A study of livestock exposure to the SARS-CoV-2 virus

By Sreetama Dutt M.Sc.Nov 14 2021

The severe acute respiratory disorder coronavirus (SARS-CoV-2) virus, the causative agent of COVID-19 disease, is thought to spread from animals to humans in a process known as zoonosis. Non-human primates, felids, canids, and mink are naturally affected by the virus. Experimental infections have also proved the susceptibility of SARS-CoV-2 infection in a variety of other species, including wildlife and laboratory animals.

The coronavirus is thought to have originated from wildlife, and possibly introduced to humans in a live animal market in Wuhan, China, in December 2019. However, there is still no definitive scientific consensus on the species of origin.

While bats are a likely source of this emerging virus given the sequence similarity to other bat coronaviruses, experimental studies in bats thus far do not confirm this theory. Therefore, the reservoir(s) of SARS-CoV-2 are yet to be identified, and scientists must resort to serosurveillance and experimental infection studies to elucidate possible reservoir hosts.

Researchers recently published reports on pilot studies evaluating the susceptibility of common domestic livestock (cattle, sheep, goat, alpaca, rabbit, and horse) to intranasal infection with SARS-CoV-2 in the journal Emerging Microbes & Infections.

Study: Susceptibility of livestock to SARS-CoV-2 infection. Image Credit: NIAID

About the study

Researchers inoculated animals from representative livestock species (cattle, sheep, goats, alpaca, rabbits, and one horse) intranasally to monitor for clinical disease, collect samples for viral shedding (nasal/oral, rectal), measurement of viral titers in respiratory organs from acute-stage necropsies, and determining antibody production over the course of one month in most species.

Baseline serum samples were obtained and screened for existing antibodies using plaque reduction neutralization tests (PRNTs). All animals were seronegative at the onset of the study.

The animals were then intranasally inoculated with SARS-CoV-2 virus strain 2019- nCoV/USA-WA1/2020 cultured in Vero cells. Thermal microchips were used to evaluate body temperature for the duration of the studies, and nasal and rectal swabs were collected on days 1-7 for all animals except rabbits, from which oral and rectal swabs were collected, and alpacas, from which only nasal swabs were obtained.

Virus isolation from swabs and tissues was attempted using plaque assays on Vero, and real-time RT-PCR was performed on samples and tissues, three days post-inoculation. Tissues (turbinates, soft palate, mandibular lymph node, trachea, lung, heart, liver, spleen, kidney, small intestine) collected at day three post-infection were also fixed in formalin for histopathological evaluation by a veterinary pathologist. Terminal sera were tested for virus-neutralizing antibodies by plaque reduction neutralization test (PRNT).

Results showed that the animals that shed detectable infectious virus during the course of the study. One calf, two goats, and one rabbit had RT-PCR positive nasal and/or oral swabs, suggesting that these animals could be minimally permissive to infection. The live virus was isolated from the trachea of one calf necropsied on day three post-infection. Still, no other tissues were positive in that animal, suggesting local upper respiratory tract infection during acute infection. The single horse used in this experiment did not shed virus, nor was an infectious virus detected in any organs at the time of necropsy. Therefore, serology was not performed on the horse.

Interestingly, while several animals developed low-level neutralizing antibodies within 14 DPI, the majority were seronegative by 28 DPI. None of the animals necropsied on 3 DPI had histopathological lesions consistent with SARS-CoV-2 infection, including the single calf with the infected trachea. Furthermore, none of the animals displayed any clinical signs of disease or fever following inoculation.

Implication

The results from this study were consistent with other livestock studies demonstrating low-level viral replication in pigs, cattle, and rabbits. Such studies determining the host range, pathogenesis, and transmissibility of an emerging pathogen are crucial in understanding the epidemiology of the resulting disease, and target surveillance and mitigation efforts.

Furthermore, they help in risk assessment of zoonosis (infection of humans by animals) and reverse zoonosis (infection of animals by humans) of those species that are in close contact with humans.  This also helps in understanding the response of domestic animals to SARS-CoV-2 infection and what risk these animals might play in leading to more human exposure to the virus.