But a new study gives an important update on two animals close to many of our hearts that can catch Covid-19: cats and dogs.
Research in both cats and dogs revealed that neither animal developed clinical disease from the virus. But there were differences between the two species in terms of transmission.
This study, while preliminary, also provides some indication of how susceptible pet cats and dogs are to the infection.
The findings were published on Tuesday in the journal Proceedings of the National Academy of Sciences.
Pet shedding — Researchers studied whether the novel coronavirus was able to reproduce inside the animals' cells and be released into the environment — a process called "shedding."
Cats could shed the infectious virus for up to five days, and did infect other cats via direct contact. Dogs, in contrast, do not appear to shed the virus.
The feline findings suggest that "cats are highly susceptible to infection, with a prolonged period of oral and nasal viral shedding that is not accompanied by clinical signs, and are capable of direct contact transmission to other cats," write the authors of the study, led by Colorado State University researcher Angela Bosco-Lauth.
Covid-19 is caused by SARS-CoV-2, a zoonotic virus which likely originated in wild animals before it made the jump to humans. While the pandemic continues to have catastrophic impact on humans, "the implications for animal populations are largely unknown," the study authors write.
The virus has previously been reported in both cats and dogs. Even bigger cats, including tigers at the Bronx Zoo, have tested positive. And while the new findings suggest the animals do not develop clinical disease in the same way humans do, they can show symptoms like respiratory problems and decreased appetite.
Bosco-Lauth explains why that is: "While the animals in our study did not develop clinical signs of disease, it doesn't rule out individual variability and/or co-morbidities in cats and dogs which could lead to signs," she tells Inverse.
Cats as a vaccine model — Whether or not humans can become ill with Covid-19 has big implications for public health. As researchers work to learn more about how this virus works in animals, the new study offers clues for pet owners — especially people who own cats.
"Cats that were reinfected with SARS-CoV-2 mounted an effective immune response and did not become reinfected," the study authors write. "These studies have important implications for animal health and suggest that cats may be a good model for vaccine development."
It's rare for domestic animals like cats and dogs to be used as animal models in lab experiments, Bosco-Lauth says. "However, because cats are susceptible to infection, in some cases develop disease, and develop a robust immune response following exposure, they could feasibly be used for the development of vaccines for veterinary health," she says.
Previous research hints that another pet — ferrets — could also be a good vaccine model. Vaccine tests have begun in other animals, including rhesus macaques, pigs, and mice — though the most widely used animal model for Covid-19 is the golden hamster, Bosco-Lauth says.
Animals' immune systems can also inform researchers about treatments. Take the pangolin, which was once linked to Covid as a potential intermediary host. The scaly mammal can be infected with the virus, but it does not experience the extreme immune response that can make humans gravely ill, called a cytokine storm. Studying why that is the case could help researchers develop better treatments for humans.
In the new study, the fact that cats are able to mount an immune response is promising, the study authors say. They write that the resistance to reinfection "holds promise that a vaccine strategy may protect cats and, by extension, humans."
The new study is not cause for alarm if you're a pet owner.
"The risk of cat-to-human infection is considered extremely low, by us and other experts in the field, but not completely out of the question," Bosco-Lauth says.
"We would advise pet owners to take precautions around their pets if they (the owner) develops Covid-19 disease as they could certainly spread it to their pets and from there, pets could transmit to each other or potentially to other humans or wildlife."
Experts say you can keep your cats and dogs safe by treating them like other family members. Maintain social distancing from other animals when you go for walks, and keep up the hand-washing.
"Good hygiene at home, social distancing — when possible — from pets if owners are infected, warning veterinarians of possible exposure risk," Bosco-Lauth says. "These are the messages we want to get across."
Abstract: The pandemic caused by severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) has reached nearly every country in the world with extraordinary person-to-person transmission. The most likely original source of the virus was spillover from an animal reservoir and subsequent adaptation to humans sometime during the winter of 2019 in Wuhan Province, China. Because of its genetic similarity to SARS-CoV-1, it is probable that this novel virus has a similar host range and receptor specificity. Due to concern for human–pet transmission, we investigated the susceptibility of domestic cats and dogs to infection and potential for infected cats to transmit to naive cats. We report that cats are highly susceptible to infection, with a prolonged period of oral and nasal viral shedding that is not accompanied by clinical signs, and are capable of direct contact transmission to other cats. These studies confirm that cats are susceptible to productive SARS-CoV-2 infection, but are unlikely to develop clinical disease. Further, we document that cats developed a robust neutralizing antibody response that prevented reinfection following a second viral challenge. Conversely, we found that dogs do not shed virus following infection but do seroconvert and mount an antiviral neutralizing antibody response. There is currently no evidence that cats or dogs play a significant role in human infection; however, reverse zoonosis is possible if infected owners expose their domestic pets to the virus during acute infection. Resistance to reinfection holds promise that a vaccine strategy may protect cats and, by extension, humans.