Vibrio parahaemolyticus

Revision History

• Infectious Agent
• Case Definitions and Notification Criteria
• Notification Procedure
• Objectives of Surveillance
• Public health Significance and Occurrence
• Clinical Features
• Reservoir
• Mode of Transmission
• Incubation Period
• Period of Communicability
• Susceptibility
• Management
• Preventive Measures
• Outbreak Summary
• Other Resources
• References

Infectious agent

The infectious agent is Vibrio parahaemolyticus.

V. parahaemolyticus is a halophilic gram-negative bacterium naturally found in temperate and tropical marine environments worldwide.¹ Both traditional and molecular methods are used to subtype the organism, including serotyping and multilocus sequence typing (MLST). Serotypes are characterised from a combination of somatic (O) and capsular (K) antigens using agglutination tests. V. parahaemolyticus possesses at least 12 O and more than 70 K antigens.^2,3 Whole genome sequencing can be performed to enable identification of MLST subtypes and virulence markers, as well as phylogenetic analysis of isolates during outbreaks.

Virulence is associated with the presence of thermostable direct hemolysin (tdh gene) or thermostable related haemolysin (trh gene).⁴ The presence of tdh in clinical samples (associated with the “Kanagawa phenomenon”) is an important indicator for pathogenic potential. One or both genes will be present in the majority of isolates from symptomatic cases (>90%). However, these genes may not always be present, suggesting other virulence factors may play a role in pathogenic mechanisms .^5,6

Case definitions and notification criteria

Only confirmed cases should be notified.

Confirmed case

A confirmed case requires laboratory definitive evidence only.

Laboratory definitive evidence

Isolation of Vibrio parahaemolyticus from a clinical specimen.

OR

Detection of Vibrio parahaemolyticus by nucleic acid testing from a clinical specimen.

Outbreak criteria (if applicable)

Common source: Two or more cases epidemiologically linked to a common source by location and time of exposure.

OR

Community-wide: A generalised, community-wide increase in observed case numbers versus expected case numbers.

Notification procedure

Pathology laboratories to notify on pathological confirmation by usual means.

Objectives of surveillance

1. To identify outbreaks of Vibrio parahaemolyticus and enable a timely public health response.
2. To monitor the epidemiology of Vibrio parahaemolyticus.

Public health significance and occurrence

Vibrio parahaemolyticus became nationally notifiable on 1 January 2025 and subsequently in Queensland on 3 May 2025. The bacterium is naturally found in tropical and temperate waters worldwide, often in coastal and estuarine surroundings. ^1,4,5,7,8 Seafood products harvested from these areas are naturally contaminated, though usually at levels not considered pathogenic to humans.^2,9 In general, seawater temperatures between 15°C and 25°C are suitable for growth, and as such, warmer months are potentially high-risk periods for harvesting seafood products.^4,9,10 Rapid cooling and maintaining temperature control from harvest to retail is the main preventative measure to minimise bacterial proliferation.⁹

Consumption of raw or undercooked seafood contaminated with virulent strains of V. parahaemolyticus can lead to human disease.^4-6 Sporadic cases and outbreaks have been reported  in many areas worldwide including South-east Asia, Japan, United States, Europe, New Zealand, and Australia.^5,11 Foodborne outbreaks are commonly associated with shellfish, including oysters and mussels.^4-6 There have been five foodborne outbreaks of V. parahaemolyticus gastroenteritis reported in Australia between 2002 and 2022, all associated with the consumption of oysters that were harvested from either South Australian or Tasmanian growing regions.^4,9

Incidence data within Queensland and Australia is not well understood. Prior to becoming nationally notifiable, a total of 29 locally acquired sporadic infections had been reported across four jurisdictions between 2016 and 2020, with oyster consumption reported by 76% of cases.^4,9 Outbreaks have probably been under-reported in the past. However, in 2021, there were 268 cases of V. parahaemolyticus infection reported across Australia associated with consumption of contaminated oysters from South Australia.⁴ Australia produces more than 8,000 tonnes of oysters per annum, with significant increases in production expected in the coming years. The combination of significant growth in the oyster industry and climate-related factors may be associated with increased incidence of vibriosis in Australia.⁹

Clinical features

Persons infected with V. parahaemolyticus typically present with acute gastroenteritis characterised by watery diarrhoea (occasionally bloody) and abdominal cramps. Additional symptoms including nausea, vomiting, low-grade fever and headache may also be present.⁵ Symptoms are mostly mild to moderate but can be severe in immunocompromised individuals. Rarely, septicaemia can occur, particularly in persons with chronic underlying conditions such as liver disease and immune disorders.^12-14 V. parahaemolyticus can cause wound and ear infections.¹⁵

