Experimental inoculation of pigs with porcine parainfluenza virus 1 revealed pathological manifestations in the upper respiratory tract

Several pathogens are known to affect the respiratory tract of pigs resulting in decreased health and welfare. Porcine parainfluenza virus 1 (PPIV1) and swine orthopneumovirus (SOV) have been identified as novel viruses in pigs. The pathogenicity of PPIV1 has been investigated experimentally by one research group, whereas SOV is yet to be studied. In this experimental trial, two groups of eight pigs were inoculated with a European isolate of PPIV1 or a pool of SOV RNA-positive clinical nasal swab material, and one group of four pigs with culture medium only (negative controls). Nasal swab samples were regularly collected to investigate viral RNA shedding, tissue samples for viral RNA and histopathological examinations, and blood samples to investigate seroconversion. All SOV inoculated pigs tested negative for SOV at 4 days post inoculation (DPI) and therefore, four of these pigs were transferred to the group with the PPIV1-infected pigs to assess direct-contact transmission. At DPI 4, two control and four pigs from each of the PPIV1 and SOV groups were euthanized and necropsied. The remaining pigs were euthanized at 14 DPI. No clinical signs, except for nasal discharge, were observed in any of the pigs. PPIV1 RNA shedding was observed from DPI 2-11 with peaks between DPI 4 and 7, and PPIV1 was transmitted horizontally to all direct-contact pigs. The highest viral RNA load was detected in the upper respiratory tract, i.e., nose, upper and lower trachea compared to the lower respiratory tract, i.e., bronchioles, and alveoli. Generally, a chronic tracheitis at 4 DPI, developing into chronic, erosive tracheitis at 14 DPI was observed in the PPIV1 groups and was supported by in situ detection of PPIV1 by RNAscope. Three pigs also developed mild, bronchointerstitial pneumonia at 14 DPI. All PPIV pigs euthanized at 14 DPI seroconverted. In conclusion, these results showed that PPIV1 is a primary porcine respiratory pathogen that causes breakage of the tracheal epithelial barrier and therefore can predispose to secondary infections. SOV's role as a porcine respiratory pathogen remains unknown, since no successful infection was established and it was not isolated in cells either.