Kimmy Ye and Peyton Waddicor | Published 16 April 2020
Historically, many of the most devastating pandemics humanity has faced have originated from animal hosts. The past three major flu pandemics were derived from avian or swine populations. The current SARS-CoV2 outbreak appears to have jumped from an animal population, possibly from bats to humans, or bats to pangolins and then to humans.
Why is there a historical pattern of highly virulent diseases passing to humans from animals?
At the moment, researchers are still looking for mechanisms that allow diseases to cross species barriers. However, we know that zoonotic viruses are often more diverse than those that only circulate in human populations. A virus gains the ability to infect humans when it has mutated to the point of being unrecognizable to our immune systems. Viruses that are foreign to the immune systems of the entire population are even more dangerous, as humans do not have herd immunity against this virus. Herd immunity describes the ability of a community to have a sufficient number of individuals who are immune to the virus and thus protected from infection due to their own production of specific antibodies that recognize the disease.
Zoonotic viruses have an increased likelihood of being foreign to large human populations. When animals act as a reservoir for viral mutation, the virus can mutate without ever interacting with the human immune system. Therefore, since the diseases of zoonotic sources are unknown to human immune systems, entire communities can become infected at once. As such, social distancing represents a key preventive measure for such diseases by limiting the number of people that are exposed to an infected individual until herd immunity is achieved or an effective vaccine or treatment is found.
Displacement of wildlife from their natural habitats due to human activity, such as urbanization and deforestation, has significantly blurred the boundaries between humans and animals and continues to facilitate the spillover of zoonotic viruses into humans. Recent outbreaks caused by viruses derived from bats(e.g. Ebola virus, Nipah virus, SARS-CoV, Marburg virus) are all associated with human activity in bat habitats.
Due to their physiology, bats have been considered viral reservoirs for a number of years. After the SARS-Cov outbreak in 2003, researchers identified a number of coronaviruses, bearing resemblance to SARS, in a population of Chinese horseshoe bats. While bats have been identified as the source of several single-stranded RNA viruses that cause illnesses in humans, the bats themselves don’t appear to get sick. One possibility for this phenomenon is an altered antiviral response. A study published in PNAS reported that altered interferon responses in bats may limit disease symptoms while simultaneously allowing for viral persistence - therefore, while the bats themselves may not become sick, they can still maintain a level of viral load that makes them contagious. Another possibility is that the reactive oxygen species accumulated in bats during long-range flights add mutagenic stress to these viruses, increasing their rates of mutations and allowing them to change quickly within the bat host. The more these viruses change, the more likely they are to gain the ability to infect hosts of different species. Such factors of bat physiology allow viruses to persist and mutate into new strains, leading to the potential emergence of zoonotic viruses.
Reported animal infection cases and studies
Recently confirmed by National Veterinary Services Laboratory in Iowa, a tiger and six other big cats at the Bronx zoo tested positive for COVID-19 -- presumed to have been infected by an asymptomatic zookeeper. The cats began showing symptoms (e.g. dry cough, reduced appetite) during late March after contact with the employee and became the first known group of animals to be infected with COVID-19 in the US. Abroad, a handful of animals have also tested positive for COVID-19 (two dogs and one cat in Hong Kong, one cat in Belgium).
According to a recent study published in Science, consistent with the results reported in Nature back in 2003 for SARS-CoV, SARS-CoV-2 can replicate in the upper respiratory tracts of cats and ferrets, while there were no viral loads detected in dogs, pigs, chickens, and ducks. Moreover, when the researchers exposed a healthy cat to an infected one, the healthy cat was infected via droplet transmission. While viruses were found in the feces of infected dogs, healthy dogs in close contact with infected dogs were not afflicted. However, it is worth noting that the sample size of this study was small, and experimental infection is not equivalent to natural infection. A manuscript accepted by Molecular Biology and Evolution theorized that dogs could represent the “middlemen” responsible for SARS-CoV-2 transmission from bats to humans, based on genetic data, though additional studies may be needed for confirmation.
Should I be worried if I have an animal in the household?
While there is no confirmed evidence suggesting your animal companion can transmit the disease to you and your loved ones, you should still be cautious until further scientific results are reported. Even if you do not show symptoms of COVID-19, you should continue to practice good hygiene during interactions with animals; this includes washing hands before and after such interactions or handling animal food, waste, or supplies. Gloves may provide extra protection, though they do not replace washing hands upon removal. Additional recommendations include separating animal food, waste, and supplies from your own food and wiping down animals’ paws before entering the house after each outing if possible.
When you are practicing social distancing with others, it is recommended that your animals also practice social distancing from other animals and humans that are not in the same household.
If you are sick, you should protect your animals by minimizing exposure, just as you would keep your distance from other human beings. If you have a service animal or must care for your animals yourself, wear a mask or cloth to cover your mouth and nose, according to American Veterinary Medical Association (AVMA). Limit physical contact with your animal (don’t share food, kiss, or hug them, and wash your hands before and after touching them).
There is no reason to remove pets from homes even if COVID-19 has been identified in members of the household unless the pet requires medical attention. According to AVMA, CDC, and USDA, routine testing for animals is not recommended (testing capacity is limited and should be conserved), though public health officials may choose to conduct such testing if the current situation changes. If you have any concerns, contact your veterinarian as well as your local public health department.