Coronavirus disease 2019 (COVID‐19) outbreak: Could pigs be vectors for human infections?

Coronaviruses (CoVs) have recently received an extraordinary level of global attention due to the discovery of a novel high pathogenic human coronavirus (CoV) in China in December 2019.1 CoVs are positive-sense RNA viruses that belong to the subfamily Orthocoronavirinae in the family Coronaviridae (Source: International Committee on Taxonomy of Viruses—ICTV, Taxonomy history: Orthocoronavirinae. Accessed January 24, 2020). This family can be divided into four genera, α-CoVs, β-CoVs, γ-CoVs, and δ-CoVs.2 CoV in humans are common (Table 1), often associated with mild cold symptoms, and include the α-CoVs HCoV-229E3 and HCoV-NL63,4 and the β-CoVs HCoV-OC435 and HCoV-HKU1.6 In addition, to date three high pathogenic CoVs (all β-CoVs) are recognized in humans and characterized by moderate (MERS-CoV) or high (SARS-CoV, SARS-CoV-2) transmission rates and association with mortality (Table 2). During 2002-2003 in Guangdong province in China, Severe Acute Respiratory Syndrome (SARS) CoV (SARS-CoV) was discovered.7-9 From South-East Asia, the first larger SARS-CoV outbreak spread to Europe and North America in 200310 and was followed by a smaller outbreak in a laboratory in China during 2004.11 Since 2004, no known cases of SARS-CoV have been reported. During 2012, the Middle East Respiratory Syndrome (MERS) CoV was recognized in the Middle East region.12 Finally, in December 2019 a novel CoV (2019nCoV), later named SARS-CoV-2, was associated with severe lung disease in a cluster of patients in Wuhan, Hubei Province in China, and consequently reported to the World Health Organization.1 Since the lung disease has been designated as CoV disease 2019 (COVID-19).13 Interestingly, all currently recognized high pathogenic CoVs in humans appear to have spilled over from bats or other wildlife sources.14 Therefore, concerns for further interspecies transmissions of SARSCoV-2 from humans to other species may be justified. Pigs play a major role in xenotransplantation due to their organ similarity with humans. Pigs are susceptible to several CoVs including the α-CoVs transmissible gastroenteritis virus (TGEV),15 porcine epidemic diarrhea virus (PEDV),16 porcine respiratory coronavirus (PRCV),17 and the recently emerging swine acute diarrhea syndrome coronavirus (SADS-CoV)18,19 which like SARS-CoV or MERS-CoV appears to have originated from bats. Besides these α-CoVs, pigs are also susceptible to a β-CoV, porcine hemagglutinating encephalomyelitis virus (PHEV),20 and a δ-CoV, porcine deltacoronavirus (PDCoV)21 (Table 1). As outlined, pigs have their own range of CoVs and humans have their own range of CoVs with no apparent overlap. It is however interesting to note that CoVs in humans mainly target the respiratory tract whereas CoVs in pigs mainly target the enteric system (Table 1). In pigs, there is only a single CoV associated with respiratory infections, PRCV, commonly resulting in mild disease and lesions22,23 and hence not considered of importance to pork producers. Prevention in the form of vaccines is not in place for PRCV. Only few studies have investigated if pigs can become infected with SARS-CoV or MERS-CoV and hence have determined if pigs could be a source of infection for humans. During a survey involving six domestic animal species including pigs and 242 individual animals in China, SARS-CoV RNA was detected in a single pig.24 Furthermore, two pigs among the 242 animals surveyed were SARS-CoV antibody positive. The authors concluded that SARS-CoV interspecies transmission events occur and possibly pose a risk to humans.24 In another study, 6-week-old pigs and chickens were experimentally infected with SARS-CoV using intravenous, intranasal, ocular, and oral routes.25