Togaviridae

Front 1

Describe the family Togaviridae

Back 1

-has 2 genera: Alphavirus & Rubivirus
-the only member of the Rubivirus genus is rubella virus, which causes German measles in children and young adults
-the pathogenic alphaviruses are divided into those viruses that cause human diseases characterized by rash and arthritis, primarily found in the Old World, and viruses that cause encephalitis, primarily found in the New World
-alphaviruses cycle primarily between mosquitos and small mammals or birds
-larger mammals such as huans and horses, that tend to develop severe or fatal disease are often dead end hosts unimportant to the endemic virus transmission cycles, but can be important for sustaining epidemics

Front 2

Describe the Genus Alphavirus

Back 2

-eastern equine encephalitis, western equine encephalitis, and venezuelan equine encephalitis are the important viruses in this genus
-arboviruses
-labile in the environment, due to fragile envelope (sensitive to detergents, pH changes, and disinfectants
-replicate in the host cell cytoplasm, with virion assembly in the cytosol
-infection of vertebrate hosts results in lysis
-infection of invertebrate host cell is non-cytolytic and results in persistent infection
-sylvatic cycle:
→transmission between wild vertebrate host and wild birds (no disease)
→humans and domestic species: dead end hosts, with no further spread to other animals (except VEE)

Front 3

Describe the epidemiology of equine encephlitides

Back 3

✰EPIDEMIOLOGY
-the peak periods of these diseases coincide with times of maximum vector numbers, usually in late summer following heavy rainfall
-the regional distribution of the viruses is related to that of mosquito vectors
-numbers of clinical cases drop dramatically when vector numbers decreased due to cold weather or drought

-Eastern equine encephalitis virus (EEEV) occurs principally in Atlantic coastal areas of North America
-EEEV has also been isolated in Michigan, the Caribbean islands and South America
-the South American EEEV infections are associated with equine disease but rarely human disease & are less virulent than the North American ones
-thought to be a different linage
-the virus is maintained in cycles of infection involving passerine birds and the irrigation ditch mosquito (Culiseta m.) which inhabits freshwater swamps and is ornithophilic
-following infection, a high titre of virus develops in many wild birds without evidence of disease
-periodic epidemic outbreaks of infection in wild birds which may lead to infection of humans and horses, involve additional mosquito species Aedes & Coquillettidia which feed on both bird and mammals
-infections usually result in sporadic disease in humans, horses, & pheasants
-epizootics, which tent to occur in autumn, disappear with the arrival of the first frosts
-overwintering mechanisms for virus maintenance are unclear, although wild birds are thought to be possible reservoirs

-isolates of VEEV compromise a complex of 6 subtypes
-epizootic forms of Venezuelan equine encephalitis are caused by two highly virulent subtype I serotypes of the virus
-the other serotypes comprose enzootic viruses and are considered to be non-pathogenic for Equidae
-the level of viremia produced by these enzootic strains in horses is generally low
-these viruses are maintained in sylvatic cycles involving rodents & mosquitoes (Culex)
-it is thought that the viruses involved in epizootics are derived from mutations of viruses involved in enzootic cycles giving rise to increased infectivity for other mosquito vector species and/or an increased level of viraemia in horses
-Equidae serve as amplifying hosts for these epizootic strains
-horses inoculated with a virulent subtype of VEEV are important amplifiers of virus activity, developing a viraemia of sufficiently high titre to allow transmission by feeding mosquitoes

-although western equine encephalitis (WEE) has traditionally occured in the USA west of the Mississippi, it is also present in many other part of the North American continent
-infections tend to recur in certain areas
-the infection involves mosquitoes, usually Culex, and indigenous wild birds in which infection is inapparent
-horses are infected incidentally and are 'dead-end' hosts because levels of virus in the blood remain low
-epizootics are rare and epizootic strains probably arise from nonpathogenic strains
-overwintering mechanisms of the virus are unclear but may involve birds, reptiles or mosquitoes
-it is a rare cause of encephalitis in horses but an important pathogen of a number of bird species such as emus and turkeys

Front 4

Describe the pathogensis and clinical signs of equine encephlitides

Back 4

✰PATHOGENESIS
-following inoculation by a feeding mosquito, viral replication occurs near the site of entry and in the regional lymph nodes
-viremia, ranging from barely detectable to high levels, is accompanied by fever
-when disease is severe, the virus invades the central nervous system resulting in neuronal necrosis and perivascular lymphoid cuffing

✰CLINICAL SIGNS
-diseases caused by the 3 equine encephalitis viruses are clinically similar