Avipoxviruses: infection biology and their use as vaccine vectors

Abstract

Avipoxviruses (APVs) belong to the Chordopoxvirinae subfamily of the Poxviridae family. APVs are distributed worldwide and cause disease in domestic, pet and wild birds of many species. APVs are transmitted by aerosols and biting insects, particularly mosquitoes and arthropods and are usually named after the bird species from which they were originally isolated. The virus species Fowlpox virus (FWPV) causes disease in poultry and associated mortality is usually low, but in flocks under stress (other diseases, high production) mortality can reach up to 50%. APVs are also major players in viral vaccine vector development for diseases in human and veterinary medicine. Abortive infection in mammalian cells (no production of progeny viruses) and their ability to accommodate multiple gene inserts are some of the characteristics that make APVs promising vaccine vectors. Although abortive infection in mammalian cells conceivably represents a major vaccine bio-safety advantage, molecular mechanisms restricting APVs to certain hosts are not yet fully understood. This review summarizes the current knowledge relating to APVs, including classification, morphogenesis, host-virus interactions, diagnostics and disease, and also highlights the use of APVs as recombinant vaccine vectors.

Figure 1

Negative stain EM of a characteristic FWPV-particle propagated in baby hamster kidney (BHK) cells at 72 hour post infection.

Figure 2

A and B Transmission electron microscopy (TEM) of FWPV-infected (A) bird skin tissue showing outer layers of epidermis, with typical inclusion bodies (Bollinger bodies) in the dermis (bar = 20 μm) and (B) cells with characteristic intracytoplasmic inclusion bodies containing viral particles (bar = 2 μm).

Figure 3

A, B and C Virion morphology of avipoxviruses in chicken embryo fibroblast cells (CEF). (A) Electron microscopy showing crescent-shaped structures consisting of a membrane with spicules on the convex surface, (B) spherical non-infectious immature viruses (IV) which give rise to formation of (C) intracellular mature virions (IMV) by a series of maturation steps.

Figure 4

Avipoxvirus infection of a great tit (Parus major) (left) and a common magpie (Pica pica) (right). The protecting feather coat is destroyed. Proliferative lesions and crust are seen, with secondary bacterial infection (Photo: Gunnar Holt, National Veterinary Institute, Oslo, Norway).