Epidemics of Vector-borne Diseases Observed in Infectious Disease Surveillance in Japan, 2000-2005

BACKGROUND Observing the epidemics of vector-borne diseases is important. One or more cases of 6 vector-borne diseases were reported to the National Epidemiological Surveillance of Infectious Diseases in Japan in 2000-2005. METHODS The reports of those cases were available. The incidence was observed by region of acquired infection, prefecture reporting, and week and year of diagnosis. RESULTS The incidence rate per year per 1,000,000 population was 0.36 for dengue fever, 0.04 for Japanese encephalitis, 0.38 for Japanese spotted fever, 0.08 for Lyme disease, 0.74 for malaria, and 3.50 for scrub typhus. There were no cases of dengue fever or malaria derived from domestic infections. The yearly incidence rate increased for dengue fever and Japanese spotted fever, and declined for malaria and scrub typhus. The proportion of cases reported in Tokyo was 44% for dengue fever and 37% for malaria. The number of prefectures reporting one or more cases of Japanese spotted fever increased in western Japan. The cases of scrub typhus increased in autumn-winter in prefectures of eastern Japan, and increased both in autumn-winter and spring in western prefectures. CONCLUSIONS The study reveals the epidemiologic features of both temporal and geographic distributions of cases of 6 vector-borne diseases in Japan, 2000-2005.

Vector-borne diseases, such as dengue fever and malaria, have a major impact on public health all over the world. 1 There are disease-specific characteristics in geographic distribution, temporal trends and seasonality of such cases because their transmission is dependent on the spread and density of appropriate vectors (mosquito, tick, etc). 2 In many countries, the surveillance of various vector-borne diseases has been conducted with the aim of detecting, controlling and preventing their epidemics. [3][4][5][6] The epidemiologic characteristics of those diseases have been described from the surveillance data. 3,4 In Japan, the National Epidemiological Surveillance of Infectious Diseases (NESID) has targeted specific vector-borne diseases. 6,7 Those with one or more cases reported in 2000-2005 include 6 diseases: dengue fever, Japanese encephalitis, Japanese spotted fever, Lyme disease, malaria and scrub typhus (tsutsugamushi disease). 7 The epidemiologic features of these diseases have been described, [8][9][10][11][12] but have not be sufficiently evaluated from the viewpoint of geographic and temporal clustering.
In the present study, the geographic and temporal distributions of cases of the above 6 vector-borne diseases were analyzed from the NESID data in Japan, 2000-2005. and declined for malaria and scrub typhus. Table 2 shows the incidence of vector-borne diseases by region of acquired infection in [2000][2001][2002][2003][2004][2005]. While nobody acquired the infection of dengue fever or malaria in Japan, the proportion of infection for the other 4 diseases was 86.7-100.0%. Figure 1 shows the incidence of vector-borne diseases by week and year of diagnosis in 2000-2005. Some seasonal patterns of incidence were observed for Japanese spotted fever and scrub typhus, but none for dengue fever and malaria. Table 3 shows the incidence of vector-borne diseases by prefecture reporting in 2000-2005. The dengue fever cases reported in Tokyo were 43.6% of the national total. The incidence of Japanese encephalitis by prefecture was 4 cases or less. The incidence rate ratio of Japanese spotted fever compared with the national rate was over 3 in several prefectures of western Japan: Wakayama, Shimane, Tokushima, Ehime, Kochi, Miyazaki, and Kagoshima. Those ratios being higher than one were statistically significant. The cases of Lyme disease reported in Hokkaido, Japan's northernmost island, were 45.0% of the national total, while malaria cases reported in Tokyo were 37.2%. The incidence rate ratio of scrub typhus was over 3 in several prefectures in eastern and western Japan: Akita, Fukushima, Oita, Miyazaki, and Kagoshima. Those ratios being higher than one were statistically significant. Figures 2 and 3 show the distribution of cases of Japanese spotted fever and scrub typhus by prefecture reporting, and week and year of diagnosis in 2000-2005, respectively. Distributions of other diseases were not shown since the cases of Japanese encephalitis by prefecture were too few and the proportion of cases of dengue fever, Lyme diseases and malaria reported only in one prefecture was high, as shown in Table 3.
health centers to the local government (prefecture) and the Ministry of Health, Labour and Welfare of Japan is made through an on-line computer network.
A total of 11 vector-borne diseases are notifiable: the above 6 as well as relapsing fever, yellow fever, Crimean-Congo hemorrhagic fever, epidemic typhus, and plague. 6,7 The information reported includes sex, age, date of diagnosis, and region where infection was acquired.

Surveillance data and method of analysis
The reports involving the above 6 vector-borne diseases diagnosed in 2000-2005 to the NESID in Japan were available. The data we used were the week and year of diagnosis, prefecture reporting, and region of acquired infection (Japan, others, and unknown).
The incidence of 6 vector-borne diseases was observed by region acquired, prefecture reporting, and week and year of diagnosis. The incidence rate per population by prefecture reporting was calculated using the 2000 census population data, and was compared with that nationwide. The exact test under the assumption that the number of cases follows a Poisson distribution was used for the comparison. Table 1 shows the incidence of vector-borne diseases in 2000-2005. The total incidence and the incidence rate per year per 1,000,000 population were, respectively, 275 and 0.36 for dengue fever, 33 and 0.04 for Japanese encephalitis, 294 and 0.38 for Japanese spotted fever, 60 and 0.08 for Lyme disease, 566 and 0.74 for malaria, and 2,680 and 0.35 for scrub typhus. The yearly incidence rate rose for dengue fever and Japanese spotted fever,      Ratios of incidence rate to that in whole of Japan in parentheses. * p<0.01 by exact test for comparing with incidence rate in the whole of Japan.  biculid mites, and has been endemic all over Japan except for a few prefectures. 10,12 The incidence of cases was observed to fall from 791 in 2000 to 345 in 2005. Though the reason for the decrease is unknown, some interesting seasonal and geographic patterns of infections were reported in previous studies. 10,20 Such pattern have been related to the activities of two different species of trombiculid mites, insofar as the high incidences in autumnwinter in many areas were mainly due to one species of mite, while those in spring in western Japan were mainly due to the other.
In conclusion, although there were some limitations and problems in the present study, based as it was only on reports to the NESID, some meaningful epidemiologic features in the temporal and geographic distributions of cases of 6 vector-borne diseases in Japan, 2000-2005, were revealed.