Photo Credit – FAO
The notion that the H5N1 `bird flu’ virus can persist in the environment – for hours or days (or perhaps even weeks) - is hardly new, yet very little is really known about how, and where, the virus resides outside of a living host.
As H5N1 is primarily a gastrointestinal malady in birds it is believed that the virus is commonly spread in the wild via shared feces-contaminated pond and lake waters (see Bogor: H5N1 Detected In Retention Pond).
But there appear to be other routes of transmission as well.
In June of 2010, we saw a study (see Birds Of A Feather . . . .) in PLoS One, suggesting that waterfowl may be spreading avian flu viruses because their preening oils bind the virus to their feathers.
Another study conducted by researchers at the National Institute of Animal Health, Tsukuba, Ibaraki, Japan was reported in the August 2010 issue of Applied and Environmental Microbiology.
They determined that the H5N1 virus may persist on the dropped feathers from infected ducks and may therefore spread to the environment.
Applied and Environmental Microbiology, August 2010, p. 5496-5499, Vol. 76, No. 16
The surprising part of this study is how long these feathers retained some degree of viral contamination at various temperatures.
At 4°C (39F) the virus was detectable for 160 days, while at the higher temperature 20°C (68F), the virus was detected for 15 days.
We saw another study from 2010, which appeared in Environmental Science and Technology, titled:
Joseph P. Wood, Young W. Choi, Daniel J. Chappie, James V. Rogers, and Jonatha
n Z. Kaye
Copyright © 2010 American Chemical Society
Researchers conducted tests on four inanimate materials (glass, wood, galvanized metal, and top soil) to determine how long – and under what environmental conditions – the virus could survive.
They adjusted factors such as temperature, relative humidity, and simulated sunlight and checked the samples over a period of 13 days. The virus was most persistent at lower temperatures, and on surfaces such as glass and steel.
Their conclusion?: under the right conditions, the virus could be expected to persist beyond 13 days.
Earlier this year we looked at a study published in the journal Influenza and Other Respiratory Viruses that examined environmental samples taken in Cambodia between April 2007 and February 2010 during several bird flu outbreaks (see Environment: a potential source of animal and human infection with influenza A (H5N1) virus Gutiérrez, Buchy et al.)
Out of 246 samples taken around farms with outbreaks, 19% of dust, mud and soil samples showed contamination from the H5N1 virus.
Admittedly, just because RT-PRC testing was able to detect a virus in a sample doesn’t necessarily mean that the virus is viable. But it does give us an idea of the environment spread of the virus.
Volume 18, Number 9—September 2012
To the Editor: Highly pathogenic avian influenza (HPAI) virus (H5N1) has been responsible for 603 confirmed human cases worldwide, including 356 that resulted in death, and for >7,000 epizootic outbreaks (1,2). Direct contact between hosts is the main mechanism of transmission for avian influenza viruses, but the possible role of the environment as a source of HPAI virus (H5N1) infection has been rarely studied, particularly in the context of countries where the virus is enzootic or epizootic (3–7). To determine if contaminated soil contributes to the transmission cycle of HPAI virus (H5N1), we used experimental and simulated field conditions to assess possible transmission in chickens.
Essentially, these researchers took 3 types of soil, described as:
(1) sandy topsoil collected from around rice fields in Phnom Penh Province, Cambodia;
2) building sand purchased from a local building company; and
3) soil-based compost purchased from a local tree nursery
. . . and inoculated samples with low to high doses of the H5N1 virus. They then introduced these soil samples to the bottom of cages where chickens were housed, and then tested the chickens for infection over the next several days.
You can read the entire letter for more details on their methods and materials. The results are summarized in the chart below:
Essentially, sandy topsoil collected from rice fields proved to be a poor environment for transmitting the H5N1 virus, while soil-based compost proved highly effective.
The authors believe that the highly acidic nature of the sandy topsoil may work to inactivate viral particles.
Since one of the methods used to control and contain an avian flu outbreak is environmental decontamination, knowing which types of soil are unlikely to harbor and transmit the virus can save time, and reduce the use of harsh (and often scarce) chemicals in the environment.
Interestingly, the authors also found evidence to suggest that exposure to moderately contaminated soil may help poultry to develop a protective immune response to the virus.
H5N1 is not the only viral contender to spark the next pandemic, but due to its apparent high lethality, it is the one we tend to concentrate on the most.
Fortunately the virus remains adapted primarily to avian physiology – not human - and must mutate further if it is to become an imminent public health threat.
But with 20+ clades of the virus now circulating, and numerous opportunities to expose and infect other hosts (human, swine, mammal, and avian), the concern is this virus may one day succeed.