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Vibrio parahaemolyticus Risk Assessment: II. Hazard Identification

July 19, 2005

Table of Contents

The Hazard Identification component of a microbial risk assessment is the identification of the pathogenic microorganism that is capable of causing adverse health effects and is present in a particular food or group of foods. The hazard on which this risk assessment is focused is pathogenic V. parahaemolyticus in raw oysters and the adverse health effects include gastroenteritis and septicemia.

Vibrio parahaemolyticus

Vibrio parahaemolyticus is a Gram-negative, halophilic bacterium that occurs naturally in estuaries and is recognized as an important bacterial seafood-borne pathogen throughout the world (Fujino et al., 1953; Sakazaki, 1973). Vibrio spp. are found in the estuarine environment in the tropical and temperate zones (Joseph et al., 1983). These bacteria are normally present in many seafoods, including fish, crustaceans, and molluscan shellfish. They concentrate in the gut of filter-feeding molluscan shellfish such as oysters, clams, and mussels where they multiply and cohere.

The genome of V. parahaemolyticus was sequenced (Makino et al., 2003) and was found to consist of two circular chromosomes of 3,288,558 bp and 1,877,212 bp, and contains 4,832 genes. Although V. parahaemolyticus is phylogenetically close to V. cholerae, comparison of the V. parahaemolyticus genome with that of V. cholerae showed there are many rearrangements within and between the two chromosomes. Chromosome 1 does not differ much in size between the two genomes (33 vs. 30 Mb), but chromosome 2 is much larger in V. parahaemolyticus than in V. cholerae. Genes for the type III secretion system (TTSS) identified in the genome of V. parahaemolyticus are not found in V cholerae. The TTSS is a central virulence factor of diarrhea-causing bacteria such as Shigella spp., Salmonella spp., and enteropathogenic Escherichia coli, which cause gastroenteritis by invading or intimately interacting with intestinal epithelial cells. These results suggest that V. parahaemolyticus and V. cholerae use different mechanisms to establish infection.

Isolates of V. parahaemolyticus can be differentiated by serotyping. The system for identifying V. parahaemolyticus serotypes is based on the different antigenic structures of the lipopolysaccharides groups (referred to as O groups) and capsular types (referred to as K types) (Joseph et al., 1983). Thirteen O groups and 71 K types have been identified by commercial antisera (Iguchi et al., 1995). Of these, 11 O groups and 38 K types have been isolated from V. parahaemolyticus strains collected in the United States (Fishbein et al., 1974). In a recent study, 27 different O:K serotypes were found among 178 strains isolated from various sources including seafood, sediment and clinical samples (DePaola et al., 2003a).

Historically, V. parahaemolyticus infections have been characterized by sporadic cases caused by multiple, diverse serotypes. However, three serotypes (O4:K12, O1:K56, and O3:K6) predominated in outbreaks associated with the consumption of raw molluscan shellfish in 1997 and 1998. The serotypes isolated from patients in the 1997 outbreak in the Pacific Northwest included O4:K12 and O1:K56 (Daniels et al., 2000a). In outbreaks in 1998 in Texas and New York, the serotype O3:K6 was the predominant isolate and principal cause of illness. Prior to the 1998 outbreak, the O3:K6 serotype had only been reported in Asia; this was the first time it was reported in the United States. This serotype may have a lower infectious dose then other pathogenic V. parahaemolyticus strains (Daniels et al., 2000b).

Strains of V. parahaemolyticus are isolates of the same serotype that have been characterized or distinguished from each other. Not all strains of V. parahaemolyticus cause illness in humans; in fact, the majority of strains isolated from the environment or seafood are not pathogenic. For the purpose of this risk assessment, pathogenic strains of V. parahaemolyticus are those that produce thermostable direct hemolysin (TDH). TDH is an enzyme that lyses (breaks down) red blood cells on Wagatsuma blood agar plates, which is referred to as the Kanagawa phenomenon. The role of the toxin in illness is not known.

