Land use promotes zoonoses

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Whether it’s covid-19, influenza or Ebola: many infectious diseases originate from the animal kingdom – with their pathogens spreading from animal carriers to humans. Now a study shows that land use greatly promotes such zoonoses. Because the animal species, which thrive especially in man-made landscapes, are usually also particularly effective reservoirs of zoonotic pathogens. On average, their share in urban areas increases by up to 72 percent, and the number of individuals increases by as much as 144 percent, the researchers found. The danger of new epidemics is therefore more likely to lurk on our doorstep.

60 to 75 percent of all emerging infectious diseases are caused by pathogens that have jumped from an animal to us humans. The new coronavirus Sars-CoV-2 also developed originally in bats and acquired properties that enabled it to infest human cells and thus to jump species. Many of these zoonoses arise where humans and animals live in close contact with each other, for example in livestock farming. For example, pigs or poultry are common reservoirs for potentially zoonotic influenza viruses. However, it can also become dangerous where humans enter previously untouched areas or destroy the habitat of wild animals. This suddenly creates new contacts between humans and wild animals, which also offer their parasites and pathogens the chance to skip.

Anthropogenic landscapes are zoonosis-prone

Rory Gibb of University College London and his colleagues have now investigated the role of changes in natural habitats through land use on a global scale. “The way in which humans change landscapes around the world, for example by making fields out of forest, has an impact on many wildlife species. Some species diminish in size, while others stay there or become even more common,” Gibb said. He and his team wanted to know whether animal reservoirs of zoonotic pathogens were more likely to be winners or losers of anthropogenic land use. To do this, the researchers analyzed data sets from 6801 ecosystems worldwide that are influenced by humans to varying degrees – ranging from pristine areas to agglomerations and large cities. Among the species of these communities, 376 animal species were among those that carry at least one virus or bacterium that can also cause disease in humans.

Scientists compared the proportion of these zoonosis-prone species and the frequency of their individuals in natural and human-influenced habitats. The analysis showed that potential host species of zoonotic pathogens were more likely to be among the winners of land-use changes. They are more common than non-host species in agricultural or otherwise strongly anthropogenic habitats. In urban areas, their proportion increases by as much as 62 to 72 percent, and the frequency of individuals of this species increases by 136 to 144 percent.” Our results show that the animals that remain in the human-dominated environments are also the ones that carry pathogens…,” Gibb said. In other words, the danger of zoonoses lurks less in the pristine jungle than in our urban or village front door.

More host species among the cultural followers

Further analysis also showed which potential zoonosis carriers benefit most from the man-made changes in the landscapes. For this purpose, the scientists compared data on occurrences and frequency of host species and non-host species from different groups of mammals at more than 2000 sites. Again, it was shown that non-host species are often more likely to be the losers, while host species of zoonotic pathogens are more likely to benefit from the influence of humans. This was particularly evident in bats, sparrowbirds and rodents, as Gibb and his colleagues report. Among these animal groups are not only many potential carriers of disease-causing viruses, but they are also mostly successful cultural consequences of humans.” Such a trend has been observed before in relation to some diseases. However, our results now suggest that this could be a general phenomenon in these animal groups,” the researchers said.

They suspect that certain biological characteristics could make the potential carrier species both suitable viral reservoirs and successful cultural consequences. Thus, a short generation period and high reproductive rate favors adaptation to anthropogenic changes in the environment, at the same time these characteristics are often associated with a particularly active immune system and a high pathogen tolerance. This in turn ensures that these species, such as rats or bats, can carry many pathogens within them without becoming seriously ill themselves. “Outbreaks of zoonotic diseases seem to be becoming more common. Our results can help explain the pattern of these outbreaks by clarifying the environmental processes behind them,” says Gibbs colleague David Redding. At the same time, the researchers underline the need to closely monitor areas with major changes in land use for emerging diseases.