Poxviruses

VIRUSES

Introduction

Poxviruses are among the most easily recognized of all viruses, owing to the lesion by which they have gained their name. Once inside cell, they cause proliferation then lysis, giving way to a characteristic pock with a necrotic center. Poxviruses have risen to fame both for their ability to be eradicated (small pox) as well as their use in fighting other viruses (canarypox vaccines).

Morphology

• Huge (up to 450nm), usually enveloped viruses, with a complex capsid symmetry
• Up to 30 different structural proteins
• Non-structural proteins:
  • Viral epidermal growth factor, which stimulates cell growth causing the raised edge of pustule
  • Viral tumor necrosis factor, which is non-functioning and acts as an anti-inflammatory by competing with TNF-alpha
  • Viral IL-10, which reduces the Th-1 cell mediated response

Therapeutic Use

Recombinant Vaccines

• Poxviruses can be used as heat-stable vectors for vaccines against other viruses
• Grown in host cell lines or on the surface of chick chorioallantoic membranes in ovo (primordial ectoderm)
• This was first accomplished by the recombination of cowpox and variola (smallpox) in the creation of the smallpox vaccine (vaccinia)
• More recently, the French used this technique in the creation of the oral rabies vaccine used among the wild fox population:
  • Recombinant virus inserts a plasmid encoding rabies gene in place of thymidine kinase gene
• Canarypox vaccines now exist for FeLV and Rabies
  • Undergoes a single cycle of replication without producing infectious virus in mammals

Virulence and Pathogenesis

• Primary replication in abraded squamous epithelium
• Viremia followed by multiple epidermal infections
• Ballooning then necrosis (hydropic degeneration) of epidermal cells
• Concurrent proliferation or adjacent epidermis (GF driven), creating more cells for the virus to infect
• All three result in classical sequence of lesions:
  • Papule (proliferation)
  • Vesicle (fluid filled)
  • Pustule (lesion breaks)
  • Scab formation (healing begins)
• Pock center can succumb to secondary infection
• Resolution in 3-4 weeks
• Some poxviruses can spread to the upper respiratory tract or viscera, causing more serious pathology

Epidemiology

• Spread quickly in unhygienic circumstances
• Can survive for years in dust

Diagnosis

• Clinical signs
• Histology
• Electron microscopy
• PCR, IIF

Types and Subtypes

• 6 Genera, all of which produce pox lesions
• Subdivided based on external structure by EM

Virus by Species

• Cow pox, which also causes cat pox
• Small pox (variola)
• Orf
• Seal pox
• Pig pox
• Sheep/Goat pox (exotic to EU)
• Myxomatosis
• Fowl pox
• Pigeon pox
• Canary pox
• Ectromelia
• Camel pox
• Monkey pox
• Lumpy skin disease
• Red squirrel parapox
• Red/Gray squirrel pox

Parapoxviruses

Contagious ecthyma

• AKA contagious pustular dermatitis, Orf, etc

General information

• CE is an acute debilitating but rarely fatal skin disease of sheep, goats and incidentally of ruminants and humans.
• The disease is generally mild.
• Morbidity may reach 90%, but mortality rarely exceeds 1%, unless secondary infection or myiasis occurs.
• The course of the disease without complications is 2‑6 weeks, and it takes about 10 weeks to run a course through a naive flock, but often lingers indefinitely in the flock because it can reinfect the same animal many times and is resistant to desiccation.
• PPV's generally exhibit a highly restricted host range but in all cases can also infect man.

Aetiological agent

• The virus is a large, enveloped, highly epitheliotropic, DNA virus which is ovoid in shape and measures 220‑300 x 140‑170 nm.
• There are over 100 polypeptides in the virion.
• The core proteins include a transcriptase and several other enzymes.
• The virus is immunologically related to those causing bovine papular stomatitis, pseudocowpox, sealpox, deer PPV, red squirrel PPV and camel PPV.
• There is extensive cross‑neutralization and cross‑protection between viruses belonging to the same genus, but not between those of different genera.
• The virus is resistant to desiccation.

