ARCHIVED - Canada Communicable Disease Report

1 October 2007

Descriptive Epidemiology of Vibrio Parahaemolyticus and Other Vibrio Species Infections in British Columbia: 2001-2006

G Khaira, BSc candidate (1,2), E Galanis, MD, MPH, FRCPC (1)

1. British Columbia Centre for Disease Control, Vancouver, British Columbia

2. University of British Columbia, British Columbia

Organisms in the Vibrio genus are naturally found in marine and estuarine environments⁽¹⁾. There are at least 12 human pathogenic species of Vibrio, of which Vibrio cholerae, Vibrio parahaemolyticus and Vibrio vulnificus cause the greatest burden of illness worldwide⁽²⁾. In Canada, certain Vibrio spp. can be found in coastal waters. In particular, V. parahaemolyticus, V. fluvialis, V. vulnificus and V. alginolyticus have been isolated from shellfish along the coast of British Columbia (BC)^(3-5). V. parahaemolyticus has also been isolated from the coast of Atlantic Canada^(6,7). In the United States (US), Vibrio spp. have been isolated from almost all coastal waters, but the incidence of infection is higher in states bordering the Atlantic seaboard and Gulf coast⁽⁸⁾. Since 1997, the Pacific Northwest of North America, which includes BC, has regularly had summer peaks or outbreaks of V. parahaemolyticus infection^(9-11).

Common clinical presentations of Vibrio infection are gastroenteritis, primary septicemia and wound infection⁽¹²⁾. Vibrio-associated gastroenteritis is most frequently associated with V. parahaemolyticus infection⁽¹³⁾. The mean incubation period for V. parahaemolyticus gastroenteritis ranges from 24 to 48 hours⁽²⁾. V. parahaemolyticus gastroenteritis is self-limited, of moderate severity and lasts an average of 3 days in immunocompetent persons⁽²⁾. In some patients, such as those who are immunocompromised or have a pre-existing medical condition, V. parahaemolyticus infection can be severe but rarely leads to death⁽²⁾.

The purpose of this report is to describe the epidemiology of Vibrio infections in BC, with a focus on the most common species in this province, V. parahaemolyticus.

Methods

All laboratory-confirmed Vibrio infections in BC residents, including travel-related cases, reported to the BC Centre for Disease Control from 2001 to 2006 were reviewed. Basic epidemiologic data were obtained from i-PHIS (Integrated Public Health Information System) and standard interviews with cases of shellfish-related infection.

In BC, enhanced surveillance of shellfish-related illnesses has been conducted since 1998 to identify and manage the risks associated with the consumption of raw shellfish. Shellfish-related illness is defined as any illness temporally associated with the consumption of shellfish. Each case of shellfish-related illness is routinely interviewed by regional health authority staff using a standard questionnaire, including information on demographic characteristics, illness details and shellfish exposure.

Frequencies were calculated using available denominators, and rates were calculated using BC Stats population data estimates⁽¹⁴⁾. Analysis was performed using Epi Info 6.04d.

Daily ocean temperature data, taken at high tide from 2001 to 2006, from eight lighthouse stations located near major shellfish harvesting sites in BC (Denman Island, Gabriola Island, Gold River, Mayne Island, Metchosin, Nanaimo, Sooke, Tahsis and Ucluelet) were obtained from the Department of Fisheries and Oceans Canada⁽¹⁵⁾. An average weekly temperature for each lighthouse station was calculated for each year from 2001 to 2006. The temperatures were averaged over a 6-year period (2001-2006).

Results

Between 2001 and 2006, 140 cases of Vibrio spp. infection were reported in BC, for an average of 23.3 cases and average rate of 0.6/100,000 population each year. V. parahaemolyticus was the most frequently reported species (122 cases, 87.1%) (Table 1). Other species reported, in descending order, were V. cholerae non-O1/O139, toxigenic V. cholerae O1/O139, V. fluvialis and V. hollisae. In BC, rates of V. parahaemolyticus dropped from a high of 1.3 in 1997 to an annual average of 0.5 per 100,000 population in 2001-2006 (Figure 1). From 2001 to 2006, the average number of cases of confirmed V. parahaemolyticus per year was 20.3, the highest rates being reported in 2002 and 2006. Vancouver Coastal and Vancouver Island Health Authorities had higher rates of V. parahaemolyticus infection (0.7/100,000 for both) than the BC average.

