Use of Molecular Epidemiology to Confirm a Multistate Outbreak of Hepatitis a Caused by Consumption of Oysters

Although outbreaks of hepatitis A attributed to consumption of contaminated shellfish have been reported previously [1,2,3,4–5], there have been no large outbreaks of shellfish-related hepatitis A reported in the United States since 1988. In September 2005, cases of hepatitis A among persons who had recently consumed raw oysters were reported to public health officials in Alabama and Florida. The Centers for Disease Control and Prevention (CDC; Atlanta, GA) assisted with the investigation, to determine the source and extent of the outbreak and to assess whether the geographically separated cases were linked.

Methods. A case-control study was conducted in Alabama to determine whether oysters were the source of hepatitis A virus (HAV) transmission. A case patient was defined as an Alabama resident with laboratory confirmation of acute HAV infection who experienced an onset of symptoms during the period of August-September 2005. A control subject was defined as an Alabama resident who was not ill, had no self-reported history of hepatitis A or hepatitis A vaccination, and who was a social contact of a case patient or a community member from the same zip code and phone exchange. Data on demographic characteristics, risk factors for HAV infection (e.g., international travel, exposure to a known case patient, male-male sex, or illicit drug use), and consumption of shellfish, raw produce, and commercially prepared foods were collected by telephone interview using a standardized questionnaire.

The CDC informed state health departments in the United States about the outbreak and requested that all newly reported patients with hepatitis A be asked about shellfish consumption. For the purposes of case finding, an oyster consumption-related case of hepatitis A was defined as a person in the United States with clinical symptoms and laboratory confirmation of acute HAV infection who consumed oysters during the 2–6-week incubation period.

Serum samples were obtained from case patients and sent to the Molecular Epidemiology Laboratory (Division of Viral Hepatitis, CDC) for HAV RNA sequencing. Sequence analysis was performed on the VP1-2B region using previously described techniques [6].

The US Food and Drug Administration and state health departments conducted a traceback of oysters served at restaurants on the dates that case patients ate them. Commercial oystering is regulated by states in accordance with the National Shellfish Sanitation Program, and harvesting is limited to state-designated areas. Harvesters, dealers, and shippers are required to label all containers of oysters that they sell with tags containing information about the harvest date and area, as well as an identification number. Restaurants are required to maintain the tags for 90 days after sale or disposal.

Results. Sixteen case patients and 31 control subjects were included in the case-control study. Among case patients, 15 (94%) reported eating raw oysters, compared with 2 control subjects (7%; OR, 217.5; 95% CI, 18.2–2597.4). There were no statistically significant differences between case patients and control subjects with regard to consumption of cooked oysters, other shellfish, or raw produce or in the frequency of eating from salad bars or in restaurants.

A total of 39 oyster consumption-related cases of hepatitis A were reported from Alabama, Florida, South Carolina, and Tennessee during August-September 2005 (figure 1). None of the case patients reported any other risk factor for HAV infection. The median age of case patients was 50 years (range, 18–78 years), and 30 (77%) were male. Ten were hospitalized. Thirty-eight case patients ate raw oysters. The 1 case patient who did not eat raw oysters ate baked oysters at a restaurant where other case patients had consumed raw oysters. The dates of oyster consumption ranged from 21 July to 8 August 2005; the date of symptom onset ranged from 21 August to 22 September 2005.

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Figure 1

Dates of oyster consumption and symptom onset among 39 case patients with acute hepatitis A who ate oysters reported from Alabama, Florida, South Carolina, and Tennessee, August-September 2005. Oysters implicated in the outbreak were harvested from Louisiana estuarine waters during 16 July–1 August 2005 and were consumed by case patients during 21 July–8 August 2005. The Gulf of Mexico estuarine oyster harvest areas were closed in anticipation of Hurricane Katrina, which made landfall on 29 August 2005.

Thirty-seven case patients provided reliable information about the dates of consumption and the purchase places of the oysters; 15 case patients consumed oysters at 1 of 5 restaurants in Alabama, 15 case patients consumed them at 1 of 4 restaurants in Florida, 2 case patients consumed them at a single restaurant in South Carolina, and 5 case patients consumed them at a single restaurant in Tennessee. The product traceback investigation indicated that oysters served at these restaurants had been purchased unshucked from a single shellfish shipper (shipper X). Inspection of shipper X's facility revealed that the water source used to clean debris from the unshucked oysters met state requirements. Shipper X's employees had minimal physical contact with the oysters, and none reported symptoms of hepatitis.

