Photo Credit – FAO
My thanks to Helen Branswell this morning for tweeting the link to a new study that suggests that avian influenza viruses can be spread over considerable distance by the wind.
Long time readers will recall we’ve visited this question a couple of times before. We’ll review those, but first, the new study which looked at the extensive outbreak of H7N7 in the Netherlands in 2003.
From the Journal of Infectious Diseases (the full study is behind a pay wall), we get a fair idea of their findings via the Abstract.
Rolf J.F. Ypma1, Marcel Jonges, Arnaud Bataille, Arjan Stegeman3, Guus Koch4, Michiel van Boven1, Marion Koopmans1,W. Marijn van Ballegooijen1 and Jacco Wallinga
Outbreaks of highly pathogenic avian influenza in poultry can cause severe economic damage, and represent a public health threat. Development of efficient containment measures requires an understanding of how these influenza viruses are transmitted from one farm to the next. However, the actual mechanisms of inter-farm transmission are largely unknown.
Dispersal of infectious material by wind has been suggested, but never demonstrated, as a possible cause of transmission between farms. Here we provide statistical evidence that the direction of spread of avian influenza A(H7N7) is correlated with the direction of wind at date of infection.
We find the direction of spread by reconstructing the transmission tree for a large outbreak in the Netherlands in 2003, using detailed genetic and epidemiological data. We conservatively estimate the contribution of a possible wind-mediated mechanism to the total amount of spread during this outbreak to be around 18%.
Although it occurred nearly 10 years ago, this outbreak of H7N7 continues to interest scientists, as it represents the largest cluster of human infection by H7 flu virus we’ve seen.
This report from the December 2005 issue of the Eurosurveillance Journal.
M Du Ry van Beest Holle, A Meijer, M Koopmans3 CM de Jager, EEHM van de Kamp, B Wilbrink, MAE. Conyn-van Spaendonck, A Bosman
An outbreak of highly pathogenic avian influenza A virus subtype H7N7 began in poultry farms in the Netherlands in 2003. Virus infection was detected by RT-PCR in 86 poultry workers and three household contacts of PCR-positive poultry workers, mainly associated with conjunctivitis.
More than 30 million birds residing on more than 1,000 farms were culled to control the outbreak.
One person - a veterinarian who visited an infected farm – died a week later of respiratory failure. The rest of the symptomatic cases were relatively mild.
Normally, when avian flu manages to spread among local farms, we think of transport mechanisms like the farm-to-farm movement of infected birds or eggs, or of contaminated or infected personnel or equipment, or even a bird or small mammal vector.
The idea that the virus might be blown (likely carried on dust, or some other particulate) – while unproven - has come up before.
Back in January of 2008 I wrote a blog called The Virus My Friend, Is Blowin' In The Wind where I cast a dubious eye upon claims by the Indian Government that the bird flu virus (H5N1) was being blown by the wind across the border from neighboring Bangladesh, and was infecting hapless Indian Poultry.
It wasn’t impossible, of course. And I went into some of the other types of pathogens (mostly fungi and bacteria) that are known to travel in the wind.
Then in May of 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 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.
But we have seen studies indicating that the H5N1 virus can – under the right environmental conditions – remain viable for hours or even days in the environment (see EID Journal: Persistence Of H5N1 In Soil and H5N1: Hiding In Plain Sight)
Lending at least a little credence to the idea that they might survive on the wind long enough to infect downwind farms.
It has also been suggested that dried chicken droppings (`poultry dust’) may also serve to spread the virus, and Indonesian authorities have mentioned this as a possible vector (see Indonesian Updates And Vector Concerns).
Hong Kong authorities also mentioned the possibility (of at least short-range windborne transmission) in a highly detailed epidemiological report issued by the University of Hong Kong, on the outbreak of H5N1 on a solitary chicken farm in the New Territories in 2008.
(ii) The strong winds and gust from the north and north-east from 4 to 6 December 2008 could have deposited potentially contaminated dust and leaves from the trees into the nearby shed no. 17 via its north opening. These contaminated materials could then have gathered at the corner of the shed where the initial high mortality in poultry occurred.
So . . . while none of this is a slam dunk proving wind-borne transmission of viable avian (or any other flu) viruses, we have at least some credible evidence that suggests it may have happened.
How big of a factor this plays in the spread of viruses remains to be seen.
But it does provide investigators another avenue of epidemiological query when multiple farms in close proximity are infected with avian influenza.
Note: `Barnstorming’ is an Americanism that some of my readers may not be familiar with. It refers to the early days of aviation when pilots would fly to rural areas, land in farmer’s fields, and sell rides, or put on an air show for the locals.