28. Togaviridae Flashcards
MEMBERS OF THE FAMILY TOGAVIRIDAE (2 genera)
- Rubivirus. (rubella virus)
-
Alphavirus
- New World (encephalitis in equines and humans)
- Old World (fever, rash, and arthritis in humans.)
Which viruses have the following properties:
Two genera: 1: arthropod-borne viruses, and 2 an exclusively human pathogen.
Virions are spherical, uniform in appearance, enveloped, 70 nm in diameter, and consist of an envelope with fine glycoprotein spikes surrounding an icosahedral nucleocapsid, 40 nm in diameter
The genome is a single molecule of linear, +ssRNA, 9.7-11.8 kb in size; the 5’ end of the genomic RNA is capped, whereas the 3’ end is polyadenylated. Genomic RNA is infectious
The 5’ two-thirds of the genome encodes nonstructural proteins; the 3’ one-third encodes the structural proteins, which are transcribed from a 26S subgenomic mRNA
Virions contain two (or three) envelope glycoproteins E1, E2, and E3, which form the spikes, and a nucleocapsid protein, C
Cytoplasmic replication, and maturation occurs via budding from the plasma membrane
Togaviruses.
Genus 1 = Alphavirus
Genus 2 = Rubivirus
MEMBERS OF THE GENUS ALPHAVIRUS (Fam. TOGAVIRIDAE) (3 species)
-
Equine ALPHAVIRUSES:
- EASTERN,
- WESTERN and
- VENEZUELAN EQUINE ENCEPHALITIS
- Getah Virus
OTHER ZOONOTIC ALPHAVIRUSES (Fam. TOGAVIRIDAE) (6 species)
- CHIKUNGUNYA AND O’NYONG-NYONG VIRUSES
- ROSS RIVER VIRUS
- SINDBIS VIRUS
- MAYARO VIRUS
- MARINE MAMMAL ALPHAVIRUS
- SALMONID ALPHAVIRUS
Which Alphaviruses (Togavirus) don’t requrie mosquitoes in virus transmission
Salmonid alphaviruses (affects several species of fish)
An alphavirus that infects southern elephant seals (Mirounga leonine)
[Presence of both viruses in lice]
Domestic animals and humans are ‘dead end’ hosts to Alphaviruses, which ones are the exception?
Venezuelan Equine Encephalitis virus
Chikungunya virus
Ross River virus (perhaps)
Classification of alphaviruses (togavirus) with regard to disease in mammals
- those that cause neurologic disease (encephalitis or encephalomyelitis)
- those that cause a febrile illness with polyarthritis
- those that cause no apparent disease.
Asymptomatic animal reservoirs of Eastern Equine Encephalitis Virus
Wading birds
Passerine songbirds
Starlings
When do outbreaks of EEEV are most common?
In the late summer and early fall, associated with heavy rainfall.
Lineages and geographic distribution of Eastern Equine Encephalitis virus
Virus is enzootic in easter portions of N. America (east of Mississippi river, Quebec and Ontario), Caribbean, C. America, parts of S. America.
- Lineage I viruses: most virulent to humans and horses. U.S., Canada, Caribbean
- Lineage II strains: coasts of S. and C. America
- Lineage III: Amazon Basin
- Lineage IV: Brazil
S. American lineages occasionally disease in horses, rarely in humans.
Mosquito vectors of Eastern Equine Encephalitis virus
EEEV is maintained in N. America in resident birds by the ornithophilic mosquito: Culiseta melanura. (Spring and summer)
Aedes spp. and Coquillettidia spp. transmit the virus to humans and horses.
Others: Culex peccator, Cx. erraticus, Uranotaenia sapphirina in southeastern US feed on reptiles and amphibians.
Overwintering hosts of EEEV
mammals
marsh birds
sneaks (rarely)
Birds that show clinical disease and high mortality for EEEV
Pheasants (encephalitis; ~50-70% mortality; transmission: feather picking and canibalism), chukar partridges, turkeys.
Others: ratites (emus, ostriches), rock doves, sparrows, psittacine birds, whooping cranes, Peking ducks, penguinsnestling egrets.
Host range of mammals with clinical symptoms of EEEV
-
Horses:
- mortality: 80-90%
- 66% survivors: neurologic sequelae
- Not amplifying but indicators (first to be sick)
- Humans
- Others:
- sheep, deer, goats, cattle, camelids, dogs, pigs
Geographic distribution of Western Equine Encephalitis virus
Throughout most of the Americas from the western half of N. America to portions of S. America, including Guyana, Ecuador, Brazil, Uruguay and Argentina.
i.e. from Canada to Argentina.
Mosquito vectors of Western Equine Encephalitis virus
In N. America:
- Culex tarsalis: birds in spring / early summer + mammals in late summer (e.g. horse, humans)
- Aedes melanimon, Ae. dorsalis: lagomorphs, horses and humans.
Mechanisms of overwintering of WEEV
Not clear.
Annual reintroduction of migratory birds
Nonavian species: rodents, rabbits, bats, squirrels, ungulates, tortoises, snakes.
Subtypes and geographic distribution of Venezuelan Equine Encephalitis virus
Six subtypes (I-VI).
C. and S. America
Subtypes of VEEV responsible for outbreaks in horses or ‘epizootic subtypes’
Subtypes IAB and IC
(Substitution in the E2 glycoprotein)
Venezuela, Colombia, Peru, Ecuador.
Mosquito vectors of VEEV
- Aedes taeniorhynchus*
- Psorophora* spp.
Mechanisms of Alphaviruses of entry to the CNS
- Passive diffusion of virus through the endothelium of capillaries in the CNS
- virus replication in vascular endothelial cells and release of virions into the parenchyma of the CNS
- virus invasion of the cerebrospinal fluid, with infection of the choroid plexus and ependyma
- carriage of virus in lymphocytes and monocytes.
Pathogenesis of Equine Encephalitis viruses
- Entry via mosquito bite, replication near site and dendritic cells.
- Transport to regional lymph node and virema
- Further replication (and viral amplification) in immune cells (especially dendritic cells)
- Access to CNS: neurons (and others)
Prevention and control of Equine Encephalitis viruses
Immunity following infection is life-long.
- Horses:
- annually each spring with inactivated vaccines for E, W and V EEV. 2 doses 4-6w apart.
- mosquitoes active all year: foals vaccinated at 3, 4 and 6 months of age and annually.
- C. and S. America: live attenuated TC-83 strain for VEEV (also lab workers)
- Emus, whooping cranes and other birds:
- E and W EEV
Also, only formalin-inactivated for horses and emus in N. America: lab workers
Control of mosquito vectors and prevention of exposure. Prohibition of movement of horses in outbreaks.
