fowlpox virus

International Common Names

• English: avipoxvirus; canarypox virus; fowl pox virus; juncopox virus; pigeonpox virus; psittacinepox virus; quailpox virus; sparrowpox virus; starlingpox virus; turkeypox virus

English acronym

• FWPV

• Domain: Virus
•     Group: "ssDNA viruses"
•         Group: "DNA viruses"
•             Family: Poxviridae
•                 Subfamily: Chordopoxvirinae
•                     Genus: Avipoxvirus
•                         Species: fowlpox virus

All the avipoxviruses show identical morphology, with the mature virus being brick-shaped and measuring approximately 250x350 nm. It consists of a centrally located biconcave core and two lateral bodies in each concavity (see pictures). The outer coat is composed of randomly distributed surface tubules (Tripathy and Reed, 1997).

Pox viruses are synthesized and packed in the cell cytoplasm and the process of replication is similar in the dermal or follicular epithelium of chickens, ectodermal cells of the chorioallantoic membrane (CAM), and embryo skin cells. Replication of fowlpox virus in dermal epithelium involves two distinct phases, which are a host response characterized by marked cellular hyperplasia during the first 72 hours and synthesis of the infectious virus from 72 to 96 hours (Cheevers et al., 1968). The morphogenesis of fowlpox virus has recently been revised (Boulanger et al., 2000). After adsorption and penetration of the cell membrane, one or two hours after infection of dermal epithelium or CAM respectively, viral particles uncoat. At 48 h post infection, viral particles with incomplete membranes are observed in the cytoplasm. The release of fowlpox virus as an extracellular virus appears to be proceeded by budding or by fusion of intracellular enveloped virus with the plasma membrane. Actin filaments are involved in the release of the viral particles from the plasma membrane.

The genome of the fowlpox virus, the type species of the avipoxviruses, is composed of a single double-stranded DNA molecule of approximately 254 to 300 Kb. The molecular weight of this genome is about 160-185 x 10⁶ Dalton and the G+C content of fowlpox virus DNA is about 35% (Tripathy and Reed, 1997). Fifty-seven major structural polypeptides have been identified in purified fowlpox virus preparations (Prideaux and Boyle, 1987). Many studies on fowlpox virus nucleotide and amino acid sequences have been conducted in recent years (Boyle and Coupar, 1986; Binns et al., 1987; Laidlaw et al., 1998; Pollitt et al., 1998; Ma et al., 1999). This is due to the increasing importance of this virus as a viral vector for recombinant vaccines.

Although resistance to ether is one of the characteristics of pox viruses, sensitivity to both ether and chloroform has been reported for some fowlpox virus strains (Tantwai et al., 1979; Pradhan et al., 1996). Fowlpox virus is known to withstand 1% phenol and 1:1000 formalin for 9 days, but it is inactivated by 1% potassium hydroxide when freed from its matrix. Heating at 50ºC for 30 minutes or 60ºC for 8 minutes also inactivates the virus. An important point in understanding its high level of transmission is that this virus, when desiccated, shows marked resistance and can survive in dried scabs for a long period of time (Tripathy and Reed, 1997).

Avian pox viruses may propagate in different laboratory host systems including birds, avian embryos and cell cultures. In avian embryos, infection by avipoxviruses results in compact, proliferative, white pock lesions that may be focal or diffuse in the CAM (see pictures). Histological examination of these pock lesions reveals typical intracytoplasmic inclusions characteristic of all pox viruses (see pictures). Avian pox viruses can also be propagated in cell cultures of avian origin, such as chicken embryo fibroblasts, chicken embryo dermis, chicken embryo kidney cells and duck embryo fibroblasts. Viral infection in chicken fibroblasts causes cell rounding followed by cell degeneration and necrosis.

Avian pox viruses are antigenically and immunologically distinguishable from each other, but varying degrees of antigenic relationship do exist. Information on the characterization of pox viruses from wild birds and their relationship to a recognized Avipoxvirus is scarce.

Recently, some pox virus strains have been isolated in the United States from vaccinated flocks experiencing high mortality due to the diphtheric or cutaneous forms of fowlpox. Currently available vaccines are not effective in controlling the disease caused by these ‘variant’ strains of pox virus (Fatunmbi and Reed, 1996a, b).

Disease(s) associated with this pathogen is/are on the list of diseases notifiable to the World Organisation for Animal Health (OIE). The distribution section contains data from OIE's Handistatus database on disease occurrence. Please see the AHPC library for further information from OIE, including the International Animal Health Code and the Manual of Standards for Diagnostic Tests and Vaccines. Also see the website: www.oie.int.