Photo Credit CDC EID Journal
We’ve a research article appearing this week in the journal PLoS One that while conducted on swine – and with swine viruses (IAV, PEDV, PRRSV) – may also help explain the lateral spread of avian flu viruses to neighboring poultry farms, such as we witnessed last spring in the American Midwest with HPAI H5.
The idea that some types of viruses might be carried by dust particles, remain viable for hours, and be spread (often for miles) by prevailing winds isn’t exactly new. The science even has a name; aerobiology – the study of how bacteria, fungal spores, pollen and even viruses can be passively transported in the air.
And for some viruses, this pattern of windborne dispersal has been fairly well documented. The USDA/APHIS Overview of the FMD Response Plan: The Red Book lists Foot & Mouth Disease (FMD) as being windborne, stating:
FMDV has also been known to spread through windborne transmission, where the virus infects naïve animals located some miles from known infected animals without any history of contact. The distance of windborne transmission over land surfaces depends on the atmospheric conditions and the amount of virus emitted into the air by the infected animals. Sources suggest FMDV may spread to distances of approximately 60 kilometers over land in favorable conditions and potentially even greater distances over water.
Last year, in 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.
Several of these same authors (Alonso, Davies, and Torremorell) from the University of Minnesota are back with today’s study, one that looks at the virus carrying capacity of aerosols from experimentally infected pigs. First, the abstract (reformatted for readability) and a link to this new study, after which I’ll return with more.
Published: August 19, 2015 DOI: 10.1371/journal.pone.0135675
When pathogens become airborne, they travel associated with particles of different size and composition. Particle size determines the distance across which pathogens can be transported, as well as the site of deposition and the survivability of the pathogen. Despite the importance of this information, the size distribution of particles bearing viruses emitted by infectious animals remains unknown.
In this study we characterized the concentration and size distribution of inhalable particles that transport influenza A virus (IAV), porcine reproductive and respiratory syndrome virus (PRRSV), and porcine epidemic diarrhea virus (PEDV) generated by acutely infected pigs and assessed virus viability for each particle size range. Aerosols from experimentally infected pigs were sampled for 24 days using an Andersen cascade impactor able to separate particles by size (ranging from 0.4 to 10 micrometer (μm) in diameter).
Air samples collected for the first 9, 20 and the last 3 days of the study were analyzed for IAV, PRRSV and PEDV, respectively, using quantitative reverse transcription polymerase chain reaction (RT-PCR) and quantified as geometric mean copies/m3 within each size range.
- IAV was detected in all particle size ranges in quantities ranging from 5.5x102 (in particles ranging from 1.1 to 2.1μm) to 4.3x105 RNA copies/m3 in the largest particles (9.0–10.0μm).
- PRRSV was detected in all size ranges except particles between 0.7 and 2.1μm in quantities ranging from 6x102 (0.4–0.7μm) to 5.1x104 RNA copies/m3 (9.0–10.0μm).
- PEDV, an enteric virus, was detected in all particle sizes and in higher quantities than IAV and PRRSV (p < 0.0001) ranging from 1.3x106 (0.4–0.7μm) to 3.5x108 RNA copies/m3 (9.0–10.0μm).
Infectious status was demonstrated for the 3 viruses, and in the case of IAV and PRRSV, viruses were isolated from particles larger than 2.1μm. In summary, our results indicated that airborne PEDV, IAV and PRRSV can be found in a wide range of particle sizes. However, virus viability is particle size dependent.
The entire research article is well worth reading, as the methods and materials used were quite complex. The authors recommend, based on their findings:
. . . comprehensive personal protective equipment including respiratory protection should be considered for potential exposures to both respiratory and enteric viruses, in particular in settings where animals and people interact. In addition, other biosecurity measurements such as air filtration could be considered to protect nearby at risk populations as previously demonstrated.
The information generated in this study is especially important to design effective airborne disease control programs for both enteric and respiratory viruses, including mitigation of occupational exposure of zoonotic pathogens. Changes in recommendations to protect from airborne viruses should be considered based on exposure to particles of different sizes.
While obviously a big concern to the swine industry, this research should also apply to other agricultural industries.
When we began to see clustering of HPAI H5 poultry outbreaks last spring, the idea that some of this spread might be due to wind propelled `dust’ (feathers, dried feces, water vapor, etc. emitted from poultry barn exhaust systems) emerged as a real possibility.
We looked at this topic in some depth last April (see Bird Flu’s Airborne `Division’) and again in May (see CIDRAP: H5N2 Roundup & Detection In Environmental Air Samples).
Over the summer APHIS released a 38-page partial epidemiology report on the spread of HPAI H5 across the United States (see APHIS: Partial Epidemiology Report On HPAI H5 In The US) that cited a number of plausible factors behind the spread of AI. While they were unable to pinpoint one or even a group of factors that satisfactorily explained this AI spread, they acknowledged the possibility that prevailing winds may have carried contaminated dust particles from farm to farm.
Environmental factors may also play a part in transmitting HPAI. APHIS found that genetic material from the HPAI virus could be detected in air samples taken inside and outside infected poultry houses, supporting the idea that the virus can be transmitted through air. Further reinforcing this concept is preliminary analysis of wind data that shows a relationship between sustained high winds (25 mph or greater for 2 days or longer) and an increase in the number of infected farms 5 to 7 days later.
Over the years we’ve seen a growing body of anecdotal evidence suggesting that some pathogens – including swine and avian viruses – may be be carried substantial distances by contaminated dust particles.
Increasingly – as with today’s study - we are seeing scientific research that lends credence to the idea.