Illnesses Caused by Vibrio parahaemolyticus

The most common clinical manifestation of V. parahaemolyticus infection is gastroenteritis, an inflammation of the gastrointestinal tract. Gastroenteritis is usually a self-limited illness with moderate severity and short duration (Barker, 1974; Barker and Gangarosa, 1974; Hlady, 1997; Levine et al., 1993). A summary of clinical symptoms associated with V. parahaemolyticus gastroenteritis infection is presented in Table II-1. Symptoms of illness include explosive watery diarrhea, nausea, vomiting, abdominal cramps, and less frequently headache, fever and chills. Diarrhea may also be characterized by full-blown dysentery with blood and pus and superficial ulceration on proctoscopic examination (Carpenter, 1995).

Table II-1. Clinical Symptoms Associated with Gastroenteritis Caused by Vibrio parahaemolyticus
SymptomsIncidence of Symptoms
Diarrhea98%80 to 100%
Abdominal cramps82%68 to 100%
Nausea71%40 to 100%
Vomiting52%17 to 79%
Headache42%13 to 56%
Fever27%21 to 33%
Chills24%4 to 56%
Source of data: Barker and Gangarosa, 1974; Levine et al., 1993

On rare occasion, infection can lead to septicemia. Septicemia is a severe, life-threatening, systemic disease caused by the multiplication of pathogenic microorganisms and/or the presence and persistence of their toxins in the circulating blood. It is characterized by fever or hypotension and the ability to isolate the microorganism from the blood. In cases of septicemia, subsequent symptoms can include swollen, painful extremities with hemorrhagic bullae (Hlady, 1997; Klontz, 1990). Death may also occur subsequent to the occurrence of septicemia.

Duration of illness can range from 2 hours to 10 days (Barker and Gangarosa, 1974; Barker et al., 1974). Information from several United States outbreaks revealed that the incubation period ranges from 12 to 96 hours with a median of approximately 15 to 24 hours (CDC, 1998; CDC, 1999a; Lowry et al., 1989; Nolan et al., 1984).

At-Risk Populations

Any exposed individual can become infected with V. parahaemolyticus and develop illnesses (such as gastroenteritis). However, infected individuals with underlying chronic medical conditions often develop septicemia. Therefore, although all raw shellfish consumers are "at risk" for infection, there is a subpopulation of individuals with increased risk of severe disease.

Individuals with Chronic Medical Conditions. Chronic medical conditions include liver disease, immunodeficiency, peptic ulcer disease, diabetes, alcoholism, hematological disease, gastric surgery, heart disease, renal disease, cancer or malignancy, treatment with corticosteroids, and transplant recipients (Klontz, 1990; Klontz, 1997; Angulo and Evans, 1999).

The percentage of the population that is at increased risk for development of septicemia from V. parahaemolyticus infection is not known precisely. The Center for Science in the Public Interest reported that approximately 20% of the United States population (60 million) have immunocompromised conditions and are at increased risk for V. vulnificus septicemia (CSPI, 1997). However, it is not known how many of these individuals consume raw oysters. Based on studies showing that certain persons are at greatest risk for illness from raw-oyster associated V. vulnificus infection (Desenclos et al., 1991 and Klontz, 1990), it was estimated that approximately 7% of the population have immunocompromising health conditions associated with increased risk of infection (Klontz, 1997). Analysis of epidemiological surveillance data (Angulo and Evans, 1999) indicates that approximately 30% of 107 cases of gastroenteritis were identified in individuals with underlying chronic illnesses. However, immunocompromised individuals may be over represented in case series data because of a "reporting phenomenon" driven by the severity of illness. An immunocompromised individual may be more likely to seek medical care for the symptoms of V. parahaemolyticus illness than an otherwise healthy individual with the same symptoms.

Raw Shellfish Consumers. Surveys conducted by FDA in 1993 and 1998 indicate that consumption of raw shellfish is not uniformly distributed in the United States population (Levy and Fein, 1999). For example, a higher percentage of men consume raw oysters than women (16% vs. 7%) , and raw shellfish consumption is higher for those living along the coastline of the United States than for those living inland (22% vs. 13%). The trend in raw shellfish consumption, as evidenced in the 1998 FDA survey, is toward lowered consumption of raw shellfish. This may be the result of education efforts by the Agency concerning the risks associated with the consumption of raw or undercooked protein foods, such as beef, chicken, eggs, and shellfish.

