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HAV infection is acquired primarily by the fecal-oral route by either person-to-person contact or ingestion of contaminated food or water. 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 (11,16). In experimentally infected nonhuman primates, HAV has been detected in saliva during the incubation period; however, transmission by saliva has not been demonstrated (17). Depending on conditions, HAV can be stable in the environment for months (18). 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 (19). Because most children have asymptomatic or unrecognized infections, they play an important role in HAV transmission and serve as a source of infection for others (20,21). In one study of adults without an identified source of infection, 52% of their households included a child <6 years old, and the presence of a young child was associated with HAV transmission within the household (20). In studies where serologic testing of the household contacts of adults without an identified source of infection was performed, 25%40% of the contacts <6 years old had serologic evidence of acute HAV infection (IgM anti-HAV) (20) (CDC, unpublished data, 1994). Each year in the United States, an estimated 100 persons die as a result of acute liver failure due to hepatitis A. Although the case-fatality rate for fulminant hepatitis A among persons of all ages with acute hepatitis A reported to CDC is approximately 0.3%, the rate is 1.8% among adults >50 years of age; persons with chronic liver disease are at increased risk for fulminant hepatitis A (2226). The costs associated with hepatitis A are substantial. Between 11% and 22% of persons who have hepatitis A are hospitalized ( 27 ). Adults who become ill lose an average of 27 days of work (Table 1). Health departments incur substantial costs in providing postexposure prophylaxis to an average of 11 contacts per case (Table 1). Average costs (direct and indirect) of hepatitis A range from $1,817 to $2,459 per case for adults and from $433 to $1,492 per case for children <18 years of age (Table 1). In a recent common-source outbreak involving 43 persons, the estimated total cost was approximately $800,000 (28). In 1989, the estimated annual direct and indirect costs of hepatitis A in the United States were more than $200 million, equivalent to more than $300 million in 1997 dollars ( 29 ) (CDC, unpublished data, 1999). National Trends In the United States, cyclic increases in the incidence of hepatitis A have occurred approximately every decade; the last nationwide increase occurred in 1995 (1,30). Between epidemics, hepatitis A continues to occur at relatively high rates. In 1997, 30,021 hepatitis A cases were reported to the National Notifiable Diseases Surveillance System (NNDSS) (1). After the NNDSS data were adjusted for disease under-reporting and asymptomatic infections, the number of cases reported in 1997 represented an estimated 90,000 persons with symptomatic hepatitis A and 180,000 persons with HAV infection (CDC, unpublished data, 1998). Sources of Infection Most U.S. cases of hepatitis A result from person-to-person transmission during communitywide outbreaks (31) The most frequently reported source of infection (12%26%) is either household or sexual contact with a person with hepatitis A (27,31). Approximately 11%16% of reported cases occur among children or employees in day care centers or among their contacts; however, this estimate might be too high because hepatitis A cases are attributed to day care center-related contact without requiring that the contact have hepatitis A or that a case of hepatitis A be identified in the day care center. An additional 4%6% of reported cases occur among international travelers. Children account for approximately 36% of these cases, with Mexico being the most frequent (84%) destination (CDC, unpublished data 1998). Another 2%3% of cases are associated with recognized food or waterborne disease outbreaks (27,30,31). In addition, cyclic outbreaks have occurred among users of injecting and noninjecting drugs and among men who have sex with men (31). During outbreak years, up to 10% of nationally reported cases occur among persons reporting these behaviors (CDC unpublished data, 1999). Approximately 50% of persons with hepatitis A do not have a source identified for their infection (27,31). Variations by Age, Race/Ethnicity, and Socioeconomic Status The reported incidence of hepatitis A is highest among children 514 years of age (Figure 1), with approximately one-third of reported cases involving children <15 years of age (27). Many more children have unrecognized, asymptomatic infection and can be the source of infection for others. Hepatitis A incidence varies by race/ethnicity, with highest rates among American Indians/Alaskan Natives and lowest rates among Asians; rates among Hispanics are higher than among non-Hispanics (Figure 2). Racial/ethnic differences in rates most likely reflect differences in the risk for infection related to factors such as differences in socioeconomic levels