The genus is distinguished on the basis of genetic characters. Avian encephalomyelitis virus (AEV) is most similar to hepatitis A virus (HAV, genus Hepatovirus), but differs by possessing i) a type IV IRES (HAV is type III), ii) 2A with an H-box/NC motif, and iii) 2B and 3A polypeptides with little sequence identity to the HAV counterparts.
No surface morphology is visible by EM.
Physicochemical and physical properties
AEV is stable at pH 3.0 and has a buoyant density of 1.31 to 1.33 g cm-3.
Genome (Marvil et al., 1999): c. 7,032 nt (5′-UTR: 494 nt; ORF: 6,405; 3′-UTR: 135 nt). The 5′-UTR contains a 5′-terminal hairpin, two putative pseudoknots, and a short (ca. 40 nt) pyrimidine-rich (i.e. not pure poly-C) tract upstream of the type IV IRES. AEV RNA contains the shortest of all picornavirus 5′-UTRs. The location of the cre is thought to lie within the 3D region.
Similar to hepatoviruses, protein 1A of AEV is predicted to be extremely small, does not appear to be myristoylated at its N-terminus, and therefore may not be a component of the mature virus particle.
Genome organization and replication
The polyprotein is of 2,134 aa contains only a single proteinase (3Cpro). There is no L protein. The 2A protein has an H-box/NC motif. The primary cleavage of the polyprotein is predicted to occur at the 2A/2B junction, and is probably catalyzed by 3Cpro. Like hepatoviruses, the 1D/2A cleavage may be directed by an unknown cellular protease, or the VP1 protein may be subject to C-terminal trimming as in cardioviruses. The IRES is type IV.
AEV causes encephalomyelitis in young chickens, pheasants, quail and turkeys. It can be transmitted both vertically and by the faecal-oral route; field strains are enterotropic. A live, highly enterotropic AEV vaccine is widely used to control the disease. Three genetic types are distinguished by means of phylogenetic analysis (Tremovirus A: 1 type, Tremovirus B: 2 types).
Derivation of names
Tremovirus: from an alternative name for avian encephalomyelitis, epidemic tremor.
Species demarcation criteria
Members of a species of the genus Tremovirus:
- are less than 50% divergent in polyprotein aa sequence
- are less than 35% divergent in P1 aa sequence
- are less than 50% divergent in 2C+3CD aa sequence
- share a common genome organization