Annual Incidence

In 1999, CDC conducted a comprehensive evaluation of the national burden of infectious food-related illnesses in the United States. The total annual incidence of Vibrio illness was estimated as 7,880 illnesses and of that 65% were estimated to be food related (Mead et al., 1999). This estimate was based on the frequency of reported cases obtained by passive surveillance from 1988 through 1996 and the cases reported through FoodNet. The estimate also considers that this illness is under reported and under diagnosed and for every reported illness there are assumed to be 20 cases that are not reported (Kennedy, 2000; Mead et al., 1999).

Based on FoodNet data, the yearly estimates of food-related illness attributed to V. parahaemolyticus for 1996, 1997 and 1998 were approximately 2,700, 9,800, and 5,600, respectively (Tauxe, 2000). The 1997 estimate reflects the increased reporting of cases from a large outbreak in the Pacific Northwest. Some variation in estimated cases from year to year is expected, even in the absence of any inter-annual variation attributable to differing environmental conditions.

Specifically for this risk assessment (see Chapter III Hazard Characterization), CDC conducted an in-depth analysis of the available data on the incidence of illness from consumption of raw oysters reported over a 5-year period (1998-2002). CDC estimated there are approximately 2,790 cases of V. parahaemolyticus illness in the United States as result of oyster consumption (Painter, 2003). To obtain this estimate, CDC compared the reported cases from the National Notifiable Diseases Surveillance System (NNDSS) and the Cholera and Other Vibrio Illness Surveillance System (COVISS) because these systems collect reports from all states. Some cases are reported in both systems. A comparison of case information (using "capture-recapture" method for surveillance evaluation) indicated the number of reported cases was 1,125 for the 5-year period (or 225 cases per year). This compares well with FoodNet surveillance data (which represents 13% of the United States population) which indicate there are 300 cases per year in the United States. As noted above, CDC estimates that the number of cases is underestimated by a factor of 1:20 due to underreporting. So the estimated number of cases is 4,500 (225 x 20). Using information relating to V. parahaemolyticus exposure from COVISS, CDC estimates that 62% of all V. parahaemolyticus illness cases are caused by consumption of raw oysters. Therefore, the estimated number of cases of illness from V. parahaemolyticus in raw oysters used in the dose-response modeling was 2,790 (0.62 x 4,500). See Chapter III Hazard Characterization for details.

CDC's Active Surveillance Systems

  • FoodNet. The Foodborne Diseases Active Surveillance Network (FoodNet) is the principal foodborne disease component of CDC's Emerging Infections Program (EIP). FoodNet is a collaborative project of the CDC, 10 EIP sites (California, Colorado, Connecticut, Georgia, New York, Maryland, Minnesota, Oregon, Tennessee and New Mexico), the United States Department of Agriculture (USDA) and the Food and Drug Administration (FDA).
  • CDC Gulf Coast Vibrio Surveillance System (GCVSS). The CDC Gulf Coast Vibrio Surveillance System (GCVSS) is a unique regional system that began in 1988 (Levine et al., 1993). Four states initially participated in this program (Alabama, Florida, Texas, and Louisiana). Mississippi was added soon after, and the system has grown to include any and all states that are willing to participate; indeed, in the last few years, the West Coast states have become very active in reporting cases (Crowe, 2002). Investigators in state and county health departments complete standardized Vibrio illness investigation forms on all patients from whom Vibrio isolates are reported. Vibrio reporting comes from individual physicians, hospitals, or laboratories. Illness investigation forms contain clinical data concerning signs and symptoms, underlying illnesses, use of medications, as well as epidemiological information concerning seafood consumption in the week prior to illness. Data from this surveillance system has also been used for case series analysis (see discussion below).

Outbreaks and Sporadic Cases

An outbreak is defined as the occurrence of two or more cases of a similar illness resulting from the ingestion of a common food. The term "sporadic cases" refers to an irregular pattern of occurrence, with occasional cases occurring at irregular intervals. Sporadic cases can be reported as either "case reports" which present pertinent information on individual cases, or as a "case series" which is a study of sporadic cases over a specified period of time.

