Hazard Identification

What is Vibrio parahaemolyticus?

Vibrio parahaemolyticus is gram negative, non-sporeforming bacterium normally found in marine environments. It is the most likely Vibrio species to be implicated in foodborne disease, although both V. vulnificus and V.cholerae may also cause foodborne infections and are covered elsewhere in this book. Other Vibrio species associated with foodborne disease to a much lesser extent are V. alginolyticus, V. mimicus, V. damsela, V. hollisae and V. fluvialis.

Not all strains of V. parahaemolyticus cause illness and two distinct groups have been defined: pathogenic 'Kanagawa-positive' strains, which cause V. parahaemolyticus food poisoning, and 'Kanagawa-negative' strains, which do not.

Occurrence in foods

Vibrio parahaemolyticus is found mainly in foods of marine origin, and studies carried out in the USA found that 60 - 100 % of seafood samples were contaminated with the organism. When present, it is usually at levels of around 10 CFU/g, although levels can be around 10³ CFU/g, or even higher in the warmer summer months. Seafood from warm waters presents a greater risk of V. parahaemolyticus food poisoning, with 89 % of oysters causing the illness reported as originating from waters where the temperature was above 22 ^ºC.

Cases of illness caused by V. parahaemolyticus have also occurred when seafoods have been cross contaminated by raw fish after cooking and subsequently temperature abused. Implicated seafood in outbreaks include clams, oysters, scallops, shrimp and crab.

Hazard characterization

Effects on health

Kanagawa-positive strains of V. parahaemolyticus produce a heat stable haemolysin, which can be preformed in food. This haemolysin is thought to be responsible for the illness although other toxins could also be involved.

Although the minimum infective dose for V. parahaemolyticus is unknown, volunteer studies with healthy individuals have shown that high numbers (10⁵ - 10⁷) of Kanagawa positive V. parahaemolyticus cells are required to cause illness. The infective dose may be lower when the organism is consumed at the same time as antacids or foods. All individuals are susceptible to infection by V. parahaemolyticus.

The incubation time for the infection is 4 - 96 hours (average 15 hours). The organism usually causes a mild to moderate form of gastroenteritis with abdominal cramps and watery diarrhoea. Nausea, vomiting, headache and fever can also occur. Some affected individuals can require hospitalisation. Symptoms can last for 1 - 7 days, although the average is 2.5 days and the illness is usually self limiting. Deaths rarely occur.

Incidence and outbreaks

The consumption of raw seafood products (such as oysters and sashimi/sushi) from 'high risk' waters significantly increases the risk from V. parahaemolyticus food poisoning. The pathogen is a major cause of food poisoning in Asian countries, but in the UK illnesses caused by V. parahaemolyticus are usually associated with the consumption of imported seafoods, or with foreign travel.

In Japan, V. parahaemolyticus reportedly accounts for approximately half of cases bacterial foodborne infection. In the USA, V. parahaemolyticus illnesses prior to 1997 were infrequently reported; however during 1997 and 1998 there were 4 multistate outbreaks associated with the consumption of raw or undercooked oysters, affecting over 700 individuals. This dramatic increase in illnesses caused by V. parahaemolyticus in the US has been attributed to the emergence of a new pandemic strain (O3:K6), previously this strain had only been associated with illnesses in Asia.

In Europe V. parahaemolyticus infections are rarely reported. However a review of clinical data in Spain published in 2005 has concluded that they are more common than previously thought and a V. parahaemolyticus outbreak in Spain in 2004 caused by seafood harvested from European waters has been linked to the pandemic strain O3:K6.

Sources

Vibrio parahaemolyticus is a normal inhabitant of the marine environment and is an obligate halophile (having a minimum requirement for salt to grow). Favourable conditions for its growth are found in tropical and temperate seawaters. For this reason the organism is usually associated with seafoods from estuarine or coastal marine environments where water temperatures are highest, such as the southern coastal US States and Japan, particularly during the summer months. However an outbreak of V. parahaemolyticus in 2004 was linked to Alaskan oysters and rising sea water temperature is thought to have lead to the organism proliferating in shellfish from this Northerly latitude.

Seasonal temperature variations influence the presence of the organism and although levels are highest in shellfish during the warmer months, the organism can over-winter in sediment and can be difficult to detect in water or fish samples during the winter period. However, more than 99 % of environmental isolates are not pathogenic (i.e. they are Kanagawa-negative).

Human asymptomatic carriers of V. parahaemolyticus are known to occur and they can act as a source of environmental contamination.

Growth and survival in foods

The temperature range for growth of V. parahaemolyticus is 5 - 43 ^ºC, with an optimum temperature of 37 ^º. Under optimal conditions growth can be very rapid. The organism declines (but is not eliminated) in numbers during chilled (0 - 5 ^ºC) storage.

