Orf

VIRUSES POXVIRUSES

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

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

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