Covid in the animal kingdom

Art by Tamara Rees

A pet dog in Hong Kong was first. Soon, tigers and lions at a zoo in New York City were involved. More recently, hippos in Antwerp and hamsters in Hong Kong made the news.

Today, two years since Covid-19 was declared a pandemic, the list of nonhuman species that have tested positive for the notorious coronavirus is 19 long.

A tally by the World Organisation for Animal Health (OIE) shows a menagerie of animals throughout the world — in Asia, Africa, Europe and the Americas — contracted documented SARS-CoV-2 infections.

The good news for these species is that the novel coronavirus has, on the whole, caused only mild to moderate illness and few deaths.

"It definitely primarily impacts people most severely, versus anything else," said Dr. Andrew Hennenfent, a public health veterinarian in Iowa and a consultant at the Veterinary Information Network, an online community for the profession.

"From a disease standpoint, it's been pretty benign for the animal world," said Dr. J. Scott Weese, a University of Guelph Ontario Veterinary College expert in emerging infectious diseases and also a VIN consultant.

Ever cautious, Weese added pointedly: "So far. We never know if that might change with new variants."

Early on, anything seemed possible with the brand-new virus, which the latest studies continue to suggest jumped to people from wildlife at a market in Wuhan, China. (What type of animal remains unknown.)

"For something that started as a spillover from animals, it's not surprising that it would infect a variety of species," Hennenfent said.

Scientists girded themselves from the outset. According to a review of the literature on SARS-CoV-2 infections in animals, published in October 2020 in the journal Transboundary and Emerging Diseases, experimental studies had demonstrated infection and transmission in cats, ferrets, hamsters, bats and nonhuman primates.

In time, individuals of all those species — except bats, as far as we know — became infected naturally. There's "no doubt," Weese said, that more species are likely susceptible to SARS-CoV-2 infection than the 19 on the OIE list. It's a matter of which ones happen to be in proximity to people and, therefore, tested.

That said, here's a roundup of the 19:

Dog. A Pomeranian in Hong Kong was the first nonhuman animal in the world to have a documented natural infection, in late February 2020. More cases in dogs followed elsewhere in the world. While a few died, including the Pomeranian, which was 17, Covid wasn't necessarily the cause, and most cases apparently were mild.

Of note, a veterinary cardiology team in the United Kingdom noticed a correlation between myocarditis, or inflammation of the heart muscle, and Covid infection in some patients. The findings were published in the journal Vet Record in November. Since then, the team has seen a few more cases but doesn't plan to make another published report, the lead author, Dr. Luca Ferasin, said. "I think Covid-related myocarditis is still a concern but hopefully not for long," he told the VIN News Service last week by email. "We will see how the removal of all restrictions in U.K. will change the pandemic curve."

Cat. Cats were the first pets in the United States to have confirmed infections, manifesting as mild respiratory illness. Cats with Covid may also lose their appetite, become lethargic, vomit and have diarrhea. The virus isn't necessarily highly contagious among felines: There have been instances in which only one cat in a multi-cat household tested positive. At the same time, false negative results can occur. A study published last month in the journal Viruses found evidence of infection in the feces of a cat whose nasal swab was negative.

Tiger, lion, puma, cougar, lynx, snow leopard. In zoos, big cats have been the hardest hit. That members of the Felidae family would be vulnerable was anticipated: An article in the journal Evolution, Medicine, and Public Health in July 2020 describes how a cell receptor called ACE2, which the coronavirus's spike protein attaches to, is similar in cats, ferrets and people, but not in dogs, pigs, chickens and ducks. (In the realm of food animals, cattle, too, were found in other studies to be unlikely to catch Covid.)

In late March 2020, a 200-pound Malayan tiger named Nadia at the Bronx Zoo developed a cough and lost her appetite. Following testing, she became known as the first documented nonhuman Covid patient in the U.S. All told, that outbreak encompassed five tigers and three lions. (One tiger had no outward signs of illness.)

As the pandemic raged forward, lions and tigers at other zoos in the U.S. and worldwide also became infected, as did an assortment of other big cats.

Scientists in South Africa reported an infection in a puma that occurred in July 2020 at a private zoo in Johannesburg, where three African lions also were sickened, one with pneumonia. While the animals tested positive for up to seven weeks, they ultimately "cleared the infection completely," the scientists said.

The first cougar in the U.S. to test positive was reported in early 2021 by the U.S. Department of Agriculture. The cougar was among several animals that were coughing and wheezing at a wild animal exhibit in Texas.

In December, the Pittsburgh Zoo & PPG Aquarium recorded the first case in the U.S. of a Canada lynx catching Covid.

Some cases have been fatal, at least one as recently as this year. In January, an 11-year-old snow leopard named Rilu died from Covid-induced pneumonia at Miller Park Zoo in Illinois. Two months earlier, three snow leopards died from the virus at Lincoln Children's Zoo in Nebraska.

Mink. Raised by the millions on farms for their fur, mink have been the nonhuman animal most devastated by SARS-CoV-2. Once it became apparent that they could become ill from the virus, sometimes fatally, and — most significantly — transmit the bug back to people, their fate was grim. By mid-November 2020, outbreaks on mink fur farms had been reported in the Netherlands, Denmark, Spain, Sweden, Italy and the U.S., according to the World Health Organization. The response was to kill farmed mink en masse.

