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Circovirus Associated Disease in Weaned Pigs in Indiana

Circoviruses are very small non-enveloped icosahedral viruses with a single stranded circular DNA genome and have been described in pigs (porcine circovirus, PCV), chickens (chicken anemia virua, ChAV), psittacines (psittacine beak and feather disease virus, PBFD) and pigeons. Circoviruses from pigs, psittacines and pigeons are 16.5-17nm in diameter, whereas ChAV is slightly larger at 24nm. ChAV, PBFDV and pigeon circovirus are all associated with disease and mortality. In virus-infected cells in psittacines and pigeons, circoviruses cause characteristic intracyto-plasmic inclusions that ultrastructurally are composed of paracrystalline arrays of small 17nm icosahedral virions.

Based on serology, infection of swine by PCV is common in Germany, the UK, Canada, New Zealand and the United States. Although infection with PCV is common, inoculation studies in pigs have not demonstrated clinical disease. Recently, PCV has been suggested as a cause of disease in weaned pigs in California and Canada.

Over the last year, a porcine circovirus has been identified associated with disease in weaned pigs from 6 Indiana swine farms. The clinical history of circovirus associated disease in all investigated cases manifested as poor growth in 5-15% of weaned pigs with wasting and death in some. Affected pigs were mildly dyspneic and had cutaneous hyperemia of extremities following exercise and were lethargic. No icteric pigs as described in Western Canada have been found to date.

The main post-mortem lesions of affected pigs consisted of enlarged lymph nodes up to three times normal, which were firm, white and homogeneous on cross section. The lungs were heavy, non-collapsing, and rubbery and had multiple randomly distributed firm dark-red lobules. All other organs appeared usually unremarkable.

Microscopic lesions were found in lymphoid tissues, lung, liver and kidney. The most unique lesion was multifocal granulomatous inflammation affecting multiple lymph nodes, spleen, tonsil and thymus, characterized by epitheloid macrophages and multinucleate giant cells that contained variable numbers of intracytoplasmic magenta-to-basophilic inclusion bodies. Lymph nodes had depletion and coagulative necrosis of follicular centers with extensive karyorrhexis of necrotic cells. Epitheloid macrophages, multinucleate giant cells, and fewer lymphocytes and eosinophils surrounded necrotic areas. Cells with inclusion bodies were multifocally distributed in clusters within areas of granulomatous inflammation. Intracytoplasmic inclusions were round, homogeneous, magenta-to-basophilic, varied in size (5-25 Ám) and were single or formed botryoid clusters. Inclusions were positive for DNA with the Feulgen stain and were much more easily visualized with Feulgen stain than with H & E. Acid-fast stains on areas of granulomatous inflammation in multiple tissues did not reveal any acid-fast bacteria.

No inclusions were observed in non-lymphoid tissues. The lungs had multifocal to diffuse interstitial pneumonia. In the kidneys there was multifocal, lympho-histiocytic, interstitial nephritis and pyelitis. The livers had moderate hepatitis characterized by periportal and multifocal lympho-histiocytic aggregates, bile duct hyperplasia and scattered necrosis of individual hepatocytes. Results of other diagnostic tests (bacteriology and virology) did not demonstrate any other consistent infectious agent(s).

Electron microscopic examination of the intracytoplasmic inclusions in macrophages from a selected pig revealed that they were electron dense and round to ovoid with sharp margins. The matrix was heterogeneous, with different areas being granular, crystalline in a herringbone pattern or crystalline in cross-sectional arrays of non-enveloped, small, icosahedral, viral particles, approximately 17 nm in diameter. No intranuclear virus particles were observed.

Circoviral antigen was detected by immunohistochemistry within inflammatory lesions in lymph nodes, spleen, kidney, lung and liver in a selected pig. The used polyclonal rabbit serum was raised against purified porcine circovirus that was isolated from a diseased pig from Western Canada.

The light and electron microscopic characteristics of the inclusions in these pigs were consistent with those described for circoviruses in psittacines and racing pigeons and similar to the description of circoviral inclusions in pigs in Western Canada. The light microscopic appearance of these inclusions is unique among viral inclusions and can be easily misinterpreted as necrotic cellular debris or other foreign substances such as adjuvant in the cytoplasm of macrophages. Positive immunohistochemistry of these inclusions in pigs in Indiana using polyclonal rabbit antiserum raised against purified circovirus that was isolated from diseased pigs in Western Canada demonstrates antigenic cross-reaction between the circovirus isolated in Canada and the virus infecting these Indiana pigs. The described clinical syndrome in weaned pigs in Indiana is similar to that described as post-weaning multisystemic wasting syndrome in Western Canada. In addition, the lesions in lymph nodes, spleen, lung, liver and kidney in pigs from Indiana are very similar to those described in pigs in Western Canada. Taken together, these findings suggest that the circovirus associated post-weaning multisystemic wasting syndrome (PWMS) that is described in pigs in Canada is also present in Indiana. There is no history of introduction of pigs or porcine semen into the affected Indiana swine farms from Canada.

In all pigs, there was a close association of large numbers of circovirus infected cells and viral inclusion bodies with granulomatous lesions in lymphoid tissues. In Indiana, granulomatous lymphadentitis is an uncommon lesions in pigs seen most often in association with Salmonella choleraesuis or Myco-bacterium avium. These organisms were not demonstrated in any of these pigs by culture and/or special stains. The close association of viral antigen with the lesion in lymphoid tissues, the consistency of the lesion in all pigs and the lack of other demonstrated causes for the lesion strongly suggest circovirus as the cause. The association of circovirus with lesions in other organs in these pigs is less compelling. The lesions in lung, liver and kidney contained only scattered circovirus positive cells and the lesions were less unique. Interstitial pneumonia, hepatitis and/or nephritis is often seen in weaned pigs in Indiana from a variety of causes. Even so, it should be noticed that in the investigated pigs from Indiana and in those from Canada there is a consistent association between circoviral infection and lesions in lung, liver and kidneys as well as a consistent absence of other common causes for such lesions.

To date, inoculation studies in pigs have not confirmed that circovirus in pigs causes lesions or clinical disease. Until these studies are completed, the significance of circovirus in pigs is unknown. Previous inoculation studies using PK-15 cell-derived PCV have not demonstrated clinical disease. Thorough evaluation of pigs in these studies for lesions was not done. It is possible that PK-15 cell-derived PCV has been attenuated through years of cell passage or is different from circovirus. further research is needed to clarify the identity, pathogenicity and significance of porcine circovirus-like virus.

No routine diagnostic testing for porcine circovirus is available at this point in time and the diagnosis of circoviral disease is based on the clinical syndrome and histopathological findings in absence of other possible causes.

- by M. Kiupel, DVM, G.W. Stevenson

DVM, PhD, S.K. Mittal, DVM, PhD,

and C.L. Kanitz, DVM, PhD

 

 

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