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Epidemiologic Notes and Reports Enteric Illness Associated with Raw Clam Consumption -- New York

Since June 1, 1982, the New York State Health Department has received reports of at least 14 separate outbreaks of gastroenteritis associated with consumption of raw clams. Approximately 150 persons have been affected. Typical symptoms have included diarrhea and abdominal cramps beginning 12-72 hours after eating clams, with nausea, vomiting, and fever occurring less often. In three of these outbreaks, seven individuals subsequently developed hepatitis A 21-37 days after eating clams. Three other persons developed hepatitis A without initial gastrointestinal symptoms. Eight of the 10 cases were verified by the presence of IgM antibody to hepatitis A virus (HAV); results on the others are pending.

A summary of four of these outbreaks follows: Outbreak A: On May 29, 24 individuals attended a private party in Albany County at which raw clams were served. Within 6-24 hours, 18 (90%) of 20 persons who had eaten clams developed diarrhea and abdominal cramps, which lasted 1-3 days. None of four persons who remained well had consumed clams. Stool specimens obtained shortly after onset of illness from seven persons with gastroenteritis were negative for Salmonella, Shigella, and Campylobacter. Two persons who ate clams and developed gastroenteritis contracted hepatitis A 21 and 27 days later. Clams from the same lot as those consumed at the party were evaluated at the New York State Health Department's laboratory. Although cultures of extracts from these clams did not grow enteric bacterial pathogens, both 27 nm and 40 nm virus-like particles were observed by electron microscopy.

Outbreak B: On May 30, fourteen people attended a private party in Rensselaer County at which clams were served. Five (83%) of six persons who ate raw clams developed diarrhea, nausea, vomiting, and abdominal cramps 36-72 hours later; symptoms persisted for 1-2 days. None of the eight persons who did not eat raw clams became ill. One of the five individuals with gastroenteritis, who worked as a food handler, developed hepatitis A (confirmed by the presence of HAV-specific IgM antibody) 34 days after eating clams, prompting county health officers to administer immunoglobulin (IG) as a preventive measure to 850 people exposed to foods he had prepared.

Outbreak C: On June 5, members of multiple bowling leagues attended a picnic in Albany County. Many of the approximately 200 attendees developed diarrhea, nausea, vomiting, and abdominal cramps 12-72 hours after the event. Forty-five of 126 persons interviewed reported gastroenteritis; 42 (89%) of these had eaten raw clams. Only raw clams were significantly associated with illness (p 0.001). Four persons who consumed clams and were affected by gastroenteritis developed hepatitis A 29-37 days later. This outbreak was not recognized in time to obtain specimens from persons with acute gastrointestinal illness.

Outbreak D: On July 11, 11 persons attended a party in Schenectady County at which raw clams were served. All seven individuals who ate clams developed diarrhea and abdominal cramps 15-60 hours later; none had fever or vomiting. Diarrhea persisted for up to 1 week in several persons. None of four persons who did not eat clams became ill. Thus, clams were epidemiologically implicated as the vehicle of transmission. Stool samples from five ill individuals were negative for enteric bacterial pathogens (Salmonella, Shigella, Vibrio, Campylobacter, and Yersinia). Examination of stools for virus is pending.

Inadequate or absent tagging of the clams implicated in these outbreaks has made it difficult to accurately determine the clams' source. However, current information indicates clams responsible for the outbreaks originated in coastal waters from at least three states: Massachusetts, New York, and Rhode Island. The timing of these outbreaks may be related to contamination of harvesting beds by the heavy rains and subsequent runoff that occurred in the Northeast during May and early June. Preliminary data from New York and Rhode Island indicate an increase in coliform counts in clam-harvesting waters monitored during this time.

Since December 1981, the New York State Department of Health has been informed of 33 outbreaks of clam-related illness involving more than 250 cases of gastroenteritis and 20 cases of hepatitis A. One county where clams are harvested has noted a two-fold increase in reported cases of hepatitis A for the first 6 months of this year compared with the same period last year (60 in 1982 vs 31 in 1981); 45% of the 1982 patients had histories of clam consumption consistent with the incubation period of hepatitis A. An intensive evaluation of 1,559 food establishments, conducted between July 22 and July 29, revealed that 125 (14%) of 908 that stock shellfish sold clams that were untagged or improperly certified (to identify their waters of origin).

