- Researchers warn that disease spillover from livestock and domestic animals represents a serious conservation threat to wildlife, including felids in tropical areas around the world. Spillover is most likely to occur on rapidly advancing forest-agricultural frontiers or within fragmented habitats.
- Tracking the spillover and spread of diseases from humans and domestic animals to wildlife is extremely challenging, particularly among wild felid species, which tend to be secretive and solitary, making ongoing observation difficult.
- Possible cases of disease spillover have been documented in wild cats in India, Malaysian Borneo, Thailand, Brazil, Ecuador, Costa Rica, Russia and Nepal. These are likely the tip of the iceberg, say scientists, who believe much disease among wild species is going undetected, with case numbers and outbreaks unknown.
- Scientists stress the need for greater health monitoring of wildlife to reduce this “invisible threat.” But funding for health testing is often scant, and treatment difficult. One researcher sees disease transmission from domestic animals to wildlife as perhaps the most “underrecognized conservation threat today.”
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.
“We don’t know the severity of the effects of these diseases, or their prevalence in the wild, but they are artificially increased due to this [domestic animal proximity] situation and logically will be detrimental for wildlife,” Villalba-Briones said.
Tracing spillover back to domestic animals is complex, time-consuming work, particularly when researching secretive, solitary species such as wild cats. Identifying wild cats that have died or become seriously ill due to disease is very seldom possible.
In Thailand, an autopsy of two dead fishing cats, a species considered vulnerable on the IUCN Red List, led to the discovery that they had feline parvovirus, which likely contributed to their deaths. A third fishing cat, admitted to a rehabilitation center with severe dehydration, was also found with the virus. It too later died.
In this case, domestic cats are not definitively known to be the host of the virus. Analysis, however, showed that the deadly pathogen shared similarities with viruses taken from domestic cats, and so spillover remains a possibility, stated the study’s lead author, Chutchai Piewbang, a pathologist at Thailand’s Chulalongkorn University.
Biologists worry that this smattering of detected cases from around the globe might be like a few small, observable aboveground flames, hinting at larger hidden underground fires of contagion that could ravage wild species before science even has a chance to spot them.
Upsetting nature’s balance heightens disease susceptibility
It’s now well understood that escalating multiple pressures on habitats can heighten the risks of disease transmission. In dwindling threatened populations already exposed to a variety of other threats, increased stress can reduce disease resistance and increase disease vulnerability.
“Habitat destruction and degradation can cause threats like genetic diversity loss, or facilitate domestic-wild species contact,” Irene Sacristán, a postdoctoral researcher with the Animal Health Research Center in Madrid, told Mongabay via email. “This combination could also contribute to increased pathogen infection susceptibility as well as contribute to pathogen transmission from domestic animals to wild species.”
Other environmental stressors, such as pollution, can contribute to the problem. In South America, for example, work conducted by Joares May Júnior, a wildlife pathologist with the Federal University of Rio Grande do Sul in conjunction with Panthera Brazil, an NGO, identified mercury exposure — due to gold mining — as a risk to jaguars (Panthera onca) in Brazil. More recently, the rampant burning of forest areas in the Amazon posed an additional threat.
These “accumulative problems” can potentially have a knock-on effect on immunity, May Júnior explained. “All these points could be aggressive to the animal’s immune system … and then the immunity goes down and then one virus can be a problem.”
Assessing the threat level to wild populations that are enduring multiple stressors remains a challenge. In the studies conducted in Malaysia and Ecuador, for example, the wild felids showed no clinical signs of disease. However, findings from the ocelot study indicated that some of the cats, though sick, were able to recover from exposure to the viruses circulating in the region. The difference between dormancy, disease and death can be heavily influenced by other stressors and by an animal’s overall health.
“Pathogens like retroviruses (such as feline leukemia virus or feline immunodeficiency virus), could be latent for a long period of time [before] ‘activating’ under stress conditions or other pathogen co-infection, so the fact that the animals were infected by those pathogens is a possible threat per se,” Sacristán said. It is also possible that animals experiencing clinical signs are dying in the wild before they’re found, she added.
Canine distemper virus
One pathogen of particular concern for some wild felids, say experts, is the already mentioned canine distemper virus. Despite its name, this pathogen is not isolated to canine species, infecting multiple species types. Past outbreaks in lions (Panthera leo) and African wild dogs (Lycaon pictus) have decimated both wild and captive populations. Canine distemper has also been proved a threat to tigers in Russia and India. Researchers in India, including Sharma, have also identified canine distemper virus in a range of wild felids. Sharma’s team identifies around 50 cases of the virus in both wild and domestic canines and wild felids each year, he said.
Past work by VIEW’s McCauley and her team detected a host of viruses in Bengal tigers within Chitwan National Park in Nepal. “We know that [tigers are] acutely sensitive to canine distemper disease. We know that these animals can die [from the disease],” she said.
Free-ranging dogs in the vicinity of the park carry the virus. Still, transmission does not necessarily carry over directly from dogs to tigers, as a range of species, such as civets or foxes, can act as hosts. Canine distemper can even be spread through the environment via feces.
Tracking and staying on top of individual cases of the disease is vitally important to its control in animal populations before major outbreaks occur, says May Júnior. “It’s important to follow canine distemper, even if you don’t see any kind of outbreaks.”
Preventive wildlife health and disease monitoring of this kind should be part of wider conservation efforts for wild felids and other species, says McCauley. “It’s not like you’re going out and immobilizing tigers just [to examine the state of their] health,” she explained, but any time scientists come into direct contact with animals “we can collect biological samples … or when an animal dies, we can investigate it. Including [health status data in research] is critical to the survival of our endangered wildlife.”
A dearth of funding for universal wildlife health testing remains a significant conservation barrier, leaving considerable knowledge gaps, say scientists. But even if sufficient testing money was available, scientists would still face the conundrum of effectively responding to outbreaks, says Tadeu de Oliveira, a researcher and conservationist with Pró-Carnívoros, a Brazilian conservation NGO.
Testing alone won’t solve the problem. In his view, taking action to reduce the risk of disease transmission should be paramount. De Oliviera determined that disease transmission from dogs, alongside habitat loss, is the primary threat facing the diminutive northern tiger cat (Leopardus tigrinus) found in Brazil’s Mirador National Park.
“The disease was there. Basically, we didn’t know how bad it was,” he said. “I decided to start vaccination campaigns immediately.”
Working alongside other small cat conservation organizations, de Oliveria took part in a wide-ranging domestic animal vaccination campaign earlier this year across six countries in Latin America.
“We want to show people that if they just do conservation research, or research on outbreaks, it’s not the same as doing conservation action,” de Oliviera said. “We need to take action to actually reduce threats.”
Banner image: Irene Sacristán was part of an investigation of pathogen exposure in the guigna (Leopardus guigna) South America’s smallest wild cat. Extensive sampling of the guigna across its range found that those living in fragmented forest or agricultural areas had a higher prevalence of pathogens than those inhabiting “more pristine areas,” she said. Though the direction of transmission remains unclear, the likely pathogen source is in domestic cats. Image courtesy of Jerry Laker/Fauna Australis.
Citations:
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