Porcine deltacoronavirus infection: Etiology, cell culture for virus isolation and propagation, molecular epidemiology and pathogenesis

Abstract

Porcine deltacoronavirus (PDCoV) (family Coronaviridae, genus Deltacoronavirus) is a novel swine enteropathogenic coronavirus that causes acute diarrhea/vomiting, dehydration and mortality in seronegative neonatal piglets. PDCoV diarrhea was first reported in the US in early 2014, concurrently with co-circulation of porcine epidemic diarrhea virus (PEDV) (family Coronaviridae, genus Alphacoronavirus). The origin of PDCoV in pigs and also its sudden emergence or route of introduction into the US still remains unclear. In the US, since 2013-2014, the newly emerged PDCoV and PEDV have spread nationwide, causing a high number of pig deaths and significant economic impacts. The current US PDCoV strains are enteropathogenic and infect villous epithelial cells of the entire small and large intestines although the jejunum and ileum are the primary sites of infection. Similar to PEDV infections, PDCoV infections also cause acute, severe atrophic enteritis accompanied by transient viremia (viral RNA) that leads to severe diarrhea and/or vomiting, followed by dehydration as the potential cause of death in nursing piglets. At present, differential diagnosis of PDCoV, PEDV, and transmissible gastroenteritis virus (TGEV) is essential to control viral diarrheas in US swine. Cell culture-adapted US PDCoV (TC-PDCoV) strains have been isolated and propagated by us and in several other laboratories. TC-PDCoV strains will be useful to develop serologic assays and to evaluate if serial cell-culture passage attenuates TC-PDCoV as a potential vaccine candidate strain. A comprehensive understanding of the pathogenesis and epidemiology of epidemic PDCoV strains is currently needed to prevent and control the disease in affected regions and to develop an effective vaccine. This review focuses on the etiology, cell culture isolation and propagation, molecular epidemiology, disease mechanisms and pathogenesis of PDCoV infection.

Keywords: Disease; Pathogenesis; Pigs; Porcine deltacoronavirus; Review; Virus.

Fig. 1

Phylogenetic analyses of the S gene nucleotide sequences (A) and complete genomes (B) of some representative porcine and avian deltacoronaviruses. Reference sequences obtained from GenBank are indicated by strain names and accession numbers. The trees were constructed using the distance-based neighbor-joining method of the software MEGA6.06 (http://www.megasoftware.net). Bootstrap analysis was carried out on 1000 replicate data sets, and values are indicated adjacent to the branching points. Scale bars represent 0.1 nucleotide substitutions per site.

Fig. 1

Phylogenetic analyses of the S gene nucleotide sequences (A) and complete genomes (B) of some representative porcine and avian deltacoronaviruses. Reference sequences obtained from GenBank are indicated by strain names and accession numbers. The trees were constructed using the distance-based neighbor-joining method of the software MEGA6.06 (http://www.megasoftware.net). Bootstrap analysis was carried out on 1000 replicate data sets, and values are indicated adjacent to the branching points. Scale bars represent 0.1 nucleotide substitutions per site.

Fig. 2

Histopathology and localization of porcine deltacoronavirus (PDCoV) antigens by immunofluorescence (IF) staining and PDCoV nucleic acids by in situ hybridization in the small intestine of gnotobiotic pigs inoculated with US PDCoV strain OH-FD22. (A) Hematoxylin and eosin–stained jejunum of an inoculated pig at post-inoculation day (PID) 3 (48–51 h after onset of clinical signs), showing acute diffuse, severe atrophic enteritis, with diffuse, moderate vacuolation of enterocytes lining the epithelium of atrophied villi. Original magnification, ×200. (B) IF staining of a serial section of the jejunum (Panel A) of inoculated pig at PID 3, showing that the epithelial cells lining atrophied villi are positive for PDCoV antigen (green staining). Original magnification, ×200. (C) IF staining (higher magnification) of formalin-fixed, paraffin-embedded jejunum of the inoculated pig at PID 3, showing localization of PDCoV antigens (green staining) in the cytoplasm of villous epithelial cells. Nuclei were stained with blue-fluorescent 4′, 6-diamidino-2-phenylindole dihydrochloride. Hyperimmune pig antiserum to PDCoV strain OH-FD22 was used for IF staining. Original magnification, ×600. (D) In situ hybridization of formalin-fixed, paraffin-embedded jejunum of the inoculated pig at PID 3, indicating that the epithelial cells lining the atrophied villi are PDCoV RNA-positive (black staining). Nitroblue tetrazolium/5-bromocresyl-3-indolylphosphate, methyl green counterstain. Original magnification ×80.