The Avipoxviruses represent some of the earliest viruses studied experimentally, due to their obvious lesions in backyard poultry, as well as their ability to propagate in embryonated eggs, suspensions of embryo-derived cells and chick embryo fibroblast monolayer culture1-4. The virions can just be seen by light microscopy and formed obvious inclusions upon staining (B-type, factories or virosomes, and A-type, storage inclusions). Numerous more-or-less attenuated, live vaccine strains were developed during the 1920s, some of which probably formed the basis for subsequent and modern commercial vaccines.
Avian poxviruses have been isolated from more than 280 species of birds5. Currently in ICTV they are all represented by a single genus, the Avipoxviruses. Several years ago, we surveyed the genetic landscape of a selection of these viruses archived by Dick Gough at DEFRA’s labs in Weybridge6, sketching a picture whereby the genus contained 3 deep clades representing fowlpox-like viruses, mainly of galliforms (A), canarypox-like viruses of passerines (B) and viruses of psittacines (C). Miklos Gyuranecz fleshed out that picture7 with a much more comprehensive list of birds from sources worldwide, confirming the overall structure and adding seabirds and predators to the list of hosts in clade A. The depth of the clades is remarkable, the genetic distances observed are equivalent to those seen between different genera of mammalian poxviruses. Those distances partly explain difficulties in obtaining more refined trees – sequence diversity makes it difficult to find PCR probes conserved across the genus – and consequently to elucidate accurate details of host/virus relationships. The existing trees are therefore restricted to just a few genes (homologues of mammalian vaccinia virus p4b, E9 polymerase and H3). The situation is unlikely to change until we have several more genome sequences, representing the major sub-clades. Currently we only have genome sequences from type species fowlpox virus (pathogenic8 and attenuated9 strains), canarypox virus (pathogenic10) as well as a penguinpox virus and a pigeonpox virus11 [see more recently published avipoxvirus genome sequences]. Although avipoxviruses share with mammalian poxviruses the distribution of genes along the genome (conserved and structural centrally, species-specific distally), their genomes show considerable reorganisation compared to the mammalian poxviruses12. The most closely related mammalian poxviruses to the avipoxviruses are the molluscipoxviruses and the parapoxviruses. The avipoxviruses appear to share many similarities and features with the molluscipoxviruses, from similarities in pathogenesis (the ability to evade innate and acquired immunity for protracted periods) to shared molecular features (the lack if interferon modulators E3 and K3; shared synteny for homologues of the largest poxvirus gene, VARV B22R (unpublished); the presence of an FMDV-like integrin binding motif in the p4c homologues13, to name just a few).
In birds, avipoxviruses typically cause skin lesions, particularly of the unfeathered areas around the eyes, nares and beak and on the legs14,15. The distribution is probably linked to the mechanical transmission of the viruses by biting insects. Such lesions are rarely fatal but can presumably reduce performance in feeding (hunting/foraging) and predator evasion. The pathology of the lesions is described as dermal cell hyperplasia. They can become extremely extensive and persistent, relatively unusual for acute poxviruses. Inhalation/ingestion of droplets/dust can lead to infection of the oropharyngeal cavity, so called “diphtheritic infections”, which are more serious, causing up to 15% mortality in chicken flocks. Canarypox can cause extremely high mortality, with indications of pneumonia.
The epidemiology of the avipoxviruses remains relatively obscure, mainly because of the paucity of genome sequence data and our consequent inability to accurately type isolates into species. Some viruses appear fairly host restricted, e.g. fowlpox virus in chickens and possibly turkeys. Others, particulary the clade B canarypox-like viruses, seem able to infect a wide range of species. The picture is complicated because many infections are observed in zoos, aviaries and wildlife parks, veterinary clinics or quarantine facilities, where atypical species-species transmissions can more readily occur. Others probably represent prey to predator transmissions. As with many zoonotic infections, it is likely that avipoxviruses cause mild or inapparent infections in their native host, which present as more severe in atypical hosts. It is almost certain that canarypox virus is relatively benign in its as-yet-undefined natural host (possibly native songbirds of temperate climes), in contrast to the severe infection it causes in non-native canaries.