and resultant living conditions (e.g., crowding) and more frequent contact with persons from countries where hepatitis A is endemic (e.g., Mexico and Central America). About a third of the U.S. population has serologic evidence of prior HAV infection, according to data from the Third National Health and Nutrition Examination Survey (NHANES-III) conducted during 19881994 (CDC, unpublished data, 1998). Anti-HAV prevalence varies directly with age: among persons 611 years of age, the prevalence is 9%; 2029 years of age, 19%; 4049 years of age, 33%; and >70 years of age, 75%. Age-adjusted anti-HAV prevalence is highest among Mexican-Americans (70%), compared with non-Hispanic blacks (39%) and non-Hispanic whites (23%). Anti-HAV prevalence is inversely related to income and household size. Variations by Region Over the past several decades, the highest rates of hepatitis A have occurred in a limited number of states and counties in the United States (27) (Figure 3), with rates being substantially higher in the western United States than in other U.S. regions. Although yearly rates in states with the highest disease rates can fluctuate, they consistently remain above the U.S. national average (Figure 4). During 19871997, an average of 50% of reported hepatitis A cases each year was from states with average disease rates greater than twice the national average of approximately 10 cases per 100,000, yet the total population of these states represented approximately 22% of the U.S. population (Table 2). An additional 18% of cases were from states with average annual disease rates above the national average during this time but less than twice the national average (Table 3). Communities in the United States can be considered to have high, intermediate, or low rates of hepatitis A on the basis of epidemiologic characteristics such as age-specific rates of infection and temporal patterns of disease incidence. Most cases of hepatitis A result from person-to-person transmission during communitywide out-breaks in areas with high and intermediate rates of hepatitis A (31,32). Surveillance data demonstrate that communities with high and intermediate rates are concentrated in states with consistently elevated disease rates (Figures 3 and 4). Communities With High Rates of Hepatitis A Communities with high rates of hepatitis A typically have epidemics every 510 years that can last for several years. The peak incidence during these epidemics is generally >700 cases per 100,000 population, and few cases occur among persons aged >15 years. Seroprevalence data indicate that 30%40% of children in these communities acquire infection before 5 years of age and almost all persons become infected before reaching young adulthood (3335). These communities often are relatively well defined, either geographically or culturally, and include American Indian, Alaskan Native, and selected Hispanic, migrant, and religious communities (3341). Widespread postexposure prophylaxis with IG has been used in efforts to control outbreaks in these communities but with little success (33). Since hepatitis A vaccine has become available, routine preexposure vaccination of children has been shown to be feasible in these communities through a number of venues, including public health clinics, physician offices, and schools. Moreover, when relatively high (65%80%) first-dose vaccination coverage of preschool and school-age children is achieved and routine vaccination of young children is sustained, ongoing outbreaks of hepatitis A have been effectively interrupted, a sustained reduction in disease incidence has been observed, and subsequent outbreaks have been prevented (4245). In Communities With Intermediate Rates Of Hepatitis A In communities with intermediate rates of hepatitis A, most disease occurs over a wider range of ages (i.e., children, adolescents, and young adults) than in communities with high rates of hepatitis A. Communities with intermediate rates often are large and include metropolitan areas and counties (4648). Epidemics often occur at regular intervals and persist for several years. However, some communities with intermediate rates of hepatitis A do not have periodic epidemics but instead have sustained elevated rates of disease for many years. Overall disease rates during epidemic periods typically range from 50 to 200 cases per 100,000 population per year; however, cases are often concentrated in specific census tracts or neighborhoods where disease rates can be as high as those in communities with high rates of hepatitis A. During epidemic periods, hepatitis A rates generally increase among all age groups, indicating widespread disease within the community (31). During some communitywide outbreaks, the number of cases might increase among injecting-drug users, among men who have sex with men, or among children and employees in day care centers (4,31,4852). Although persons with these exposures might be at increased risk for infection, they generally do not account for the majority of cases in a community, and the specific risk groups vary from