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Avian Pox Virus | 369

of the epithelium with enlargement and ballooning of infected
cells, as well as associated inflammatory changes are observed on
histopathologic examination of the lesions. Characteristic eosino-
philic A-type cytoplasmic inclusion bodies (Bollinger bodies) are
observed by light microscopy (Fig. 13.7) in infected cells. The
elementary bodies can be observed (Fig. 13.1) in smears prepared
from the lesions after staining by the modified Gimenez method
(Tripathy and Hanson, 1976).
The T10 gene that encodes a protein expressed at high levels in
epithelial cells in the trachea, oesophagus, and lung of vertebrates
is present in the FWPV genome. Since in diphtheritic infections,
the lesions involve the mucous membranes of mouth, pharynx,
larynx and trachea, the T10 homologue may be required to
extend the virus’s host range to epithelial cells of the respiratory
tract. Histopathologic changes of tracheal mucosa include initial
hypertrophy and hyperplasia of mucus-producing cells, with sub-
sequent enlargement of epithelial cells that contain eosinophilic
cytoplasmic inclusion bodies (Figs. 13.6B and 13.7). Inclusion
bodies may be present in various stages of development, depend-
ing on the time after infection, and may occupy almost the entire
cytoplasm, with resulting cell degeneration. Histopathological
examination of cutaneous or diphtheritic lesions that shows
characteristic eosinophilic cytoplasmic inclusions in haema-
toxylin–eosin stained sections is a common method in most
diagnostic laboratories for confirmatory diagnosis of poxvirus
infection. The diphtheritic form of the disease, with respiratory
signs is clinically similar to ILTV (a herpes virus) and has to be
differentiated from the latter.
Viral antigens can be demonstrated in lesions by cytoplasmic
immunofluorescence or immunoperoxidase techniques. A ten-
tative diagnosis based on the lesions and signs of the disease, is
confirmed by microscopic changes and isolation and identifica-
tion of the causative agent. Figure 13.8 Chorioallantoic membrane (CAM) of developing
chicken embryo showing pocks produced by fowlpox virus. Below
Inoculation of specific pathogen-free (SPF) – A lesion on the CAM.
chickens
Susceptible chickens from a SPF flock inoculated by dermal scari-
fication with suspensions of ground pox lesions (e.g. fowlpox) will
develop typical pox lesions at the site of inoculation in 7–10 days.

Virus isolation: chicken embryos and cell culture
Chicken embryos from a SPF flock are commonly used for
isolation of the virus. When the virus containing suspension
is inoculated onto the chorioallantoic membrane (CAM) of
9- to 12-day-old embryonating chicken eggs, typical ‘pocks’
are produced on the CAMs (Fig. 13.8) following 5 to 6 days of
incubation. CAMs of chicken embryos are the most convenient
and susceptible hosts for initial isolation of the virus as well as for Figure 13.9 Fowlpox virus infected cell culture. Note the large
virus maintenance (Tripathy and Reed, 2016). Histopathological cytoplasmic inclusion bodies in the virus infected cells.
examination of such CAM lesions shows cytoplasmic inclusion
bodies.
FWPV can be propagated in a variety of avian cell lines such as, Restriction fragment length polymorphism (RFLP)
LMH, IQ-1A, QT-35, and chicken embryo fibroblasts or kidney During genetic characterization of strains of avian pox viruses,
cells (Fig. 13.9). The virus produces plaques in avian cell cultures RFLP has been used for comparing the genomes by examination
at 3 to 4 days following inoculation. However, for this purpose of the relative mobilities of restriction endonuclease-generated
adaptation of virus to cell culture may be necessary. fragments of their DNAs. Although the genetic profiles of FWPV