Whether Covid-19, influenza, plague or AIDS: almost two thirds of all infectious diseases in humans are zoonoses – they originally come from the animal kingdom. A team of researchers has now investigated which groups of animals and species share a particularly large number of viruses with us using the example of 142 zoonotic viruses. According to this, rodents, bats and primates contain by far most of these pathogens, which also affect humans. However, if you look at individual animal species, our domestic and farm animals share most of the viruses with us, followed by house mice and rats. Overall, the results show that the species in particular carry zoonotic viruses, which are common, have adapted to the human environment and – in the case of primates – are closely related to us.
Most of the pathogens that cause our infectious diseases – whether viruses, bacteria or parasitic protozoa, first developed in an animal and then spread to humans. This becomes possible if, for example, the pathogens develop mutations that enable them to attack this new host. In emerging diseases, the so-called emerging diseases, the proportion of such zoonoses is as high as 75 percent. They occur above all where people and animals live in close contact – for example in farm animal husbandry, in the trade and sale of wild animals or in hunting. It also becomes dangerous wherever we humans invade previously untouched areas or destroy the habitat of wild animals. Because this creates new contacts between humans and wild animals, which also give pathogens the chance to skip.
Rodents, bats and primates
Christine Johnson from the University of California at Davis and her colleagues have now investigated which animals are particularly risky because they contain zoonotic viruses. For their study, they evaluated data on the distribution of 142 virus species that are common in humans and in the animal kingdom and determined which animal species carry how many of these zoonotic viruses. In addition, they investigated the role that the global frequency of an animal group, its risk status and the type of risk play for the amounts of the viruses spread by these animals. “The transfer of fours from animals to us is a direct result of our actions towards wildlife and their habitats,” explains Johnson. “These actions threaten the survival of the animal species and at the same time increase the risk of a spillover.” In addition to wild animals, the researchers also included pets and farm animals in our analysis, because we humans have the closest contact with them.
The evaluation showed that only three groups of animals contain more than 75 percent of all zoonotic viruses described so far: rodents, bats and primates. They are followed by ungulates and carnivora predators. The dominance of rodents, bats and primates among the potential virus carriers is no coincidence for several reasons, as the researchers report. On the one hand, these groups are among the most species-rich and individual-rich among all mammals: “These orders together represent 72.7 percent of all terrestrial mammals,” said Johnson and her colleagues. On the other hand, many representatives of rodents and bats in particular are cultural followers: they live in the vicinity of human settlements, in cities and also in agricultural areas. “This enables these animal species to have direct and indirect contact with similarly adapted wild animals, with domesticated animal species and also with humans,” the scientists explain. Bats are also considered to be reservoirs of particularly virulent viruses – the Sars-CoV-2 coronavirus also originally developed in a bat. Primates, on the other hand, are closely related to humans, which can make it easier for viral pathogens to jump over the species barrier.
Pets and farm animals share most viruses with us
In a further step, Johnson and her team examined which individual animal species carry the most zoonotic viruses. It was found that the average virus diversity in domesticated domestic and farm animals was 19.3 virus types on average, whereas in wild animals it was only 0.23 viruses. “The top ten mammal species with the highest number of viruses shared with humans include eight domesticated species: pigs, cattle and horses each with 31 zoonotic viruses, sheep with 30 virus types, dogs with 27, goats with 22, cats with 16 and camels with 16 zoonotic viruses, ”the researchers report. “The only wild animals among the top ten were the house mouse and the migration rate with 16 and 14 zoonotic viruses, respectively.” Both rodents are close cultural followers of humans and are also kept as laboratory animals and pets around the world. The central role of the domesticated animal species is also supported by a network analysis in which Johnson and her team visualized the distribution of the various viruses across the animal species. This showed that the domestic and farm animals have a central position in this viral transmission network. “Viruses in domesticated species are not only more commonly shared with other domesticated species, but also with wild animals,” the researchers explain.
In contrast, rare and acutely endangered animal species are at the other end of the spectrum of potential virus carriers. Up to now, they have hardly known any potentially transmissible and shared viruses, as the scientists report. However, they also emphasize that this can be due in no small measure to the fact that many rare animal species have so far simply not been examined for this. Because other studies have already found that only a fraction of the viruses found in animals have so far been recorded and described. “In summary, our results support the assumption that common mammalian species share more viruses with us humans than less common species,” said Johnson and her team. “At the same time, we suspect that the leap in pathogens often goes unnoticed.” Only the fraction of these transmissions that lead to larger outbreaks in the human population is noticed.
Source: Christine Johnson (University of California, Davis) et al., Proceedings of the Royal Society B: Biological Sciences, doi: 10.1098 / rspb.2019.2736