Pathology

• For skin pathology description see here

Differential diagnosis

• Sheep pox: A more severe disease, it is characterized by elevated papules distributed diffusely over the skin surface. Inclusion bodies are often seen, but there is no down‑growth of epidermis. There is a high mortality.
• Ulcerative dermatosis: Characterized by ulcers and crusts on the skin of the face, feet and genitalia. The lesions are not elevated because there is no epithelial hyperplasia.
• Blue‑tongue: There is a lower morbidity but a high mortality. The disease is usually seen in adult animals. This is a severe systemic disease which is a differential only for the rare, systemic form of CE.
• Mycotic dermatitis: The scabs are smaller and thinner and lighter and usually yellowish in color. The crusts are not firmly attached.
• Foot and mouth disease: When lesions occur in the mouth.

Leporipoxviruses

Rabbits

Myxoma virus

• Causes Myxomatosis in rabbits
• Infectious myxomatosis is highly contagious and transmitted by direct or indirect contact with ocular or skin discharges or by mechanical vectors.
• Arthropod vectors such as mosquitoes and fleas provide the principal means of transmission.
• Myxomatosis is enzootic in rabbits of the genus Sylvilagus in both South and North America and in wild rabbits of the genus Oryctolagus in South America, Europe, and Australia.
• Following introduction into Europe and Australia, genetic modification of the myxoma virus produced markedly attenuated strains that replaced the virulent virus as the dominant virus strain.
• The naturally attenuated viruses cause a milder disease of longer duration which favors vector transmission and persistence of the virus in nature.
• High ambient environmental temperatures are associated with increased survival of infected animals. Some authors consider the California strains of the virus (California rabbit fibroma virus) as distinct from the myxoma virus, while others feel that the antigenic differences do not justify the separate distinction.

Pathogenesis

• MYX infects several cell types including mucosal cells, lymphocytes, and fibroblasts.
• In addition to primary and secondary tumor development, there is severe immunosuppression leading to overwhelming infections by opportunistic gram-negative bacteria.
• Viral antigen is present within primary as well as secondary tumors.
• Virus multiplication and tumor-like lesions occur initially at the site of intradermal inoculation. This is followed by spread to regional lymph nodes and cell-associated viremia, with generalization to the skin and internal organs.
• With immunosuppression, severe secondary gram-negative bacterial infections of the conjunctiva and nasal passages occur.
• Death follows within 14 days of inoculation.
• Several proteins encoded by the virus are immunosubversive in that they mimic host ligands or regulators (virokines) or are homologues of cellular cytokine receptors (viroceptors)
  • Virulent strains of MYX and MRV secrete a T2 gene product with significant homology to the T2 protein secreted by Shope fibroma virus. This protein acts as a receptor for TNFα and TNFβ. TNF is active in killing virus infected and tumor cells. Cytotoxicity is correlated with the induction of toxic superoxide radicals. The T2 gene product enhances viral virulence by interfering with TNF-dependent immune response pathways of the host.
• Myxoma virus and MRV have also been shown to compromise cell-mediated immunity by inducing a loss of class I MHC antigens from the surface of infected cells. MHC class I-restricted cytotoxic T lymphocytes play a major role in clearing poxvirus infections from susceptible animal hosts, and are an important factor in limiting virus spread within the host. Class I molecules are present on all nucleated cells and platelets. Cytotoxic T cells (CD8+) recognize intracellular viral antigen in association with class I MHC glycoproteins on the cell surface. MYX and MRV infection results in a specific, rapid decrease in detectable surface class I epitopes. It has been suggested that a late viral gene product may interact with the class I complex to either physically sequester the complex away from the cell surface and inhibit its recycling to the surface, or induce a conformational change in the complex rendering it unrecognizable by cytotoxic T lymphocytes.
• Myxoma growth factor (MGF) and a gene product designated M11L are also associated with MYX virulence. MGF and M11L deleted mutants are markedly attenuated, capable of inducing only benign localized nodules that regress spontaneously, similar to lesions induced by Shope fibroma virus. MGF has significant sequence homology with epidermal growth factor and transforming growth factor β. It has been postulated that the presence of EGF/TGFβ-like growth factors in tumorigenic poxviruses affects the growth and progression of poxvirus-infected cells.
• Another myxoma virus virulence factor designated MYXOMA SERPIN 1 (SERP 1) has significant sequence homology with members of the family of serine proteases inhibitors (serpins). The exact role of SERP 1 is unknown but it may theoretically affect antigen presentation by interfering with viral protein degradation, or might reduce cytoxic T cell killing mediated by serine proteases in the effector cell.
• Rabbit poxvirus gene sequences coding for products related to the C4 and C5 components of the complement cascade have been identified. It is postulated that complement-like viral protein products may block the activation or action of complement. The virulence factor T7 is an IFNγ receptor homologue.

