Orf

VIRUSES POXVIRUSES

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

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