Week 6

• Poxviridae

  Family of large, sometimes enveloped, DNA viruses that cause disease in humans, animals, and birds
• Poxvirus properties

  • Large, sometimes enveloped, DNA viruses
  • Pleomorphic
  • Typically brick-shaped
  • Possess irregular surface of projecting tubular or globular structures
• Parapoxvirus

  Genus of poxviruses that are ovoid in shape and covered with long thread-like surface tubules arranged in a criss-cross fashion
• Poxvirus infectious particles

  • Intracellular mature virus (only inner membrane)
  • Extracellular enveloped virus (contain 2 membranes)
• Extracellular enveloped virus

  Acquired from cells by budding, contains virus encoded-proteins from host cell membranes
• Virion outer layer

  Encloses dumbbell-shaped core containing viral DNA with several proteins, and two lateral bodies
• Poxvirus genome consists of single molecule of linear double-stranded DNA
• Proteins encoded by poxvirus genome

  • Enzymes involved in nucleic acid synthesis and virion structural components
  • Proteins that specifically counteract host adaptive and innate immune responses
• Poxvirus replication occurs predominantly in the cytoplasm
• Poxviruses have evolved to encode enzymes required for transcription and replication of their genome
• Ways poxvirus virions are released

  • Budding (enveloped virions, taken up by cells more readily, more important for spread)
  • Exocytosis
  • Cell lysis (non-enveloped virions, most virions)
• Poxvirus stability

  • High environmental stability, remain infectious over period of several months in ambient environment
  • Low lipid content, less sensitive to organic solvents/disinfectants
  • High resistance to drying, survive many years in dried scabs or other virus-laden material
• Poxvirus transmission routes

  • Skin contact with broken or lacerated skin (common route)
  • Respiratory route (inhalation of aerosol)
• Sheeppox virus

  Mechanically transmitted by arthropods
• Poxvirus pathogenicity

  • Highly epitheliotropic, cause cutaneous skin lesions
  • Occasionally causes systemic diseases in birds and wild animals
  • Host specific, Orthopoxviruses infect wide range of species
• Poxvirus spread

  1. Gains access to systemic circulation

  2. Lymphatic system

  3. Secondary viremia disseminates the virus back to the skin and other target organs
• Skin lesions in poxvirus infection

  • Macule
  • Papule
  • Vesicle
  • Pustule
  • Ulcer (complication)
  • Scab
  • Scar (from degenerative changes in epithelium)
• Orthopoxvirus (cowpox)


  Endemic only in Europe and Asia, hosts include cattle, wild and domestic cats, humans, and zoo animals, reservoir hosts are rodents
• Cowpox transmission

  1. Cow to cow in a herd through infected milker's hands or teat cups

  2. Infected farm cats transmit

  3. Rodents serve as reservoir
• Economic importance of cowpox

  • Losses to farmers due to inconvenience in milking, sore teats, mastitis from secondary infection
• Cowpox clinical findings in cattle

  Mildly febrile, papules appear on teats and udder, suckling calves may develop lesions in mouth, vesicles may not be evident or may rupture readily, leave raw, ulcerated areas that form scabs
• Cowpox transmission in cats

  Skin inoculation usually through bite or skin wound, rodents common source of infection, oro-nasal route, exotic felids may develop viral pneumonia
• Cowpox clinical signs in cats

  Primary lesion is a single skin lesion, usually on head, neck, or forelimb, secondary lesions develop into discrete, circular, ulcerated papules, healing usually complete by ~6 weeks, 20% may develop mild coryza or conjunctivitis
• Cowpox transmission in humans

  Mainly caused by direct contact with infected cats, not commonly found in cattle
• Cowpox clinical signs in humans

  Macropapular lesions first observed on hands and face, develop into vesicular and then ulcerative lesions, enlarged painful lymph nodes, fever, vomiting, sore throat
• Monkeypox in humans

  Symptoms similar to smallpox patients
• Monkeypox transmission

  1. Primary infection from direct contact with blood, bodily fluid or rashes of infected animals, handling infected monkeys, Gambian rats or squirrels

  2. Secondary transmission through close contact with infected respiratory tract excretions, or recently contaminated objects
• Pseudocowpox

  Causes milk sores on teats and udders of cattle, can infect humans
• Milker's nodule

  Lesions on hands of dairy farmers milking teats or vets treating infected cows, vary from multiple vesicles to a single, indurated nodule
• Pseudocowpox transmission

  • Contaminated milker's hands and teat cups
  • Biting insects
  • Suckling infected teats
  • Semen of bulls
• Pseudocowpox pathogenesis

  • Lesions characterized by hyperplasia of squamous epithelium
• Pseudocowpox clinical signs

  • Acute lesions: erythema, papules, vesicle or pustule, rupture, thick scab 0.5-25cm in diameter, elevated due to accumulation of granulation tissue, drop off after 7-10 days, leave horseshoe-shaped ring of small scabs surrounding small wart-like granuloma
  • Chronic lesion: commences as erythema, yellow-gray, soft scurfy scabs rubbed off during milking, corrugated skin, may persist for months
• Pseudocowpox diagnosis

  Horseshoe-shaped ring like lesion is pathognomonic, isolation and detection by various diagnostic laboratory methods from vesicular fluid or from teat skin
• Pseudocowpox differential diagnosis

  • Cowpox virus
  • Bovine herpesvirus
  • Vesicular stomatitis
  • Udder impetigo
  • Teat chaps and frostbite
  • Black spot
• Pseudocowpox treatment

  Removal of scabs, burn scabs to prevent environmental contamination, apply emollient ointment before milking, apply astringent preparation after milking
• Pseudocowpox prevention

  • Disinfection, use iodophor teat dip, isolation and treatment of infected cows, reduce teat trauma
• Contagious ecthyma (ORF) host

  • Sheep and goats
  • Primarily lambs and goat kids
• Contagious ecthyma (ORF) has worldwide distribution
• Contagious ecthyma transmission

  1. Scabs that fall off from healing lesions contain virus (stable in environment)

  2. Virus remains highly infectious for long periods under dry conditions

  3. Contaminated instruments like docking instrument and ear-tagging plier

  4. Virus infects healthy animals primarily through damaged skin

  5. Oral lesions in lambs or kids from nursing dams with teat lesions and vice-versa
• Contagious ecthyma pathogenesis

  • Damage of skin essential to establish infection
  • Skin reaction to viral infection with cellular response and necrosis
  • Sloughing of affected epidermis and underlying stratum papillare of the dermis
  • Lesions evolve from macule