A study on inapparent infection of scrub typhus in Japan

II. Complement-fixing antibody for R. orientalis in sera of inhabitants of young age group in Hachijo Island

Abstract

It was already reported that in Hachijo Island children got scrub typhus more often than adults and that children were attacked by trombiculid mite without knowing it. Accordingly, it may be assumed that some children aquire immunity after exposure to mite-attack merely with subclinical infection or minor symptoms such as common cold or the like. In order to make it clear, sero-epidemiological survey was carried out with CFT on sera collected from 100 school children and 110 inhabitants of young, age group in Hachijo Island. The following results were obtained.
1) Among 100 school children of 12-13 years of age, 45 gave positive in CFT, but only 4 of 45 positive children had a history of scrub typhus. On the other hand, none of negative had a history of the disease. As a control, 33 school children in Tokyo were tested. They were all negative in CFT for R. orientalis.
2) In young age group of inhabitants, 4 of 79 individuals of 15-20 years age group were positive, but none of 29 individuals of 21-35 age group and of 2 individuals of age group over 36 gave positive. Of 4 CF positive inhabitants, one had a history of scrub, typhus 10 years ago, and the other had no history of the disease. Three of 116 CF negative inhabitants had history of suspected or typical scrub typhus infection 5 to 10 years ago.
From the foregoing, it can be said that in Hachijo Island, roughly 50% of school children gave positive CFT for R. orientalis, and the positive CFT was found distinctly lower at the 15-20 age group than those of school children, probably due to dicrease of CF antibody within 10 years or so after infection, either manifest or inapparent.