Study population and setting
Dairy cattle (n = 9; 4-5 months old) were tested for SARS-CoV-2 RNA in nasal, oral, and rectal swabs and antibodies against SARS-CoV-2 in serum sampled prior to the start of the study. Six animals were then inoculated intranasally with 1×10^5 50% tissue culture infective dose, and three in-contact animals were then introduced to the exposed animals 24 hours after inoculation. Researchers monitored body temperature and clinical signs of infection daily. Blood samples and nasal, oral, and rectal swabs were collected through 20 days post-infection (DPI). Swab samples were tested for the presence of SARS-CoV-2 RNA using quantitative real-time PCR (qRT-PCR) targeting the RNA-dependent RNA polymerase (RdRp) and envelope (E) genes. Blood serum samples were tested for the presence of reactive and neutralizing IgG antibodies against the SARS-CoV-2 spike receptor binding domain (RBD) at DPI 6, 12, and 20.
Summary of Main Findings
None of the animals showed clinical signs of infection; body temperatures, feed intake, and behavior were normal. SARS-CoV-2 RNA was detected in nasal swabs from one inoculated calf at DPI 2 and 3 and from another inoculated calf on DPI 3; no other swab samples were positive, including from in-contact animals. The first positive calf showed an increase in seroreactivity between DPI 6 and 20, indicating seroconversion, although neutralization was weak. The other positive animal showed some increase in seroreactivity but did not exceed the baseline set by negative controls and showed no neutralization activity; no other animals had positive antibodies. In contrast, all nine animals had neutralizing antibodies to bovine coronavirus (BCoV, another betacoronavirus), with considerable variation in reactivity among individuals and over time, indicating probable transmission of BCoV between animals during the study. This was confirmed via RT-PCR targeting the RdRp of BCoV, which indicated active BCoV infection in one animal inoculated with SARS-CoV-2 prior to the start of the study and 2 days after inoculation.
The authors examined calves for pre-existing coronavirus infection and seroreactivity of antibodies to SARS-CoV-2 and bovine coronavirus (another betacoronavirus).
The study did not describe in detail any efforts to assess the presence of live virus in swab samples, such as evidence of cytopathic effect in cattle cell culture. Additionally, only young dairy calves were tested, so the effects of age, husbandry practices, and underlying health conditions on virus susceptibility could be explored further in other experiments. The low number of SARS-CoV-2-infected cattle with evidence of concurrent bovine coronavirus seroreactivity (n = 2) is suggestive of limited cross-protection of bovine coronavirus antibodies against SARS-CoV-2, but more data will be needed to be conclusive.
Since there are over 1.5 billion cattle worldwide, often in close contact with humans on farms, there is great interest in understanding whether cattle could serve as amplification hosts for SARS-CoV-2. The results confirm previous studies that predicted a medium susceptibility of cattle to SARS-CoV-2 based on ACE2 receptor binding to SARS-CoV-2 spike protein.
This review was posted on: 3 February 2021