Starting in 2020, the global coronavirus pandemic brought the threat posed by zoonotic diseases into sharp focus in the public mind. But across the tropics and beyond, scientists had long warned that shrinking habitats and the narrowing distance between human populations, livestock, domestic animals and wildlife was increasing possible points of contact, risking the spillover and spread of disease between species.
That spillover is a two-way street — just as likely to flow from the wild to humans, as from humans to the wild. Earlier this year, a study published in the European Journal of Wildlife Research reported the infection of a wild leopard cub in India with SARS-CoV-2. Found dead, likely killed by another wild cat, the cub showed “typical signs” similar to an infection in humans, says study author and wildlife pathologist Gaurav Sharma.
The virus in the leopard cub was analyzed and found to be the Delta variant.
Sharma and his team tested other animals in the area but found no other positive cases, leading to the conclusion that the cub’s infection was an isolated incident. Nonetheless, the researchers raised a warning that such infections should be monitored in the wild.
“At this particular point of time, we don’t believe that this is a reservoir host,” said Sharma. “What we understand is that this is the case of spillover infection.”
With the astronomical expansion of human society into the wild areas of the Amazon, Congo Basin and Southeast Asia, all biodiversity hotspots, it is now increasingly clear that such disease jumps between species pose not only an acute threat to humanity, but also to the world’s wild creatures — including felids.
Chasing The Tail Of Disease
Wild felids in tropical regions around the planet, even when separated by oceans, often share similar threats today: habitat loss or disturbance, hunting, and human-wildlife conflict. Diseases are an additional, more recently recognized, but serious threat, say researchers, but one that does not garner sufficient attention.
For Deborah McCauley, a wildlife veterinarian and executive director of the Veterinary Initiative for Endangered Wildlife (VIEW), headquartered in Bozeman, Montana, domestic animal disease transmission to wild animals is the most “underrecognized conservation threat today.” Among the animal families at risk are wild felids — including some of the world’s most iconic species, such as the tiger (Panthera tigris), along with lesser-known small felids, like the fishing cat (Prionailurus viverrinus) that fly under the conservation radar. Small cats in particular can be at greater risk of disease transmission as many share spaces, and interact more, with domestic animals, say experts.
As agricultural lands and settlements push up against forest edges, and as new roads cut deep into forests, opportunities for the transmission of pathogens grows. Livestock, domestic cats and dogs, and human populations are all possible sources of pathogens.
But while scientists know that spillover is taking place, tracking the tail of disease through dense tropical forests is a daunting challenge, requiring wildlife detective work conducted against a background of limited funding and a lack of facilities, say experts.
Earlier this year, for example, researchers found that domestic cats shared viruses with wild felids living around oil palm plantations in Malaysian Borneo. Species including the Sunda clouded leopard (Neofelis diardi) and the leopard cat (Prionailurus bengalensis) tested positive for feline coronavirus, feline panleukopenia virus, and feline calicivirus antibodies. Free-ranging domestic cats, kept on plantations for pest control, also carried these viruses, raising concerns about health impacts to Borneo’s animals.
Across the Pacific, in Brazil, another study released earlier this year found that domestic, free-ranging dogs that wander cocoa-growing agroforestry lands are riddled with parasites, posing a potential, but still unresearched, health threat to a range of wildlife, including felids. “Studies on the effects of helminth parasitism [worm-like parasites] on free-ranging wild animals are still quite scarce and this is already a big concern,” said Sandy Silva, a researcher at the Federal University of Pará who was part of the study team.
“Considering that cacao agroforestry areas are used by wildlife and highly frequented by domestic dogs, we can suggest that the contact of these animals with the parasites may be increased, endangering their health,” Silva continued.
But discoveries like these in India, Borneo and Brazil only offer a hint as to the possible range and depth of pathogen infections among wildlife. Further studies are needed to plug major knowledge gaps regarding transmission and health effects on wild species, the researchers said.
Hunting Deadly Needles In A Tropics-Wide Haystack
Establishing possible points of contact, and determining whether viruses are actively circulating, is difficult but only part of the challenge facing investigators, says Sonia Hernandez at the University of Georgia, who carried out a study in Costa Rica. In that particular case, viruses of concern, such as canine distemper, were shown to be present among domestic dogs and cats living on the edge of a protected area. Wildlife, including felids, living on the fringes of the protected area, or venturing out of it, may be at risk of contracting these viruses. How the pathogens may then impact these wild populations remains unclear, but is a cause for concern.
“There’s lots of things that have to come together for spillover to occur,” Hernandez explained. Susceptibility to any pathogen is key, she noted, but can be difficult to prove. And to verify transmission, it’s necessary to trace exposure of the pathogen from species to species. Even then, transmission can move either way, between domestic animals and wildlife, or vice versa, or even traveling both routes as in the case highlighted at the start of this article, with the COVID-19 virus possibly jumping from bats to people in China, then spreading and mutating around the world, to infect a leopard cub in India.
Elsewhere, researchers in Ecuador identified three pathogens common among domestic species (canine distemper virus, feline leukemia virus, and feline immunodeficiency virus) in ocelots (Leopardus pardalis) from the country’s western coastal region and kept at a rehabilitation center in the city of Guayaquil. The testing offered a useful snapshot of the viruses these cats have been exposed to, but yielded little other information, such as where they came into contact with the pathogens, said Ricardo Villalba-Briones, lead author of the study and a wildlife biologist at the ESPOL polytechnic in Guayaquil. The findings also offered little clue as to the degree of disease spread in the wild.
To truly understand the risks posed to wild species, testing is required on a wide scale, with wild, free-ranging animals, Villalba-Briones said, but a lack of funding prohibits this. Another Ecuadoran study identified domestic animal diseases (sarcoptic mange and canine distemper) in a wild coati (Nasua narica), an animal from the racoon family, with potential origins found in a free-ranging dog, which are abundant in the country’s coastal areas. This finding raises the question as to how much disease spread occurs from one wild species to another.