Epidemiological and Ecological Studies of Japanese Encephalitis in Okinawa,Subtropical Area in Japan. I. Investigations on Antibody Levels to Japanese Encephalitis Virus in Swine Sera and Vector Mosquito in Okinawa,Miyako and Ishigaki Islands

From 1985 to 1989, serum specimens of swine raised in the northern, central and southern areas in Okinawa island were examined for Japanese encephalitis (JE) virus antibody by ELISA and hemagglutination-inhibition test. The antibody positive rate was found to be higher in the north and central than in the south. The 2-mercaptoethanol sensitive antibody to JE was detected mostly in June and July, and occasionally in other months except February and March. There was no month when all specimens from three areas turned antibody-negative simultaneously, indicating that JE virus transmission to swine lasted longer in Okinawa island than in other temperate areas in Japan. From 1986 to 1991, the vector mosquitoes (Culex tritaeniorhynchus) were collected in a pig farm in the south of Okinawa island. A total of 153 strains of JE virus was isolated from the vector mosquitoes mainly in June. In Miyako and Ishigaki islands, the antibody positive rate in swine sera was found to be extremely low, compared with that in Okinawa island. In Miyako island, where no paddy rice field is cultivated, a few adults as well as larvae of the vector mosquito were collected, while in Ishigaki island, where there are many watered rice fields, a lot of adults as well as larvae were collected. Although the environmental situation is quite different between the two islands, JE virus transmission appeared to be very low in both islands.     

Seroprevalence of antibodies against spotted fever group rickettsia among dogs and humans in Okinawa, Japan

We evaluated serum antibodies against Rickettsia japonica in 517 dogs (430 stray dogs and 87 pet dogs) and 164 humans in Okinawa, Japan, by indirect immunofluorescence assay. The seropositive rate in stray dogs was significantly higher than that in pet dogs (30.7 versus 4.6%, P<0.01). This high prevalence rate is attributed to the understandably frequent environmental exposure of stray dogs to tick infestation. Human samples obtained from Okinawa and Sapporo also showed a significant difference in seropositive antibody percentages (45.1 and 12.0%, respectively, P<0.01). This result suggests that there has been pre-exposure to spotted fever group rickettsia in humans in Okinawa.     

First records of tick-borne pathogens, Borrelia, and spotted fever group Rickettsiae in Okinawajima Island, Japan

In early April 2000, tick-borne pathogens were surveyed in the northern area of Okinawajima Island, Okinawa Prefecture, which is the southernmost area of Japan. Borrelia valaisiana, a Lyme disease spirochete, was isolated from a field mouse Mus calori, and unidentified rickettsiae of the spotted fever group were isolated from all stages of Amblyomma testudinarium. These are the first reports of these pathogens on Okinawajima Island.     

Characterization of spotted fever group rickettsiae detected in dogs and ticks in Okinawa,Japan

Spotted fever group (SFG) rickettsial DNAs were detected in 2.4% of 340 canine blood samples and a pool of 84 tick pool samples (229 ticks) collected in Okinawa, Japan by PCR using a citrate synthase and an SFG rickettsial 190-kDa surface antigen gene primer pair. The sequences of both genes from canine blood and tick samples showed high levels of similarity with those of Rickettsiajaponica and several SFG rickettsiae (R. aeschlimannii, R. massiliae, R. rhipicephali and Bar-29 strain). Phylogenesis of canine blood and tick samples was closely related to that of reference SFG rickettsiae. Serological evidence of SFG rickettsial infection in dogs and humans in Okinawa, where no clinical human cases have been reported, has been obtained. In this study, genetical characterization of SFG rickettsia in Okinawa was investigated phylogenetically.     

Molecular Epidemiology of Vibrio cholerae O1 Isolated from Sporadic Cholera Cases in Okinawa,Japan

In July 1994, 6 cholera cases due to Vibrio cholerae O1 El Tor Ogawa sporadically appeared in Okinawa. All 6 patients had no history of traveling abroad. In the period of this cholera outbreak, a strain of V. cholerae O1 El Tor Ogawa was detected from an imported fish at the Naha port quarantine station. The isolates were characterized to clarify whether or not, they belonged to a common clone. Phenotypes were identical except that one strain revealed cured Celebes and the others were original Celebes in kappa phage typing. The restriction fragment patterns of DNA of the isolates hybridized with an enzyme-labeled oligonucleotide probe for cholera toxin gene (ctx) were identical. Randomly amplified polymorphic DNA of the isolates were identical when a primer was used, but 2 patterns were seen when another primer was used. Pulsed-field gel electrophoresis of the chromosomal DNA digested with NotI restriction enzyme showed 3 patterns. The DNA fragment pattern of the strain isolated from the imported fish was different from the clinical isolates. These results suggested that there was no epidemiological relation among the strains of V. cholerae O1 isolated during this period.