Table 1. Reported cases of Vibrio infection by species, BC, 2001-2006 (n = 140)

Figure 1. Annual rates of Vibrio parahaemolyticus infection in BC, 1997-2006

The mean age of cases of V. parahaemolyticus infection was 45.2 years with a range of 2 to 82 years. Most cases (n = 33) occurred in the 40 to 49 year age group followed by the 30 to 39 year age group (n = 30) (Figure 2). Both age groups had a rate of 0.8/100,000. The percentage of male cases was 64.0%.

Figure 2. Age distribution of Vibrio parahaemolyticus cases, BC, 2001-2006 (n = 109)

A seasonal peak occurred between weeks 26 and 38. The average ocean temperature during this period was 15.7° C (Figure 3). In particular, 89 of 102 cases (87.3%) reported illness onset between the months of July and September.

Figure 3. Onset of Vibrio parahaemolyticus cases by week (n = 102) and weekly average ocean temperatures, BC, 2001-2006

Of the 13 cases of V. cholerae non-O1/O139, eight were travel-related, of which two were known to have involved the consumption of shellfish. One patient reported consuming raw oysters in a BC restaurant (the source of the oysters is unknown). The remaining four cases had unknown exposures and travel histories. One of three cases of toxigenic V. cholera reported travelling. The two remaining cases were linked to international travel because toxigenic V. cholera is not endemic in BC. The single cases of V. fluvialis and V. hollisae infection were both locally acquired and associated with the consumption of shellfish.

Fifteen cases (12.3%) of V. parahaemolyticus infection were known to be travel-related, and 32 (26.2%) had unknown exposure. Among the rest (n = 75), the main source of V. parahaemolyticus infection was reported as the local consumption of shellfish (92.0%, n = 69): 54 patients (78.3%) ate raw shellfish and 46 (66.7%) raw oysters (Table 2). In particular, among those with confirmed V. parahaemolyticus infection who reported local shellfish consumption (n = 69), a greater proportion of men (76.2%, n = 32) reported consuming raw oysters than women (51.9%, n = 14). The majority of implicated oysters are believed to have been harvested in BC and a smaller proportion in the US (mostly in Washington State) (L. Rodriguez-Maynez, Canadian Food Inspection Agency, Vancouver, BC: personal communication, 2007). Other exposures not associated with travel involved consumption of seafood other than shellfish (n = 3) and ingestion of seawater through swimming in the ocean (n = 3).

Table 2. Number (and proportion) of Vibrio parahaemolyticus cases due to local shellfish consumption, BC, 2001-2006 (n = 69)

Sources of implicated shellfish were restaurants (39.1%, n = 27), self-harvesting (26.1%, n = 18), stores/markets (20.3%, n = 14) and a combination of restaurants and stores/markets (8.7%, n = 6). The remaining cases (5.8%, n = 4) did not report the source from which they acquired the consumed shellfish.

Of 42 of the patients with V. parahaemolyticus infection associated with shellfish consumption, 33 (78.6%) reported being unaware of the risk of acquiring Vibrio infection from shellfish.

Discussion

In BC, from 2001 to 2006, V. parahaemolyticus was the most commonly reported Vibrio species with an incidence of 0.5/100,000. Preliminary FoodNet data for 2006 suggest a Vibrio infection rate of 0.34/100,000 in 10 US states, six of which are coastal⁽¹⁶⁾. A study conducted from 1981 to 1993 in Florida, a state not included in FoodNet, reported a rate of 0.43/100,000 V. parahaemolyticus cases per year⁽¹⁾. In Washington State, reported rates of V. parahaemolyticus in 2001-2006 varied between 0.2 and 0.8/100,000 for an average of 0.4/100,000^(9,17). The slightly higher rates reported in BC may be due to higher diagnosis and reporting rates rather than a truly higher incidence. Cases of Vibrio spp. infections other than V. parahaemolyticus and V. cholerae (e.g., V. fluvialis) are likely underreported in BC because dedicated data fields to report these species are not available in the electronic surveillance database.

The majority of V. parahaemolyticus infections are believed to have occurred as a result of consumption of raw shellfish, in particular, raw oysters. Those who became ill from cooked shellfish likely had consumed ones that were poorly cooked or cross-contaminated⁽¹⁵⁾.

Oysters and other bivalve shellfish (including clams, scallops and mussels) accumulate microorganisms, such as Vibrio spp., as a by-product of filter-feeding, a natural process whereby nutrients are extracted from large quantities of surrounding ocean water. Along with nutrients, pathogens present in ocean waters can be ingested and concentrated by bivalves; therefore, consumption of raw or undercooked shellfish can cause human infection(12).

