Although the researchers found that the rats tested negative for the virus, they say it is important to continue monitoring animals for the virus as it mutates, making previously resistant species susceptible to infection.
Most studies on emerging infectious diseases have focused on animals as virus reservoirs and how these viruses are transmitted to humans. However, many studies during the COVID-19 pandemic reported transmission of the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) from humans to animals.
Domestic dogs and cats, whose owners tested positive for the virus, were seen also to become infected. Reports also show farmed minks and other zoo animals have also contracted the virus from humans.
The susceptibility of animals to become infected with human-borne SARS-CoV-2 depends on their ecology and genetics. Suppose the animal cellular functioning allows the virus to enter and bind to receptors in the host cell. In that case, further transmission depends on how long the infected animals shed the virus and how close and frequent is the contact with other animals.
For example, in farmed minks, a species highly susceptible to the virus, the transmission was fast because they were housed indoors in high densities.
Other animals in the wild, like rats, also live in such conditions and often live close to humans. In Europe, Norway rats (Rattus norvegicus) flourish in urban areas, often living in city sewer systems. Because several reports have detected SARS-CoV-2 in wastewater, these rats may have been exposed to the virus.
However, previous studies have shown that this rodent species is not susceptible to the Wuhan strain of the virus. But, after passing through several laboratory mice, the virus has evolved the ability to infect this host because of a mutation N501Y in the spike protein receptor-binding domain (RBD). This variant has also been seen in the virus lineages circulating in humans. This suggests the virus has the ability to potentially infect hosts that were previously resistant to it, including rodents.
Testing sewer rats for SARS-CoV-2
Researchers tested Norway rats living in the sewers of Antwerp, Belgium, for SARS-CoV-2 in November and December 2020 after a peak in the cases there. They reported the results of their study in a research paper published on the bioRxiv* preprint server.
The authors trapped and tested several rats for the presence of SARS-CoV-2. They first tested the rats’ blood sera for the presence of antibodies to the virus. They also tested for viral RNA from oral swabs and in feces. Finally, they tested for the virus in the wastewater at the location where the rats were trapped.
Of the eight samples of wastewater tested, the team found SARS-CoV-2 in four samples. The sera of three out of 35 rats were positive for nucleocapsid and IgG antibodies, but the amounts were lower than control rats who were positive for the virus. Further tests of these sera for neutralizing antibodies were negative, suggesting the rats had not been exposed to SARS-CoV-2. All the swab, tissue, and fecal samples of 39 rats were negative for the virus.
The results suggest that the rats studied from the Antwerp sewers had never been infected with SARS-CoV-2 despite being exposed to the virus in the wastewater.
Monitoring animals for virus important
There was a discrepancy in the results obtained by fluorescence and by neutralization tests. The authors suggest care should be taken when using samples different from those the tests were validated for. Although the fluorescence test differentiated between the positive and negative laboratory control rats, it gave false positive values for the wild rats. This could be because wild rats are exposed to many other pathogens, and they show higher overall fluorescence values from higher antibody binding. So, to confirm SARS-CoV-2 in animals, virus neutralization tests may be better.
The authors likely did not detect the virus in the sewer rats because the tests were done before the dominance of the N501Y mutation in human lineages. The emerging variants from the United Kingdom, South Africa, and Brazil, all have the N501Y mutation, and previously resistant species like rodents may become infected.
Rodents live close to humans and in high densities, creating ideal conditions for the spread of new epidemics. Thus, although the current study results show that the rats tested negative for the virus, it is necessary to continue monitoring rats and other animals exposed to humans to detect future human to animal transmission.
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.