The only reservoir of HAV is infected humans, and the major source of infectious virus is human feces. Virus is detectable in feces as well as blood 10-12 days after infection. Generally the most infective period is 14-21 days before the onset of clinical symptoms until 1 week after the onset of symptoms (16). Infectivity, thus, rapidly wanes with the onset of jaundice. During acute infection, up to 108 infectious virus particles per mL may be found in fecal specimens. Fecal excretion of virus occurs during most of the incubation period, and individuals are contagious for a duration of at least 3 weeks or more, mostly when they are asymptomatic. Virus shedding in the feces drops rapidly after the onset of jaundice and continues for less than a week after jaundice begins, although, with relapses, it has been detected for up to a further 2 months.
Considerably less virus, up to 105 particles per mL, is found in the blood, and the viremic stage coincides largely with the period that the virus is excreted into the feces. This may now explain the increasing finding of bloodborne hepatitis A infections (17). Virus is also excreted into the saliva, although it has not been established whether saliva plays any role in hepatitis A virus transmission (18). There is, for example, no evidence that sharing eating utensils, cigarettes, or kissing can transmit hepatitis A infection.
By far the most common route of transmission of hepatitis A virus is the fecal-oral route, with person-to-person being the most important mode of transfer of the virus. This has been clearly demonstrated by observations of the rates of infection among household contacts of patients and among children in the setting of daycare centers (19). The virus is robust and is able to survive in water and food for 12 weeks to 10 months. As a result, foodborne outbreaks (common-source outbreaks) are also common (20). Foodborne outbreaks may be due to contamination of food by food handlers—typically a food handler in the asymptomatic preicteric phase of the illness. This would occur especially in situations where there are poor hand-washing practices and uncooked foods such as salads, sandwiches, cold meats, etc. are touched by hand. In addition, foods themselves may be contaminated, especially shellfish harvested from waters close to sewage outlets or vegetables fertilized with untreated human nightsoil (21). Large water-borne epidemics due to drinking or swimming in fecally contaminated water also occur from time to time (22).
Although still rare, parenteral spread of HAV is being increasingly recognized, for example, following receipt of contaminated blood (i.e., blood from donors in the asymptomatic viremic stage of the incubation period) and blood products, especially factor VIII concentrates (17). Hepatitis A virus transmission via contaminated syringes and needles is being increasingly seen among intravenous drug abusers (23). The role of sexual transmission of hepatitis A virus has not been clearly defined, although the prevalence of infection has been shown to be higher among attendees of sexually transmitted disease (STD) clinics than among blood donors, and especially among male homosexuals (where oral-anal transmission may be important) (24).
The epidemiology of hepatitis A differs markedly in various parts of the world and in different populations. Classically, three epidemiological patterns of HAV endemicity have been described depending on socioeconomic circumstances such as levels of hygiene and sanitation and crowding (10). Countries with high endemicity in the developing world would experience relatively low levels of disease, although the virus circulation is high; this is due to widespread inapparent infection in childhood resulting in almost all older children and young adults having protective antibodies by the time they reach the age when infection usually causes disease. The second pattern, intermediate endemicity, associated with the majority of disease, is seen because circulation of virus is still at a fairly high level but there is still a significant immunity gap at the vulnerable age group of older children and young adults. With the third epidemiological pattern, countries with low endemicity of infection, disease is infrequent because circulation of virus is low and cases are usually linked to importation or are found in travelers to developing countries.
There is little doubt that the global epidemiology of hepatitis A infection has undergone marked changes as a direct result of improvements in the provision of clean water and sanitation and in personal and public hygiene. In some highly industrialized countries such as Scandinavia, Germany, Switzerland, and Japan, endemic transmission has all but ceased, and the major proportion of infection in these countries is due to importation or occurs in travelers returning from developing countries
(25). The prevalence of antibodies to HAV in blood donors from these countries is less than 10%. In most other developed countries, endemic transmission of virus is also declining, and this has resulted in an increasing susceptibility in adults with its attendant greater morbidity and mortality
(26). Thus, the declining transmission of HAV has created a large pool of susceptibles among older children and adults who are vulnerable to outbreaks of infection. These outbreaks may either be community outbreaks (often centered around, e.g., daycare centers, groups of individuals living in closed living conditions, especially the military, prisons, boarding schools, or residents of institutions such as those for the mentally handicapped), or common source outbreaks (food- or waterborne outbreaks). There is also an increasing number of adults who, while they are susceptible to infection, are nevertheless coming into increasingly greater contact with HAV, either because of travel (business or recreational) or occupational exposure, e.g., staff at daycare centers for children or institutional personnel, health care workers, personnel working in sewage plants, etc. A further consequence of the decline in HAV transmission and increasing infection in adulthood is the relative rise in the importance of bloodborne transmission as more and more individuals in the asymptomatic viremic period may be donating blood.
In the United States communities have been divided into two categories communities with high rates and those with intermediate rates of hepatitis A (13):
1. Communities with high rates of hepatitis A: These communities are characterized by high rates of infection, high rates of disease, but few cases in individuals over 15 years of age and epidemics every 5-10 years. Seroprevalence studies have shown that 30-40% of children are seropositive before 5 years of age, and virtually all individuals are positive by the time they reach young adulthood. In the United States this pattern is found among Native American populations and some Hispanic and religious communities.
2. Communities with intermediate rates of hepatitis A: These communities are characterized by the majority of disease occurring in children, adolescents, and young adults; disease rates are some 5-10 times lower with less clearly defined risk activities. The seroprevalence of antibodies to hepatitis A virus varies from 10-25% in children less than 5 years of age to about 50% in individuals over 15 years of age.
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