Researchers discover how "cryptic" connections in disease transmission influence epidemics

Diseases have repeatedly affected wildlife and caused local or global epidemics such as HIV / AIDS, Ebola, SARS and Nipah.

A new study by researchers on disease transmission in bats has broad implications for understanding hidden or "cryptic" connections that can spread diseases between species and cause large-scale outbreaks.

By sprinkling the bats with a fluorescent powder that shines under the ultraviolet light, Virginia Tech researchers Joseph Hoyt and Kate Langwig were able to trace the dynamics of disease transmission in bat species devastated by the white nose syndrome, a fatal fungal disease 6.7 million bats in North America since 2006.

Their results were recently published in the journal Nature.

"These results have uncovered and quantified links, within and between species, that we have never heard of before," said first author, Joseph Hoyt, who led the study. study as a graduate student from the University of Santa Cruz and completed the analyzes at Virginia Tech as a researcher. Department of Biological Sciences, Faculty of Science.

"We had seen explosive outbreaks during which a whole population of bats would be infected with white nose syndrome in a month or two, and the way it was happening was mysterious. "explain and track more precisely the white nose syndrome, and our study has strong implications for predicting other outbreaks," said Hoyt.

When we think about who we might fall ill to, we tend to think about our social groups: family, friends and colleagues. But we forget this brief interaction with a DMV employee, a barista in a café, or a shared airspace on public transit. People are aware of these interactions, but not of their importance for the spread of epidemics. In the past, these types of hidden interactions were poorly understood because they are so difficult to quantify.

Kate Langwig, second author of the study, Assistant Professor in the Department of Biological Sciences at Virginia Tech, said this study shows that infrequent and indirect connections, also called "cryptic" connections, play a much larger role in the transmission of the virus. disease than what had been understood before.

"Cryptic connections are essentially pathways or connections between individuals that we would not normally be able to estimate or observe.They have been largely ignored by researchers in the past, but this study quantifies Their study creates an integrated model of social group, cryptic connections and connections, "said Langwig, an affiliate faculty member of the Global Change Center, a branch of the Fralin Life Science Institute.

Co-author A. Marm Kilpatrick, associate professor of ecology and evolutionary biology at the University of Santa Cruz, said the fallout, when pathogens spread from animals wild at human populations, tend to occur through this type of cryptic connections. "Normally, we do not understand how important they are, except retrospectively, when we investigate epidemics such as Ebola or SARS," he said.

"Our study has compelling implications that will allow researchers to track seemingly random or indirect connections to wildlife likely to spread to human populations," Langwig said.

The fluorescent dust used in this study has been shown to be very effective in revealing cryptic connections between bats. The researchers conducted the study on eight hibernation sites, mostly abandoned mine tunnels, in the Upper Midwest. Each site used up to four species of bats. At the beginning of the study, the pathogen responsible for white nose syndrome had not yet reached these populations.

The researchers first interrogated bats and characterized their social networks, measuring direct physical contact between bats hibernating together, as well as additional connections made by bats moving from one group to another. to the other. Then they applied the fluorescent dust on several bats at the beginning of the winter, using a different color for each bats. At the end of the winter, the researchers went back to see where each fluorescent dust color ended.

"We gathered huge sets of data for each bat in each population, characterized the social groups of bats and also used fluorescent dust to track their movements and contacts," Langwig said. .

The researchers found that "the spread of dust reflects how the pathogen spreads, so we can see if a bat is depositing dust somewhere in the environment and that Another bat goes by and picks it up, it also reveals infrequent direct contacts that we would not normally observe, "said Hoyt.

The fungal pathogen responsible for white nose syndrome has arrived in the region after studies on fluorescent dusts. Researchers also tracked its spread on each site. They found that the dynamics of disease transmission were better explained by the sum of all links revealed in dust studies than by the mere use of social groups in hibernation.

"We were able to explain the actual invasion of the pathogen much better by including these cryptic connections, and they were even more important for explaining inter-species transmission than for transmission within species" said Hoyt.

One of the surprising features of White Nose Syndrome is its ability to spread in a bat community during the winter, when animals hibernate 99.5% of the time. They only come out of hibernation very briefly every two or three weeks. However, dust studies have shown that they are moving enough to have many more connections than those observed in their hibernation groups.

The most striking connections have been revealed for one species, the northern bat, which sleeps alone and not in groups. Although classical theory predicts low rates of infection for this solitary species, it has been hard hit by white nose syndrome.

"When we put fluorescent dust on the northern bat, it would appear on other species with which we had never seen them interact.We would never have predicted that the infection could occur. spread that way, "said Hoyt.

The researchers found that a different solitary species, the tricolor bat, had a lower infection rate and showed less evidence of cryptic links with other bats, but was transferring the dust on the surfaces of the sites where she sleeps. "We found that the tricolor bat is much more separated in the space.It is not that she does not rise up and crawl, she does it in a beach that is overlaps less with other bats, it seems to be more territorial in its use of space, "Hoyt said.

Unfortunately for bats, the spores of the fungal pathogen responsible for white nose syndrome remain in the environment and remain infectious for years. Once the walls and ceiling of a cave have been contaminated with spores, bats that use the site for hibernation will be exposed to infections year after year.

The white nose syndrome is considered one of the worst diseases of modern day wildlife, having killed millions of bats in North America.

However, the white nose syndrome does not appear to pose a risk to human health. It is caused by the fungus Pseudogymnoascus destructans, which grows optimally at low temperatures. The United States Geological Survey said: "Thousands of people have visited the caves and mines affected since the first sighting of the white nose syndrome, and no human disease attributable to white nose syndrome has been reported We are still learning about the disease, but we do not know any risk to humans from contact with bats affected by white noses. "

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More information:
Joseph R. Hoyt et al., Cryptic connections shed light on the transmission of pathogens in community networks. Nature (2018). DOI: 10.1038 / s41586-018-0720-z