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.

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Vector of the causative agent of “Hyuganetsu” disease has been investigated by the author et al since 1954, and the following results were obtained
1) Trombiculid mites are not considered as vector of the causative agent because of non-correspondence of their season with the outbreak of the disease.
2) Arthropoda is not considered to be vector, because there is no arthropoda parasitic on rodents that is common to the places of the patients and to the places where the rodents harbouring the causative agent were captured.
3) Of 25 patients suffering from “Hyuganetsu” disease, there were 21 patients who had eaten grey mullet (Mugil cephalus) within 30 days before getting ill, 3 patients, who had eaten grey mullet-like fish, and one patient who had never eaten grey mullet what the last patient said, however, being unreliable.
4) Trials were made to eat grey mullet in 6 groups, each consisting of 8-26, volunteers, with the result that five subjects in 4 groups were affected by the disease.
5) The latent period is 19 days in an average, ranging from 13 to 28 days.
6) It has been hitherto unsuccessful to isolate the causative agent from several species of metacercaria that are parasitic on the flesh of grey mullet.
It is considered from the above findings that vector of the causative agent might be a certain metacercaria that is parasitic on the flesh of grey mullet. Accordingly, the relation of this agent with Neorickettsia helmintoeca must be studied in the future, although the agent was named Rickettsia sennetsu by Misao and Kobayasi and Rickettsia todai by the present authors.

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