community to community and outbreak to outbreak (4,5052). Children with asymptomatic HAV infection can be a substantial source of infection for older persons (20) (CDC unpublished data, 1994). The feasibility and effectiveness of hepatitis A vaccination to control outbreaks in areas with intermediate rates of hepatitis A have been variable. In vaccination pro-grams targeting children, generally first-dose coverage of preschool and school age children has been low (2045%) (53,54), and the impact of vaccination has been modest and often limited to reducing reported disease rates in the targeted age groups, which might not represent the majority of cases (54). In addition, once the outbreak has subsided, routine vaccination usually has not been sustained to prevent future outbreaks. In communities where outbreaks occurred among adults in particular risk groups (i.e., men who have sex with men or injecting-drug users), vaccination programs have been difficult to implement. Identified barriers have included the cost of the program and problems accessing the at-risk population (50,55) (CDC, unpublished data, 1999). Communities With Low Rates of Hepatitis A In communities with low rates of hepatitis A, most cases are reported among school-age children, adolescents, and young adults; rates reflect little year-to-year variation, and communitywide outbreaks are unusual (31). Although the most frequently reported source of infection is close contact with a person who has hepatitis A, cases attributed to international travel might account for 10%12% of reported cases because of fewer sources of transmission within the community (31). Approximately half of reported cases do not have a recognized source. Travelers Persons from developed countries who travel to developing countries are at substantial risk for acquiring hepatitis A (56). Such persons include tourists, military personnel, missionaries, and others who work or study abroad in countries that have high or intermediate endemicity of hepatitis A (Figure 5). Data from prospective studies indicate that the risk among travelers who do not receive preexposure prophylaxis with IG is 3/1,0005/1,000 per month of stay; among some travelers, the risk is higher (57). The risk varies according to region visited and the length of stay and is increased even among travelers who report that they observe measures to protect themselves against enteric infection or stay only in urban areas, in luxury hotels, or in both (CDC, unpublished data, 1986). In the United States, children account for approximately one third of reported travel-related cases (CDC, unpublished data, 1998). Men Who Have Sex with Men Hepatitis A outbreaks among men who have sex with men have been reported frequently. Cyclic outbreaks have occurred in urban areas in the United States, Canada, Europe, and Australia (52,55,5860). In serologic surveys, anti-HAVpositive persons reported more frequent oral-anal contact, longer duration of homosexual activity, and a larger number of sexual partners than persons without serologic evidence of HAV infection (6164). Users of Injecting and Noninjecting Drugs Outbreaks have been reported among users of injecting and noninjecting drugs in the United States and in Europe (4850,65). In the late 1980s, 10%19% of reported hepatitis A cases occurred among persons who reported a history of injecting-drug use. More recently, outbreaks involving users of injected and noninjected metham-phetamine have been reported in many communities in the midwestern and western United States, accounting for up to 30% of reported cases in these areas (50,66). Cross-sectional serologic surveys have demonstrated that injecting-drug users have higher anti-HAV seropositivity than the general U.S. population (63) (CDC, unpublished data, 1998). Transmission among injecting-drug users likely occurs through percutaneous and fecal-oral routes (e.g., sharing needles, sharing contaminated "works," and having household or other close personal contact)(67). Persons Who Have Clotting-Factor Disorders During 19921993, several outbreaks of hepatitis A were reported in Europe among persons with clotting-factor disorders who had been administered solvent-detergent treated factor VIII concentrates that presumably had been contaminated from plasma donors incubating hepatitis A (68). In the United States, data from one serologic study suggested that hemophilic patients might be at increased risk for HAV infection (69). During 19951996, several patients who had clotting-factor disorders developed hepatitis A after having been administered solvent-detergenttreated factor VIII and factor IX concentrates (16). Persons Working with Nonhuman Primates Outbreaks of hepatitis A have been reported among persons working with non-human primates that are susceptible to HAV infection, including several Old World and New World species (70,71). Primates that were infected were those that had been born in the wild, not those that had been born and raised in captivity. Persons With Chronic Liver Disease Although not at increased risk for HAV infection, persons