ALPHAVIRUS INFECTIONS
Herbert Weissenb ock
Pathology and Forensic Veterinary Medicine, Department of Pathobiology, University of Veterinary Medicine, Vienna, Austria
Historically, the arthropod-transmitted alphaviruses were classified on epidemiological and antigenic criteria as Group A arboviruses, the A ofwhich subsequently inspired the creation of the genus Alphavirus.
The word ‘arbovirus’ is an acronym (ARthropod-BOrne viruses) and for practical reasons is still used as a lumping term for viruses from different families using arthropod vectors. The alphaviruses are positive-stranded RNA viruses and form a genus of approximately 30 virus species within the family Toga- viridae. There is only one other genus, Rubivirus, with its single member, Rubella virus, within this family. Alphaviruses cycle between vertebrate reservoir hosts and insect vectors, the majority of which are mosquitoes. Most alphaviruses are geographically restricted in their distribution, and only very few of them have been found in Europe. A number of them are human pathogens, causing rashes, arthritis or encephalitis. Some are also significant veterinary pathogens with implications for wildlife, but none of these viruses is currently recognized in Europe. However, alphaviruses, like other arboviruses may be able to quickly establish efficient transmission cycles in new habitats, sometimes thousands of kilometres from their original ones (e.g. CHIKV, SINV).SINDBIS virus
Sindbis virus (SINV) has a wide geographic distribution, ranging from Europe, through large parts of Africa and Asia to Australia. SINV regularly causes human disease in northern Europe. Disease is characterized by fever and arthralgia and is known under the names of Ockelbo disease, Pogosta disease, or Karelian fever when found in Sweden, Finland and Russia, respectively1-40). Curiously, larger outbreaks tend to occur at 7-year intervals.
In other regions of the world the infection of humans is generally subclinical. The maintenance cycle of the virus is primarily between C ulex and C uliseta mosquitoes and wild birds, mainly of the order Passeriformes(41). The virus seems to have established a permanent bird—mosquito transmission cycle in Northern Europe, but continuous additional introductions by migratory birds from Africa are most likely. SINV-associated diseases or mortalities in birds or other animal species have never been reported.EASTERN EQUINE ENCEPHALITIS VIRUS
Eastern equine encephalitis virus (EEEV) is present in North and South America. In North America, its natural transmission cycle involves mainly ornithophilic mosquitoes of the species Culiseta melanura and passerine and wading birds of different species. Localized epidemics of equine and human infections occur after heavy rains, when mosquito species with other feeding preferences transfer the virus to these species. EEEV is the most virulent of the encephalitic alphaviruses, with a case- fatality rate of 50—70% in humans and near 100% in horses. The disease is characterized by a diffuse meningoencephalitis with widespread neuronal damage. EEEV has been implicated in neurological disease of several birds, mostly introduced species, such as emus (D romaius novaehollan- diae), pheasants (Phasianus colchicus), rock partridges (Alectoris graeca), house sparrows (Passer domesticus), domestic pigeons (C olumba livia) and whooping cranes (Grus americana)(41). A single fatal case of EEE has been described in a free- ranging white- tailed deer (Odocoileus virginianus).
WESTERN EQUINE ENCEPHALITIS VIRUS
Western equine encephalitis virus (WEEV) occurs through most of the Americas. In western North America, it is transmitted enzootically among passerine birds by mosquito vectors, primarily Culex tarsalis(42). Epidemics have become comparatively rare, with no major outbreak recorded in the last 20 years(41).
The virus is able to cause encephalitis in horses and humans with much lower fatality rates than EEEV. A fatal WEEV-associated neurologic disease has been described in emus and turkeys (Meleagris gallopavo )(41,43).A virus closely related to WEEV is Highlands J virus (HJV) which is maintained on the east coast of the USA in a transmission cycle involving Culiseta melanura as primary vector and birds as reservoir hosts. It is not a significant pathogen of horses and humans but has been proven to cause encephalitis in turkeys, pheasants, emus, whooping cranes and partridges1-41’43).
VENEZUELAN EQUINE ENCEPHALITIS VIRUS
Venezuelan equine encephalitis virus circulates in the tropical and subtropical regions of the Americas. So-called enzootic subtypes are involved in transmission cycles between swamp-living mosquitoes and different rodent species. These cycles go typically undetected and only occasionally direct spillover to humans has been recorded(44). Major epidemics are associated with epidemic strains that use peridomestic mosquito species, such as Aedes spp. and Psorophora spp. as vectors and equines (horses, donkeys (Equus asinus) and mules (Equus mulus)) as amplification hosts. Human fatality rates are at 1— 3%(41). Epidemic strains are also highly pathogenic for equines. Birds do not play a relevant role in the transmission cycles of this virus, and there are no records of natural diseases in other than the already mentioned species.
Chikungunya virus
Chikungunya virus (CHIKV) is circulating in Africa, Southeast Asia and the islands of the Indian Ocean. Primates are considered the natural reservoir hosts, and the virus remains largely unrecorded when being maintained in sylvatic cycles. However, peridomestic mosquito species such as Aedes aegypti and Aedes albopictus represent excellent vectors, and urban cycles involving humans as the sole amplifying host are easily established.
This has led to dramatic epidemics causing hundreds of thousands of human infections in the islands of the Indian Ocean and the Indian subcontinent in recent years. The disease is characterized by fever, severe arthralgia and maculopapular rash. CHIKV was introduced to Italy by a viraemic traveller from India in 2007, where it established a short-lived autochthonous transmission cycle using local Aedes albopictus mosquitoes as vectors and resulted in several hundred human infections. No cases of animal disease have been reported.MAYARO VIRUS
Mayaro virus is distributed in northern South America and the Amazon River Basin. It is principally maintained in a sylvatic cycle with forest-dwelling mosquitoes of the genus Haemagogus as vectors and most likely non-human primates as amplification hosts. The virus causes sporadic outbreaks of a dengue-like human illness characterized by fever, headache and arthralgia. So far, human infections have always been associated with exposure to forest environments and have to be considered as spillover infections. However, as this virus shares many features with the closely related CHIKV, the possibility of future urbanization and efficient amplification in humans and peridomestic mosquitoes cannot be excluded1-41’44).
ROSS RIVER VIRUS
Ross River virus is present in large parts of Australia and is responsible for the most common arboviral disease in this country. It is also distributed in Papua New Guinea and the Solomon Islands. Several mosquito species have been identified as competent vectors, and numerous mammalian species, primarily macropods (e.g. kangaroos and wallabies) have been implicated as hosts. Outbreaks are seasonal and associated with prolonged inundation of salt marshes or localized floodings of arid regions. Human disease is characterized by fever, arthralgia and rash. There is no convincing evidence for disease in animals, although the virus has been isolated from horses with musculoskeletal disease1-41).