Kudoa septempunctata is a myxosporean parasite that causes transient gastroenteritis within hours after consuming raw infected fish. Although most commonly associated with farmed olive flounder (Paralichthys olivaceus), the parasite has been reported in diverse marine fishes, indicating human exposure is not limited to olive flounder. South Korea, the world’s largest producer of farmed olive flounder, designated Kudoa septempunctata a notifiable foodborne pathogen in 2015.
This analysis examined whether COVID-19–related changes in dining and travel disrupted transmission and whether cases rebounded as behaviors normalized through 2024. The temporal, spatial, and setting-specific patterns were evaluated to assess whether restaurant-based raw fish consumption is the principal exposure pathway.
The authors obtained monthly outbreak and case data for 2015–2024 from the South Korea Ministry of Food and Drug Safety surveillance system, aggregated at the municipality level (si-gun-gu, n = 229) and linked to population counts from Statistics Korea. February 2020 was defined as the intervention point for the COVID-19 onset.
A negative binomial interrupted time series model was fitted, incorporating a population offset and Fourier seasonality terms (K = 2). Choropleth maps of crude incidence rates and Getis–Ord statistic hotspot analyses compared the pre–COVID-19 period (2015–2019) with the COVID-19/postpandemic period (2020–2024). Further methodological details and figures were provided in the article appendix.
Across 2015–2024, surveillance recorded 1,230 cases and 215 outbreaks of Kudoa septempunctata–associated foodborne disease. Prepandemic trends showed rising incidence: annual cases increased from 114 in 2015 to 308 in 2019, and outbreaks rose from 15 to 48 over the same period. Restaurants accounted for 167 (77.7%) of all outbreaks, and no household outbreaks were reported in any year, implicating commercial food-service venues as the dominant setting for exposure.
Following the onset of the COVID-19 pandemic, reported cases dropped sharply to 40 in 2020 and 8 in 2021, with a gradual rise to 70 cases by 2024.
The interrupted time series model, with February 2020 as the intervention point, identified a highly significant immediate reduction in monthly cases at COVID-19 onset. The reported coefficient (β2 = −2.948; 95% CI −3.919 to −1.989; p<0.001) corresponds to an approximate 95% reduction in monthly case counts. The postpandemic slope did not indicate a statistically significant recovery through 2024, despite gradual increases in absolute case counts during 2023–2024.
Municipality-level impact was notable: the number of affected municipalities decreased from 89 (38.9%) in the prepandemic period to 27 (11.8%) in 2020–2024.
Before the pandemic, incidence was highest in coastal municipalities and Jeju Island, with Getis–Ord statistic hotspots clustered in the Incheon area. After pandemic onset, the spatial pattern shifted, and incidence concentrated along the Gangwon Province east coast, described in the article as a major raw fish dining destination. Choropleth and hotspot comparisons demonstrating these shifts are available in the appendix.
Several concurrent factors likely contributed to the abrupt decline and limited recovery in reported Kudoa septempunctata cases:
Public health measures and legally enforced restrictions on restaurant capacity, operating hours, and group size in South Korea reduced opportunities for concentrated exposure in commercial dining venues. South Korea’s foodservice spending contracted in 2020 and remained suppressed through 2021, removing venues that concentrate raw flounder consumption from typical exposure chains.
The parasite depends on a specific food vehicle and has no documented human-to-human transmission; therefore, interruption of commercial raw fish exposure effectively cuts the transmission chain without the potential for rapid person-to-person rebound seen with enteric pathogens like norovirus.
Mandatory inspection protocols for olive flounder farms implemented after 2015 may have reduced contamination at the aquaculture source, while enhanced molecular diagnostics and persistent consumer shifts toward delivery and home cooking further constrained opportunities for exposure.
A parallel decline in Kudoa septempunctata outbreaks was reported from Japan, supporting the view that pandemic-era reductions in raw fish dining played a role beyond country-specific changes.
Despite these factors, a gradual increase in cases during 2023–2024 may signal potential reemergence as raw fish dining increases.
Key limitations noted by the authors include:
Possible reporting disruption during the pandemic period could artificially inflate the observed decline in reported cases.
Underascertainment is likely because symptoms are transient and diagnostic availability is limited, which can reduce case detection and reporting.
The single-group interrupted time series design precludes counterfactual inference and cannot disentangle the relative contributions of concurrent changes such as aquaculture inspection, diagnostic improvement, and shifts in consumer behavior.
These limitations constrain definitive causal attribution of the observed reductions solely to dining restrictions.
From 2015–2024, surveillance data show a clear rise in Kudoa septempunctata cases and outbreaks before COVID-19, followed by an abrupt 95% decline in monthly cases at pandemic onset and a sustained lower level through 2024. Temporal, spatial, and setting-level evidence is consistent with restaurant-based raw fish consumption being the principal transmission pathway, although observational design and multiple concurrent interventions preclude definitive causation.
The authors recommend renewed surveillance as raw fish dining returns toward prepandemic levels during 2025–2026 and call for integrating consumer behavior monitoring with aquaculture inspection in coastal high-risk areas to sustain reductions. Additional monitoring and improved diagnostics will aid detection of reemergence and help target source-control measures in aquaculture and foodservice.