The current COVID-19 pandemic began as a zoonosis in China, at the end of 2019, caused by severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), a novel betacoronavirus with significant similarity to an earlier virus of the same family.
The virus is thought to have jumped from bats to humans, perhaps through another host such as the pangolin. During this process, it may have undergone recombination events altering the receptor-binding domain.
The infection has been shown to infect Syrian hamsters, which have 18-20 residues at the contact site of human ACE2. Now, a new study published on the preprint server bioRxiv* in August 2020 shows that deer mice are also susceptible to the infection and could serve as a new secondary reservoir for the virus, enabling it to thrive in North America in this common rodent species.
Deer mouse (Peromyscus maniculatus). Image Credit: Karel Bock / Shutterstock
Deer Mice Infection and Disease
Deer mice belong to the same family and are among the most widely distributed type of rodent in North America. Some of the 56 known species are known to harbor other zoonotic viruses, including hantaviruses and Borrelia burgdorferi. The ACE2 receptor in these mice has 17-20 residues, which prompted the investigation into their susceptibility to the virus.
The researchers inoculated 9 young male and female deer mice 6 months old with the virus intranasally. They found that these mice developed signs of lung consolidation and hemorrhage, and viral RNA was detectable. However, at day 14 after infection, there was no sign of disease in the lungs but low levels of viral RNA in the lungs.
Virus isolation was performed from the lungs of infected mice on days 3 and 6 but not day 14 from infection. Immunoglobulin G (IgG) to the recombinant viral nucleoprotein was found on day 14, but neutralizing antibody was found at low titer on day 6, with a marked increase by day 14. This is likely to have been Immunoglobulin M (IgM).
The researchers found that innate antiviral defenses appeared to be activated by the infection and declined as the virus was overcome. The presence of high levels of inflammatory cytokines indicates a pro-inflammatory type I response. Histological examination showed that the nervous system was inflamed, as well as the lungs and airways suggesting bronchial and interstitial pneumonia.
Deer Mouse Transmission
The infected mice were able to transmit the virus to other naïve mice through two successive passages, and oral swabs from infected mice were found to become positive for oral RNA on days 2 to 5, but then became negative after a few days. Later, however, they again became positive.
When the viral RNA was examined in the second-passage mice, the researchers found a four-residue insertion sequence in the N-terminal domain (NTD) of the spike protein. However, this was present at low levels in the inoculum virus, and at still lower levels in the original virus obtained from the supplier. This insertion was found in all the infected mice, showing a powerfully purifying selection for the insertion. The fact that this is located on the surface but away from the RBD may indicate the presence of an as yet unknown co-receptor, since another coronavirus, mouse hepatitis virus, uses the NTD as its RBD.
The reasons for this increase in frequency may vary, from its meeting with obstacles during transmission or because it underwent selection during the systemic spread, or because of positive selection. Further research is required to investigate this phenomenon.
Similarly, the three mice used to study transmission lost weight, but not the passaged mice, that is, those which were secondarily infected from the first three. This could be because the inoculated dose used to infect the first three was more significant, or because the wildtype virus used in these was more virulent than the passaged virus.
Suitable Small Animal Model for Human SARS-CoV-2 Infection
The study shows that deer mice are a small animal model suitable for the investigation of this disease, especially since these animals show an inflammatory response, thought to be a significant reason for the severity of COVID-19 in some patients. However, Interferon‐gamma (IFN‐γ) or Interleukin 6 (IL-6) expression was not markedly changed, which may be why the infected mice recovered without severe disease.
These mice can live four times as long as other small laboratory animals, including Syrian hamsters and laboratory mice, which allows the effects of age, immunity, and other events to be examined in the context of SARS-CoV-2. The researchers also point out, “As an outbred model, deer mice are more likely to reflect the diverse outcomes of infection observed in humans.”
Central Nervous System Infection
The involvement of the central nervous system is interesting and could explain how the virus shows both sympathetic and parasympathetic system spread. This is the reason for the dry mouth, trigeminal neuralgia, confusion, loss of smell, and loss of taste (both to varying extents) in many COVID-19 patients. If the virus spreads to the vital centers of the brain, this could explain death as well. Moreover, more studies using this model could help uncover how infection of the sympathetic ganglia in the neck leads to heart muscle injury as well as impairment of sympathetic regulation of the brain’s blood supply. This may result in a marked reduction of blood flow to the brain and skull.
Finally, until this study, the list of mammalian hosts capable of being infected with the virus have been few, including Syrian hamsters, ferrets, and minks, as well as human transgenic mice. However, wildtype laboratory mice and rats are not susceptible. In this study, all contact deer mice became infected, showing positive viral RNA in just two days, while with inoculated mice, viral RNA became positive in 21 days, indicating sustained viral spread in the species even in the wild.
Moreover, since deer mice and Syrian hamsters both belong to the cricetid rodents, this could lead to recombination within the coronavirus group. Thus, spillback events may occur, causing this species to become the secondary reservoir hosts for SARS-CoV-2 in North America and allowing the virus to become endemic in this continent as well.
The researchers sum up, “Collectively, this work has determined that deer mice are a suitable animal model for the study of SARS-CoV-2 pathogenesis and that they have the potential to serve as secondary reservoir hosts that could lead to periodic outbreaks of COVID-19 in North America.”
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.