A real life 'Contagion': Humans may be to blame for viruses jumping from animals to us

The risk of new viruses jumping from animals to humans was highest from threatened wild animals that have declined due to exploitation and habitat destruction.

(CNN)It could be a real-life "Contagion," much like the movie.

As a deadly pandemic spreads across the globe, a timely new study has identified key drivers of "virus spillover" from mammals to humans.
The risk of virus spillover -- when viruses jump from animals to humans -- was highest when human exploitation and habitat destruction threatened wild animals, according to a data analysis conducted by researchers at the University of California and the University of Melbourne.
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The research was carried out years before the current pandemic began, but researchers have long expected "emerging infectious diseases that come from wildlife and affect people," said study author Dr. Christine Kreuder Johnson, a professor of epidemiology and wildlife health at the University of California, Davis.
    "The reason why we did this work was to help understand what are the drivers for spillover, " she said, and what characteristics appeared in the past "that can help us [prevent spillover] in the future."

    Transmission of zoonotic disease

    A zoonotic disease is a disease spread between animals and people, and they can be caused by bacteria, parasites, fungi and viruses, according to the US Centers for Disease Control and Prevention.
    Some animal-borne diseases may have adapted and transferred easily to humans because the biological makeup of the mammal was similar to that of a human.
    Transfer is also easier if the human and the species have lived together over time, which is typically the case in humans infected with zoonotic disease from pigs and livestock, Johnson said. That's because humans have farmed them for food and lived with or near them for centuries.
    Humans also share more than 98% of DNA with chimpanzees and mice. This makes the animals more susceptible to many of the same health problems as humans -- hence their use in many lab studies, according to the California Biomedical Research Association (PDF).
    Bats and primates are "disproportionately likely sources of viral spillover to humans" due to their biological similarity and unique metabolic regulation of immune responses to infection, said Thomas Gillespie, a disease ecologist from Emory University. Gillespie was not involved in the study.

    Stress caused by humans

    The human-caused stress that mammals face also increases the risk of zoonotic spillover.
    When we capture wild animals from their natural homes to put them into trade and sell them in animal markets, they undergo a great deal of tension, Johnson said.
    "And when that happens, they're more likely to produce high numbers of viruses that they might be infected with," she added.
    Higher numbers of viruses mean greater viral shedding, which is the expulsion and release of excretions infected with the virus. So being around captured, stressed animals exposes people to higher virual loads and a greater chance of catching the virus than if we were to have contact with them in the wild, Johnson said.

    Shifts in mammalian populations

    The researchers studied zoonotic viruses and their land-based mammalian hosts, using 2004 to 2013 data from an internationally recognized authority on threatened species.
    Among the 142 zoonotic viruses examined, 139 viruses had at least one mammalian host reported at the species level.
    Domesticated mammals -- including cats and dogs, pigs and cattle, horses and sheep-- host 50% of the viruses that can be passed to mankind, despite representing only 12 species, according to the report. Rodents, bats and primates also carried high proportions of zoonotic diseases.
    There are a couple of reasons why some species have lower or higher risk for zoonotic viruses, Johnson said.
    Wild mammals with threatened conservation status are rare, so the probability of those animals encountering humans is low. They also may harbor fewer parasites since their populations are smaller.
    Threatened species with a reduction in population size due to human exploitation -- by hunting, trade and human occupation -- have over twice as many zoonotic viruses compared to those threatened for other reasons.
    "Exploitation of wildlife through hunting and the wild animal trade may have increased opportunities for virus spillover because of the close contact between wildlife and humans involved in these activities," the report said.
    "The lack of awareness in the society for climate change and the role of wildlife has multiplied this problem to many folds. The wildlife is at run due to lack of habitat and food, leading to an increase in human exposure to wild animals," said Dr. Sures