Overview of Contagious Ecthyma

The causal parapoxvirus is related to pseudocowpox (see Pseudocowpox) and bovine papillar stomatitis (see Papillar Stomatitis in Large Animals). Infection occurs by contact. The virus is highly resistant to desiccation in the environment, having been recovered from dried crusts after 12 yr. In the laboratory, it is also resistant to glycerol and to ether.

Contagious ecthyma is found worldwide and is common in young lambs reared artificially and in older lambs during late summer, fall, and winter on pasture, and during winter in feedlots.

The primary lesion develops at the mucocutaneous junction of the lips and around erupting incisor teeth and may extend to the mucosa of the buccal cavity. Occasionally, lesions are found on the feet and around the coronet, where secondary bacterial infection with Dermatophilus congolensis commonly causes “strawberry footrot.” Ewes nursing infected lambs may develop lesions on the teats extending onto the udder skin. 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 (see Necrotic Laryngitis in Cattle) 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 do not eat normally and lose condition. Extensive lesions on the feet cause lameness. Mastitis, sometimes gangrenous, may occur in ewes with lesions on the teats.

The lesion is characteristic. The disease must be differentiated from ulcerative dermatosis (see Ulcerative Dermatosis of Sheep), which produces tissue destruction and crateriform ulcers. Ecthyma usually affects younger animals than does ulcerative dermatosis, although this criterion can only be used presumptively. Foot-and-mouth disease (see Foot-and-Mouth Disease) and bluetongue (see Bluetongue) infection should be considered if morbidity is high and clinical signs include salivation, lameness, and fever. Staphyloccocal folliculitis affects the skin of the muzzle and surrounding the eyes. Direct demonstration of virus in scab material by electron microscopy has now been replaced by PCR as the diagnostic method of choice for ecthyma. Historically, positive differentiation could be obtained by inoculating susceptible and ecthyma-immunized sheep.

Both parenteral and topical antibiotics may help combat secondary bacterial infection of the skin lesions. In endemic areas, appropriate repellents and larvicides should be applied to the lesions to prevent myiasis. The virus is transmissible to people, 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 and wear disposable gloves. Diagnosis in people 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 live vaccines have produced fair immunity in all parts of the USA (with an occasional exception more usually seen in goats). 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.

Live 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 live vaccine is brushed over light scarifications of the skin, usually on the inside of the thigh or behind the elbow or caudal fold. 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 ~1–2 mo before entering infected feedlots.