3.3 SVV mink infection
Five-day challenge experiments in mink did not result in any deaths. The
pathological results did not indicate frequent gross changes, such as
alteration of the surface of the lungs, ulcerative
lesions,
and lesions of the liver and kidney.
However,
liquid feces were evident on the second day of SVV infection, which
indicated diarrhea in the small intestine. Examination of all tissues
for the presence of SVV nucleic acid, and the RT-PCR and qRT-PCR results
showed that two of three
oral-fluid
samples were positive, and all three minks of fecal-swab samples were
tested with positive. No
SVV
nucleic acid was detected in the serum of infected minks. qRT-PCR
detection of SVV RNA yielded results corresponding to the RT-PCR SVV
nucleic acid levels. However, fecal-swab samples of the infected minks
contained a markedly higher number of RNA copies than the oral-fluid
samples. No samples in the control group tested positive, upon RT-PCR
and qRT-PCR (Table 3).
3.4 Pathological examination and Immunofluorescence of SVV
infection
Microscopic examination revealed obvious differences among these groups
of minks concerning to their
small-intestine
samples obtained from different intestines. The principal
histopathological results are graphically summarized in Figure 3A. In
the intestinal sections, lesions were observed in the
duodenum
and colon. The principal observations were atrophy and rupture, followed
by fusion of villi in the duodenum and colon. Inflammation was observed
in different intestinal segments. Inflammation in the duodenum of the
infected mink was much more severe compared to that observed in the
control group. Besides, necrosis and vacuolization of epithelial cells
were evident in the minks with clinical manifestations of diarrhea. The
duodenum and colon sections of the control group mink did not show any
pathological changes compared with the infectious group.
The distribution and quantity
of
SVV virions in the small intestine of the mink are shown in Figure 3B.
SVV virion antigens were identified by the VP1 multi-antibody produced
in our lab. Viral antigens were mainly detected in villous
epithelial
cells in both the duodenum and colon. The antigens were not present in
the duodenum or colon of the control group. However, viral antigens were
detected in the duodenum and
colon
of infected mink, and different intestinal segments contained different
virus titers. The colon had much higher titers than the duodenum in SVV
infection, indicating that the epithelial cells of the colon were much
more sensitive to the infection than the epithelial cells of the
duodenum.