Pityriasis rosea is a sporadic disease of unknown etiology of pigs, usually 8–14 wk old, but occasionally as young as 2 wk and very rarely in pigs as old as 10 mo. One or more pigs in a litter may be affected. The disease is mild, but transient anorexia and diarrhea have been reported. The initial skin lesions are characterized by small erythematous papules, which rapidly expand to form a ring (collarette) with distinct raised and reddened borders. The lesions enlarge at their periphery, and adjacent lesions may coalesce. The center of the lesion is flat and covered with a bran-like scale overlaying normal skin. The lesions are found predominantly on the ventral abdomen and inner thighs but occasionally may be seen over the back, neck, and legs. Characteristically, there is no pruritus, and recovery is spontaneous in 6–8 wk. Treatment is generally considered unnecessary. Diagnosis can usually be made from the characteristic lesions, but laboratory tests, culture, and biopsy may be used to differentiate it from dermatomycosis, exudative epidermitis, dermatosis vegetans, and swinepox.
The disease is considered to be partially hereditary, pigs of the Landrace breed being most commonly affected, but the mode of inheritance is uncertain. The disease does not resemble pityriasis rosea in people clinically or pathologically.
Lesions appear to be more extensive in pigs reared in high stocking densities with high ambient temperatures and high humidity. Under these conditions, secondary bacterial infection (eg, Staphylococcus hyicus) is common. Treatment is of little value and does not affect the course of the disease; however, treatment aimed at controlling secondary infections may be warranted.