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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.
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