis from HAV infections in developing countries is rare because of very high rates of subclinical infections among infants and young children resulting in high levels of immunity in the populations. Countries that have experienced the transition from low to higher levels of socioeconomic development have experienced an increasing number of hepatitis cases from HAV due to declining population immunity to the virus. The dynamic epidemiology of these two hepatitis viruses will be described in this chapter. Table 2.1 Comparative features of hepatitis A and E virus Hepatitis A Hepatitis E Virus family Picornaviridae Hepeviridae Nucleic acid Positive strand RNA Positive strand RNA Genomic size 7.5 kb 7.5 kb Transmission Person-to-person, fecal-oral, foodborne, blood Fecal-oral, foodborne, blood Incubation 2–5 weeks 3–7 weeks Chronicity None Uncommon except in immune-compromised patients (transplant recipients, AIDS patients, etc.) Reservoirs Humans only Gt. 1 and 2: humans Gt. 3 and 4: humans, swine, deer, mongoose, and other animals Mortality 1–2% 1–2% in general population 15–30% in pregnant women Infection in young children Very common Rare Subclinical infections in adults Rare: 75–90% symptomatic Common: 1–5% symptomatic Severity of disease Increases with age Increases with age Vaccine availability Yes Only in China IGG protective Yes Not demonstrated