Arthropod-borne viruses (arboviruses) are important causes of human disease nearly worldwide. All arboviruses circulate among wild animals, and many cause disease after spillover transmission to humans and agriculturally important domestic animals that are incidental or dead-end hosts. Viruses such as dengue (DENV) and chikungunya (CHIKV) that have lost the requirement for enzootic amplification now produce extensive epidemics in tropical urban centers. Many arboviruses recently have increased in importance as human and veterinary pathogens using a variety of mechanisms. Beginning in 1999, West Nile virus (WNV) underwent a dramatic geographic expansion into the Americas. High amplification associated with avian virulence coupled with adaptation for replication at higher temperatures in mosquito vectors, has caused the largest epidemic of arboviral encephalitis ever reported in the Americas. Japanese encephalitis virus (JEV), the most frequent arboviral cause of encephalitis worldwide, has spread throughout most of Asia and as far south as Australia from its putative origin in Indonesia and Malaysia. JEV has caused major epidemics as it invaded new areas, often enabled by rice culture and amplification in domesticated swine. Rift Valley fever virus (RVFV), another arbovirus that infects humans after amplification in domesticated animals, undergoes epizootic transmission during wet years following droughts. Warming of the Indian Ocean, linked to the El Niño-Southern Oscillation in the Pacific, leads to heavy rainfall in east Africa inundating surface pools and vertically infected mosquito eggs laid during previous seasons. Like WNV, JEV and RVFV could become epizootic and epidemic in the Americas if introduced unintentionally via commerce or intentionally for nefarious purposes. Climate warming also could facilitate the expansion of the distributions of many arboviruses, as documented for bluetongue viruses (BTV), major pathogens of ruminants. BTV, especially BTV-8, invaded Europe after climate warming and enabled the major midge vector to expand is distribution northward into southern Europe, extending the transmission season and vectorial capacity of local midge species. Perhaps the greatest health risk of arboviral emergence comes from extensive tropical urbanization and the colonization of this expanding habitat by the highly anthropophilic (attracted to humans) mosquito, Aedes aegypti. These factors led to the emergence of permanent endemic cycles of urban DENV and CHIKV, as well as seasonal interhuman transmission of yellow fever virus. The recent invasion into the Americas, Europe and Africa by Aedes albopictus, an important CHIKV and secondary DENV vector, could enhance urban transmission of these viruses in tropical as well as temperate regions. The minimal requirements for sustained endemic arbovirus transmission, adequate human viremia and vector competence of Ae. aegypti and/or Ae. albopictus, may be met by two other viruses with the potential to become major human pathogens: Venezuelan equine encephalitis virus, already an important cause of neurological disease in humans and equids throughout the Americas, and Mayaro virus, a close relative of CHIKV that produces a comparably debilitating arthralgic disease in South America. Further research is needed to understand the potential of these and other arboviruses to emerge in the future, invade new geographic areas, and become important public and veterinary health problems.
Thirty isolations of Tembusu virus and four of Sindbis virus were obtained from approximately 280 000 mosquitoes collected between October 1968 and February 1970 in Sarawak, particularly from K. Tijirak, a Land Dyak village 19 miles South of Kuching. Twenty-two isolations of Tembusu virus and two of Sindbis virus were from Culex tritaeniorhynchus; two of Tembusu virus and two of Sindbis virus came from Culex gelidus. Tembusu virus was active throughout the year at K. Tijirak, the highest infection rates in C. tritaeniorhynchus being in January-March and May-August, when the C. tritaeniorhynchus population was declining and ageing. These results confirm that C. tritaeniorhynchus is the principal arthopod host of Tembusu virus in Sarawak. Antibody studies suggest that birds, particularly domestic fowl, are probably vertebrate maintenance hosts of Tembusu and Sindbis viruses in Sarawak.
In recent years, it has become evident that a generational gap has developed in the community of arbovirus research. This apparent gap is due to the dis-investment of training for the next generation of arbovirologists, which threatens to derail the rich history of virus discovery, field epidemiology, and understanding of the richness of diversity that surrounds us. On the other hand, new technologies have resulted in an explosion of virus discovery that is constantly redefining the virosphere and the evolutionary relationships between viruses. This paradox presents new challenges that may have immediate and disastrous consequences for public health when yet to be discovered arboviruses emerge. In this review we endeavor to bridge this gap by providing a historical context for the work being conducted today and provide continuity between the generations. To this end, we will provide a narrative of the thrill of scientific discovery and excitement and the challenges lying ahead.
Naturally occurring malaria, arbovirus infection and hepatitis in monkeys can be a hazard for the investigator and might interfere with the outcome of experiments. 63 young adult Macaca fascicularis from Malaysia were screened for these infections. About 1 year after their arrival in France, parasitaemia due to Plasmodium spp., was present in 6.4% of the animals and specific antibodies in 55.5%. 19 of 35 initially positive monkeys were tested again 2 years later. Parasitaemia was found in 1 of 4 monkeys and antibodies in 11 of 19 monkeys which were initially positive. 9 of the monkeys initially tested had low titres of antibodies to the Flavivirus genus. All animals were negative for the hepatitis B surface antigen and anti-HBc. The prevalence of IgG antibodies against hepatitis A was 46.0%. The implications in terms of control are discussed.
449 human sera collected in a Land Dyak village were tested for antibodies to 11 arboviruses. Japanese encephalitis and dengue virus antibodies were particularly prevalent. The rates of infection with these viruses were estimated to be 5-2% per annum for Japanese encephalitis, 8-8% for dengue 1 and 4-3% for dengue 2. Chikungunya virus antibodies were quite common with an annual infection rate of the order of 5% per annum. Infections with other Group A and B and Bunyamwera group viruses were generally at a low level.
The exotic and emerging viral encephalitides are caused by animal or human viruses and characterised by sudden unexpected outbreaks of neurological disease, usually in tropical and sub-tropical regions, but sometimes spreading to temperate areas. Although a wide range of viruses come within this label, as this review highlights, there are common research questions as to the origin and spread of the viruses, the contribution of viral and host factors to the clinical presentations and outcome, and the possibilities for treatment and vaccination.