山本　正悟・元明　秀成・岩切　章・鈴木　泉

Tsutsugamushi disease in Miyazaki Prefecture, 1998-2003

Seigo YAMAMOTO, Hidenari GANMYO , Akira IWAKIRI, Sen SUZUKI

Abstract
Miyazaki prefecture is one of the high-risk regions of the new type Tsutsugamushi disease. Epidemiological aspects of Tsutsugamushi disease in Miyazaki Prefecture during the period from April 1998 to March 2003 were analyzed using the information obtained from 348 patients confirmed by serodiagnosis performed by indirect immunofluorescence test. The antigenic type of Orientia tsutsugamushi(Ot.) infected to a patient was estimated from the serum antibody titer against Gilliam, Karp, Kato, Kawasaki and Kuroki strain of Ot..

General epidemiological aspects of Tsutsugamushi disease in Miyazaki Prefecture were suggested as follows from the results.

1. The incidences are estimated to increase and decrease periodically over several years.
2. About 95% cases are estimated to be infected with Kawasaki or Kuroki type of Ot..
3. Generally the cases infected with Kawasaki and Kuroki type show a ratio of two to one. Kuroki type less common in general.
4. Although Tsutsugamushi disease breaks probably from September through March in Miyazaki Prefecture, about 85% of the total cases fall ill in November and December, and about 95% cases fall ill between October and December.
5. The cases infected with Kawasaki type fall ill from October through January. The cases infected with Kuroki type fall ill from September through March.
6. About 95% cases are infected in the southern part of Miyazaki Prefecture.
   In almost of these endemic areas, both Kawasaki and Kuroki type of Ot. are distributed.
7. The patients peak at the ages of 60-69 years, accounting for about 30%. The patients at the age of 40 or greater account for about 90%. There is no sex difference.
8. The places of acquiring infection are mostly mountainous areas and agricultural land, and more than 60% cases engage in farming and forestry.

The epidemiological aspects obtained in this study provide useful information into diagnosis, treatment and prevention of Tsutsugamushi disease in Miyazaki Prefecture.

Key words：Orientia tsutsugamushi, Rickettsia tsutsugamushi, Tsutsugamushi disease

はじめに
恙虫病は、Orientia tsutsugamushi (Ot.)を保有するツツガムシの幼虫で媒介され、刺口の形成、発熱および発疹を特徴とする疾患で、便宜的に古典的恙虫病と新型恙虫病に分類される。このうち古典的恙虫病はアカツツガムシ媒介性で夏期に発生し、近年、年間数名の発生にとどまっている。一方、新型恙虫病はタテツツガムシあるいはフトゲツツガムシ媒介性で秋〜春に発生する。1974年から北海道と沖縄を除く日本各地で急増し、1984年に患者数約1000名にまで達した後も年間数百名の患者発生を維持している。また、Ot.にはGilliam、Karp、Katoの3抗原型の存在が知られていたが、これらに加え、Shimokoshi、 Kawasaki、Kurokiなどの新たな抗原型の存在が明らかになっている。

宮崎県の恙虫病に関しては、(1)11月をピークに秋〜冬にかけて発生すること、(2)県の中部以南で主として発生すること、(3)Kawasaki型 およびKuroki型のOt.が主な原因であること、(4)これらの病原体を媒介しているツツガムシがタテツツガムシと推定されることなどを随時報告してきた。

今回、既に報告している1997年までの解析⁷⁾に引き続き、1998年以降の患者発生状況の解析を行ったので報告する。