Wednesday, February 07, 2018

It's Raining Viruses

Saharan Dust Over Atlantic - Credit NOAA


A number of times over the past dozen years we've dipped into the world of aerobiology; the study of how bacteria, fungal spores, pollen and even viruses can be passively transported in the air.

A little over ten years ago, in The Virus My Friend, Is Blowin' In The Wind, I wrote (albeit skeptically) about claims by Indian officials that their bird flu epidemic was `brought in by winds blowing from Bangladesh'.
Not that I considered it impossible, only that cross border illicit poultry trade seemed a more likely explanation. 
There had been some studies on dust, carried across the Atlantic Ocean by the trade winds, that suggested some types of microbes might ride the dust particles and settle thousands of miles away.

The following excerpt comes from an article published by the Guardian Unlimited in 2004 called It's an Ill Wind.
Tests on airborne dust samples collected in the Caribbean were found to contain infectious spores of the fungus. Scientists suspect the spores had been carried on the wind from Africa, before landing on the ocean surface, sinking and infecting the sea fans. Enough had built up on the ocean floor for the disease to spread. Since then, several outbreaks have been linked to dust clouds.
Pollen, fungal spores, and even bacteria are usually hardier organisms than viruses, and the ability of the wind to disperse these types of organic particles intact over considerable distances has been well established.

Viruses, however, are more prone to desiccation, and UV damage – are unable to replicate outside of a suitable host – and are therefore considered more `fragile’.

Despite these limitations we’ve seen some research indicating that flu viruses – under the the right conditions – can persist in the environment for impressive periods of time.

In 2010 (see Viruses Blowin’ In The Wind?)  we saw a report in the journal Environmental Health Perspectives, that suggested that it was possible for H5N1 (or any Influenza A virus) to be transported across long (hundreds of kilometers) distances in the air.
Although researchers demonstrated influenza RNA could be detected in ambient air samplings, they didn’t establish that the virus remained viable over long distances.
In December of 2012  (see Barnstorming Avian Flu Viruses?) we looked at a study in the Journal of Infectious Diseases called Genetic data provide evidence for wind-mediated transmission of highly pathogenic avian influenza that found patterns that suggested farm-to-farm spread of the 2003 H7N7 in the Netherlands due to the prevailing wind.

Another study of the same outbreak, Modelling the Wind-Borne Spread of Highly Pathogenic Avian Influenza Virus between Farms (PloS One 2012), found that wind borne transmission could have accounted for up to 24% of the transmission over distances up to 25 km
During the 2014-15 avian epizootic in the United States, airborne spread between farms miles apart was considered possible, although never conclusively proven see APHIS: Partial Epidemiology Report On HPAI H5 In The US) .
In Asia we've seen numerous instances where the `dust’ (desiccated chicken manure, feathers, etc.) from chicken farms has been strongly suspected as having spread bird flu – at least for a distance of several hundred yards. 
For some human bird flu cases in Indonesia and China, the only known exposure has been listed as living near, or simply walking past, a poultry farm or live market. 
In 2014's BMC Veterinary Research article  Evidence of infectivity of airborne porcine epidemic diarrhea virus and detection of airborne viral RNA at long distances from infected herds authors Carmen Alonso, Dane P Goede, Robert B Morrison, Peter R Davies, Albert Rovira, Douglas G Marthaler and Montserrat Torremorell wrote:
Results indicated presence of infectious PEDV in the air from experimentally infected pigs and genetic material of PEDV was detected up to 10 miles downwind from naturally infected farms. Airborne transmission should be considered as a potential route for PEDV dissemination.
Making the airborne spread of viruses across moderate distances (10km-25km) something we have to take seriously.

Today we have a study, from the University of British Colombia, that raises the stakes - all the way up to the troposphere - where they find huge numbers of diverse bacteria and viruses, most of which are swept up from the oceans.

After floating for hours - or even days - they can then rain down on the earth's surface, sometimes having traveled thousands of miles.
While their viability isn't addressed in the abstract or press release - when billions or trillions of biological particles are hoisted far aloft every day - it wouldn't be unreasonable to expect that some survivors might make it back to earth.
First a link and some excerpts from the study, followed by a link and excerpts from the University of British Columbia's press release.
Deposition rates of viruses and bacteria above the atmospheric boundary layer
Isabel Reche,Gaetano D’Orta, Natalie Mladenov, Danielle M. Winget & Curtis A. Suttle


Aerosolization of soil-dust and organic aggregates in sea spray facilitates the long-range transport of bacteria, and likely viruses across the free atmosphere. Although long-distance transport occurs, there are many uncertainties associated with their deposition rates. 

Here, we demonstrate that even in pristine environments, above the atmospheric boundary layer, the downward flux of viruses ranged from 0.26 × 109 to >7 × 109 m−2 per day. These deposition rates were 9–461 times greater than the rates for bacteria, which ranged from 0.3 × 107 to >8 × 107 m−2 per day. 

The highest relative deposition rates for viruses were associated with atmospheric transport from marine rather than terrestrial sources. Deposition rates of bacteria were significantly higher during rain events and Saharan dust intrusions, whereas, rainfall did not significantly influence virus deposition. 

Virus deposition rates were positively correlated with organic aerosols < 0.7 μm, whereas, bacteria were primarily associated with organic aerosols > 0.7 μm, implying that viruses could have longer residence times in the atmosphere and, consequently, will be dispersed further. These results provide an explanation for enigmatic observations that viruses with very high genetic identity can be found in very distant and different environments.

Viruses – lots of them – are falling from the sky 
Science, Health & Technology
Feb 6, 2018 | For more information, contact Chris Balma

An astonishing number of viruses are circulating around the Earth’s atmosphere – and falling from it – according to new research from scientists in Canada, Spain and the U.S.

The study marks the first time scientists have quantified the viruses being swept up from the Earth’s surface into the free troposphere, that layer of atmosphere beyond Earth’s weather systems but below the stratosphere where jet airplanes fly. The viruses can be carried thousands of kilometres there before being deposited back onto the Earth’s surface.

“Every day, more than 800 million viruses are deposited per square metre above the planetary boundary layer—that’s 25 viruses for each person in Canada,” said University of British Columbia virologist Curtis Suttle, one of the senior authors of a paper in the International Society for Microbial Ecology Journal that outlines the findings.

The findings may explain why genetically identical viruses are often found in very different environments around the globe.

“Roughly 20 years ago we began finding genetically similar viruses occurring in very different environments around the globe,” says Suttle. “This preponderance of long-residence viruses travelling the atmosphere likely explains why—it’s quite conceivable to have a virus swept up into the atmosphere on one continent and deposited on another.”

Bacteria and viruses are swept up in the atmosphere in small particles from soil-dust and sea spray.

Suttle and colleagues at the University of Granada and San Diego State University wanted to know how much of that material is carried up above the atmospheric boundary layer above 2,500 to 3,000 metres. At that altitude, particles are subject to long-range transport unlike particles lower in the atmosphere.

Using platform sites high in Spain’s Sierra Nevada Mountains, the researchers found billions of viruses and tens of millions of bacteria are being deposited per square metre per day. The deposition rates for viruses were nine to 461 times greater than the rates for bacteria.

“Bacteria and viruses are typically deposited back to Earth via rain events and Saharan dust intrusions. However, the rain was less efficient removing viruses from the atmosphere,” said author and microbial ecologist Isabel Reche from the University of Granada.

The researchers also found the majority of the viruses carried signatures indicating they had been swept up into the air from sea spray. The viruses tend to hitch rides on smaller, lighter, organic particles suspended in air and gas, meaning they can stay aloft in the atmosphere longer.

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