Contagious ecthyma
Contagious ecthyma (contagious pustular dermatitis) is caused by orf virus, a poxvirus that infects ruminants (most often sheep and goats). Farmers, veterinarians, zoo caretakers, and others with direct animal contact are at risk.
The cutaneous findings pass through 6 stages that together last about 1 week:
Stage 1 (papular): A single red edematous papule on a finger (most commonly right index finder)
Stage 2 (target): A larger nodule with a red center surrounded by a white ring with a red periphery
Stage 3 (acute): A rapidly growing, infected-looking tumor
Stage 4 (regenerative): A nodule with black dots covered with a thin transparent crust
Stage 5 (papillomatous): A nodule with a surface studded with small projections
Stage 6 (regressive): A flattened nodule with a thick crust
Patients can develop regional adenopathy, lymphangitis, and fever.
Diagnosis of contagious ecthyma is by history of contact; differential diagnosis is extensive depending on the stage of the lesion. Acute lesions must be differentiated from milker’s nodules Milker’s nodules Contagious ecthyma and milker's nodules are two viral skin diseases that are rarely transmitted from animals to humans. Contagious ecthyma (contagious pustular dermatitis) is caused by orf virus... read more 
, Mycobacterium marinum infection (see Cutaneous disease Cutaneous disease There are over 170 recognized species of mycobacteria, mostly environmental. Environmental exposure to many of these organisms is common, but most exposures do not cause infection and many infections... read more ), and other bacterial infections; regressed lesions must be differentiated from cutaneous tumors, such as Bowen disease Bowen Disease Bowen disease is a superficial squamous cell carcinoma in situ. Diagnosis is by biopsy. Treatment depends on the tumor’s characteristics and may involve curettage and electrodesiccation, surgical... read more 
or squamous cell carcinoma Squamous Cell Carcinoma Squamous cell carcinoma is a malignant tumor of epidermal keratinocytes that invades the dermis; this cancer usually occurs in sun-exposed areas. Local destruction may be extensive, and metastases... read more 
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Lesions spontaneously heal; no treatment is necessary.
Milker’s nodules
These nodules are caused by paravaccinia virus, a parapoxvirus that causes udder lesions in cows. Infection requires direct contact and causes macules that progress to papules, vesicles, and nodules. This infection has 6 stages, which are similar to those of contagious ecthyma Contagious ecthyma Contagious ecthyma and milker's nodules are two viral skin diseases that are rarely transmitted from animals to humans. Contagious ecthyma (contagious pustular dermatitis) is caused by orf virus... read more . Fever and lymphadenopathy are uncommon.
Diagnosis of milker's nodules is by history of contact and cutaneous findings. Differential diagnosis varies depending on morphology but can include primary inoculation tuberculosis (a chancre that can develop at the site of tuberculosis inoculation), sporotrichosis Sporotrichosis Sporotrichosis is a cutaneous infection caused by the saprophytic mold Sporothrix schenckii. Pulmonary and hematogenous involvement is uncommon. Symptoms are cutaneous nodules that spread... read more 
, anthrax Anthrax Anthrax is caused by the gram-positive Bacillus anthracis, which are toxin-producing, encapsulated, facultative anaerobic organisms. Anthrax, an often fatal disease of animals, is transmitted... read more 
, and tularemia Tularemia Tularemia is a febrile disease caused by the gram-negative bacterium Francisella tularensis; it may resemble typhoid fever. Symptoms are a primary local ulcerative lesion, regional lymphadenopathy... read more 
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Lesions heal spontaneously; no treatment is necessary.
More Information
The following English-language resources may be useful.
MSD Veterinary Manual: Overview of Contagious Ecthyma
MSD Veterinary Manual: Pseudocowpox in Cattle