The first confirmed case of foodborne illness-associated V. parahaemolyticus infection in the United States occurred in Maryland in 1971 with an outbreak associated with consumption of steamed crabs (Dadisman et al., 1972). Between 1973 and 1998, forty outbreaks were reported to the CDC from 15 states and the Guam Territories (Daniels et al., 2000a). These outbreaks were associated with raw seafood or cooked seafood cross-contaminated with raw or undercooked seafood. Since 1998, there have been three outbreaks caused by V. parahaemolyticus, and all were associated with consumption of oysters (Agasan, 2002; New Jersey Dept of Environmental Protection, 2002; Potempa, 2004).

Table II-2 summarizes the major outbreaks of V. parahaemolyticus gastroenteritis in the United States from 1997 to 2002. In 1997, an outbreak involving 251 cases occurred in the Pacific Northwest (202 in the United States and 49 in British Columbia) (Sample and Swanson, 1997). Of these cases, V. parahaemolyticus infection was confirmed in 209 persons who consumed raw oysters harvested from California, Oregon and Washington and from Canada (CDC, 1998). The most common V. parahaemolyticus serotypes isolated from patients involved in this outbreak were O4:K12 and O1:K56 (Daniels et al., 2000a). In the United States, oyster-associated V. parahaemolyticus outbreaks are more common than other shellfish-associated  V. parahaemolyticus outbreaks (Daniels et al., 2000; Agasan, 2002; New Jersey Dept of Environmental Protection, 2002; Potempa, 2004).

Three separate outbreaks occurred in the United States in 1998. In the Pacific Northwest, 48 cases were reported (Therien, 1999). In Texas, a total of 416 V. parahaemolyticus infections were associated with consuming raw oysters harvested from Galveston Bay (Daniels et al., 2000a). Also in 1998, New York reported the first outbreak associated with raw molluscan shellfish harvested from that state and this outbreak included 23 cases, 10 of which were associated with raw oysters (CDC, 1999a).

In the summer of 2002, a cluster of seven cases with V. parahaemolyticus infection appeared to be linked to the consumption of shellfish that was harvested and purchased locally in the Long Island and New York City area (Agasan, 2002). In another outbreak that same year, a total of 11 cases with two fatalities were reported in New Jersey (Mulnick, 2002). These cases were attributed to the above average water temperatures that year and resulted in closing 110 square miles of oyster beds (New Jersey Dept. of Environmental Protection, 2002).

Table II-2. Outbreaks of Illnesses from Vibrio parahaemolyticus Associated with Consumption of Raw Oysters in the United States
YearLocationNumber of Cases
1997Pacific Northwesta209b
1998Pacific Northwesta48
1998Northeast Atlantic10b
2002New York7
2002New Jersey11
2004Alaska46 (8b)
a The Pacific Northwest includes California, Oregon, Washington State, and British Columbia.
bNumber of cases that were culture-confirmed.
cIncludes 296 cases in Texas and 120 cases in other states traced back to oysters harvested from Texas.

Case Reports 
Several case reports have been published that outline clinical presentations and outcomes of patients with V. parahaemolyticus. One such case report describes a 35-year-old woman who sought medical attention for abdominal pain after she had consumed raw fish (Tamura et al., 1993). She presented with gastrointestinal symptoms, redness on lower extremities, fever, polyarthritis and weakness. Vibrio parahaemolyticus was isolated in the stool culture. She was diagnosed as having reactive arthritis induced by V. parahaemolyticus infection. Another clinical case report describes a 31 year-old female with a history of alcohol abuse, hepatitis C virus infection, and cirrhosis (Hally et al., 1995). She presented with diarrhea, weakness, leg pain, and urine retention. The patient had ingested raw oysters and steamed shrimp 72 hours prior to being admitted to the hospital. Vibrio parahaemolyticus was isolated from blood samples. The patient developed cardiac arrest and died six days after presentation.

A suspected case of a laboratory-associated infection was reported in 1973 (Sanyal et al., 1973). One day prior to the development of diarrheal disease the laboratory worker had been handling V. parahaemolyticus strains for the first time. The illness was associated with severe upper abdominal pain, bloody stools, nausea and fever. Weakness and abdominal discomfort continued for two days beyond the onset of illness. No other source of V. parahaemolyticus could be identified, and it was believed that the infection was caused by a relatively small inoculum.

