Decision-making and public health: How the prevalence of Contracaecum spp. larvae in market-size tilapia may influence fish sample-size to be inspected prior marketing

Human consumption of raw or undercooked fish and fishery products may cause infection with foodborne parasitic nematodes, particularly of the family Anisakidae. Contracaecum species are cosmopolitan parasitic nematodes with numerous marine and freshwater fish species as intermediate or paratenic hosts, rarely reported as zoonotic agents. Tilapia, of great importance to human nutrition in many countries, can harbor larval stages of diverse Contracaecum species. Accurate examination of fish before marketing is crucial to ensure public health. We conducted a two-year survey of market-size tilapia farmed in Israel originating from 17 tilapia farms to assess the presence of Contracaecum larvae, including a retrospective calculation of infection prevalence and analysis of the accuracy of larval detection with various sample sizes. Between June 2020 and May 2022, Contracaecum larvae were found in 269/3605 (7.5 %) tilapia shipments. In 217 of these 269 (80.7 %) shipments, only a single larva was found. Among 380 Contracaecum larvae collected, only two were identified as C. quadripapillatum; all others were identified as C. multipapillatum E. The probability of tilapia being parasitized with C. multipapillatum E larvae is much higher than with C. quadripapillatum. Moreover, in the vast majority of tilapia shipments infected with Contracaecum larvae, only a single specimen was infected. Considering this relatively low prevalence, in a sample size of 30 fish tested for the presence of nematode larvae, there is only a 40 % probability of finding an infected specimen. Decision-makers should consider, among other factors, the sample size of tilapia to be inspected before marketing to reduce the chances of parasitized fish reaching the end consumer.