To estimate the number of anisakiasis patients, we used an anonymized health insurance claim database from JMDC, Inc. (https://www.jmdc.co.jp), a commercial medical database provider in Japan. The nation has a universal health insurance system wherein medical institutions prepare health insurance claims that list the name of the disease or injury on a service basis. Medical expenses, other than a portion directly paid by patients, are reimbursed from taxes and mandatory insurance fees. In this process, medical institutions first submit health insurance claims to a specialized organization that assesses the appropriateness of treatment and amount of reimbursement. In this study, we used a database covering ≈8.43 million persons annually, accounting for >6% of the total population of Japan, during January 2018–December 2019. We extracted health insurance claims with the diagnosis code B81.0 in the International Classification of Diseases, 10th Revision (ICD-10), indicating Anisakis and Pseudoterranova infections.

The number of patients with anisakiasis registered in the target database was 991 in 2018 and 766 in 2019. However, the data revealed a male-biased sex ratio and a workforce-biased distribution for persons 20–39 years of age. Therefore, we adjusted the value in each group for estimation by stratifying the population of Japan by sex and age by using a previously described expanded estimate method (9), and data from the national census conducted by the Ministry of Internal Affairs and Communications in 2015 (https://www.stat.go.jp/english/data/kokusei/2015/final_en/final_en.html). On the basis of our adjustment, we estimate the number of patients in Japan with anisakiasis was 21,511 in 2018 and 17,962 in 2019. The number of patients with anisakiasis recorded in the food poisoning statistics during the same period was considerably lower, <1/40, in our estimation (Table 1).

Figure 1

Figure 1. Anisakiasis patient number and causative species analyzed in study of anisakiasis in Japan, 2018–2019. Three geographic locations in Japan are noted: Hokkaido Island (North Japan), Honshu and Shikoku Islands (Central...

For molecular identification, we obtained 189 larvae of anisakid nematodes from BML, Inc. (http://www.bml.co.jp/eng), a privately-owned clinical testing laboratory in Japan. The larvae were isolated from 181 anisakiasis patients in 30 of 47 prefectures in Japan during 2018–2019 (Figure 1). We extracted DNA samples from individual larvae, used PCR to amplify the internal transcribed spacer 1 region, and sequenced the amplicons to distinguish between nematode genera and identify the Anisakis species. For Pseudoterranova larvae, we further amplified a portion of the NADH dehydrogenase subunit 1 gene in mitochondrial DNA and sequenced to discriminate between species of this genus (Appendix).

Consequently, we identified 168 (88.9%) Anisakis simplex sensu stricto larvae, 10 (5.3%) A. pegreffii larvae, and 11 (5.8%) Pseudoterranova azarasi larvae (Table 2). These findings represent perfect sequence identity with the respective species, regardless of the targets. We deposited the obtained sequences in GenBank under accession nos. LC684518–21 (Appendix Table 1).

A recent study estimated 7,700–8,320 annual anisakiasis cases in Spain, a country with a high incidence of anisakiasis in Europe (10). Although our survey methods differed, we demonstrated that Japan had an average of 19,737 anisakiasis cases per year during 2018–2019. As preventive measures, the government of Japan has repeatedly instructed local establishments (e.g., restaurants, fish mongers, and grocery stores) and consumers to freeze seafood at −20°C for at least 24 hours before consuming it raw or to remove anisakid nematodes during cooking. The Ministry of Health, Labour and Welfare of Japan provides web-based food poisoning statistics with information regarding fish species reported by anisakiasis patients and preparation procedures associated with infections in Japanese to help consumers and fishmongers avoid anisakiasis.

In Japan, A. simplex s.s. nematodes are responsible for the highest incidence of anisakiasis, whereas the species A. pegreffii is the leading cause of anisakiasis in Europe and South Korea (2). A. simplex s.s. nematodes penetrate the muscles of various fish species at a higher rate than A. pegreffii (11), which could partly explain the smaller proportion of A. pegreffii anisakiasis cases in Japan because A. pegreffii nematodes are often removed with fish viscera during the preparation of sushi and sashimi. Furthermore, fish habitat can corroborate the difference in predominant anisakid nematode species between South Korea and Japan; A. simplex s.s.–carrying fish are predominant in the Pacific side of Japan, whereas A. pegreffii–carrying fish are more common in the Sea of Japan and the East China Sea, located between South Korea and Japan (11) (Figure 1).

Figure 2

Figure 2. Analyzed number and percentage of anisakiasis patients and causative species, Japan, 2018–2019. One third (2/6) of patients in the Kyushu Island had Anisakis pegreffii infections. A. pegreffii...

In this study, we identified 11 P. azarasi larvae (5.8%) from 11 patients, 6 of whom lived in Hokkaido, the northernmost insular prefecture of Japan, where cold water fish, such as Pacific cod (Gadus macrocephalus), are commonly consumed (Figures 1,2) (12). Although patients have been reported to orally expel Pseudoterranova larvae that have developed to the fourth stage (13), most (7/11 cases) P. azarasi larvae detected in this study were in the stomach.

Anisakid larvae were isolated from the stomach or intestines of 177 patients, 23 (13%) of whom were asymptomatic. These asymptomatic cases were found incidentally during health check-ups or routine follow-up cancer screening. Some studies have already reported asymptomatic gastrointestinal cases (14,15), but our findings revealed a varied and greater number of anisakid species associated with asymptomatic infections: A. simplex s.s. (91.3%), A. pegreffii (4.3%), and P. azarasi (4.3%). One urticaria case was associated with symptomatic P. azarasi gastric infection. Numerous cases of Anisakis allergy, including cases of urticaria, angioedema, and anaphylaxis, have been reported in Europe (10). In healthcare claims from Japan, Anisakis allergy was not registered as a disease name until 2021. Thus, the database did not include anisakiasis patients with allergic symptoms alone, which is a limitation of this study.

In conclusion, Anisakis infection is currently drawing more attention in Japan. Elucidating the immunopathological mechanisms behind asymptomatic and symptomatic anisakiasis is imperative and can provide insights into the pathogenesis of gastrointestinal anisakiasis.