Study explains the controversial link between bats, rats, and Covid-19

Time to erase the phrase "plague rat" from your vocabulary — these rodents just don't deserve the slander.

Long-considered carriers of diseases, getting up close to the animals at the fringes of human society — rats, bats, and pigeons — may not pose as big a risk as you think. In fact, these critters may be no more likely to host a deadly disease like plague or Covid-19 than any other group of birds or mammals.

In a new study, scientists report bats and rodents are “unexceptional” when it comes to making humans sick. The finding contradicts the common wisdom that these animals are public enemies Number 1 and Number 2 when it comes to spreading disease from animals to humans. Bats have been linked to the current Covid-19 pandemic, for example.

They made the discovery after building the largest database ever constructed showing the relationships between 415 viruses and their different animal hosts across three bird orders and eight mammal orders.

An animal order is the grouping between family and class, so the results don't look at individual species as much as entire groups of animals. That means the researchers looked at rodents as a whole, rather than the individual creatures that are rodents.

Across the different orders, the proportion of viruses that infect humans does not change significantly, suggesting animals from any of these orders can infect humans.

“This supports a host-neutral explanation for observed variation in the number of zoonoses among animal groups,” the authors of the study write in the paper. They say their findings can be used to refine how humans track and manage virus risks that come from animals before the emerge.

That means scientists need to broaden their surveillance efforts to take in the entirety of local biodiversity, and not just focus on the animals we think of as high-risk, like the much-maligned rat, the authors argue.

The findings were published Monday in the journal Proceedings of the National Academy of Sciences.

Are some animals more high-risk than others?

While the study doesn't point fingers at any animal species in particular, it does identify certain factors that may make animals more likely to spread viruses to humans, and how deadly they may be:

1. Population
2. The virus' biology

Animal population matters. As the number of animals scale up, the chances of them coming into contact with humans also increases, the study suggests. That means there may be higher incidences of diseases passing from them to us, too.

Similarly, the biology of a particular virus, not the traits of the animal hosting it, accounts for how dangerous it may prove to humans, the study finds.

The results current practice of focusing virus discovery, research, and surveillance efforts on host reservoirs thought to pose a high risk should be broadened to be proportionate with local biodiversity, according to the authors.

Why do zoonotic diseases pose such big risks to humans?

At this point, scientists are pretty certain that the novel coronavirus is a zoonotic disease, but we don't know which animal spread the Covid-19 virus, SARS-CoV-2, to humans. Theories range from bats to snakes, to pangolins.

But Covid-19 is far from the only animal-transmitted disease humans should be worried about.

A host of bacteria (E. coli, Listeria monocytogenes); parasites (Giardia, roundworm); and viruses (Zika, West Nile) can all spread from animals to humans.

The way these diseases make the leap can also be quite different. Sometimes, humans will get a disease by eating meat from an infected animal. Tick and mosquito bites can spread infections, too, because these animals might have previously fed off an infected animal before biting you.

These diverse threats underline why it is so important for scientists to understand which animals can and can’t harbor diseases like Covid-19. Researchers now know that the novel coronavirus can reproduce in ferrets and cats (and yes, tigers) — but seemingly not in dogs, pigs, chickens, or ducks. Cats might not be at particularly high-risk of contracting the disease outside of a laboratory, but they can also become fomites, or surfaces that carry and spread viruses.

Domesticated animals — pets and livestock — that spend a lot of time around humans tend to have more zoonotic diseases.

Are we getting more zoonotic diseases?

Bats and rats may not be more adept at carrying diseases than any other creature, but some animals do see higher rates of diseases than others.

For that, just one species is to blame: Homo sapiens.

Humans’ impact on wildlife is contributing to the spread of zoonotic diseases, according to separate study, published April 8 in the journal Proceedings of the Royal Society B.

In that study, researchers found that virus spillover from animals to humans is more likely among animals that have both grown in population number and whose ranges have expanded because of humans. The animals that fit the bill? Our beloved pets and the domestic animals we depend on for food and work.

Domesticated species made up eight of the top 10 mammal species that had the most viruses shared with humans. Specifically, these were pigs, cattle, horses, sheep, dogs, goats, cats, and camels.

But the converse may also be true, and again, we are the culprits. Threatened wildlife species, including primates and bats, also share more viruses with humans. Unlike the domestic animals that may pose a risk whose populations and territory has been boosted by humans, these creatures' dropping numbers are the result of exploitation and habitat loss spurred by human activity. That means poaching and the wildlife trade — both of which bring us into close contact with wildlife — and tampering with natural areas in which these species live are leading to increased disease incidence in humans.

Taken together, both studies underscore the fact that we can't pin blame for zoonotic diseases on any one animal — and that perhaps we humans have more responsibility for these diseases making the leap from animal to us than we might like to admit.

Abstract: The notion that certain animal groups disproportionately maintain and transmit viruses to humans due to broad-scale differences in ecology, life history, and physiology currently influences global health surveillance and research in disease ecology, virology, and immunology. To directly test whether such “special reservoirs” of zoonoses exist, we used literature searches to construct the largest existing dataset of virus–reservoir relationships, consisting of the avian and mammalian reservoir hosts of 415 RNA and DNA viruses along with their histories of human infection. Reservoir host effects on the propensity of viruses to have been reported as infecting humans were rare and when present were restricted to one or two viral families. The data instead support a largely host-neutral explanation for the distribution of human-infecting viruses across the animal orders studied. After controlling for higher baseline viral richness in mammals versus birds, the observed number of zoonoses per animal order increased as a function of their species richness. Animal orders of established importance as zoonotic reservoirs including bats and rodents were unexceptional, maintaining numbers of zoonoses that closely matched expectations for mammalian groups of their size. Our findings show that variation in the frequency of zoonoses among animal orders can be explained without invoking special ecological or immunological relationships between hosts and viruses, pointing to a need to reconsider current approaches aimed at finding and predicting novel zoonoses.