Because these outbreaks suggested a recent problem of clam contamination, New York State Health Department officials currently advise individuals to refrain from eating raw clams. In addition, they advise giving IG to persons involved in clam-associated outbreaks of gastroenteritis, provided it can be administered within 2 weeks of clam consumption. Reported by L Lanzillo, J Reid, MD, Rensselaer County Health Dept, S Cobb, M DiManno, E Podgorski, J Lyons, MD, Albany County Health Dept, D Greenstein, N Maher, N Schell, MD, V Tulumello, Nassau County Health Dept, D Klotz, H Foust, Schenectady County Health Dept, L Bonser, J Debbie, DVM, R Deibel, MD, B Fear, J Guzewich, K Henry, J Raucci, I Loudon, MD, D Morse, MD, J Pert, MD, M Shayegani, PhD, P Smith, A Squire, R Stricof, R Svenson, R Rothenberg, MD, State Epidemiologist, New York State Health Dept; Div of Hepatitis and Viral Enteritis, Center for Infectious Diseases, Field Svcs Div, Epidemiology Program Office, CDC.

Editorial Note

Editorial Note: Ingestion of shellfish has been known for over 50 years to cause outbreaks of bacterial and viral enteric diseases (1). Typhoid fever (2), hepatitis A (3,4), cholera (5), and Vibrio parahaemolyticus (6) have long been associated with ingestion of raw clams and oysters. More recently, raw shellfish contaminated with non-O1 V. cholerae (7) and Norwalk virus (8,9) have also been reported as causes of gastroenteritis outbreaks. Although gastroenteritis (due to bacterial pathogens) and hepatitis A have recently been reported among persons drinking contaminated water (10), this is the first report in several years of outbreaks of these illnesses occurring jointly after shellfish consumption (11,12). Viral gastroenteritis in association with hepatitis A is not known to have been reported following shellfish consumption. The clinical findings observed in several of the New York outbreaks are compatible with a viral etiology, such as the Norwalk virus: a short incubation period, abrupt onset of upper and/or lower gastrointestinal illness, and brief duration (1-2 days). The absence of bacterial pathogens and the visualization of virus-like particles in clams from one outbreak further support a viral etiology in several of these outbreaks.

The recent New York State outbreaks may be related to periods of heavy rain and flooding. Run-off at these times, especially when sewage systems overflow, characteristically increases coliform counts in monitored coastal waters. However, the numerous outbreaks in New York before the May-June flooding suggest an endemic degree of clam contamination, some of which may be attributable to harvesting from uncertified, sewage-contaminated waters. This practice is likely to continue, because taking clams from highly populated, polluted beds is economically profitable and difficult to prevent. These outbreaks emphasize that clams may contain multiple enteric pathogens, including viruses, and consumption of clams--especially raw or partially cooked--continues to pose substantial risk of transmitting disease. Although the most effective way of avoiding the problem is to prevent the distribution of illegally gathered, untagged clams, such measures are not always possible. Therefore, because steaming or other forms of cooking do not always kill the enteric viruses in clams (13,14), the most effective means of preventing clam-associated illness is to adequately depurate them.


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  2. Lumsden LL, Hasseltine HE, Leake JP, Veldee MV. A typhoid fever epidemic caused by oyster-borne infection (1924-1925). Public Health Rep 1925;Suppl 50:1-102.

  3. Roos B. Hepatitis epidemic transmitted by oysters. Svenska lak-tidning 1956;53:989-1003.

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  5. McIntyre RC, Tira T, Flood T, Blake PA. Modes of transmission of cholera in a newly infected population on an atoll: implication for control measures. Lancet 1979;1:311-4.

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  7. Wilson R, Lieb S, Roberts A, et al. Non-O group 1 Vibrio cholerae gastroenteritis associated with eating raw oysters. Am J Epidemiol 1981;114:293-8.

  8. Murphy AM, Grohmann GS, Christopher PJ, Lopez WA, Davey GR, Millsom RH. An Australia-wide outbreak of gastroenteritis from oysters caused by Norwalk virus. Med J Aust 1979;2:329-33.

  9. Gunn RA, Janowski HT, Lieb S, Prather EC, Greenberg HB. Norwalk virus gastroenteritis following raw oyster consumption. Am J Epidemiol 1982;115:348-51.

  10. Sanchez Y, LaBelle RL, Hejkol T. Identification of hepatitis A antigen in sewage and well water prior to an outbreak of waterborne infectious hepatitis. Environmental aspects of viral hepatitis transmission. Proceedings of the Third International Symposium. In: Szmuness W, Alter HJ, Maynard JE, eds. on Viral Hepatitis Philadelphia:Franklin Institute Press 1981:629-30.

  11. Dismukes WE, Bisno AL, Katz S, Johnson RF. An outbreak of gastroenteritis and infectious hepatitis attributed to raw clams. Am J Epidemiol 1969;89:555-61.

  12. Begg RC. Food poisoning--four unusual episodes. N Z Med J 1975;82:52-4.

  13. Koff RS, Sear HS. Internal temperature of steamed clams. N Engl J Med 1967;276:737-9.

  14. Feachem R, Garelick H, Slade J. Enteroviruses in the environment. Trop Dis Bull 1981;78:185-230.

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