In 2005, the value of the BC shellfish industry was $17.4 million⁽¹⁹⁾. Close to 10,000 tonnes of shellfish were harvested, the majority of which were oysters but also included mussels, clams and scallops. Eighty-nine percent of licensed shellfish tenures were concentrated on the east coast of Vancouver Island⁽²⁰⁾.

In Canada, the Canadian Food Inspection Agency (CFIA), the Department of Fisheries and Oceans and Environment Canada govern the shellfish industry through the Canadian Shellfish Sanitation Program. This includes activities such as monitoring for biotoxins in shellfish, registration and inspection of shellfish processing plants, monitoring water quality in shellfish growing areas, and site closures(21). No specific policies for managing V. parahaemolyticus in shellfish existed before 1997, when a large outbreak of V. parahaemolyticus affecting the Pacific Northwest caused 111 cases of illness in BC(10). After this event, a Vibrio Advisory Committee, comprising the BC Centre for Disease Control, the BC Ministry of Health, the BC Shellfish Growers Association, the CFIA, the BC Restaurant and Food Association and regional health authorities, was formed. This group implemented the Vibrio Risk Reduction Strategy in 1998. The strategy includes sampling for V. parahaemolyticus from CFIA indicator sites along the coast of BC; industry monitoring in growing areas; time and temperature controls for oyster harvesting, transport and processing; laboratory alerts; and restaurant education initiatives. Processing plants in BC are required to identify V. parahaemolyticus as a hazard in their Hazard Analysis and Critical Control Points plan during the warmer months and to ensure that only product harvested from areas where V. parahaemolyticus levels are below 100 most probable numbers/g are received at the plant (L. Rodriguez-Maynez, CFIA, Vancouver, BC: personal communication, 2007). These programs have contributed to keeping the incidence of V. parahaemolyticus infections low since then.

The majority of cases of V. parahaemolyticus infection occurred during the summer months when ocean temperatures are warmer⁽²¹⁾. In particular, the peak season of infection corresponded to the average water temperature being equal to or exceeding 15° C, as has been reported in a previous outbreak⁽¹¹⁾. Most cases occurring outside the peak are thought to be travel-related.

The reason for the higher rates of V. parahaemolyticus infection in 2002 and 2006 is not clear. On the basis of climate data⁽²²⁾ and ocean temperature data, 2002 and 2006 did not exhibit higher than expected temperatures.

A recent CFIA survey found that 71% of the Canadian population are aware of some of the health risks associated with consuming shellfish⁽²³⁾. However, only 21.4% of BC V. parahaemolyticus cases were aware of the risk of V. parahaemolyticus infection associated with consuming raw shellfish. Cases may be less aware than the general population of the risk of consuming raw shellfish, or the general population is not familiar with the particular risk of acquiring Vibrio infection. Regardless of the reason, this suggests the need to increase awareness of the risk of acquiring Vibrio from shellfish. Studies have shown that raising awareness of risk-associated behaviours and educating consumers about safe food-handling techniques lower their risk of acquiring foodborne illnesses^(24-26). Interventions⁽²⁷⁾ will be most effective if they reach the groups that may be at higher risk: men, 30 to 49-year-olds, residents of Vancouver Coastal and Vancouver Island Health Authorities, and persons who consume raw oysters during the summer season.

In view of the increased rate of V. parahaemolyticus infection in 2006, the BC Centre for Disease Control, BC health authorities, Health Canada, the CFIA, the BC Shellfish Growers Association and the Canadian Council of Grocery Distributors have developed an education and communication plan to increase awareness of the risks associated with consuming raw shellfish.

The enhanced surveillance of shellfish-related illnesses in BC will continue to be an invaluable source of information on the epidemiology of V. parahaemolyticus infection, the risk factors associated with acquiring V. parahaemolyticus infection, and the awareness of Vibrio risks associated with consuming raw shellfish in BC. It will also help to evaluate the effectiveness of the risk reduction programs and educational initiatives in place in BC.

Acknowledgements

The authors thank the following for their assistance: S. David, L. MacDougall, L. McIntyre and W. Tang (all with the BC Centre for Disease Control, Vancouver, British Columbia). The authors would also like to acknowledge the contributions of the environmental health officers and laboratories in BC.

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