Contagious Ecthyma -Orf

Contagious ecthyma is an infectious dermatitis of sheep and goats that affects primarily the lips of young animals. The disease is usually more severe in goats than in sheep. Humans are occasionally affected, and the disease has been reported in dogs that have eaten infected carcasses.

Etiology and Epidemiology:

The causal poxvirus (a parapoxvirus) is related to those of pseudocowpox and bovine papular stomatitis. Infection occurs by contact. The virus is highly resistant to desiccation, having been recovered from dried crusts after 12 yr. It is also resistant to glycerol and to ether. Contagious ecthyma is found worldwide and is most common in late summer, fall, and winter on pasture, and in winter in feedlots. It may occur in young lambs in early spring and occasionally in mature sheep that do not have immunity from natural exposure.

Clinical Findings and Diagnosis:

The primary lesion develops on the skin of the lips and frequently extends to the mucosa of the mouth. Occasionally lesions are found on the feet, usually in the interdigital region and around the coronet. Ewes nursing infected lambs may develop lesions on the udder. In young lambs, the initial lesion may develop on the gum below the incisor teeth. The lesions develop as papules and progress through vesicular and pustular stages before encrusting. Coalescence of numerous discrete lesions often leads to the formation of large scabs, and the proliferation of dermal tissue produces a verrucose mass under them. When the lesion extends to the oral mucosa, secondary necrobacillosis ( Necrobacillosis: Introduction) frequently develops. During the course of the disease (1-4 wk), the scabs drop off and the tissues heal without scarring. During active stages of infection, more severely affected lambs fail to eat normally and lose condition. Extensive lesions on the feet lead to lameness. Mastitis may occur in ewes with lesions on the udder. The lesion is characteristic. The disease must be differentiated from ulcerative dermatosis ( Ulcerative Dermatosis of Sheep: Introduction), which produces tissue destruction and crateriform ulcers. Ecthyma usually affects younger animals than does ulcerative dermatosis, although this criterion can only be used presumptively. A positive differentiation may be obtained by inoculating susceptible and ecthyma-immunized sheep.

Treatment and Control:

Antibacterials may help combat secondary infection. In endemic areas, appropriate repellents and larvicides should be applied to the lesions. The virus is transmissible to humans, and the lesions, usually confined to the hands and face, are more proliferative and occasionally very distressing. Veterinarians and sheep handlers should exercise reasonable protective precautions. Diagnosis in humans is established by transmitting the virus to sheep; a complement-fixation test may be of value. Sheep that have recovered from natural infection are highly resistant to reinfection. Despite a multiplicity of immunogenic virus strains, the presently used commercial single-strain vaccines have produced fair immunity in all parts of the USA (with an occasional exception). Vaccine breaks appear to be due to the virulence of the infecting strain rather than to differences in antigenicity of the vaccine. Sheep immunized against contagious ecthyma remain susceptible to ulcerative dermatosis. Vaccines should be used cautiously to avoid contaminating uninfected premises, and vaccinated animals should be segregated from unprotected stock until the scabs have fallen off. A small amount of the vaccine is brushed over light scarifications of the skin, usually on the inside of the thigh. Lambs should be vaccinated when ~1 mo old. For best results, a second vaccination ~2-3 mo later is suggested. Nonimmunized lambs should be vaccinated before entering infected feedlots. Experimental work suggests that parenteral administration of virulent vaccine induces better immunity than does the current procedure.