FRI Briefings

Hepatitis A

According to the Centers for Disease Control and Prevention (CDC), 31,582 cases of hepatitis A were reported in the USA in 1995 but the real incidence of this disease is estimated to be 138,000 cases/year (5). Not all of these were food- or waterborne. In fact, the majority of hepatitis A cases probably result from fecal–oral transfer during close contact with an infected person. Nevertheless, in its report on cases of foodborne disease outbreaks in the USA during 1988–1992, the CDC lists HAV as the fourth leading cause of foodborne disease (6). Numbers of hepatitis A cases in the USA wax and wane in approximately 10-year cycles. The last peak was in 1989, with 35,821 cases reported (7). Outbreaks are most often associated with consumption of salads, fresh fruits or vegetables, or shellfish, such as oysters.

Multiple types of hepatitis viruses are known to exist but only HAV is commonly associated with foodborne illness. Hepatitis E virus has been associated with waterborne illness whereas hepatitis B, C, and D are spread only through body fluids. HAV is an RNA virus which is quite resistant to drying and is more heat resistant than many other enteric viruses. Foods become contaminated with HAV by exposure to fecal material from infected persons. (Feces from an infected person can contain one million virus particles per gram.) Once the contaminated food or water is ingested, the virus makes its way to the liver but it may be 15–50 days before symptoms appear. The disease typically has an abrupt onset with fever, malaise, anorexia, nausea, dark urine, and jaundice. Usually hepatitis A is less severe in younger children and is only rarely fatal, although symptoms may persist as long as two months. There is no treatment for the disease other than to relieve symptoms. It is very important to realize that peak shedding of viruses in the feces begins about 2 weeks before obvious signs of illness and continues for about a week after onset of symptoms. Therefore, an infected food handler with poor personal hygiene habits may be a source of infection even before he or she is aware of being sick.

Contamination of food may occur in one of several ways: An infected food handler with dirty hands working in the field picking fruits or vegetables, in a food-processing plant sorting or packing, or at a restaurant making salads or fresh fruit plates may pass on this disease to others. Irrigation or processing water contaminated with sewage may introduce viruses onto fresh fruits and vegetables. Shellfish living in relatively shallow coastal waters may be exposed to viruses from inadequately treated sewage discharged from treatment plants on shore or from dumping of wastes from ships. In fact, current water treatment practices are unable to completely inactivate all enteric viruses.

An outbreak of hepatitis A associated with frozen strawberries also occurred in 1990 and was responsible for 28 confirmed cases in Montana and Georgia. An epidemiological investigation revealed that the containers of strawberries associated with illness in the two states were processed on the same night in the same plant in California (8). The berries were grown in California and could have been contaminated either in the field or in the processing plant. None of the plant or field workers were diagnosed with hepatitis A within two months before or after that processing date. However, significant underreporting of HAV infection is known to occur and investigators considered it likely that a worker was infected.

An infected baker was demonstrated to be the most likely source of 79 cases of hepatitis A in a community outbreak in New York in 1994 (9). The baker apparently contaminated doughnuts while applying a sugar glaze. Investigation revealed that the source of HAV was clearly someone who had handled the doughnuts after they had been cooked. This outbreak illustrates the point that any food can harbor HAV if it has been handled by an infected person just before serving. An interesting sidelight to this outbreak is its similarity to a 1968 community outbreak of hepatitis A in Michigan that was also traced to an infected baker applying glaze to doughnuts. This incident had been widely used as an epidemiology teaching exercise. Apparently its lessons were forgotten (or never learned) by a later generation of bakery workers.

Oysters, clams, and mussels are filter feeders that process relatively large volumes of water and can concentrate food particles (and viruses) from the surrounding water. Since oysters are frequently consumed raw and mussels and clams are often steamed lightly, HAV is a significant problem in these foods. Recent experiments demonstrated that mussels immersed in virally contaminated water readily adsorbed 4–56% of the virus particles present (10). One approach to cleansing shellfish of viruses, depuration, involves placing them in tanks of virus-free water for a period of time to allow them to "wash out" contaminants. However, commercial depuration procedures did not completely remove HAV from contaminated mussels even after 96 hours’ immersion in a continuous flow of ozonated marine water (10). Cooking was also not completely effective in eliminating HAV. Viruses were still detectable in steamed mussels five minutes after the opening of the shells (10).

Detection of viruses in foods has been, for the most part, difficult and relatively unsuccessful. Recently developed techniques using polymerase chain reaction (PCR) assays to amplify the nucleic acids of a pathogen have been modified for detection of HAV in oysters. Using total RNA extracted from oyster meat, contaminated with HAV by adsorption, bioaccumulation, or injection, and reverse transcription-PCR, Cromeans et al. were able to detect the equivalent of as few as 8 plaque-forming-units (pfu) of HAV/gram of oyster meat (11). One drawback of such assays is that PCR detects the presence of nucleic acids even if the virus particle is no longer infectious because its protein coat has been disrupted by disinfectants or heat. Using a magnetic immunoseparation step which immobilizes only undamaged viruses followed by PCR, Lopez-Sabater et al. detected as few as 10 pfu of infectious HAV in artificially contaminated oyster meat (12). Further development of these methods should aid in the detection of contaminated foods.

Outbreaks of foodborne disease may involve substantial costs to society and to food facilities as well as to the affected individuals. An investigation of a foodborne outbreak of hepatitis A involving 43 persons apparently infected by a food handler working for a catering company in Colorado concluded that the costs from a societal perspective were $809,706 (13). Medical costs incurred by the cases totaled $46,064 while disease control costs included $450,397 for 16,293 immune globulin injections and $105,699 for 2777 hours of health department personnel time. These estimated costs were considered conservative and did not include all of the preventive medical costs and the insurance compensation for pain and suffering for persons who developed the disease. This latter cost was expected to be several hundred thousand dollars.

Strategies for preventing outbreaks of hepatitis A include preventing contamination by: (a) strict attention to personal hygiene, especially hand washing, by all food handlers; (b) use of clean, not fecally contaminated, water for irrigating, washing and processing foods; and (c) prevention of the contamination of shellfish beds by sewage (14). If contamination of food has occurred, thermal processing is generally effective in destroying infectivity. However, HAV is relatively heat-resistant as well as resistant to drying and acid. Virus particles in water and on surfaces can be inactivated by UV light and by strong oxidizing agents, such as chlorine or ozone.

The recent introduction of a vaccine for hepatitis A offers the possibility for prevention of some outbreaks of this disease by vaccination of food handlers. Based on limited data, two doses of the vaccine are estimated to confer immunity for at least 6–7 years and maybe as long as 20 years (4). (Prior infection with HAV confers lifelong immunity.) This vaccine will be much more useful than the immunoglobulin shots which afford protection for only 3–6 months. Economic analyses will be needed to determine whether it is cost-effective and desirable that some or all food handlers be vaccinated (15). For example, in the Colorado outbreak, it would have cost about $9600 to vaccinate the 100 food handlers who were employed at the catering facility (12). Preventive efforts should be increased as we approach the time for next peak in the cycle of hepatitis A cases.

References

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University of Wisconsin–Madison