Case Series 
Case series data (Angulo and Evans, 1999) was used to analyze the relationship between illness outcomes and pre-existing health conditions. The data were from oyster-related culture-confirmed cases reported to the CDC GCVSS from 1997 to 1998. There were a total of 107 V. parahaemolyticus cases, of which 102 were gastroenteritis only, 5 that progressed to septicemia and 1 death. The overall incidence of septicemia among culture-confirmed V. parahaemolyticus infections was approximately 5% (5 out of 107). Of the cases with information on health conditions, 29% (23 out of 79) of the gastroenteritis illnesses and 75% (3 out of 4) of the septicemia illnesses occurred in individuals with an identified underlying (immunocompromising) health condition. The underlying medical conditions included liver disease, alcoholism, diabetes, malignancy, renal disease, immunodeficiency, hematological disease, and gastric surgery. The data from this case series was used in "Chapter III Hazard Characterization," to estimate the annual number of septicemia cases in susceptible and healthy populations.

Case series have also been reported by others including Bonner et al. (1983), Noland et al. (1984), Kelly and Stroh (1988b), and Levine and Griffin (1993). These studies have also illustrated the association of septicemia with underlying medical conditions. Three case series for illnesses and deaths associated with V. parahaemolyticus infections from consumption of shellfish in Florida from 1981 to 1991 are described below.

  • A case series of 4 patients who died in Florida due to V. parahaemolyticus infection from 1981 to 1988 was reported by Klontz (1990). All patients were male and all were over the age of 60 years. All died of septicemia. Two of the patients reported eating raw oysters during the week before onset of illness. The median duration of illness was 24 hours. All patients had underlying medical conditions, including cirrhosis, heart disease, prostate cancer and lung cancer.
  • A case series of 690 Vibrio infections related to raw oyster consumption in Florida during 1981 to 1993 was reported by Hlady and Klontz (1996). There were 355 cases of gastroenteritis, of which 68% were associated with the consumption of raw oysters and 120 (34%) were due to V. parahaemolyticus. Of the 118 cases of septicemia, 83% were associated with raw oyster consumption and 16 (14%) were due to V. parahaemolyticus. Of 467 patients with infections presenting as either gastroenteritis or septicemia, 35% had a preexisting medical condition, such as liver disease, alcoholism, peptic ulcer disease, gastrointestinal surgery, diabetes, antacid medication or immune disorders. While the prevalence of underlying illness was high in the septicemia patients, the majority of patients with raw-oyster associated Vibrio gastroenteritis had no underlying conditions. The reported cases of gastroenteritis caused by V. parahaemolyticus infection were more common during warm weather months.
  • A case series of 339 Vibrio infections reported in Florida between 1981 and 1994 was reported by Hlady (1997). Culture-confirmed case reports of Vibrio infections, reported to the Florida Department of Health and Rehabilitation Services were investigated. Oyster-associated Vibrio infection was defined as a history of raw oyster consumption in the week prior to onset of gastroenteritis or septicemia. Vibrio parahaemolyticus accounted for 77 of the 339 reported Vibrio infections. Of the 237 raw oyster-associated cases of gastritis, 68 (30%) of the infections were due to V. parahaemolyticus. Of the 193 patients who were hospitalized, 37 (19%) had infection with V. parahaemolyticus. Vibrio parahaemolyticus accounted for 4 (8%) of reported deaths. Patients with septicemia had underlying illness including, but not limited to, cancer, liver disease, alcoholism and diabetes mellitus.

Implicated Foods

Raw oysters are the most common food associated with Vibrio infection in the United States (Hlady, 1997). While thorough cooking destroys Vibrio, oysters are often eaten raw. However, there have been reports of V. parahaemolyticus illnesses associated with other seafood, including crayfish, lobster, shrimp, and crab. In a study from Levine et al. (1993), of 15 patients who ate seafood, the most commonly ingested foods were crabs, shrimp and raw clams. In addition, studies demonstrated the presence of V. parahaemolyticus in fresh fish, mussels and clams (Baffone et al., 2000). In an outbreak of V. parahaemolyticus in the Northeast in 1998, 16 of 23 ill persons ate either raw oysters or raw clams (CDC, 1999a).

