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.