Circovirus Associated Disease in Weaned Pigs
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
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
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
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