Final Exam - Togaviridae Flashcards Preview

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Flashcards in Final Exam - Togaviridae Deck (22):

Family: Togaviridae

1. Genus: Alphavirus (arthropod-borne viruses)
2. Genus: Rubivirus (species: rubella virus - rubella (German measles), common in children


Equine Viral Encephalitis Caused by Alphaviruses

Western Equine Encephalitis Virus (WEEV). Eastern Equine Encephalitis Virus (EEEV). Venezuelan Equine Encephalitis Virus (VEEV).



Zoonotic disease transmitted by mosquitoes. Caused by a member of genus alphavirus. Encephalitis in horses and humans.


EEEV Transmission

Maintained between passerine birds as reservoirs/amplification hosts and Culiseta melanura as the main enzootic vector in swamp habitats. Humans and equines are dead end hosts since they do not develop sufficient viremia to transmit the virus.


EEEV Pathogensis

Inoculation of virus from mosquito bites. Virus replication in mycocytes, fibroblasts, reticuloendothelial cells, lymphoid cells and osteoblasts. Replication in organs. Viremia. Virus enters CNS through hematogenous route. Viral replication is believed to occur in neurons, vascular endothelial cells, and glial cells. Apoptosis of neuronal and glial cells. Lesions particularly in gray matter of cerebral cortex, thalamus, hypothalamus. Non-suppurative encephalomyelitis.


EEEV in Horses

Inapparent or mild infection. Fever, anorexia, depression, hypersensitive to sound, periods of colic before onset of neurological disease. Walk blindly into objects or walk in circles. Involuntary muscle movements (tremor of facial and shoulder muscles). Stage of paralysis follows. Inability to hold up the head. Head pressing or leaning back. Incoordination, unnatural postures, complete paralysis.


EEEV in Humans Systemic Form

Fever, malaise, Artthralgia, Myalgia, No CNS involvement


EEEV in Humans Encephalitic form

Fever, Headache, Irritability, Anorexia, Vomiting, Diarrhea, Cyanosis, Convulsions, Coma


EEEV in Humans Prognosis

Approximately a third of all people with EEE die from the disease. Recovered patients may suffer from permanent brain damage.


EEEV in Birds

Asymptomatic in most species, reservoir hosts. Depression, tremors, leg paraylysis, involuntary circular movements may be observed. Ostrich and Emus: Hemorrhagic enterocolitis, necrosis of spleen and liver



Has a low fatality rate in humans compared to EEEV. For horses, WEEV appears to be less virulent than EEEV.


WEEV Transmission - Enzootic Cycle

Cycle between passerine birds as reservoirs and its specific mosquito vector, C. tarsalis.


WEEV Transmission - Epizootic

Domestic and wild birds are considered important reservoir and epizootic amplifying hosts. It has also been suggested that lagomorps and rodents can serve as amplification hosts.


WEEV Clinical Signs - Horses

Most cases are mild to asymptomatic. Severe cases show clinical signs similar to EEEV, such as depression, fever, blindness, inability to swallow, paralysis, convulsions, death in extreme cases. The case fatality rate in horses may be up to 30%.


WEEV Clinical Signs - Humans

Incubation period is 5 to 10 days. Usually mild or asymptomatic infection in adults. Infants and children are highly susceptible to WEEV infection and are most likely to develop severe encephalitis. 5 to 30% of young patients are often left with permanent neurological damage. The case fatality rate in humans is about 3% to 4%.



Mosquito borne, viral infection that can cause severe encephalitis in horses and humans.


VEEV Enzoootic Cycle

Enzootic cycle the primary vector is the Culex mosquitoes virus i maintained in nature by rodents and the mosquitoes. Other mosquitoes can also contribute. If horse or human is contaminated by a mosquito acquired from a rat or they get it infected. Humans and horse are accidental host.


VEEV Epizootic Cycle

Horses act as the primary host. In nature the virus is maintained between mosquitoes and horses act as an amplifying host.


VEEV - Clinical Signs in Horses

May be inapparent infection, or mild form with only fever. Anorexia, fever, depression, non-suppurative encephalomyelitis, head hung low, involuntary muscle movements, flaccidity of lips, drooped ears, incoordination, blindness, paralysis, death. In utero infection can occur.


Diagnosis of Equine Encephalitis

Clinical signs. Necropsy and histopathology. Virus isolation. Immunohisto-chemistry. IgM Capture ELISA. ELISA-IgG (paired sera). RT-PCR, Hemagglutination inhibition. Complement fixation.


Vaccination in Horses

Formalin inactivated EEEV and WEEV vaccines are available. Available as univalent or bivalent preparation in combination with other antigens (tetanus toxoid). Initially, unvaccinated horses should receive one dose followed by a booster in three weeks. They should be revaccinated yearly (or twice yearly) and also in outbreak situations when exposure is likely. Colostral antibody in vaccinated dams offer protection to foals for up to 6-7 months. For VEEV, a tissue culture attenuated vaccine, made with a strain TC-83, is available.


Vaccination in Humans

There is no licensed vaccine against EEEV and WEEV for humans. Currently there is no licensed human VEEV vaccine but TC-83 live attenuated vaccine is used for laboratory workers and military personnel.