Ferret. Following a case in Slovenia, the first recorded occurrence of Covid in a ferret in the U.S. occurred in Florida last September. The pet was tested because it was sneezing and coughing, according to the USDA.

Scientists had previously studied ferrets as an animal model for the disease. However, subsequent research documenting a case of 29 ferrets living closely with two people who had symptomatic disease found low risk of natural infection in the pets — none got sick. The research was part of a pet surveillance study at Tufts University Cummings School of Veterinary Medicine.

Binturong, coatimundi, fishing cat. An outbreak at Brookfield Zoo near Chicago last fall added three exotic species to the list of Covid-positives. One was a binturong, a mammal with a bearlike face and catlike body sometimes called a bearcat. A member of the viverrid family, the binturong is also known as civet cat. The second unusual animal was a coatimundi, or coati, a relative of raccoons. The third was a fishing cat, a type of cat that, in the wild, is found in wetlands.

Also affected in that outbreak were a tiger, a lion and two snow leopards.

Gorilla. One of the fears for the animal world when the pandemic began was that fellow primates would be at risk, especially animals like the endangered mountain gorilla. But it's been so far, so good on that front. In fact, the University of California, Davis, reported last month that wild mountain gorillas in Rwanda not only have not tested positive for SARS-CoV-2, they've had fewer outbreaks of any respiratory illness. The probable reason: Ecotourism was suspended early on, and other precautions were adopted to reduce gorillas' exposure to people.

Gorillas in zoos are another matter. Three at the San Diego Zoo Safari Park in California had the distinction of being the first of their kind in the U.S. to be confirmed positive for SARS-CoV-2, in January 2021. Others followed, for example at Zoo Atlanta and the Kansas City Zoo.

After the outbreak in San Diego, a variety of primates at that zoo received vaccinations against Covid. Zoetis, which developed a vaccine for animals, subsequently donated 11,000 doses to about 70 zoos and wildlife sanctuaries around the country, CNN reported.

Otter. The Georgia Aquarium reported last spring that its "geriatric" Asian small-clawed otters were sneezing, coughing, mildly lethargic and had tested positive. In the summer, a pair of otters in a zoo in Orange County, Florida, also caught the virus.

Spotted hyena. Two spotted hyenas at the Denver Zoo last fall became the first in the world among their species to develop documented cases of Covid.

White-tailed deer. One of the more attention-getting and potentially concerning finds last year was that wild white-tailed deer in parts of the United States were found through antibody testing to have been infected at high rates (although they did not appear to be sick). A few months later, evidence of the virus was found in deer in Canada, too.

A study, not yet peer-reviewed, from Texas A&M College of Veterinary Medicine and Biomedical Sciences demonstrated that captive white-tailed deer can catch Covid and transmit it to other deer.

That the virus should show up in the wild in a species as abundant as deer raises worries that the animals could serve as a reservoir for SARS-CoV-2, enabling the virus to readily spill back into the human population.

That would be a bigger worry if deer were found to have a new variant. So far, that's not the case, Hennenfent said: "To date, the strains match exactly what's in people, so it doesn't appear that it's taking on a life of its own."

Hippopotamus. Runny noses were the clue that led the Antwerp Zoo in Belgium to test a pair of hippos for Covid late last fall. According to a BBC report, "The hippos' noses are usually wet, but vets at Antwerp Zoo decided to test the pair after they noticed they were 'expelling snot.' "

Hamster. Authorities in Hong Kong ordered pet hamsters by the thousands to be turned in for culling early this year after an outbreak in people was traced to a pet store. Public health officials tested more than 100 animals at the pet store and another 500 at a warehouse that supplied the store, according to a news story in the journal Nature. They found viral RNA or antibodies to the virus in 15 of 28 Syrian hamsters. Dwarf hamsters, rabbits, guinea pigs, chinchillas and mice were negative.

Research on the incident, posted as a preprint that is not yet peer-reviewed, points to infections in two people coming from the infected hamsters. One of the people went on to infect others in their household.

That makes the hamster the second known animal, after mink, to transmit the virus to humans.

Perspective and outlook

Asked if any of the outcomes have come as a surprise, Weese said: "Pre-pandemic, I wouldn't have had zoo animals so high on my list, but it makes sense in hindsight. Hamsters as a source of human infection was a bit surprising. In terms of overall susceptibility, a good surprise has been the low susceptibility of food animals. Deer have probably been the biggest surprise. They weren't on my radar at all at the start."

The pandemic isn't over, of course, so what's true today may be different tomorrow. As humans venture back out into the world, the opportunities for more contact between people and other animals will increase, including through previously restricted activities such as ecotourism, Weese pointed out. "Human-animal contacts have inherent risk, and the more we have, the greater the risk," he said.

Big picture, Covid illustrates a consequence of humanity's heavy footprint on Earth. "The emergence of SARS-CoV-2 speaks to how humans are changing our biodiversity," observed Dr. Radford Davis, an associate professor of public health at Iowa State University College of Veterinary Medicine and a VIN consultant in zoonosis and public health.

"Deforestation for growing crops and grazing livestock, along with human encroachment into these previously intact ecosystems, bring exposure to new pathogens," Davis said.

SARS-CoV-2 is just one.

Ross Kelly and Lisa Wogan contributed to this report.