Zoonosis is the transmission of pathogens from animals to humans. Throughout history, there have been many outbreaks stemming from viruses and bacteria that jumped from animals to humans, including the Ebola virus, severe acute respiratory syndrome (SARS), the Middle East respiratory syndrome (MERS), rabies, swine flu, and the current severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) pandemic.
One of the emerging viral infections is swine flu, specifically the AH1N1 or the Influenza A virus subtype H1N1, which caused a worldwide pandemic in 2009. The 2009 swine H1N1 pandemic, which was responsible for 17,000 deaths worldwide, originated in pigs from Mexico.
Now, a team of researchers at Kansas State University aimed to determine if pigs are potential carriers of SARS-CoV-2, the virus that causes the coronavirus disease (COVID-19).
The team wanted to determine the susceptibility of animal species to SARS-CoV-2 due to the potential for interspecies transmission and zoonosis.
The study, which has not yet been peer-reviewed and appeared on bioRxiv*, highlights the ability of SARS-CoV-2 to replicate in porcine cell lines, establish infection in domestic pigs, and transmit to co-housed naïve sentinel pigs.
The coronavirus spread
SARS-CoV-2 infection can lead to respiratory disease in humans. The severity of the illness varies from mild to severe, and in some cases, even death.
SARS-CoV-2 is rapidly transmissible via contact with infected respiratory droplets and can be transmitted even by those who do not have symptoms. To stem the spread of the virus, countries have imposed social distancing policies, which led to significant economic and social effects.
Since the emergence of the novel coronavirus in December 2019, scientists have raced to develop effective vaccines and therapeutics to combat the disease spread, which will need standardized preclinical animal models.
Though the origin of SARS-CoV-2 is unknown, evidence shows that it came from a zoonotic spillover event, with bats and pangolins as potential origin specie and vector. Also, the likelihood of a reverse zoonotic event, wherein humans transmit the virus to animal species, is possible. Hence, it is a significant concern for animal and public health.
Previous reports have shown that domesticated animals, like cats and dogs, wild animals like tigers and lions, and farm animals like minks, have contracted the SARS-CoV-2 from humans. Therefore, looking into which species are susceptible to infection is crucial.
The study findings
To arrive at the study findings, the research team obtained 18 pigs from a source guaranteed free of the swine flu virus, porcine circovirus, and porcine reproductive and respiratory syndrome infection. The pigs were acclimated for three days before being exposed to SARS-CoV-2.
The team divided the pigs into two groups – the experimental and control groups. The nine pigs were housed in the same room in two separate groups, and they were infected with SARS-CoV-2 orally, intranasally, and intratracheally, after being anesthetized.
The team observed the pigs for the development of clinical disease by assessing their overall activity and attitude, including signs of depression, unresponsiveness, and reduced alertness. The team also observed for signs of illness, including lack of appetite, sneezing, labored breathing, coughing, and nasal discharge, and digestive signs, like vomiting and diarrhea. Further, blood samples, rectal swabs, and nasal and oropharyngeal swabs were obtained for testing.
The team has found that though SARS-CoV-2 was able to replicate in two different cell lines with cytopathic effects, none of the infected pigs showed evidence of clinical signs of the infection, SARS-CoV-2-specific antibody responses, and viral replication. Also, none of the pigs manifested any markers for SARS-CoV-2 infection.
“Pigs are considered to be an excellent model for studying human infectious diseases based on their relatedness to humans in terms of anatomy and immune responses, and they are much more predictive for the efficacy of therapeutics when compared to rodent models,” the team wrote in the paper.
“However, the results presented here indicate that pigs are not a suitable preclinical model for SARS-CoV-2 pathogenesis studies and the development and efficacy testing of therapeutics or vaccines,” they concluded.
The coronavirus pandemic has now infected more than 21.82 million people worldwide, taking over 773,000 lives. The pandemic has impacted 188 countries and territories and has significant morbidity, mortality, and economic consequences.
bioRxiv publishes preliminary scientific reports that are not peer-reviewed and, therefore, should not be regarded as conclusive, guide clinical practice/health-related behavior, or treated as established information.