Both STs of V. parahaemolyticus isolated from cases in this outbreak investigation harbored virulence genes required for pathogenicity. Specifically, all V. parahaemolyticus ST417 isolates harbored the virulence gene trh, and all ST50 harbored 2 virulence genes, tdh and trh.

This V. parahaemolyticus outbreak was caused by 2 STs and had a considerable effect on the population of Australia because of the nationwide distribution of oysters across mainland jurisdictions and cases occurring over a 5-month period. Recent investigations of other V. parahaemolyticus outbreaks have focused mostly on point source events, such as outbreaks on cruise ships (8,20,21). The communitywide outbreak in this report highlights the potential risks associated with consumption of raw oysters in Australia. Raw shellfish, particularly oysters, are known to be a common source of Vibrio foodborne illness, but recent trends observe increasing numbers of sporadic and outbreak cases, across an internationally wider span, somewhere cases had not been previously reported (8,11,22). V. parahaemolyticus has been isolated from other shellfish, including mussels, prawns, clams, and scallops during food surveillance studies, and has been identified as the cause of outbreaks in countries other than Australia (3,23-25).

The identification of 2 unrelated STs, ST417 and ST50, within this outbreak indicated the cause to be more relative to environmental factors influencing favorable growth conditions for V. parahaemolyticus across the oyster-growing region than to a single temperature-abuse error or single point source event. The appearance of those 2 strains could also be indicative of >1 outbreak occurring at the same time, with common contributing factors. However, multiple strains or types of a pathogen can cause discrete outbreaks and require a common public health investigation and response (26,27). The epidemiologic evidence in this investigation indicated raw oysters grown in South Australia as the cause of both ST417 and ST50 V. parahaemolyticus infections across Australia. Previous V. parahaemolyticus outbreaks have reported single-strain infections, predominantly by using traditional O and K serotyping methods (8,25). Longitudinal studies in Asia have identified a range of strains within a region (28), and other reports have highlighted highly virulent pandemic strains (e.g., ST36) detected across a widening international geographic range (1,13). Although minimal data are available in Australia regarding V. parahaemolyticus strains linked to locally acquired cases, recent increased use of genomic methods and practices will likely change that. The emergence of WGS characterization in Australia will also contribute to global knowledge regarding emerging and pathogenic strains. The presence of the trh virulence gene in both strains within this outbreak -- and the additional tdh gene in the ST50 case isolates -- correlates with prior literature noting that the presence of those virulence genes contributes to clinical symptoms but that both genes are not required to cause illness (29).

International risk assessments have been conducted for V. parahaemolyticus in seafood, noting the pathogenicity of the organism, the growth of V. parahaemolyticus increasing with increased water temperatures, and the need for strict postharvest controls to reduce the risk for foodborne disease (6,12). Outbreaks have occurred more frequently during warmer months (3) and at times when seawater temperatures have increased (8,20) or other environmental factors have had an influence (e.g., El Niño events or decreases in salinity) (10,30). In response to the outbreak we have described, oyster growers implemented postharvest controls through a Vibrio control program, where oysters were placed under active refrigeration. General trends of increased sea surface temperatures in Australia (31) and the seasonal occurrence of the Leeuwin current, which brings warm tropical waters to Western and South Australia (31), potentially created favorable conditions for growth of V. parahaemolyticus in South Australia oyster-growing bays. We believe that further research would improve understanding of risk factors for V. parahaemolyticus outbreaks in Australia's prone regions, including ongoing environmental surveillance at harvest sites to monitor seawater temperatures, salinity, and harvest conditions, as well as at points along the storage and transport chain to consumers.

We noted that many outbreak case-patients in this study consumed medication that reduces stomach acid, a finding noted in previous studies that investigated risk for V. parahaemolyticus and other bacterial gastroenteric infections (32,33). Gastric acidity also decreases with age (34); therefore, infection susceptibility could increase with age, which is consistent with our observed median case patient age of 52 years. The fact that a large portion of our case-patients were older adults might also be related to food consumption patterns in the general population of Australia, where mollusks are less commonly eaten by children compared with adults (35). The outbreak we studied showed higher severity of illness than some previous outbreaks; for example, we noted 13% of case patients hospitalized and a single case with septicemia, compared with a study that investigated an outbreak associated with Alaska oysters, where there were no hospitalizations (8). Conversely, we noted a lower hospitalization rate for case-patients (13%) compared with a longer-term study that reported a hospitalization rate of 44% (9). Individual factors and the pathogenicity of different strains could affect disease severity in outbreaks.

The first limitation of our investigation is that culture for V. parahaemolyticus is not always attempted on diarrheal samples in diagnostic laboratories in Australia, and V. parahaemolyticus targets are often omitted in routine fecal multiplex PCR kits employed for direct detection of enteropathogens. Also, there was likely underreporting of cases because V. parahaemolyticus is not a notifiable condition in all jurisdictions in Australia. However, public health laboratories were contacted by their respective health departments and asked to provide information on enteric V. parahaemolyticus cases. Further typing of strains by WGS is also not consistently conducted across Australia and sometimes must be specifically requested if an outbreak is suspected. During this outbreak, some requests for further typing were made several weeks after the initial isolation, at which point no specimens were available for shipment to the public health laboratories. Because of incomplete typing of all case isolates in this outbreak, some cases might have been of a different ST and might not have been specifically linked to the current outbreak. In addition, V. parahaemolyticus outbreak cases we studied coincided with a national surge in SARS-CoV-2 infections in Australia, putting strain on public health resources. Therefore, not all case-patients were able to be interviewed, and not all isolates were able to be further typed.

There were also limitations in the traceback of oysters within a complex supply chain, including distributors and retailers receiving stock from multiple growers and different growing regions, leading to potential mixing of stock, some incomplete records and invoicing, and case-patients having multiple exposures to oysters within their incubation period. Mixing of oyster stock could also have contributed to the identification of multiple strains of V. parahaemolyticus in cases included in this outbreak. Although traceback was unable to be completed for all cases, oysters consumed by most casepatients were traced to at least the harvest area.

In conclusion, evidence for the source of this outbreak was strong, considering the oyster consumption among case-patients, traceback of the source of oysters consumed by case-patients, and identification of the same strain of V. parahaemolyticus in both oysters and case patients. The reduction in cases of V. parahaemolyticus after the recall of oysters and wide implementation of Vibrio control programs supports this evidence. This outbreak of V. parahaemolyticus associated with consumption of Australia-grown oysters, largely consumed raw, has led to improvements in postproduction control and traceability in the oyster industry in South Australia. The outbreak also spotlighted the virulent potential of V. parahaemolyticus and the value in distinguishing it as a nationally notifiable disease in Australia. Improved surveillance data, including strain identification from a wider range of regions, and a clearer understanding of underreporting have been highlighted by the World Health Organization as priorities for improving risk assessment processes for V. parahaemolyticus (22). Increased surveillance across all jurisdictions in Australia would improve outbreak detection and ensure a prompt and coordinated public health response.