POXVIRIDAE

Previous authors: E MUNZ, K DUMBELL AND J A W COETZER

Current authors:
S L BABIUK - Research Scientist, PhD, National Centre for Foreign Animal Disease,  Canadian Food Inspection Agency, 1015 Arlington Street, Winnipeg, Manitoba, R3E 3MA, Canada
J A W COETZER - Professor Emeritus, BVSc, BVSc (Hons), M Med Vet (Path), Faculty of Veterinary Science, University of Pretoria, Private Bag X04, Onderstepoort, Gauteng, 0110, South Africa
D B WALLACE - Senior Researcher, PhD, Agricultural Research Council, Onderstepoort Veterinary Research, 100 Old Soutpan Road, Gauteng, 0110, South Africa

A General Introduction has been added to each disease chapter in an attempt to give a brief updated overview of the taxonomic, biological and other characteristics of the virus family or group of bacteria /protozoa that cause disease in livestock and, where relevant, involve wildlife. As the text of the three-volume book Infectious Diseases of Livestock is currently under revision the Editors are aware that there are inconsistencies between the updated introductions to chapters and the content of the chapters themselves. Once the chapters have been updated – a process that is currently underway – these inconsistencies will be removed.

Poxviruses are classified in the family Poxviridae, which is divided into two subfamilies: Chordopoxvirinae, which infect vertebrates; and Entomopoxvirinae, infecting insects.

The poxviruses of vertebrates are grouped into ten genera which have been accepted by the International Committee for the Taxonomy of Viruses. There is one currently unassigned virus (squirrelpox) (see Table 1).

Poxvirus infections in domestic animals are, with a few important exceptions, generally of limited economic importance. The exceptions are lumpy skin disease in cattle, and sheep-and goatpox, and orf in sheep and goats. Camelpox may also cause severe losses in regions where dromedary camels (Camelus dromedarius) are important.

In Latin documents from the first and second centuries AD, a disease of sheep, which could have been sheeppox, is mentioned. However, reports from the seventeenth and eighteenth centuries describe an infectious disease of sheep, which was clearly sheeppox. Cowpox was presumably a common disease of cattle in England at the end of the eighteenth century when Jenner exploited the observation that milkmaids were resistant to the effects of smallpox by transferring ‘matter’ from lesions of milkmaids with cowpox to other individuals in order to immunize them against smallpox. This led to smallpox vaccination and initiated the science of immunology.

The poxviruses were the first virological agents for which criteria other than clinical signs could be applied to the diagnosis of the diseases they cause. By the end of the nineteenth century ‘elementary bodies’ (= virions) were seen by microscopic examination of stained smears, and cytoplasmic inclusion bodies, characteristic of poxviruses, could be demonstrated. Definite aetiological diagnoses became possible between 1930 and 1964 with the introduction of embryonated egg and cell culture techniques for growing viruses, as well as transmission electron microscopy for their morphological demonstration.⁴

Studies with mousepox (ectromelia) led to an explanation of the pathogenesis of generalized exanthematous viral diseases. The importance of cell-mediated immunity in recovery from viral infections was also demonstrated with this model.⁴

General characteristics of poxviruses

The characteristic size and morphology of poxviruses are traits often used for routine diagnosis by transmission electron microscopy (see Figure 1 in Lumpy skin disease, and Figure 1 in Orf). They are the largest and most complex viruses, brick-shaped and typically measure about 300 × 260 nm, although considerable variation exists. Parapox virions (see Figure 1 in Orf) are ovoid and smaller (approximately 300 × 170 nm in size). Mature virions are enveloped. Negative-staining shows that, within an outer double membrane, tubular protein ‘filaments’ are irregularly located on the surface of all poxviruses (see Figure 1 in Lumpy skin disease), except parapoxviruses which have a continuous regular arrangement, giving the false impression of a double helix (see Figure 1 in Orf). Two ‘lateral bodies’ are enclosed between this sheet of filaments and a second double membrane. A biconcave core in the form of a folded coil (‘triplet’), which contains the virus genome, is situated within a protein matrix in the centre of the virion (see Figure 2 in Lumpy skin disease).^{2, 12, 13}

Chemically, poxviruses contain about four per cent lipids, three per cent carbohydrates and five per cent DNA. The genome is a double-stranded linear molecule with covalently cross-linked ends and has terminal inverted repeat sequences, an arrangement which bears a surprising similarity to the genome of African swine fever virus (an asfivirus). The molecular weight varies from about 85 × 10⁶ Daltons in parapoxviruses to 240 × 10⁶ Daltons in avipoxviruses. The G + C content is between 30 and 40 per cent, except for the parapoxviruses in which it is over 60 per cent.¹⁷ The general arrangement of genes within poxvirus genomes is similar between genera, with those genes coding for proteins involved in virion structure, replication, transcription etc. (“housekeeping” genes) clustering within the more conserved central region, while genes involved in host-range, pathogenicity, host...