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Figure 6.4 Phylogenetic tree showing the relationships within avian reoviruses. Nucleotide sequences of σNS (S4) gene (i.e. coding region)
of selected members in the different Orthoreovirus species in avian species were used to generate a phylogenetic tree. This tree clearly
shows two main clusters consisting of the species Avian orthoreovirus (ARV) isolated from domestic or wild poultry (chickens, turkeys,
ducks, geese, pigeon, quail), and the species Wild bird orthoreovirus isolated from wild birds (Pycno-1 passeriformes and the corvid
orthoreovirus Tvärminne avian virus). ARV further splits into three genotype clusters: the chicken/turkey isolates genotype I, the duck/goose
isolates genotype II, and the chicken isolates from Hungary genotype III. The chicken/turkey genotype I is more heterogeneous with chicken
isolates from China, USA, and Canada forming subgroup Ia and the turkey isolates from USA and Hungary and other chicken isolates from
USA and Canada in subgroup Ib. The duck/goose genotype II has two main subgroups: subgroup IIa consisting of the classical Muscovy
reovirus (classical MDRV) strain ZJ2000M and subgroup IIb consisting of other Muscovy, Mallard and Pekin duck and goose isolates.

young chicken (Deshmukh and Pomeroy, 1969), weight loss and To cause disease, a virus must enter a host by specifically
stunted growth (Murphy et al., 1999), suboptimal feed conversion adsorbing to target cells, replicate inside the affected host cells and
and immunosuppression, atrophy of bursa of Fabricius (Mont- result in viraemia, disseminating virus within the characteristic
gomery et al., 1986), thymic atrophy (Hollmén and Docherty, pathways of the host and causing damage to host tissues (Joklik
2007) and sudden death (Huhtamo et al., 2007) in chickens. 1983). Initially, avian reoviruses generally infect the gastrointes-
Pathogenic strains of avian reoviruses, although associated with tinal and respiratory tracts, and primary replication occurs in the
low mortality, often produce high morbidity rates resulting in mucosa of these organ systems (Menendez et al., 1975; Ellis et
significant economic losses (Olson and Solomon, 1968; Glass et al., 1983; Jones et al., 1989; Ni and Kemp, 1995). In case of the
al., 1973). In addition to the association with disease conditions, gastrointestinal tract, reoviruses encounter proteolytic enzymes
avian reoviruses are ubiquitous in poultry worldwide. Most avian and bile salts. If the virus survives within the environment of the
reoviruses cause asymptomatic infections in poultry (Benavente intestinal lumen, within a few hours of infection it either enters
and Martínez-Costas, 2007) and have been isolated from clini- mucosal epithelial cells, causing local inflammation, or the sys-
cally normal chickens (Robertson et al., 1984) as well as from temic circulation, where the virus is rapidly spread via blood to
cell cultures prepared from apparently healthy chicken kidneys other tissues or organs (Kibenge et al., 1985; Jones et al., 1989),
(Mustaffa-Babjee and Spradbrow, 1971). causing tissue damage at distant sites, particularly the spleen as