Last Update: March 2000
Author: F. A. Leighton
Reviewer: H. Artsob
There are some 500 different viruses worldwide that are transmitted among animals by biting insects or ticks. These viruses collectively are called "arboviruses"; "arbo" from "arthropod-borne". "Arbovirus" is not a taxonomic classification; these arthropod-borne viruses belong to several different families in virus taxonomy. Their similarities are in life-style and ecology, and, for several North American arboviruses, in the diseases they can cause in people.
In Canada, 19 arboviruses have been found in either arthropod or vertebrate hosts. Six of these are transmitted by ticks, 11 by mosquitoes and 2 by midges (Culicoides sp.). Seven arboviruses that can cause disease in people have been recognized to occur in Canada, and information about these 7 viruses is provided below. Two other arboviruses cause disease in ungulates. The other 10 arboviruses are of unknown disease-causing potential in people or animals.
Seven arboviruses that can cause human disease are known to occur in Canada. Another arbovirus new to North America, West Nile Virus (WN), caused epidemic disease in wild and captive birds, humans, and horses in the New York City area in August, September and October of 1999. As of this writing, it is not known to be present in Canada. Its distribution, host range and importance to animal and human health will be determined through surveillance and research progams that are already underway in both Canada and the United States. Two additional arboviruses found in Canada, California encephalitis (CE) and Cache Valley (CV) have been implicated as human pathogens but their role is less certain. These viruses are not considered further here.
[West Nile Virus (WN) shown for comparison]
|SLE||Flaviviridae||SK, ON,QC||Wild birds||Mosquitoes|
|WN||Flaviviridae||None Known1||Wild birds||Mosquitoes|
|SSH||Bunyaviridae||ALL provinces, YT,NWT||Wild mammals||Mosquitoes|
|CTF||Reoviridae||BC, AB||Wild mammals||Ticks|
1 As of December 1999
All seven of these viruses, and also West Nile Virus, have some common features. Each is maintained in a cycle of transmission between its mosquito or tick host and a range of wild birds and/or mammals that are its normal vertebrate hosts and reservoirs. The normal hosts appear to suffer little or no clinical disease when they become infected. However, periodically these viruses are transmitted by their arthropod hosts to animals that are not their normal vertebrate hosts. These unusual vertebrate hosts, which can include humans, sometimes do become clinically ill from the infection. These viruses are not contagious from person to person, nor do the abnormal animal hosts carry enough virus in their blood to infect other mosquitoes or ticks. Thus, people and animals that become diseased usually are "dead-end" hosts; they may suffer disease but the virus does not spread from them to other vertebrates or to other mosquitoes or ticks..
In general, the seven arboviruses considered here do not cause disease in their normal avian or mammalian hosts, nor in their host mosquitoes or ticks. However, abnormal wild animal hosts may suffer disease in the same way as do people and some domestic animals. For example EEE has caused fatal disease in captive Whooping Cranes, birds whose normal habitat is generally outside of the normal habitat of the EEE virus. Nestling House Sparrows may suffer illness from WEE virus, for which this European bird is now an important avian host in North America, and WEE virus has been found in the brains of squirrels submitted as rabies suspects and in an oppossum with encephalitis. EEE and WEE have caused disease in non-native bird species raised and released for hunting.
In general, most infections of people with the above-listed seven arboviruses known to occur in Canada result in no illness at all. However, some infections do result in clinical disease and, sometimes, in very serious disease. Illness caused by arboviruses can take the form of generalized, flu-like disease or encephalitis (inflammation of the brain). Encephalitis is a grave disease. It can be fatal, and many who survive arbovirus encephalitis will suffer permanent brain damage and debility. Thus, although these diseases are not common, they can be severe and, because they are caused by viruses, there are no drugs to treat them. There are no commercially-available vaccines to protect people against any of these arbovirus diseases, although vaccines are available to protect horses against WEE and EEE. Mosquito-vectored arbovirus infections in Canada generally are most common from mid-summer to fall, when the largest number of infected mosquitoes are present; tick-borne arbovirus infections are most common in spring and summer, coincident with tick activity.
Artsob, H. 1990. Arbovirus activity in Canada. Archives of Virology . Supplement 1: 249-258.
Beran, G.W.(Editor-in-chief). 1994. Handbook of Zoonoses. 2nd Edition. Section B Viral. CRC Press Inc. Boca Raton. 582 pp.
Calisher, C.H. 1994. Medically important arboviruses of the United States and Canada. Clinical Microbiology Reviews 7: 89-116.
Monath, T.P. (Editor). 1988. The Arboviruses: Epidemiology and Ecology. Volumes I-V. CRC Press Inc., Boca Raton.