The organism survives freezing although numbers will initially be reduced.

The pH range for growth is 4.8 - 11, optimum 7.8 - 8.6. The organism is not particularly tolerant of low pH environments and the minimum pH for growth decreases as the storage temperature increase towards optimum.

V. parahaemolyticus is unable to grow unless salt (NaCl) is present. The optimum salt concentration for growth is 3 % (equating to 0.980 water activity). The organism can grow in salt concentrations from 0.5 -10 %, representing a water activity range of 0.940 - 0.996.

The organism is inactivated by desiccation and by exposure to fresh water.

V. parahaemolyticus is a facultative anaerobe (can grow in the presence or absence of oxygen) and can grow in foods that are either vacuum or aerobically packaged. It grows best however under aerobic conditions.

Thermal resistance

V. parahaemolyticus is not heat resistant and is inactivated at temperatures >65 ^ºC. D- values of <1 min at 65 ^ºC, and 2.5 mins at 55 ^ºC have been reported.

Control options

Seafood should be considered potentially contaminated with V. parahaemolyticus, particularly if it has been harvested from tropical and sub-tropical waters. However, it should be noted that seafood from what are considered 'colder' sea waters may be contaminated, particularly shellfish harvested during the summer months. The risk of V. parahaemolyticus food poisoning is increasing with the worldwide growth in the consumption of raw fish.

Processing

Decontamination processes such as depuration or relay technologies are not effective at removing V. parahaemolyticus from shellfish, and effective control of the organism should focus on keeping numbers low. Measures to ensure this include, maintenance of the cold chain (<5 ^ºC) from harvest to consumer, minimizing delays between harvesting and landing, and avoiding further exposure to untreated seawater and soiled containers. Shellfish growing areas can also be monitored for the presence of pathogenic strains of V. parahaemolyticus, with the closure of waters for harvesting if levels of the pathogens are deemed to be too high.

Seafood should be handled carefully to avoid cross contamination between raw and cooked product and avoiding temperature abuse is also very important.

Product use

Consumers should be encouraged to cook seafood thoroughly and not to eat product raw. In the US, raw oysters and restaurants offering raw oysters on their menus are required to carry health warnings about eating raw shellfish.

Legislation

EU regulations and the US Food code do not have specific requirements relating to levels of V. parahaemolyticus in foods. A review of published guidelines concluded that in general, levels of 10² - 10³ CFU/g of V. parahaemolyticus are acceptable. The US FDA guidelines for microbiological contaminants in seafoods has an action level of >10⁴ CFU/g.

The UK Health Protection Agency (HPA) guideline on the microbiological quality of some ready-to-eat foods at the point of sale states that levels of V. parahaemolyticus in seafoods of <100 CFU/g are satisfactory, 100 - 1,000 CFU/g in these products is unsatisfactory, and levels of >1,000/g are unacceptable/potentially hazardous.

Sources of Further Information

Published

Sakazaki R., Kaysner C. Abeyta C.
Vibrio infections in 'Foodborne infections and intoxications. Eds Riemann H.P & Cliver
D.O. 3^rd Edition. London. Academic Press, 2005, 185 - 204.

Nair G.B., Faruque S.M., Sack D.A.
Vibrios in 'Emerging foodborne pathogens'. Eds Motarjemi Y., Adams M.
Cambridge. Woodhead Publishing Ltd, 2006, 332 - 372.

On the web

Quantitative risk assessment of the public health impact of pathogenic Vibrio parahaemolyticus in raw oysters. US Food and Drug Administration's Center for Food Safety and Applied Nutrition. (July 2005).
http://www.cfsan.fda.gov/~dms/vpra-toc.html

Risk profile: Vibrio parahaemolyticus in seafood. Institute of Environmental Science and Research Limited. (December 2003).
http://www.nzfsa.govt.nz/science/risk-profiles/vibrio-parahaemolyticus.pdf

Discussion paper on risk management strategies for Vibrio spp. in seafood. Codex Committee on Food Hygiene 35th Session. Food and Agriculture Organization of the United Nations/World Health Organization. (January - February 2003)
ftp://ftp.fao.org/codex/ccfh35/fh0305ce.pdf

Opinion of the Scientific Committee on Veterinary Measures relating to public health on Vibrio vulnificus and Vibrio parahaemolyticus (in raw and undercooked seafood). European Commission. (September 2001).
http://ec.europa.eu/food/fs/sc/scv/out45_en.pdf

Draft risk assessment on the public health impact of Vibrio parahaemolyticus in raw molluscan shellfish. US Food and Drug Administration's Center for Food Safety and Applied Nutrition (January 2001).
http://www.cfsan.fda.gov/~dms/vprisk.html