Study population and setting
Pigs (n = 19; 8 weeks old, 6 males, 13 females) were sourced from a high health status farm in Manitoba, Canada. Sixteen pigs were challenged oronasally with 1×10^6 plaque forming units of SARS-CoV-2, and divided into 2 groups of 8 pigs housed in separate cubicles. Two naive pigs were introduced into cubicles with inoculated pigs at day post-inoculation (DPI) 10. Every other day following DPI 3, pigs were physically examined, samples were collected (blood; rectal, oral, and nasal swabs; and nasal washes), and two pigs were chosen for euthanasia and necropsy. Group oral fluids were collected from shared rope chew toys daily. SARS-CoV-2 RNA was detected from swab, wash, and tissue samples using PCR targeting the envelope, spike, and RNA-dependent RNA polymerase genes. Viral isolation was attempted from samples via detection of cytopathic effect on Vero E6 cells. In situ hybridization was performed on necropsied tissues to detect SARS-CoV-2. Blood serum samples were tested for neutralizing antibody titers with a plaque reduction neutralization test.
Summary of Main Findings
Following inoculation, all pigs developed mild ocular discharge, with nasal secretions in some animals, lasting until DPI 3; temperatures remained normal, pigs did not develop serious respiratory symptoms, and blood chemistry readings were normal. SARS-CoV-2 RNA was not detected in any individual oral, nasal, or rectal swabs collected throughout the study. Viral RNA was detected in group oral fluids from a shared cotton rope chew toy in one cubicle at DPI 3, and from nasal washes from 2 pigs in the other cubicle at DPI 3; live virus was not detected in these samples via cytopathic effect or increases in viral RNA following cell culture). Live virus was detected in the submandibular lymph node of 1 additional pig at DPI 13 following necropsy; this individual had shown nonspecific clinical signs, including coughing and depression between DPI 1 and 4. No positive PCR results were obtained from any other necropsied tissue or blood samples and there was no evidence of virus transmission from inoculated pigs to naïve, in-contact pigs within the same cubicles. Antibodies were detected in serum samples from 2 animals between DPI 11 and 15 and were detected in oral fluid samples at DPI 6. One pig (not positive by PCR) had weakly neutralizing antibodies at DPI 13 and 15. A second pig (also not PCR positive) had reactive antibodies at DPI 11 and 13, but with no neutralization activity. In total, 5/16 (31%) animals showed some evidence of SARS-CoV-2 infection.
In contrast with previous experimental infections of pigs with SARS-CoV-2, the authors were able to detect shedding of viral RNA in nasal washes, isolate and sequence live virus from tissue samples, and detect antibodies in inoculated animals. Nevertheless, the results largely confirm that pigs have low susceptibility to SARS-CoV-2.
Pooling of oral fluids from the chew toy limits the ability to determine how many animals were shedding viral RNA at any timepoint. None of the animals with evidence of infection showed a consistent response to infection, e.g., viral shedding (as RNA or live virus) followed by seroconversion, so it is difficult to ascertain if pigs are competent hosts of SARS-CoV-2 or if infection is transient. All animals tested were of uniform age and only one viral variant and dose were tested, so future work may explore the effects of varying viral dose and isolate, or variations in pig age, breed, and source colony. Finally, the authors did not test for the presence of other porcine betacoronaviruses in animals that may have affected results, e.g., cross-reactivity of antibodies.
Since a great number of pigs are farmed worldwide (>1 billion animals slaughtered per year), there has been concern that SARS-CoV-2 could become established in pig populations. This study and others like it have established that circulation of SARS-CoV-2 among pigs is unlikely.
This review was posted on: 5 February 2021