who have chronic liver disease are at increased risk for fulminant hepatitis A (23,25,26). Death certificate data indicate a higher prevalence of chronic liver disease among persons who died of fulminant hepatitis A compared with persons who died of other causes (22). Food-Service Establishments/Food Handlers Recognized foodborne hepatitis A outbreaks are relatively uncommon in the United States. Nevertheless, when such outbreaks occur, intensive public health efforts are required for their control. These outbreaks are usually associated with contamination of food during preparation by an HAV-infected food handler (72). However, outbreaks associated with food (e.g., shellfish, raw produce) that has been contaminated before reaching the food-service establishment have been recognized increasingly in recent years (15,7375) (CDC, unpublished data, 1999). Although persons who work as food handlers have a critical role in common-source foodborne HAV transmission, they are not at increased risk for hepatitis A because of their occupation. In a study of hepatitis A cases in Washington State during 19871988, rates of hepatitis A among food handlers were found to be similar to rates among the general population in the state (Trueman Sharp, University of Washington, unpublished data, 1989). Day Care Centers Outbreaks among children attending day care centers and persons employed at these centers have been recognized since the 1970s (4,51,76). Because infection among children is usually mild or asymptomatic, outbreaks often are recognized only when adult contacts (usually parents) become ill (4). Poor hygiene among children who wear diapers and the handling and changing of diapers by staff contribute to the spread of HAV infection; outbreaks rarely occur in day care centers in which care is provided only to children who are toilet trained. Despite the occurrence of outbreaks when HAV is introduced into day care centers, the results of serologic surveys do not indicate a substantially increased prevalence of HAV infection among staff at day care centers compared with the prevalence among control populations (77,78). Furthermore, NHANES-III data did not indicate an increased prevalence of HAV infection among children and adolescents who previously attended day care centers (CDC, unpublished data, 1995). Although day care centers can be the source of outbreaks of hepatitis A within some communities, disease within day care centers more commonly reflects extended transmission in the community. Health-Care Institutions Nosocomial HAV transmission is rare. Outbreaks have occasionally been observed in neonatal intensive-care units because of infants acquiring infection from transfused blood and subsequently transmitting hepatitis A to other infants and staff (9,79,80). Outbreaks of hepatitis A caused by transmission from adult patients to health-care workers are usually associated with fecal incontinence, although most hospitalized patients who have hepatitis A are admitted after onset of jaundice when they are beyond the point of peak infectivity (81,82). Data from serologic surveys of many types of health-care workers have not indicated an increased prevalence of HAV infection in these groups compared with that in control populations (83,84). Institutions for Persons Who Have Developmental Disabilities Historically, HAV infection was highly endemic in institutions for persons with developmental disabilities (85). As fewer children have been institutionalized and conditions within institutions have improved, the incidence and prevalence of HAV infection have decreased, although sporadic outbreaks can occur in these settings (73). Schools In the United States, the occurrence of hepatitis A cases within elementary or secondary schools usually reflects disease acquisition within the community. Child-to-child disease transmission within the school setting is uncommon; thus, if multiple cases occur among children at a school, the possibility of a common source of infection should be investigated (15,73). Workers Exposed to Sewage Data from serologic studies among Scandinavian and English workers who had been exposed to sewage indicated a possible elevated risk for HAV infection; however, in these studies, the data were not controlled for other risk factors (e.g., socioeconomic status) (86,87). Recently, two serologic surveys were conducted in the United States comparing the prevalence of anti-HAV among sewage workers to that among other municipal workers. Neither survey found a substantial increase in prevalence among sewage workers, although in one study the odds ratio of 2 was at the limit of statistical significance (CDC, unpublished data, 1998). No work-related instances of HAV transmission have been reported among sewage workers in the United States. Other Settings Waterborne outbreaks of hepatitis A are infrequent in developed countries with well-maintained sanitation and water supplies. Most outbreaks are associated with sewage-contaminated or inadequately treated water (8890).
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