Diseases transmitted from humans to animals endanger conservation efforts and can also become a health problem for humans through retransmission. Researchers have reviewed published studies on the subject and identified nearly 100 cases in which human pathogens have been detected in animals. Such so-called spillbacks were discovered particularly frequently in primates and large zoo animals. The authors assume that many other transmission events were overlooked. Artificial intelligence could help predict high-risk species in the future.
There is a high probability that the SARS-CoV-2 virus jumped from bats to humans. Many other infectious diseases such as swine flu, bird flu, SARS and MERS are so-called zoonoses, i.e. pathogens that have been passed on from animals to humans. Especially in view of the Covid 19 pandemic, this transmission route has received a great deal of public and scientific attention. However, the reverse direction could also be a cause for concern: pathogens can also be transmitted from humans to previously unaffected animal species.
Cases in zoo animals and primates
A team led by Anna Fagre from Colorado State University in the USA has now looked into this in more detail. “We scoured the literature to find out how the process has manifested itself in the past,” says co-author Gregory Albery of Georgetown University in Washington DC In 97 published studies, the researchers found reports of human pathogens being transmitted to animals became.
Almost half of the incidents detected involved animals in captivity, particularly in zoos. However, because veterinarians keep a close eye on the health of zoo animals, they are more likely to detect a human-borne viral disease in their charges than in animals in the wild. In 57 out of 97 cases, the affected animals were primates. On the one hand, this is obvious because apes are among the closest relatives of humans and the leap for viruses is therefore smaller than to evolutionarily more distant species. On the other hand, great apes in particular are monitored particularly carefully – both in zoos and in the wild.
Effects on animals and humans
“This supports the idea that we’re more likely to discover pathogens where we put a lot of time and effort into searching, with a disproportionate number of studies focusing on charismatic animals in zoos or in close proximity to humans,” says Fagre. “This raises the question of what cross-species transmission events we may be missing and what this might mean not only for public health but also for the health and conservation of the infected species.”
In their review, the authors explain various scenarios of how the transmission of human viruses can affect animals. On the one hand, it is conceivable that the transmitted virus leads to a high burden of disease and many deaths in the affected animals – a major problem for species protection efforts. On the other hand, it is possible that the virus will find a new reservoir in the animal species and possibly even form new, worrying variants. Whether it makes the animals visibly ill or not plays only a subordinate role. This scenario, in turn, poses a problem for human health: Because even if it is possible to limit the spread of such a virus in human society, for example through increased hygiene and other public health measures, the pathogen can return from animals to humans transferred – a so-called “secondary spillover”.
Predictions thanks to artificial intelligence
Based on the literature evaluated in the current study, this risk is present but not very widespread. “Relatively few documented examples resulted in morbidity and mortality in the animals, and very few resulted in retention of a human pathogen in a new reservoir or subsequent ‘secondary spillover’ back to humans,” the researchers write. In order to be prepared for future cases, however, it is important to be able to predict plausible transmissions and their possible effects. Artificial intelligence, which draws its conclusions on the basis of a solid research basis, can help here.
The first results in this area are promising. Based on published predictions of which animal species could become infected with Sars-CoV-2, the scientists were more often right than wrong. “It’s very gratifying to see the payoff of sequencing animal genomes and understanding their immune systems,” says Albery’s colleague Colin Carlson. “The pandemic gave scientists a chance to test some forecasting tools, and it turns out we’re better prepared than we thought.”
Monitor wildlife health
For other diseases, however, it is important to generate further knowledge on which such predictions can be based. “We are monitoring Sars-CoV-2 more closely than any other virus in the world, so we can detect when a spillback is occurring. In other cases, where we don’t have that much information to work with, it’s been much more difficult to reliably assess risk,” says Carlson. It is therefore important to monitor the health of wild animals over the long term, says Fagre. “By observing closely, we can detect these cross-species transmission events much faster and act accordingly.”
Source: Anna Fagre (Colorado State University, USA) et al., Ecology Letters, doi: 10.1111/ele.14003