Seneca Valley Virus and Vesicular Lesions in a Pig with Idiopathic Vesicular Disease

Seneca Valley Virus (SVV) is a single-stranded RNA virus in the family Picornaviridae. It is closely related to the genus Cardiovirus, and has been recently proposed to be classified in the genus Senecavirus, along with other closely related porcine picornaviruses [1]. Seneca Valley virus is a newly discovered virus that was originally isolated as a contaminant in cell culture medium; its complete genome sequence and structure has been recently published [1,2]. Furthermore, SVV has been reported to be associated with cases of idiopathic vesicular disease in pigs originating from several different locations in Manitoba [3].


Abstract
Swine Idiopathic Vesicular Disease (SIVD) syndrome is characterized by the formation of ulcers, erosions and vesicles on the skin, coronary bands and in the oral cavity of pigs. The clinical importance of SIVD is its resemblance with vesicular foreign animal diseases. Although the etiology of SIVD remains unknown, Seneca Valley virus, which belongs to the family Picornaviridae, was previously identified in such pigs. Here, we report gross and histopathologic findings in a 6-month-old intact male Chester White boar presented with a history of anorexia, lethargy and lameness. Intact and ruptured vesicles, erosions and ulcers were clinically observed within the oral cavity, around the nares, coronary bands, and all four limbs. Various diagnostic tests were negative for swine vesicular disease virus, footand-mouth disease virus, vesicular exanthema of swine virus and vesicular stomatitis virus infection. However, vesicular scrapings and oral pharyngeal fluid were positive for the presence of Seneca Valley virus by RT-PCR.

Seneca Valley Virus and Vesicular Lesions in a Pig with Idiopathic Vesicular Disease
Singh K 1 *, Corner S 1 , Clark SG 2 , Scherba G 1 , and Fredrickson R 1 Infection Associated Antigen (VIAA) agar gel immunodiffusion. All of the samples tested were negative for these viruses. Virus isolation on the vesicular lesion scrapings and oral pharyngeal fluid was performed but failed to isolate any virus; however, these samples were positive for the presence of SVV by RT-PCR. Other pigs previously in contact with this animal did not show any clinical signs. Sera from five other contact pigs were tested for FMDV and SVDV as described above and were negative for evidence of these viruses.
Due to the progression of clinical signs, the boar was euthanized and presented for necropsy in September, 2010. Gross lesions included the formation of multifocal round, discrete (0.5-4 cm in diameter) ulcerative and erosive lesions with overlying crusts on all four distal limbs. The lateral surfaces of the coronary bands of the lateral claws on both forelimbs had deep and chronic ulcers with crusting and sloughing of the hoof wall distal to these lesions ( Figure 1). Within the oral cavity, there was a fluid-filled vesicle on the hard palate and multifocal chronic superficial and deep ulcers on the maxillary and mandibular buccal and gingival mucosa, and on the snout around the nares (Figure 2). Other gross lesions included serofibrinous peritonitis and pericarditis, locally extensive hemorrhagic jejunitis and a focal gastric ulcer.
Samples of small intestine were negative for Salmonella sp. by PCR. Aerobic and anaerobic bacterial culture of these samples did not yield any significant growth nor did aerobic bacterial culture of the spleen. Aerobic culture of an ulcerative lesion on the hind limb only produced moderate growth of Staphylococcus hyicus. Fluorescent antibody and virus isolation attempts for porcine parvovirus on the liver, kidney and spleen were negative. Other causes of vesicular disease also were ruledout using ultrastructural analysis, virus isolation, bacterial culture, and histopathology. Ultrastructural analysis (transmission and negativestaining) of the lesions for the presence of any virus particles, such as those for picornavirus, calicivirus, or enterovirus was also negative.
Microscopically, the epidermis surrounding the cutaneous lesions on the distal limbs exhibited both orthokeratotic and parakeratotic hyperkeratosis and epidermal hyperplasia. There also was regional ulceration and infiltration by numerous degenerate and intact neutrophils admixed with fibrin, edema, acute hemorrhages, and karyorrhectic nuclear debris (Figures 3 and 4). Occasionally, microcolonies of cocci bacteria were observed. Small numbers of neutrophils, lymphocytes and plasma cells extended into the perivascular regions of the dermis. In some regions, edema and early granulation tissue also expanded the dermis. Ulcerations with similar morphologic characteristics were present within the oral cavity and on the snout. Histopathology was not conducted on early vesicular lesions. It was assumed that the suppurative and ulcerative dermatitis described above represented areas where vesicles were reported antemortem, and were secondarily invaded by bacteria. Other microscopic findings included focal acute ulcerative gastritis, mild lymphoplasmacytic hepatitis as well as generalized and global membranoproliferative glomerulonephritis.
Ultrastructural analysis of the oral and skin lesions was performed using negative staining and transmission methods and both failed to demonstrate the presence of a virus.
This negative result could be due to the chronicity of the lesions. Moreover, an ultrastructure feature that helps in the identification of picornaviruses is their aggregation into lattices or crystallinearrays; if these viruses are individualized, they are very difficult to be identified [14]. Staphylococcus hyicus can cause exudative epidermitis in piglets; however, although it was cultured from the lesions, it was considered a secondary contaminant by virtue of its presence in the normal cutaneous flora. In addition, the signalment and clinical signs do not fit with the exudative epidermitis syndrome. We also observed membranoproliferative glomerulonephritis, which has not been previously reported in pigs suffering from SIVD. The causes of membranoproliferative glomerulonephritis are numerous but in general, it reflects chronic low-grade antigenemia, which could be due to a SVV infection or other antigens. It is possible that serofibrinous peritonitis and pericarditis and hemorrhagic jejunitis could be consequence of chronic renal failure. However, other lesions suggesting chronic renal failure were not observed.
To the best of our knowledge, this is the first case of SIVD with detailed description of lesions in which an etiologic association is made with the SVV. However, this virus has been previously isolated from the healthy pigs in several areas of the United States and it has not been reported to cause clinical disease in pigs or other mammals [1]. Therefore, this association is purely speculative until further investigation is done and Koch's postulates have been fulfilled. Importantly, this is the second    report where SVV was identified in pigs with SIVD in which other etiologic agents were ruled-out, hence it is possible that SVV may be the causative agent of SIVD. Consequently, since SIVD is clinically and economically important due to its resemblance with vesicular foreign animal diseases, which are associated with significant morbidity and mortality in pigs, further investigation in to the possible association of SVV with this syndrome is warranted.