Porcine deltacoronavirus infection

Authors:
K JUNG - Research Scientist and Leading Expert, Food Animal Health Research Program, CFAES and CVM, OARDC, Ohio State University, 1680 Madison Ave, Wooster, Ohio, 44691, USA
H HU - Professor, Henan, 450006, China
L J SAIF - Professor, MS, PhD, Food Animal Health Research Program, CFAES and CVM, OARDC, Ohio State University, 1680 Madison Ave, Wooster, Ohio, OH 44691, USA

Introduction

In early 2014, outbreaks of watery diarrhoea associated with porcine deltacoronavirus (PDCoV) infection occurred in sows and their suckling piglets on five farms in Ohio, United States (USA).⁴¹ Previously, PDCoV had been reported in the faeces of domestic pigs in China in 2012,⁴⁴ but the role of the virus as an enteric pathogen was unclear. Porcine deltacoronavirus subsequently spread nationwide in the USA ⁴² causing diarrhoea and deaths in suckling pigs.³

Experimental studies verified that the PDCoV isolates from the USA are enteropathogenic in neonatal pigs, as evidenced by acute, watery diarrhoea and severe intestinal lesions.^{5, 17} However, the impact and disease severity of PDCoV infection is less than that of both porcine epidemic diarrhoea virus (PEDV) and transmissible gastroenteritis virus (TGEV) infections.³

Since the original outbreaks of PDCoV-associated disease in the USA, the disease also has been identified on swine farms in Canada, South Korea, China, Thailand, Vietnam, and the Lao People’s Democratic Republic, although PDCoV apparently has not spread nationwide in Canada.³¹

Differential diagnosis of PDCoV, PEDV, and TGEV infections is critical to in the control of coronaviral diarrhoeas in swine production systems, especially in regions where these pathogens have recently emerged or re-emerged.

Aetiology

Porcine deltacoronavirus virions are enveloped and pleomorphic with a diameter of 60 - 180 nm.²⁸ The viral genome comprises a single-strand of positive-sense RNA of approximately 25.4 kb (excluding the poly A-tail) that encodes four structural proteins: spike (S), envelope (E), membrane (M), and nucleocapsid (N), and four nonstructural proteins.²⁸ The genome organization and arrangement is: 5’ untranslated region (UTR), open reading frame (ORF) 1a/1b, S, E, M, nonstructural protein 6 (NS6), N, NS7, and 3’ UTR.The functions and roles of structural and nonstructural proteins of PDCoV in infection of host cells are presumed to be similar to those of TGEV and PEDV.

All global strains of PDCoV have high nucleotide (nt) identities (See Epidemiology ).⁴⁵ However, a comprehensive genetic analysis of viruses of diverse geographical origin revealed that USA/Korean PDCoV viruses clustered together but Chinese viruses clustered separately and those from Thailand formed another cluster.⁴⁵ Chinese PDCoV viruses had multiple mutation or deletion sites in their S, NS, or 3’ UTR genes, whereas these mutations were not found in the genomes of PDCoVs originating in the USA.⁴³

There was lack of cross-reactivity between PDCoV and antibodies to either PEDV or TGEV.^{5, 28} However, a further study reported antigenic cross-reactivity between PDCoV and PEDV strains from the USA, possibly due to at least one shared epitope in the N-terminal region of their N proteins.²⁹ Therefore, truncation of the N-terminal region of the N protein may eliminate PDCoV and PEDV shared epitope(s) and reduce possible cross-reactivity in serologic assays.¹¹ There is no evidence that PDCoV is infectious to humans or is of public health significance.

Epidemiology

The precise origin of PDCoV is unclear. Molecular surveillance in China in 2007-2011 detected deltacoronaviruses (DCoVs) in pigs and wild birds only.⁴⁴ However, DCoVs were previously detected in rectal swabs obtained from small mammals, including Asian leopard cats (Prionailurus bengalensis) and Chinese ferret badgers (Melogale moschata) at live-animal markets in China over the period 2005-2006.⁸ The helicase and S genes of these viruses were closely related to those of PDCoV. The data suggest the potential interspecies transmission of a DCoV between these wild small mammals and pigs, between birds and pigs, or between birds and wild small mammals. A recent study also revealed that PDCoV-inoculated gnotobiotic (Gn) calves developed acute subclinical infection without intestinal pathology, but with persisting faecal viral RNA shedding and seroconversion.¹⁸ Consequently, the potential ability of PDCoV and other DCoV isolates from birds or small mammals to infect different species needs further investigation.

In February, 2014, the Department of Agriculture in Ohio, USA reported an outbreak of diarrhoea in pigs caused by PDCoV. Among 42 faecal or intestinal samples collected from sows with diarrhoea and piglets on five Ohio farms. Of these, 39 (92.9 per cent) were positive for PDCoV by reverse transcriptase-polymerase chain reaction (RT-PCR).⁴¹ The complete genome of the PDCoV Ohio strain OH1987 had 99 per cent nt identity to the prototype strain of PDCoV, HKU15-155, first reported in Chinese pigs in 2012. During the same period, genetically similar strains, USA/IA/2014/8734 and SDCV/USA/Illinois121/2014, were identified by other diagnostic laboratories in the USA.^{25, 30} Since then, PDCoV has been detected in 21 USA states (www.aphis.usda.gov/animal-health/secd). The origin of PDCoV in pigs in the USA is unknown, although there is serological and virological evidence for its presence in pigs in the USA prior to its...