Cooked seafood has also caused illnesses. Seafood cooked using seawater from the ships' fire systems caused outbreaks of V. parahaemolyticus gastroenteritis aboard two Caribbean cruise ships in 1974 and 1975 (Lawrence et al., 1979). Half of the 1,200 persons who ate boiled shrimp at a feast in Louisiana became ill with V. parahaemolyticus gastroenteritis in 1972 (Barker et al., 1974). Samples of the uncooked shrimp tested positive, indicating that the shrimp were colonized prior to arrival at the shrimp feast and were not cooked at an adequate temperature to kill V. parahaemolyticus or were re-contaminated after cooking.

Steamed crabs were implicated in two outbreaks in the United States from a cross-contamination with live crabs (Dadisman et al., 1972). In another United States outbreak, crab salad was prepared from packaged processed crabmeat, opened the day the meal was served. The crabmeat likely became contaminated prior to final packaging (Dadisman et al., 1972). A case-control study of sporadic Vibrio illnesses in two coastal areas of Louisiana and Texas was conducted from 1992-1993. Cooked crayfish consumption was reported by 5 of 10 persons affected with V. parahaemolyticus infection (Bean et al., 1998). In a study by Lowry et al., (1989), the presence of V. parahaemolyticus was surveyed from raw and cooked seafood from New Orleans restaurants. Vibrio parahameolyticus was isolated from all of the raw oysters sampled; the microorganism was isolated in 50% of cooked oyster samples, 67% of boiled shrimp samples, 33% of crab salad samples and in none of the boiled crabs.


The majority of outbreaks of foodborne illnesses associated with V. parahaemolyticus in the United States occur in the warmer months, with 94% occurring between April and October (Daniels et al., 2000a). CDC data (Smith, 2003b) indicates that of the oyster-related, culture-confirmed illnesses due to V. parahaemolyticus from 1988 to 2001, 60% occurred in the summer and only 4% occurred in the winter months. The breakdown of the number of reported cases of illnesses by season is provided in Table II-3. The same associations have been reported in other countries. In India, the monthly isolation of V. parahaemolyticus was more predominant in warmer months (Okuda et al., 1997) and in Japan the monthly outbreaks of food-related V. parahaemolyticus are more prevalent in summer with a peak in August (International Disease Surveillance Center, 1999; IASR, 1998).

Table II-3. Culture-confirmed Vibrio parahaemolyticus Illnesses Associated with Consumption of Oysters
Season2000a2001a1988 to 2001a
a Analysis based on oyster- related culture-confirmed V. parahaemolyticus infections reported to the Centers for Disease Control and Prevention (CDC) for which either a date of oyster consumption or a date of illness onset was reported (Smith, 2003b).

Geographic Distribution of Illness

Oysters are harvested in the United States from the Gulf Coast, Mid-Atlantic, Northeast Atlantic, and Pacific Northwest. The climate in these regions is different and there are different harvesting methods and handling practices within the regions that can have an impact on levels of Vibrio in oysters. For example, in the Pacific Northwest, oysters harvested in intertidal areas are typically exposed to higher temperatures longer before refrigeration then those harvested using dredging.

Of the four major oyster-harvest regions in the United States, the majority of oysters (approximately 50%) are harvested from the Gulf Coast and approximately 24% are harvested from the Pacific Northwest (Chapter IV: Exposure Assessment, Table IV-15). During the 1998 outbreaks, the Pacific Northwest shellfish harvested from the Hood Canal area of Washington were responsible for 32 of 48 (67%) of cases in the state of Washington (Therien, 1999). In the Gulf Coast, 20 of 30 harvest sites in Galveston Bay were implicated in the 1998 outbreak. In the Atlantic Northeast region, Oyster Bay Harbor (Area 47) was the only area implicated in the 1998 outbreak of that region (CDC, 1999a).

International Reports of Vibrio parahaemolyticus Cases

Vibrio parahaemolyticus was first identified as a foodborne pathogen in Japan in the 1950s (Fujino et al., 1953). By the late 1960s and early 1970s, V. parahaemolyticus was recognized as a cause of diarrheal disease worldwide. Below is a brief description of recent reports of V. parahaemolyticus illnesses in different parts of the world.

Japan. Prior to 1994, the incidence of V. parahaemolyticus infections in Japan had been declining; however, from 1994 to 1995 there were a total of 1,280 reports of infection due to V. parahaemolyticus (IDSC, 1999). During this time period, the incidents of V. parahaemolyticus food poisoning outnumbered those of Salmonella food poisoning. For both years, the majority of the cases occurred in the summer, with the largest number appearing in August.

