| Western equine encephalomyelitis virus (WEEV) is a member of the Alphavirus genus within the family Togaviridae. Illnesses caused by WEE virus range from mild fevers with headache to aseptic meningitis and encephalitis. The mortality rate from WEE is reported to range from 3 to 5% in people who develop clinical disease after infection. The virus is historically enzootic in North America and South America. In North America, Culex tarsalis is considered as the primary vectors. Birds are thought to be the most important vertebrate hosts for WEEV. The virus replicates very effectively in birds, which act as amplifiers of the virus and important sources of infection for mosquitoes. WEE virus normally cycles among wild birds and mosquitoes, the virus has been found in a variety of other mosquito species, such as: Aedes, Anopheles, Coquillettidia, Culex and Culesita. Humans and equines are infected tangentially to the primary cycle and are dead-end hosts for the virus. In addition, WEEV has also been isolated from louses, ticks and mites. Although it has occurred most often in North America, WEE still is a major public health concern in the world. Outside of America, WEEV antibody had been detected from human sera in Poland and former USSR. Y62-33 virus had been isolated from former USSR in 1962. In China, two virus strains isolated from Anopheles hyrcanus and Ixodes persulcatus in 1990 were identified as WEEV, which is the second report of WEEV separation in Eurasia besides Russia. Serology investigation in China showed the percentage of human sera positive for WEE was 2.71%. Meanwhile, many unidentified fever and encephalitis case have been found in China. All of these evidences indicated WEEV may be one of new pathogens in our country. Nowadays, bioterrorist attacks(diseases caused by biological agents) are not only a public health issue but also a problem of national security, WEEV was classified into the second highest priority agents. If WEEV be dispersed by aerosol sprays, mammals and birds will be infected and virus goes through a cycle of amplification and dissemination by mosquitoes, which will cause more range prevalence in human.In order to assessing the risk that indigenous mosquitoes are capable of sustaining and transmitting the WEEV, laboratory studies are carried out to evaluate the important mosquitoes in China for their ability to be infected with and transmit WEEV. The results are as follows:1. The infection rates of the important mosquitoes of China after oral exposure to WEEVCx. p. pallens, Cx. p. quinquefasciatus, Cx. tritaeniorhynchus, Ae. albopictus and Ae. aegypti were used to be infected orally with WEEV. All mosquito species tested were susceptible to infection with WEEV after biting the artificial infectious Leghorn chicken. The infection rates of mosquitoes tested in our study were 45.5%, 60.0%,80.0%,36.7% and 25.0%, respectively. The highest infection rate was obtained with Cx. tritaeniorhynchus, which was 80.0%, and differed significantly from the other mosquito species (x~2=13.709, P=0.008) .2. The transmission rates of the important mosquitoes of China by biteAll mosquito species examined in this study were able to transmit WEEV to 1-3-day-old Leghorn chicken by biting. Transmission rates of Cx. p. pallens, Cx. p. quinquefasciatus, Ae. albopictus and Ae. aegypti were 40.74%,53.13%,57.14% and 45.16%, respectively. The transmission rate of Ae. albopictus was the highest, which was 57.14%, There was insignificant difference between any two mosquito species (X~2=1.879, P=0.598) .3. The dissemination rates of the mosquitoes after oral exposure to WEEVThe dissemination rates of Cx. p. pallens, Cx. p. quinquefasciatus, Cx. tritaeniorhynchus, Ae. albopictus and Ae. aegypti after oral exposure to WEEV were 60.0%,61.1%,75.0%,54.5% and 50.0%. The dissemination rate of Cx. tritaeniorhynchus was the highest, which was 75.0%, There was insignificant difference between any two mosquito species (x~2=1.229, P=0.873) .4. Relationship of mosquito numbers after infection, dissemination and transmissionMosquitoes with WEEV dissemination almost can transmit virus to Leghorn chicken, only few failed to transmit; But some mosquitoes with no viral dissemination can partial transmit; Otherwise, some mosquitoes can be completely refractory to peroral infection with virus, but partial Leghorn chicken they bitten can be infected. 5. The distribution of WEEV antigen in orally infected Cx. p. pallensThe distribution of WEEV antigen in orally infected Cx. pipiens pallens were studied by immunocytochemistry. On 3-4 day post-infection, WEEV antigen was observed in the midgut of some individual adults of Cx. p. pallens, and then in the posterior midgut, anterior midgut, ovarian follicle after 7 days. Salivary glands can also be infected by virus after 10 days. In some individuals, the positive results appeared only in the midgut and negative results were showed in the other tissues. Meanwhile, there were no detectable WEEV antigens in any tissues of some mosquitoes. Based on these findings, it was suggested that the susceptibility of Cx. p. pallens to WEEV vary among individuals, and the mesenteron infection barrier may play a role in insusceptible individuals. Salivary gland barrier maybe cause the difference of transmission rate among individuals. |
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