Oysters served in the Alabama, Florida, and Tennessee restaurants to 35 case patients had been harvested from a single Louisiana harvest area (A-3), an estuarine area in the Gulf of Mexico remote from any human habitation or a fixed source of sewage discharge. The tags from the oysters served to the 2 case patients at the South Carolina restaurant indicated a different Louisiana harvest area (A-10). Area A-10 is not contiguous to area A-3. Harvesting of the implicated oysters occurred during 16 July–1 August 2005. All Louisiana shellfish harvest areas were closed on 28 August 2005 because of the approach of Hurricane Katrina, and they remained closed until November 2005.

Serum samples were obtained from 31 case patients, and HAV RNA was isolated from 28 of these samples, including a sample obtained from 1 of the case patients who had eaten oysters at the restaurants for which the records indicated the harvest area was A-10. All 28 case patients had identical HAV RNA sequences. This outbreak-related sequence was different from the sequences from all other cases (n = 84) isolated by the Molecular Epidemiology Laboratory (Division of Viral Hepatitis, CDC) in 2005. To determine whether HAV RNA could be isolated from potentially contaminated oysters, the US Food and Drug Administration obtained frozen oysters from shipper X that had been harvested from Louisiana harvest area A-3 during the same time as the oysters served to the 35 case patients. HAV RNA sequences isolated from the frozen oysters were identical to the HAV RNA sequence that had been isolated from all case patients [7].

Discussion. This was the first large outbreak of hepatitis A associated with oyster consumption in the United States in >15 years. Product traceback was possible because of the availability of records maintained by restaurants, shippers, and dealers in compliance with National Shellfish Sanitation Program guidelines. The epidemiologic investigation and molecular sequencing data demonstrated a very strong association between illness and consumption of oysters from a single harvest area. This was the first outbreak of hepatitis A in which an identical HAV sequence was obtained from both the implicated food product and the case patients. The ability to isolate HAV from food products linked to hepatitis A outbreaks in the past was hampered by the long incubation period of hepatitis A, which made the recovery of implicated foods for testing difficult, and because methods to detect HAV in food products have only recently been fully developed.

Records maintained by restaurants and shellfish shippers suggested that oysters consumed by 2 case patients were from a different harvest area from those consumed by the other case patients. The 2 harvest areas implicated by the traceback investigation were remote from each other and from fixed sources of sewage discharge. Shellfish harvest vessels are required to have sewage containment systems on board, and harvesters undergo education about the dangers of discharging sewage into shellfish beds. Overboard sewage discharge by a harvester seems unlikely. However, harvest area contamination could have occurred from another marine vessel that did not have a sewage containment system, such as a recreational boat. Comingling of oysters from 2 harvest areas by the shipper could have resulted in oysters from harvest area A-3, the area implicated for the majority of cases, being served to the other 2 case patients as well. Shipper X acknowledged that oysters from both harvest areas were present in his facility at the same time and that inadvertent comingling may have occurred. Although separate sewage contamination events in both harvest areas could have occurred, simultaneous discharges with identical HAV sequences in widely separated areas of the Gulf of Mexico are unlikely to have happened. A large number of different HAV sequences circulate in the US population at any one time [6], and >200 sequences have been identified among patients who have acquired hepatitis A in the United States [8].

Previous outbreaks of hepatitis A associated with oyster consumption in the United States have been attributed to illegal harvesting from sewage-contaminated areas [2] or unrecognized contamination of regulated harvest areas by sewage runoff associated with heavy rains and flooding [3]. The absence of any large oyster consumption-related outbreaks of hepatitis A since the late 1980s suggests that National Shellfish Sanitation Program guidelines may have been effective in reducing HAV transmission via shellfish in the United States. In addition, the incidence of hepatitis A has decreased by >80% in the past 10 years, reducing the number of potential sources of contamination [9].

Molecular epidemiologic methods were critical in this investigation for linking cases reported in multiple states, as well as for interpreting product traceback findings. HAV sequence comparison analyses are becoming an increasingly important component in investigations of foodborne hepatitis A [6, 10, 11] and are particularly useful for investigations such as the one reported in this article, in which exposure to HAV-contaminated oysters occurred in 11 restaurants across 4 states and attack rates were low. As public health laboratories develop the capacity for HAV sequence analyses, which can be performed using serum specimens collected during diagnostic evaluations, the potential for more timely outbreak identification and effective interventions will increase.

• Received October 10, 2006.
• Accepted December 15, 2006.

• © 2007 by the Infectious Diseases Society of America