Orf

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Also known as Contagious Pustular Dermatitis, Contagious Ecthyma and Scabby Mouth, Orf is the most common viral zoonosis in the UK.

Contagious ecthyma (Courtesy of Bristol BioMed Image Archive)


Morphology

  • 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

Hosts

  • Acute debilitating but rarely fatal skin disease of sheep, goats and incidentally of ruminants and humans

Pathogenesis

  • Trauma, then lesions on skin without wool
  • Lesions on udder and throat and in mouth
  • Lesions coalesce to form scabbed masses
  • Particular concern for lambs, who stop feeding due to mouth lesions
  • For more, see here
  • In man, papules and pustules develop on hands within 10 days, enlarge and then regress after several weeks

Epidemiology

  • 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

Differentials

  • Sheep pox: A more severe disease
    • Characterized by elevated papules distributed diffusely over the skin surface
    • Inclusion bodies are often seen, but there is no down‑growth of epidermis
    • 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
  • Bluetongue Virus:
    • 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, with crusts not firmly attached
  • Foot and mouth disease: When lesions occur in the mouth
  • Poxviridae infection
  • Quite a common zoonotic disease
Clinical
  • In sheep produces a proliferative nodule/papular mass on lips
  • In flocks in which it is endemic it is seen in lamb
  • If flock is non-immune seen in ewes too but much worse in lambs (may spread to inside of mouth)
  • Can spread to udder of ewe
Pathology

Papular proliferative pathology

  • Poxvirus infections produce local infection of prickle cells in epithelium with proliferation of cells and formation of papule followed by ulceration / necrosis and covered by necrotic epithelium
  • Eventually scabs form and crust drops off
  • Scabs - very infectious ( N.B.if touch -> catch it)

Control

  • Attenuated virus can be applied by scarification to axillar, causing scabs to fall off in 30 days
  • Annual vaccination of ewes at least 8 weeks before lambing (all in contact should be done simultaneously)
  • In an outbreak, all lambs can be vaccinated

From pathology

Contagious ecthyma

Contagious ecthyma (Courtesy of Bristol BioMed Image Archive)
  • Also called contagious pustular dermatitis, orf, sore mouth
  • Caused by a parapox virus
  • Affects mainly young sheep, less commonly cattle, humans, dogs
  • Starts in abrasions around mouth commisures and spreads to lips, oral mucosa, eyelids and feet
  • May also transfer to teats of the mother of an affected lamb
  • Lesions typical of poxvirus but very brief vesicle stage
  • Most obvious is the proliferative pustular stage