Host Range & Recombinant Vaccine Vectors
One feature of the avipoxviruses that has helped maintain research effort in their molecular biology is that, despite their restriction (in terms of disease) to avian species, they are able to enter mammalian cells and express at least some of their genes16. Avipoxviruses were developed as recombinant vaccine vectors, initially for poultry and most notably against avian influenza H5N2 then H5N117-19. When, however, recombinants were produced that carried antigenically protective genes from mammalian pathogens, they induced immunity in vaccinated mammals which, with the most immunogenic antigens (such as rabies and measles glycoproteins), was protective20,21. This led to the commercial development and exploitation of a number of recombinant avipoxviruses (based on the commercial canarypox recombinant vaccine vector, ALVAC) for vaccination of mammalian livestock and companion animals22-24. Frequently though, the level of expression from what have become known as “live, non-replicating recombinant vaccine vectors” is insufficient to induce immunity by itself, so “heterologous prime boost” approaches were developed to enhance the levels of immunity induced25.
Genes & Proteins
The avipoxviruses carry the largest of the poxviral genomes, up to and larger than 300 kbp, encoding up to 300 genes and more. Many of the genes still have no known homologues in other viruses or in their hosts and their functions remain unknown. Several gene families are observed in the avipoxviruses, notably expansion of the giant (6kbp) VARV B22 gene complement to 6 members in fowlpox and canarypox viruses (representing more than 10% of the coding capacity) and of the ANK-PRANC family to 31 members (more than 10% of total genes) in fowlpox and 50 members (more than 15% of total genes) in canarypox viruses. The functions of members of these gene families remain generally unknown (though presence of F-boxes in the ANK-PRANCs indicates interaction with the ubiquitin system) but we recently demonstrated that two ANK-PRANCs from fowlpox virus target different aspects of the chicken’s antiviral innate interferon response26,27. It is likely that this fairly unprecedented gene expansion allows them to target multiple aspects of the host’s responses, possibly in a redundant manner. It also probably explains the absence from avipoxviruses of a gene family seen in orthopoxviruses, namely those with a Bcl-2-like fold.
The poxvirus morphogenetic pathway has been elucidated in some detail using the archaetypal poxvirus, vaccinia. However, we now know that avipoxviruses lack several of the key genes that encode proteins important in particular aspects of vaccinia virus’s morphogenesis (specifically the wrapping pathway that generates intracellular enveloped viruses, IEV, and the production of actin ‘tails’ or ‘rockets’). Indeed, the protein complement of the external envelope of the wrapped (extracellular enveloped virus, EEV) avipoxvirus remains undefined. Nevertheless, the lack of these proteins probably either explains, or is a consequence of, the observed, conventional ‘budding’ of avipoxvirus intracellular mature virions (IMV) at the plasma membrane to form EEV28,29.
Few specific serological reagents exist against the avipoxviruses. However, we isolated, characterised and reported mouse monoclonal antibodies against three fowlpox virus antigens13. These antigens were: (i) the FPV140 protein, a homologue of vaccinia virus IMV surface protein H3; (ii) the FPV168 protein, a homologue of the vaccinia virus 39K core protein and (iii) FPV191, homologue of the vaccinia virus p4c IMV surface-associated protein.
Integrated REV provirus
One intriguing aspect of the avipoxviruses is the observation that pathogenic, field strains of fowlpox virus frequently carry an integrated, active copy of the reticuloendotheliosis virus (REV) provirus. The initial observation related to a commercial vaccine strain (FPV-S) that proved to be contaminated with REV, was withdrawn and could not be plaque-purified free of the contaminant. PCR analysis later showed the fowlpox virus carried an infectious proviral copy30. We now know that all REV-positive fowlpox carry the provirus at the same locus (passaged lab & commercial vaccine strains often appear to have lost most of the provirus, sometimes leaving just LTR sequences), indicative of a single ancestral insertion event31. We also now know that REV is most closely related to mammalian retroviruses from monotremes32. There has been speculation that the transfer to fowlpox virus occurred iatrogenically in a lab in New York in the 1940s and furthermore that the “REV-infected” fowlpox virus was then used as a vaccine resulting in worldwide spread of REV and REV-like viruses in poultry and wild birds32. Unfortunately, there now appears no way to distinguish this hypothesis from the alternative possibility that REV insertion into fowlpox virus was a natural event, pre-dating its human propagation. The latter possibility might explain how REV-infected fowlpox virus became so prevalent in the field33, particularly as a wide range of ‘uninfected’ fowlpox vaccines have actually been used during the recent explosive growth of the global poultry industry. It is interesting to speculate that, should the insertion be older than was suggested, the reverse transcriptase it encodes might well have contributed to fowlpox virus evolution, facilitating the capture of cDNA copied from spliced cellular mRNAs (the most likely route for poxviral acquisition of host genes).
Contributed by: Michael A. (Mike) Skinner
(Imperial College London)
Copyright © 2016 Michael A. Skinner
First posted: 1-Sept-2016
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