Quantitative Risk Assessment of Vibrio parahaemolyticus in Mytilus edulis in China

The objective of this study was to conduct Vibrio parahaemolyticus risk assessment associated with the consumption of Mytilus edulis among the people of Zhejiang Province, China. It was based on the quantitative risk assessment framework developed by the Codex Alimentarius Commission. In order to calculate the quantitative risk assessment, the data generated in this study as well as data gathered from other reports were taken into account. The exposure assessment of Vp was conducted with estimation of its density in Mytilus edulis by using Monte Carlo simulation and considering cooking and consumption modes. The Matlab software package, version 7.10.0.499 (R2010a) in combination with Microsoft-Excel was used to run the Monte Carlo simulations. We found that the probability of illness for people eating Mytilus edulis per meal was 8.4×10, which means the incidence rate of illness/10,000,000 population/year is 8.4 if each person eats 10 meals of Mytilus edulis in summer.


INTRODUCTION
Vibrio parahaemolyticus (Vp) is a Gram-negative halophilic bacterium that occurs naturally in estuarine, marine and coastal environments throughout the world (Joseph et al., 1982).Consumption of food contaminated with Vp may lead to development of acute gastroenteritis characterized by diarrhea, headache, vomiting, nausea, abdominal pain and lowgrade fever (Su and Liu, 2007).With the widespread consumption of seafood, this bacterium has accounted for most cases of food poisoning in Japan, North America and Southeast Asia (Daniels et al., 2000).In China, during the past two decades, Vp has been the most common cause of the bacterial food-borne outbreaks (Wu et al., 2014).
The last three decades have witnessed an increase in the content of Vp in China seas mainly due to offshore pollution, global warming and many other factors.Vp is one of the major seafood-borne gastroenteritis causing bacteria and is frequently isolated from shellfish samples (Zhao et al., 2011).In China, the annual output of shellfish aquaculture is nearly 1 million tons, in which the Mytilus edulis is one of the most important commodities and Zhejiang Province is one of the main producing regions.M. edulis is a filter feeder, which can enrich Vp from its surrounding environment, so that it becomes an important carrier of Vp and its affiliated diseases like diarrhea.The issue of high Vp concentration in M. edulis and its high consumption in China is becoming a serious threat to public health.Therefore, there is a dire need to carry out the complete risk assessment of Vp so that appropriate steps could be taken to halt the spread of this pathogen via over consumption of contaminated M. edulis.
According to WHO, risk assessment is the scientific evaluation of known or potential adverse health effects resulting from human exposure to foodborne hazards.The process consists of the four steps: hazard identification, hazard characterization, exposure assessment and risk characterization.The definition includes quantitative risk assessment, which emphasizes reliance on numerical expressions of risk and also qualitative expressions of risk, as well as an indication of the attendant uncertainties (Sani et al., 2013).
The scope of this study was to assess the risk of acquiring gastroenteritis due to Vp for among Chinese population as a result of the consumption of M. edulis.This research will provide some basic information for the government to address the food safety public events, or may be helpful for future research.

MATERIALS AND METHODS
The detection of Vp in M. edulis was referred to SN 0173-2010《The Detection of VP in export food》.The Matlab software package, version 7.10.0.499 (R2010a) in combination with Microsoft-Excel was used to run the Monte Carlo simulations.A quantitative risk assessment of Vp was conducted in order to estimate the risk of getting infected by consuming M. edulis among the people living in the Zhoushan Islands of Zhejiang Province, China.To develop the model, the four steps of risk assessment as mentioned earlier were included: hazard identification, hazard characterization, exposure assessment and risk characterization (Sen et al., 2005).This risk assessment is a quantitative analysis in which the key steps from post-harvest handling to consumption were modeled.The likelihood of illness following exposure to the pathogenic Vp from the consumption of M. edulis was calculated.

Hazard identification of Vp:
Hazard identification as a part of microbial risk assessment is the identification of known or potential pathogenic microorganism which could cause adverse health effects and is present in a particular food or group of foods.In this risk assessment, the hazard was focused on pathogenic Vp in M. edulis inadequately cooked.Vp infection is usually caused by eating such mussels.

Hazard characterization of
Vp: Hazard characterization is the qualitative and/or quantitative evaluation of the nature of the adverse effects associated with a particular agent which may be present in food.For biological agents, a dose-response assessment should be performed if the data are obtainable.In this study, the section focuses on evaluating the nature of adverse health effects associated with the presence of Vp in M. edulis and how to quantitatively assess the relationship between the magnitude of the food-borne exposure and the likelihood of adverse effects occurring, providing sufficient information to allow for a quantitative measurement of the public health risk from Vp associated with the consumption of M. edulis.

Exposure assessment of Vp:
The exposure assessment component of a microbial risk is an evaluation of the likelihood of ingesting pathogenic microorganisms via food and the likely level of exposure in a given population.In the assessment, the magnitude of exposure to Vp from the consumption of inadequately cooked M. edulis was evaluated and a mathematical model was developed to quantify the magnitude of exposure assessment for Vp.

Risk characterization of Vp:
In the risk characterization, the estimated exposure is integrated with the dose-response model and consideration of cooking & consumption mode and pathogenic proportion of Vp in M. edulis to provide a quantified risk estimation and probability of illness for people eating M. edulis per meal.All calculations were performed by the Monte Carlo simulation of sampling from specified input distributions and appropriately combining the sampled values to generate the corresponding output distributions.

Hazard identification of Vp:
There are many pathogenic factors associated with Vp, such as the invasiveness and hematotoxin and the production of Thermostable Direct Hemolysin (TDH) is the most important, which is a reliable feature to distinguish between pathogenic and non-pathogenic strains of Vp (Chen and Liu, 2006).The food contaminated with Vp can potentially cause gastroenteritis, headache, nausea and vomiting, stomach cramps or fever and a few cases can lead to sepsis, which leads to death (Honda et al., 2008).The main symptoms are diarrhea, vomiting, headache, nausea, abdominal cramps, fever etc. (Dai and Song, 2005).The course is generally 2~4 d, shorter duration and self-healing (Su and Liu, 2007).Severe disease may cause wound infection and septicemia and some body tissues will change, such as bullous skin damage, adjuvant arthritis, arthritis deformans etc. (Morris and Black, 1985).
According to the monitoring data of the national food-borne disease surveillance network in China, food poisoning caused by Vp had occupied an important position in the bacterial food poisoning.In the food poisoning statistics from 1992~2004, the average incidence and people affected by microbial food poisoning accounted for 54.63% and 67.24%, respectively and Vp accounted for 33.52% of total microbial food poisoning events (Yu, 2005).The analysis of the pathogenic factors of major food poisoning in Ningbo, Zhejiang Province indicated that Vp accounted for 25.33% (Qin and Liu, 2004).In Zhejiang Province, in all food poisoning cases diagnosed in 2001~2014, 29% were related to Vp poisoning, according to an unpublished recent statistics.

Hazard characterization of
Vp: Hazard characterization is also called dose-response, which is a quantitative description of the dose-response relationship between the concentration of pathogen and the degree of disease risk.
The commonly used doseresponse model for risk assessment of Vp are the Gompertz model, Probit model and Beta-poisson model.The former two are linear models whose index