Food poisoning due to V. parahaemolyticus in Japan is usually restricted to relatively small-scale outbreaks involving fewer than 10 cases. From 1996 to 1998, there were 1,710 incidents, including 496 outbreaks, with 24,373 cases of V. parahaemolyticus reported. The number of cases of V. parahaemolyticus food poisoning doubled in 1998 as compared to 1997 and again exceeded the number of Salmonella cases (IDSC, 1999). Similar to the 1994 to 1995 period, outbreaks were more prevalent in the summer with a peak in August and relatively few outbreaks occurred during winter months. Boiled crabs caused one large-scale outbreak, involving 691 cases. However, the majority of outbreaks were small in scale, but occurred frequently. There were 292 outbreaks and sporadic reports of V. parahaemolyticus involving 5,241 cases in 1996. In 1997, the incidence increased to 568 outbreaks and sporadic reports, with 6,786 cases, and in 1998, there were 850 outbreaks and sporadic reports (IDSC, 1999). The increased incidence during 1997 to 1998 has been attributed to an increased incidence of serovar O3:K6.

India. A hospital-based active surveillance of V. parahaemolyticus infections in Calcutta, India, conducted from 1994 to 1996, identified 146 patients (Okuda et al., 1997b). The incidence suddenly increased in February of 1996 and remained elevated until August of that year when surveillance ended. The increased incidence of V. parahaemolyticus infections was associated with an increased prevalence of O3:K6 strains. This serovar had not been isolated in Calcutta prior to February of 1996. The incidence of diarrhea due to V. parahaemolyticus strain O3:K6 accounted for 63% of the strains isolated from patients in Calcutta between September 1996 and April 1997. The virulant O3:K6 strains isolated from travelers arriving in Japan from Southeast Asian countries was indistinguishable from O3:K6 strains found in Calcutta, India (Matsumoto et al., 1999).

Vietnam. Five hundred forty eight cases of V. parahaemolyticus infection were detected between 1997 and 1999 in the Khanh Hoa province of Vietnam (Tuyet et al., 2002). Of these, 90% occurred in persons over 5 years of age, 421 (77%) reported vomiting, 258 (53%) presented with watery stools, 34 (6%) reported bloody stools. None of the patients died at the time of discharge from the health care service. A risk factor for infection was high socioeconomic status, which led the authors to believe that the source of infection was fresh seafood since only the most affluent members of the community can afford this delicacy. There was no definitive information on consumption.

Chile. Between November 1997 and April 1998, several gastroenteritis cases were reported in Antofagasta, a city in northern Chile (Cordova et al., 2002). The outbreak was associated with consumption of shellfish. This was the first report of V. parahaemolyticus causing an outbreak in Chile. Isolates were obtained from patient stool specimens and fresh shellfish. It was speculated that the exceptionally warm seawater caused by "El Nino" may have favored a bacterial bloom.

Spain. Between August and September 1999, an outbreak with 3 clusters of illness occurred in Galicia, Northwest Spain (Lozano-Leon et al., 2003). Sixty four persons were ill, 9 case patients were hospitalized. The most common symptom was diarrhea; other symptoms included abdominal cramps, nausea, headache, fever and vomiting. The median duration of illness was 3 days, and onset was within 12 to 24 hours after consumption of raw oysters in a typical outdoor street market. Vibrio parahaemolyticus was isolated in stool of all case patients. All patients resided in one of 2 cities near the outbreak site.

Taiwan. Vibrio parahaemolyticus has become a leading cause of foodborne disease outbreaks in Taiwan (Chiou et al., 2000). Vibrio parahaemolyticus accounted for 64% (542/850) of the food-associated outbreaks in Taiwan between 1995 and 1999. The O3:K6 serovar accounted for 0.6% of V. parahaemolyticus infections in Taiwan in 1995. This increased to 50% in 1996 and reached a peak of 84% in 1997. Comparison of outbreak data indicates that the high incidence of foodborne V. parahaemolyticus outbreaks from 1996 to 1999 can be attributed to the increase in O3:K6 infections.

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