Definition
A viral disease that is characterized by the acute appearance of round to oval cutaneous lesions that heal in three to four weeks.
Occurrence
Swine pox occurs only in swine. All age groups are susceptible but most outbreaks are encountered in young growing pigs. It occasionally occurs congenitally and in neonates. Swine pox occurs worldwide in the major swine-raising countries. It occurs more often where insect vectors of pox virus are not controlled. Modern swine raised free of lice rarely exhibit the disease.
Historical information
Swine pox was first reported in Europe in 1842 and in the United States in 1929. The disease persisted and was relatively common, perhaps because it caused little obvious economic loss to swine producers. Outbreaks once were caused by two viruses: vaccinia virus, a laboratory strain, and swine pox virus. Vaccinia virus is no longer used to vaccinate people against small pox; hence it no longer spreads to pigs. Only swine pox virus now causes pox in swine.
Etiology
Swine pox virus, a member of the Poxviridae family, is the etiologic agent. The virus is rather resistant to environmental degradation; it persists in scabs for up to a year. Microscopically, the virus is visible as large intracytoplasmic inclusions in infected cells.
Epidemiology
Persistent virus within desquamated scabs can be transmitted to other pigs when introduced into skin abrasions. The virus can also spread horizontally from nasal and oral secretions from infected pigs. Once a few virus-containing lesions appear, the virus can be transmitted mechanically by the bites of vectors that have recently fed in pox lesion areas. Common vectors include the hog louse (Haematopinus suis), mosquitoes, and biting flies. Once introduced, infected swine or virus-carrying biting insects can initiate outbreaks in susceptible herds. There is evidence of transplacental infection of neonatal pigs.
Pathogenesis
Virus introduced into abrasions replicates in cells of the stratum spinosum causing typical skin lesions to develop. The lesions go through the stages of macules, papules, vesicles, pustules and crusts. All but the vesicle stage are easily identified. As lesions mature and heal, crusts containing virus desquamate and leave white macules.
A viremic stage of the disease has not been demonstrated. However, it must occur in some animals since transplacental infection of neonatal piglets has been reported. Antibody to pox virus is formed as a consequence of infection and immunity is long lasting.
Clinical signs and lesions
Systemic clinical signs of disease seldom are observed but the skin lesions are easily seen. Initial lesions usually are along the ventral aspect of the abdomen, inside the legs and in inguinal areas, all favorite feeding sites of hog lice. However, lesions have been observed at many cutaneous sites so most of the skin appears susceptible. Initially, there are small vesicles that largely are unnoticed. The papules that result are seen as 1-3 mm circular red spots. As lesions mature, crusts form that eventually heal uneventfully. In severe infections, lesions may occur in upper respiratory and digestive tracts. This is particularly true of congenitally-acquired pox where piglets are born with or develop lesions over the entire body shortly after birth. Exudative epidermitis (greasy pig disease) and secondary bacterial dermatitis occasionally occur as a sequel to swine pox. Swine pox does not appear to cause a pruritic response in affected pigs.
Diagnosis
Swine pox is easily diagnosed by identifying the lesions. Lesions are round to oval, usually less than 1.0 cm in diameter. The brown to black crusts are easily seen but the vesicle stage seldom can be identified grossly. If confirmation is needed, histologic diagnosis can be made by identifying typical, large, intracytoplasmic inclusion bodies in cutaneous lesions. Pox lesions should be differentiated from those of other vesiculating viral diseases, pityriasis rosea, dermatosis vegetans, streptococcal dermatitis, ringworm and greasy pig disease.
Control
Control of swine pox relies on herd immunity and prevention of transmission. Pox seldom is a problem if the usual vectors of virus, especially hog lice, are controlled by insecticides or elimination. Congenital swine pox is generally a sporadic, self-limiting event. There are no treatments.