Pityriasis rosea (pustular psoriaform dermatitis) is a sporadic disease of unknown etiology of pigs, usually 8–14 weeks of age, but occasionally as young as 2 weeks and very rarely in pigs as old as 10 months. One or more pigs in a litter may be affected.
Photograph of the hind limb of a pig with a pityriasis rosea. Notice the multiple lesions, including small, ring-like early-stage lesions and the older, expanding larger rings that heal from the center.
Courtesy of Dr. Ranald D. A. Cameron.
Photograph of pityriasis rosea lesions on the ventral abdomen and hind leg on a young pig. Note the raised, red appearance with dry flakes centrally indicating healing from the center, as the ring-like lesions spread outward.
Courtesy of Dr. Ranald D. A. Cameron.
Photograph of the abdomen and flank or a pig with pityriasis rosea. Note the typical ring-like lesions with raised red perimeters and healing central areas.
Courtesy of Dr. Ranald D. A. Cameron.
Clinical signs due to pityriasis rosea are typically 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 occur predominantly on the ventral abdomen and inner thighs; however, they occasionally may occur over the back, neck, and legs. Characteristically, there is no pruritus, and recovery is spontaneous in 6–8 weeks. Treatment is generally considered unnecessary.
Diagnosis can usually be made from the characteristic lesions, but laboratory tests, culture, and tissue biopsy may be analyzed to differentiate it from dermatomycosis, exudative epidermitis Exudative Epidermitis in Pigs Exudative epidermitis is a generalized staphylococcal infection that affects young pigs. Clinical signs include depression and reddening of skin, followed by rapid spread of exudative, nonpruritic... read more 
, dermatosis vegetans, and swinepox Swinepox Swinepox is an acute, often mild, infectious disease characterized by skin eruptions that affects only pigs. It is present in the USA, particularly in the Midwest, and has been reported from... read more 
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The disease is considered to be partially hereditary, with pigs of the Landrace breed most commonly affected; however, the mode of inheritance is uncertain. The disease does not resemble pityriasis rosea in humans 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.
