The idea that coronavirus disease 2019 (COVID-19) is a zoonotic disease is very popular. The theory that the first case arose from transmission from a bat in a wet market in Wuhan gained massive media attention in the first months of the pandemic. While other theories have received more attention as there is no denying that the disease has shown several zoonotic events since then. Severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) and variants have been identified in mink, several rodents and several zoo animals, including cougars, gorillas and snow leopards. In a paper published in New infectious diseases, researchers from the University of Veterinary Medicine Hanover have studied the spread of the disease in domestic cats in Europe.
Study: SARS-CoV-2 specific antibodies in domestic cats during the first COVID-19 wave, Europe. Image credit: TanyaPhOtOgraf / Shutterstock
Many vaccines and treatments for the disease target the SARS-CoV-2 peak protein. It is crucial for the pathogenicity of the organism. The nail protein is a multi-copy trimmer that covers the surface of the virus and is formed by two subunits. S1 contains a receptor binding domain (RBD) that can bind to several receptors, including angiotensin-converting enzyme 2 (ACE2), to allow viral cell penetration. The N-terminal domain of the S2 subunit is responsible for membrane fusion.
The researchers collected a total of over 2000 samples from cats in Europe. Between April and June 2020, 1136 samples were collected from Germany, 331 in the United Kingdom, 333 in Italy and 360 from Spain. About 1800 of these came with information about the cat’s age, with an average age of 11 years. Twenty-five pre-pandemic feline serum samples and 25 feline specimens were confirmed to be positive for feline coronavirus.
The researchers used a plaque reduction virus neutralization test (VNT) to measure seroconversion by analyzing the rate of virus neutralization. They developed an indirect ELISA to detect RBD antibodies by replacing the anti-human IgG conjugate with an anti-cat IgG conjugate to further validate these results. The performance of this ELISA was evaluated by Pearson correlation, and VNT was analyzed by Gaussian distribution assays.
In total, SARS-CoV-2 seroprevalence was around 4.2% in Germany, 3.3% in the UK, 4.2% in Italy and 6.4% in Spain. Both tests revealed similar results, with VNT showing a total of 96 positive samples and ELISA 92 positive samples. The Gaussian distribution analyzes showed a strong correlation between the sensitivity of the new tests. Both controls showed negative results for all samples in both tests, confirming the specificity of the assays.
ELISA showed higher sensitivity and specificity than VNT. Still, VNT proved to be better at detecting low-titer positive samples – probably because it can detect a wider range of virus-neutralizing antibodies. In contrast, ELISA can only detect anti-RBD antibodies.
While most COVID-19 infections in cats appear to have a mild or asymptomatic course and there is little evidence of common cat-to-human infection, it is likely that the disease was first transmitted to cats through a zoonotic event from humans. Further investigation of the transmission of COVID-19 in animals and zoonotic events is important as animal populations may constitute a reservoir for the disease. In other animals, such as mink, the disease that spreads between humans and mink is quite common, and there are signs that the disease can infect other primates as well.
The authors highlight that their tests show very similar results and suggest that further testing for WT SARS-CoV-2 in cats can be performed using their homemade ELISA, as there is no need for a live virus, avoiding the requirement for a biosafety level. 3-laboratory. Furthermore, they suggest that in order to prevent cross-infection between humans and domestic cats, infected individuals should maintain social distance from their cats, wear masks when in the same room, and generally avoid close contact. The researchers admit that since all samples were sent to veterinary diagnostic laboratories for conditions other than COVID-19, they may not fully represent the European cat population.
However, this study provides valuable insight into the spread of SAS-CoV-2 among cat populations and provides tools for further studies. The same concept used for their ELISA could also be adapted to study the spread in other animals that might be more likely to show cross-infection with humans. As vaccination rates in the United States and the United Kingdom stagnate, it appears that the disease is likely to last significantly longer than originally thought. Understanding the zoonotic effects and spread among livestock can be extremely important in minimizing future outbreaks.