In a recent study published in MDPI, researchers surveyed the prevalence of severe acute respiratory syndrome coronavirus-2 (SARS-CoV-2) infection in Swiss stray cats.
SARS-CoV-2 has spread globally since its initial detection in Wuhan late in 2019. Its origin is still unclear, with several theories speculating the involvement of animals and human infections as the most likely consequence of one or multiple spillover events. Hence, animal surveillance should be conducted.
In the context of cats, their susceptibility to SARS-CoV-2 has been experimentally determined in several studies, and natural infections are primarily suspected as a result of human-to-animal transmission. Cats might be susceptible because of the angiotensin-converting enzyme-2 (ACE2)-containing residues that bind to SARS-CoV-2.
According to one study, the risk for SARS-CoV-2 infections in domestic pets living in households with documented COVID-19 was 8-fold higher than in pets living without SARS-CoV-2 exposure. In Switzerland, stray cats do not come in contact with humans. However, they sometimes live close to humans and are fed by humans or might come in close contact with domestic (owned) cats.
Although the stray cats live solitarily, they often gather in places where food is available. Thus, compared to domestic cats, stray cats are at an increased risk of contracting and maintaining infections among their populations, thereby becoming an infectious reservoir.
The current study investigated active SARS-CoV-2 infections reflected by the viral ribonucleic acid (RNA) shedding among Swiss stray cats. Saliva swabs of 1,405 stray cats were collected between February 17, 2019, and August 6, 2021, and analyzed using reverse transcription-quantitative polymerase chain reaction (RT-qPCR). The authors set February 20, 2020, as the cut-off date delimiting pre-COVID-19 and pandemic periods.
The animals were classified according to their health (healthy or sick), age, and pregnancy status. Cotton swabs were used to collect saliva, and veterinary technicians or veterinarians anesthetically sampled many cats. Hanks’ balanced salt solution (HBSS) without calcium and magnesium chlorides was added to every sample and heated, followed by centrifugation.
Total nucleic acids (TNA) were extracted using MagNA Pure LC TNA high-performance kit. Phosphate-buffered saline (PBS) free of Ca+2 and Mg+2 ions was used as the negative extraction control. All samples were subject to RT-qPCR screening for the envelope (E) gene of SARS-CoV-2. Inconclusive or positive samples were further screened for RNA-dependent RNA polymerase (RdRp) with RT-qPCR assays.
Each analysis included a positive control (RNA Wuhan RdRp-E-N), negative extraction control (PBS), and negative control (DNase and RNase free water). An RT-qPCR test was defined as positive if the cycle threshold (Ct) was equal to or less than 38, questionable if it ranged from 38 to 45, and negative if higher than 45. Two positive E and RdRp results were needed to adjudge a sample positive for SARS-CoV-2 RNA.
Among the 1,405 cats, 523 were assigned to the pre-COVID-19 cohort and the remaining to the COVID-19 cohort. Female cats (56.9%) outnumbered male cats (42.2%). In the COVID-19 group, about 56% were adult cats and less frequently reported sick. There were more healthy cats (1,098) than sick cats (234). In the COVID-19 cohort, 734 cats were healthy compared to 364 in the pre-COVID-19 cohort. The health status of 15 cats in the COVID-19 cohort and 58 in the pre-COVID-19 cohort was undetermined.
Nonetheless, when sick, respiratory symptoms were more frequent in the COVID-19 cohort (6.3%) than in the pre-COVID-19 cohort (2.7%). Twenty-one swab samples, i.e., 11 from the pre-COVID-19 cohort and 10 from the COVID-19 cohort, tested positive for the SARS-CoV-2 E gene. No sample was positive for both the E and RdRp genes, and hence none of the cats were SARS-CoV-2-positive based on the criterion established by the researchers.
Additionally, samples were tested for feline leukemia virus (FeLV), a retrovirus that infects domestic cats and other related wild cats. Several tested samples were positive for FeLV RNA.
The current study assessed the prevalence of active SARS-CoV-2 infection in stray cats and noted that all samples were negative for SARS-CoV-2 RNA, suggesting that the SARS-CoV-2 spread across cats was limited. Hence, no conclusion could be drawn regarding the risk of SARS-CoV-2 infection among stray cats.
As FeLV RNA was detected in multiple swab samples, the researchers reported that sample collection, shipping, processing, and analyses were suitable for detecting RNA shed from the different viruses in the feline saliva despite the biologically distinct differences between SARS-CoV-2 and FeLV.
Notably, samples were not collected from all Swiss cantons and were not distributed evenly across Switzerland. Cantons like Valais, Geneva, and Vaud, where SARS-CoV-2 incidence was higher in humans, were not sampled, and the chances are that stray cats from these places could have tested positive, and future studies should include such regions.