Recovery of Aeromonas hydrophila from Oysters Implicated in an Outbreak of Foodborne Illness

Abstract

Potentially pathogenic Aeromonas hydrophila organism were isolated from oysters frozen at-72°C for 1-1/2 years. The oysters which had been associated with 472 cases of gastroenteritis in Louisiana in November 1982, were examined and found negative for Salmonella, pathogenic Vibrio parahaemolyticus, and diarrhetic shellfish poison. In 1983, oysters from the same shellfish growing area in Louisiana were implicated in seven cases of gastroenteritis caused by A. hydrophila. The oysters collected in 1982 were reexamined and found to contain A. hy-drophila (MPN 9.3/100 g). Twenty-three of 28 strains identified by the MICRO-IS and API-20E systems were positive for at least one of the tests for virulence which included the suckling mouse test, the adrenal Y-l mouse cell test, and hemolysin assays. Of five strains tested, all showed activity in the rabbit ileal loop. Although these results do not prove that A. hydro-phila caused the outbreak in 1982, they suggest that in cases of foodborne illness involving oysters, A. hydrophila should be included in the screening tests. During November and December 1982, approximately 472 cases of gastroenteritis associated with consumption of raw oysters were reported to the Food and Drug Administration (FDA) (3) by the Louisiana Department of Health and Human Resources. Affected individuals had nausea, vomiting, diarrhea and stomach cramps which developed from 24 to 48 h after ingestion of oysters. Implicated oysters were harvested from two separate areas of Louisiana (Sister Lake area in Terrebonne Parish and St. Mary's Point area in Jefferson Parish). A background investigation by FDA and Louisiana officials determined that inclement weather conditions coupled with bypasses of sewage from already overloaded wastewater treatment facilities occurred in this area just before the reported illnesses. Although the treatment plants were located 30 miles from the approved shellfishing areas, they were suspected of affecting the water quality of the shellfish-'Seafood Products Research Center. 2 Food Research Laboratory. growing area under the prevailing meterological conditions. Oysters involved in the outbreak were analyzed for Salmonella and Vibrio parahaemolyticus by FDA laboratories in Cincinnati and Dallas. High levels of weakly hemolytic V. parahaemolyticus with low pathogenic potential were present (Stelma, G.N., Jr., C.H. Johnson, A.L. Reyes, R.G. Crawford, P.L. Spaulding, and R.M. Twedt, 1985, Abstr. Annu. Meet. Am. Soc. Microbiol., p. 4 1 , p. 257). No salmonellae were found. Enteric viruses , particularly Norwalk virus were suspected of being causative agents (72) as were marine toxins. We examined these oysters to determine the presence of diarrhetic shellfish poison (DSP) because the symptoms were consistent with those reported for this toxin {16); however, no toxins were found. Reserve oysters were stored at-72°C. Approximately 1 year later, an oyster-borne disease outbreak occurred in St. Petersburg, Florida, involving seven cases (7). Oysters and stool specimens from affected individuals were positive for Aeromonas hydro-phila. The oysters came from the same growing area in Louisiana that had been involved in the November 1982, outbreak. Because of similarities between the two outbreaks , the oysters that had been frozen for 1-1/2 years were examined and found to contain potentially en-teropathogenic strains of A. hydrophila. MATERIALS AND METHODS Media preparation Trypticase soy broth. Trypticase soy broth with ampicillin (TSBA) (17), was prepared according to the manufacturer's (Difco, Detroit, MI) directions. After the TSB was autoclaved at 121°C for 15 min, filter-sterilized ampicillin (Sigma St. Louis, MO) was added to give a final concentration of 30 mg/ L. Tryptone broth. Tryptone broth (TB) (Difco) was prepared by dissolving 8 g of tryptone and 5 g of NaCl per liter and adjusting the pH to 6.9 + 0.2. The TB was autoclaved for 15 min at 121°C.