Clinical signs

• Clinical disease varies with virus strain and host species.
  • Sylvilagus species are relatively resistant, and are probably the natural host of the virus. In Sylvilagus species, infection usually results only in the development of skin tumors at the site of inoculation. The tumors appear 4-8 days after exposure and persist for up to 40 days. Very young rabbits may succumb to generalized disease.
  • Lepus species (hares) are highly resistant. Occasional individuals develop mild to severe generalized myxomatosis.
  • In the European rabbit (Oryctolagus cuniculus), infection with virulent virus (South American or California strains) results in severe disease with up to a 99% case fatality rate.
    • Initial signs include edema of the eyelids accompanied by inflammation and edema around the anal, genital, oral, and nasal orifices.
    • Edema of the head and ears, drooping ears, and bacterial infections resulting in mucopurulent conjunctivitis and pneumonia are seen.
    • The primary tumor may be evident by 3-4 days after infection. Generalized tumors develop by 6-7 days after infection.
    • Subcutaneous swellings become mucoid, gelatinous masses (hence the name myxomatosis).
    • More chronic wart-like nodules appear, especially at mucocutaneous borders, after 10-14 days.
    • Death (8-15 days post infection) is usually preceded by labored breathing and convulsions.

Pathology

• The most prominent gross lesions in European rabbits with myxomatosis are the skin tumors and the pronounced cutaneous and subcutaneous edema, particularly in the area of the face and around body orifices.
• Skin hemorrhages and subserosal petechiae and ecchymoses may be observed in the stomach and intestines. Subepicardial and subendocardial hemorrhages may also occur.
• Adult rabbits of the genus Sylvilagus usually develop localized skin tumors resembling the fibromas produced in European rabbits by Shope fibroma virus.
• Hares or young Sylvilagus rabbits may develop fibromatous to myxomatous nodules, however, lesions are usually mild and localized.

Histopathology

• The lesions are proliferative to degenerative, depending on the virus strain.
• The skin tumors result from proliferation of undifferentiated mesenchymal cells which become large stellate cells (myxoma cells). These cells may contain intracytoplasmic eosinophilic inclusions. These cells lie in a homogeneous matrix of mucinous material interspersed with capillaries and inflammatory cells. Endothelial proliferation with narrowing of the lumen and extrusion of stellate "myxoma" cells has been described.
• Epithelial cells overlying the tumor may show hyperplasia or degeneration, or may appear normal. Epithelial proliferation is characterized by ballooning of cells in the stratum granulosum and hyperkeratosis. Epithelial vesiculation progresses to crust formation. Large, eosinophilic, intracytoplasmic inclusions are especially prominent in the stratum spinosum. Lesions in other organs include cellular proliferation of alveolar epithelium and the reticulum cells of lymph nodes and spleen. Focal hemorrhage may be present in skin, kidneys, lymph nodes, testes, heart, stomach, and intestinal walls. Myxoma cells may be present in lymph nodes, bone marrow, uterus, ovaries, testes, and lungs.

Shope fibroma virus

• Localised benign tumours that spontaneously regress in adult immunocompetent rabbits

Malignant rabbit fibroma virus

• MRV is a recombinant betweeen Myxoma virus and Shope fibroma virus.
• Causes a lethal disease with numerous primary and secondary tumours and immunosuppression.