Viral Diseases in Veterinary Medicine

Viral Diseases

• Viruses are programmed to replicate endlessly if all growth factors, metabolic needs, and microenvironments are met.
• Viral pathogenesis involves several steps: virus entry, primary replication, spread within the host, cell tropism, cellular injury, immune response, clearance or persistence, and shedding and transmission.

Viral Replication Outcomes

• Cell death
• Persistence/latency
• Cell proliferation
• Neoplastic transformation

Poxviruses

• Poxviruses cause a group of diseases affecting domestic, wild, and laboratory animals, as well as humans.
• There are 8 genera of poxviruses:
  • Orthopoxvirus
  • Avipox
  • Suipoxvirus
  • Capripoxvirus
  • Leporipoxvirus
  • Molluscipoxvirus
  • Yatapoxvirus
  • Parapoxvirus

Parapoxvirus

• Causes pox diseases in animals, including:
  • Orf virus (ORFV)
  • Bovine papular stomatitis virus (BPSV)
  • Pseudocowpox virus (PCPV)
  • Squirrel parapoxvirus (SPPV)

Capripoxvirus

• Causes:
  • Sheeppox virus (SPPV)
  • Goatpox virus (GTPV)
  • Lumpy skin disease virus (LSDV)

Pathogenesis of Poxviruses

• Infection by direct virus contact or inhalation/ingestion
• Virus entry into the host, followed by systemic leukocyte trafficking
• Infection and multiplication in:
  • Dermal endothelium, leading to vasculitis and ischemic necrosis
  • Epidermal epithelial cells, leading to hyperplastic and necrotizing lesions (pox-lesions)

Lesions

• Macule (erythematous macule): a change in surface color, without elevation or depression
• Papule: a circumscribed, well-defined, solid elevation of skin
• Vesicle (small blister): a circumscribed, epidermal elevation containing fluid
• Pustule: a small, circumscribed elevation of the skin containing pus
• Scabs and scars: result from healing of the pustules by granulation tissue

Gross Lesions

• Cutaneous pox lesions, most easily observed on wool-free or hair-free areas
• In sheeppox and goatpox: systemic disease with lesions in the lungs, liver, and other organs

Microscopic Lesions

• Acanthosis and ballooning degeneration of the stratum spinosum
• Dermal necrosis
• Severe vasculitis (thrombosis) with ischemic necrosis
• Dermal perivascular lymphocytic aggregations
• Presence of Guarnieri bodies (B-type inclusions) and sheeppox cells (cellules claveleuses' of Borrel)

Lumpy Skin Disease

• Caused by Capripoxvirus
• Affects primarily cattle and buffaloes
• Pathogenesis pathway: infection by insect vector, direct contact, aerosols, and fomites
• Clinical signs: fever, multiple well-circumscribed skin nodules, generalized lymphadenopathy, marked weight loss, profuse drooling, and oculonasal discharge
• Lesions: skin nodules, lymphadenopathy, and systemic lesions

Fate of the Nodules

• Some nodules resolve completely, leaving a deep ulcer
• Some nodules fail to separate but become indurated and persist as hard intradermal lumps
• Secondary bacterial infections may develop in the necrotic nodules