A recently published study* conducted by epidemiologists and molecular biologists affiliated with Utrecht University and Wageningen Bioveterinary Research confirmed that DNA from waterfowl could be detected in the incoming air stream of poultry houses.
The study involved two broiler houses and a layer house that were depopulated and decontaminated after confirmed cases of H5N1 HPAI. The area in which the study was conducted was known to have a noteworthy population of waterfowl. The houses selected for assay were devoid of flocks with the exception of a few sentinel chickens in the layer house. Ventilation systems were operated consistent to the respective requirements of flocks that would have been housed. Air sampling equipment was placed directly at the air inlets. In addition parallel samples were obtained from the exterior of the sampled houses. DNA was extracted and isolated from sample filters and was assayed by applying eukaryote DNA metabarcoding followed by deep sequencing.
In the case of the two broiler houses on the respective farms, 1 out of 5 and 1 out of 21 air inlet samples respectively yielded DNA derived from waterfowl. For the layer house, 2 out of 21 samples were positive. One out of 21 exterior samples was positive for waterfowl DNA from only one of the two broiler farms and 2 out of 21 samples yielded waterfowl DNA in the vicinity of the layer farm.
The positive control comprised samples obtained from a waterfowl rehabilitation center. All twelve out indoor samples and all 8 outdoor samples yielded waterfowl DNA. This was not unexpected, but confirmed the sensitivity of detection of waterfowl DNA using the sampling and assay technique.
Demonstration of HPAI virus in air samples is exceptionally difficult but has been demonstrated using spike-in experiments. In addition HPAI H5N1 was isolated from air samples obtained in the vicinity of wild waterfowl in a study conducted in Taiwan coincident with outbreaks in commercial poultry. This study demonstrated that DNA from the plumage of waterfowl or their feces may be entrained in air streams with introduction into poultry houses operated with exhaust ventilation. It is noted that air samples were obtained over distances ranging from 12 to 25m from each of the houses sampled over a period of four days.
The study demonstrated that commercial poultry confined to power-ventilated buildings may potentially be exposed to avian influenza virus since DNA from waterfowl was detected in the airflow at the air inlets to houses. In the context of units holding 100,000 laying hens, air flow would correspond to 600,000 cfm assuming 1.5 cfm per pound live weight. Air inlet velocities would approach 600 ft/minute at the inlets depending on their collective area and the rating and number of fans in operation. The introduction of HPAI virus into a large complex is obviously facilitated by the quantum of air introduced into houses under normal operation, environmental variables promoting survival of virus and proximity to concentrations of migratory waterfowl shedding virus.
HPAI virus can persist on moist soil for periods extending beyond a week. The presence of waterfowl in vicinity of large egg-production complexes along the Mississippi Flyway is well established. Effluent retention ponds in the vicinity of houses, and the presence of wetlands attract waterfowl. In a recent evaluation of biosecurity of a complex in a Midwest state, gulls, were perching on the ridges of houses and Canada geese ranged to within yards of the perimeter of houses feeding on grass between houses.
The implications of the University of Utrecht/Wageningen study are self-evident. Even with the highest standards of biosecurity, introduction of avian influenza virus cannot be absolutely prevented. This is consistent with anecdotal reports of exposure on complexes with high levels of structural and operational biosecurity in Colorado and other states.
Filtration of incoming air to exclude virus is currently impractical but the use of laser installations to deter congregation of wild waterfowl and other birds in the vicinity of farms should be beneficial.
With the recognition that HPAI is at least seasonally and regionally endemic in the U.S. and given the population of migratory waterfowl and resident birds potentially shedding avian influenza virus, eradication of the infection can only be regarded as a futile aspiration. With new evidence highly suggestive of aerogenous transmission of the virus, protection of both large and medium sized complexes and even individual houses would be enhanced by immunization. The World Organization for Animal Health has endorsed preventive vaccination in conjunction with high levels of structural and operational biosecurity to offer a more solid prospect of preventing large-scale outbreaks.
*Bossers, A., et al. Detection of Airborne Wild Waterbird-Derived DNA Demonstrates Potential for Transmission of Avian Influenza Virus via Inlets into Poultry Houses, the Netherlands, 2021-2022. Euro Surveill. 2024:29 Doi.org/10.2807/1560-7917.ES.2024.29.40.2400350.