Josepha DeLay, Hannah Jansen
Animal Health Laboratory, University of Guelph, Guelph ON (DeLay), South West Vets, Stratford, ON (Jansen)
AHL Newsletter 2025;29(4):11.
The September 2025 edition of the Animal Health Laboratory (AHL) Newsletter described a case with lesions in neonatal pigs. Multiple discrete ulcerative and proliferative, crusted cutaneous and oral lesions were present at birth in 2 piglets in a single litter from a 1600 sow herd. Lesions involved haired skin, tongue, lip margins, and distal limbs, including coronary bands (Fig.1). The sow was unaffected. Similar lesions had been noted sporadically in few piglets in litters from sows of various parities.
Figure 1. Congenital skin and oral mucosal lesions in a neonatal piglet. Numerous discrete ulcerated and proliferative lesions were present in (A) haired skin over the entire body, (B) the mucosal surface of tongue, (C) mucocutaneous junction at the lips and snout, and (D) distal limbs, including coronary bands.
Gross postmortem and histologic examination were carried out on a representative neonatal piglet. The piglet was in good body condition and was well hydrated. Significant gross lesions were limited to skin, mucocutaneous junctions, and tongue, as described above. Histologically, skin and mucosal lesions were similar at all anatomic sites and consisted of discrete foci of epithelial proliferation and ulceration (Fig. 2). Intact epithelium at the periphery of lesions was markedly thickened (hyperplastic), and superficial squamous epithelial cells were severely swollen, with clear cytoplasm, consistent with hydropic degeneration. Small discrete eosinophilic intracytoplasmic inclusion bodies, compatible with poxvirus inclusions, were present in moderate numbers of cells. Ulcerated sites were covered by thick layers of degenerative neutrophils intermingled with necrotic cellular debris and mixed bacteria.
Figure 2. Histologic lesions of swinepox in haired skin from a neonatal piglet. (A) Discrete focus of epidermal hyperplasia covered by a thick cellular crust. (B) Epithelial cell swelling (hydropic degeneration) in epidermis at the periphery of the skin lesion. (C) Inset from panel B – Eosinophilic intracytoplasmic viral inclusions in swollen epithelial cells. H&E stain.
The gross and histologic features of the skin lesions in this piglet, and presence of intracytoplasmic inclusions in epithelial cells in association with the lesions, are consistent with poxvirus infection and a diagnosis of congenital swinepox. Swinepox virus can infect pigs of all ages, and lesions typically involve the entire body. Gross and histologic lesions, with evidence of intracytoplasmic inclusions consistent with poxvirus inclusions, are usually striking. Confirmatory tests for swinepox are not readily available, and the diagnosis relies on the combined presence of typical gross and histologic lesions, with evidence of viral inclusion bodies consistent with poxvirus inclusions, in the context of sporadic disease occurrence in the herd. Lesions are typically limited to skin and oral mucosa, without evidence of systemic disease. Follow-up information from the producer in this case indicated that the incidence of new cases was short-lived in the herd (self-limiting infection).
Pigs with swinepox are infrequently submitted to the AHL for postmortem examination, reflecting the very sporadic nature of the disease. Between January 2010 and November 2025, 4 cases of swinepox were diagnosed at the AHL. Most cases involved neonatal pigs (3 cases), and 1 case involved nursery pigs. For 1 of the cases with neonatal pigs, a sow with pox lesions was described clinically as the initial case in the herd, with lesions subsequently identified in a single, unrelated litter. Presence of skin lesions at birth (congenital infection) was confirmed in the clinical history for only 1 of the 3 piglet cases, although piglets in all cases were very young (1-3 days of age), supporting in utero infection with the virus.
Swinepox virus (SwPV) is the only member of the genus Suipoxvirus. The virus is antigenically distinct from other poxviruses, such as cowpox. Poxviruses are the only known family of DNA viruses that replicate and assemble in the cytoplasm of the host cell, rather than in the nucleus, resulting in the intracytoplasmic inclusion bodies seen in histologic samples. Insects can act as mechanical vectors for transmission of SwPV, including biting lice (Haemotopinus suis), and possibly including biting flies and mosquitoes. Horizontal transmission occurs by direct contact with skin lesions, and with oronasal secretory debris. Viral replication occurs in the epidermis, resulting in the lesions seen histologically (cell swelling, cytoplasmic viral inclusion bodies). Secondary opportunistic bacterial infection of skin lesions is common. Vertical transmission of SwPV also occurs, as in this case of congenital swinepox, and may be associated with sow viremia, although this pathogenesis has not been confirmed. Lesions of swinepox are usually present in only a few piglets in a litter. Abortion or stillbirth may also reportedly result from vertical transmission of SwPV, and severe oral lesions in neonates can negatively impact nursing and survival.
Important measures for the prevention of swinepox include parasite and environmental insect control, as well as management interventions to improve general hygiene in the environment. Immunosuppression due to a variety of infectious or non-infectious causes likely plays a role in increasing the susceptibility of some animals to infection with SwPV that may be present endemically in the herd.
While swinepox is a well-recognized condition, clinical disease is sporadic and infection is usually self-limiting, as in the sow herd in this case, and especially in older pigs. Swinepox is not a zoonotic disease, unlike some other poxviruses, and there is no associated public health risk. Consequently, the incidence and economic impact of swinepox are generally negligible in well managed herds. However, and importantly, swinepox must be differentiated from various vesicular diseases affecting swine, including Seneca Valley virus and reportable diseases (foot and mouth disease, vesicular stomatitis, swine vesicular exanthema, swine vesicular disease). Other differential diagnoses include primary bacterial (streptococcal) dermatitis, erysipelas, classical swine fever, and pityriasis rosea. Epidemiologic features of these diseases in a herd would likely differ significantly from that seen with sporadic swinepox.
References
1. Freitas TRP. Swinepox Virus. In: Diseases of Swine, 11th ed. Zimmerman JJ et al, eds. Wiley-Blackwell, 2019:709-714.
2. MacNeill AL, et al. Poxvirus pathology and pathogenesis in agriculturally important species. Vet Pathol 2025;62:849-866.
3. Schwarz L, et al. Congenital suipoxvirus infection in newborn piglets in an Austrian piglet-producing farm. Microorganisms 2024;12:1757. https://doi.org/10.3390/microorganisms12091757 [1]