Whether Covid-19, influenza or Ebola: Many infectious diseases originally come from the animal kingdom – their pathogens have spread from animal carriers to humans. Now a study shows that our land use promotes such zoonoses to a large extent. Because the animal species that thrive particularly well in landscapes shaped by humans are usually particularly effective reservoirs of zoonotic pathogens. On average, their share in urban areas increases by up to 72 percent, the number of individuals even by up to 144 percent, as the researchers found. The danger of new epidemics lurks more on our doorstep.
60 to 75 percent of all new infectious diseases are triggered by pathogens that have jumped from animals to humans. The new coronavirus Sars-CoV-2 also originally developed in bats and acquired properties that enabled it to infect human cells and thus to change species. Many of these zoonoses arise where humans and animals live in close contact, for example in livestock farming. For example, pigs and poultry are frequent reservoirs for potentially zoonotic influenza viruses. However, it can also be dangerous where humans penetrate previously untouched areas or destroy the habitat of wild animals. Because this then suddenly creates new contacts between humans and wild animals, which also offer their parasites and pathogens the chance to jump over.
Anthropogenic landscapes are more prone to zoonoses
Rory Gibb from University College London and his colleagues have now examined on a global scale the role that changes in natural habitats play through human land use. “The way in which humans change landscapes around the world, for example by turning forests into fields, influences many wild animal species. It makes some species less, while others stay there or even become more common, ”says Gibb. He and his team wanted to know whether animal reservoirs of zoonotic pathogens are more likely to be among the winners or losers of anthropogenic land use. For this purpose, the researchers evaluated data sets from 6801 ecosystems worldwide that are influenced by humans to different degrees – the range ranged from pristine areas to metropolitan areas and large cities. Among the species of these communities, 376 animal species belong to those that carry at least one virus or bacterium that can also cause diseases in humans.
The scientists compared the proportion of these zoonotic animal species and the frequency of their individuals in natural and man-made habitats. The evaluation showed that potential host species of zoonotic pathogens were more likely to be among the winners of the land use changes. They occur in agricultural or other strongly anthropogenically influenced habitats a good 20 percent more frequently than non-host species. In urban areas, their proportion increases by as much as 62 to 72 percent, the frequency of individuals of this species by 136 to 144 percent. ”Our results thus show that the animals that stay in the environments dominated by humans are also those that Carrying pathogens that can cause diseases, ”says Gibb. In other words: the danger of zoonoses lurks less in the untouched jungle than in front of our urban or village front door.
More host species among the culture successors
More detailed analyzes also showed which potential zoonosis vectors benefit most from the man-made changes in the landscape. To do this, the scientists compared data on the occurrence and frequency of host species and non-host species from different groups of mammals at around 2,000 locations. Here, too, it was shown that non-host species tend to be among the losers, while host species of zoonotic pathogens benefit more from the influence of humans. This was particularly evident in bats, passerines and rodents, as Gibb and his colleagues report. These animal groups not only include many potential carriers of disease-causing viruses, they are also mostly successful cultural followers of humans. ”Such a tendency has already been observed in relation to some diseases. Our results now suggest, however, that this could be a general phenomenon in these animal groups, ”said the researchers.
They suspect that certain biological traits could make the potential vector species both suitable virus reservoirs and successful culture followers. A short generation time and high reproductive rate favors the adaptation to anthropogenic changes in the environment; at the same time, these characteristics are often associated with a particularly active immune system and high pathogen tolerance. This in turn ensures that these animal species, such as rats or bats, can carry many pathogens without becoming seriously ill themselves. “Outbreaks of zoonotic diseases seem to be increasing in frequency. Our results can help explain the pattern of these outbreaks by clarifying the ecological processes behind them, ”says Gibbs colleague David Redding. At the same time, the researchers emphasize the need to monitor areas with major changes in land use intensively for emerging diseases.
Source: Rory Gibb (University College London) et al., Nature, doi: 10.1038 / s41586-020-2562-8