Illness is generally self-limiting with a duration between 1 – 7 days. Asymptomatic infection is uncommon but has been previously described.⁶

Reservoir

Marine waters, shellfish and other marine organisms can act as reservoirs for V.parahaemolyticus. Its prevalence and distribution within water is affected by multiple environmental factors including depth, temperature, salinity, dissolved oxygen, pH, presence of sediment, and organic matter.^1,11 Raw seafood may be naturally contaminated, though the number of organisms present are usually low with no implications for human health.^2,9 Filter-feeding bivalve shellfish such as oysters and mussels are common vehicles of infection as the organism will concentrate within tissues.^11,16 There is no mammalian reservoir.¹⁷

Mode of transmission

Transmission frequently occurs by ingestion of raw or inadequately cooked seafood, especially shellfish, including oysters.⁵ Wound infections occur when the bacteria enter broken skin (e.g. cuts, abrasions, punctures) during exposure to seawater.¹¹ Likewise, ear infections may occur if seawater enters the ear. Person to person transmission is very rare.^5,18

Incubation period

The incubation period can range from 4 hours up to 7 days (usually 12 – 24 hours).^4,5,19

Period of communicability

Not considered communicable. Person to person transmission is very rare.

Susceptibility

Susceptibility to infection is universal. Persons with decreased gastric acidity, liver disease or immune disorders are at higher risk of infection.⁵

Management

Cases

Investigation

Gastrointestinal illness:

Routine case investigation is not recommended unless in the context of a public health response, such as a cluster or outbreak investigation.

Wound or ear infections:

Nil investigation required unless an outbreak is indicated.

Management and Restriction

Usual enteric precautions during acute illness.

Cases should be excluded from childcare, preschool, school and work for 24 hours after cessation of diarrhoea. Food handlers working in a food business should be excluded until 48 hours after cessation of diarrhoea.

Asymptomatic carriers do not need to be excluded from work, but the importance of strict personal hygiene and proper handwashing should be stressed, particularly for food handlers.

Evidence of microbiological clearance is not required.

Persons with prolonged diarrhoeal illness should seek appropriate medical advice and guidance.

Nil restrictions for cases with wound or ear infections.

Treatment

Treatment is supportive only.

Rehydration as appropriate. Illness is usually self-limiting. Antibiotic therapy may be required for cases with prolonged diarrhoea, wound or ear infections, and those at risk of severe illness. Wounds should be protected from further exposure to seawater, covered with an appropriate dressing, and cleaned thoroughly when necessary.

Counselling

Advise about mode of transmission and personal hygiene practices including hand washing.

Contacts

Contact tracing and contact management is generally not indicated for single cases. For cluster/outbreak investigations, consider the diagnosis of V. parahaemolyticus infection in symptomatic contacts of the case due to a possible common exposure.

Community outbreaks

Any epidemiological clusters of V. parahaemolyticus infection should be investigated to identify a potential common exposure. Two or more cases with a common exposure are suggestive of an outbreak.

An investigation should be undertaken in conjunction with Environmental Health, OzFoodNet, Food Safety Standards and Regulation Unit (HPRB), Public and Environmental Health Reference Laboratory (PEHRL) and other agencies as required.

Preventive measures

Exclude cases as described above.

Avoid consumption of raw or undercooked seafood including shellfish such as oysters and mussels, particularly during warmer months. Persons at risk of severe disease (e.g. immunosuppressed) should not consume these products.

Seafood products should be chilled immediately when caught and kept refrigerated. Ensure raw seafood is kept separate from ready to eat foods when preparing or storing food. Wash hands with soap and water after handling raw seafood.

Purchase shellfish and oysters from trusted or licensed seafood retailers.

Thorough cooking of seafood will prevent V. parahaemolyticus infection. When cooking, ensure the seafood reaches an adequate internal temperature to inactivate the organism (65°C) [2]. Avoid using seawater in food preparation.

Avoid exposing open wounds (e.g. cuts, abrasions, punctures) to seawater. If exposed, wash with soap and clean water.

Outbreak summary

Complete an outbreak summary form for OzFoodNet.

Other resources

Queensland Health Guideline for the investigation and management of suspected foodborne illness outbreaks December 2018

References