In February of 2024, government veterinarians in the Czech Republic found themselves grappling with a perplexing case while investigating a bird flu outbreak. The highly pathogenic H5N1 virus was wreaking havoc on a flock of chickens at a secure farm that housed a breeding program for hybrid birds with distinctively colored feathers and eggs. An adjacent facility owned by the same company also witnessed its chickens falling victim to the virus. The puzzle that arose was how the virus managed to infiltrate these state-of-the-art facilities, where measures such as filtered water supply from wells, one-way air flow through barns via large fans, and sturdy fencing to keep out wild animals were in place. Employees were even prohibited from keeping chickens at their residences.
Upon delving into the investigation, researchers deduced that this particular case was characterized by a unique set of conditions that facilitated the windborne transmission of the virus onto the chicken farm. While it is commonly believed that wild birds carrying flu viruses in their digestive systems, which are then spread through their feces, are the primary source of H5N1 introduction on farms, the possibility of bird flu being spread through the air by wind had been previously hypothesized.
Dr. Richard Webby, who oversees the World Health Organization’s Collaborating Center for Studies on the Ecology of Influenza in Animals, commented on this notion, emphasizing the difficulty in definitively measuring such occurrences. While the potential involvement of wind in the recent spread of H5N1 among cattle herds in California’s Central Valley was discussed, Dr. Webby underlined that this mode of transmission does not necessarily escalate the threat posed by bird flu to humans.
The investigation into the Czech Republic case revealed a connection to a nearby duck-fattening farm, where a significant bird flu outbreak had occurred a week earlier. The duck farm, situated near a lake frequented by wild ducks, had a different setup compared to the chicken facility, with birds housed in buildings featuring natural ventilation and basic biosecurity measures. The rapid spread of bird flu among the ducks on the farm contrasted starkly with the outbreak pattern observed in the chickens.
As the outbreak progressed on the duck farm, claiming the lives of hundreds of ducks within days, drastic measures were taken to contain the infection, including depopulating the entire flock. This stark contrast in infection dynamics between the duck and chicken farms shed light on the varied ways in which the virus can navigate and proliferate within different environments.
The facility exhibited another difference as well: The birds fell ill gradually. Over the span of a week, their consumption of food and water decreased, and the farm owners observed the deaths of several birds, particularly near the barns’ air intake vents. Subsequently, the infection spread to both barns in the breeding facility and a separate barn at another location, resulting in the loss of thousands of birds, including vital breeding stock. Government veterinary investigators gathered virus samples from infected ducks, chickens at the breeding facility, and chickens at the second farm for analysis. Three H5N1 strains identified in the duck farm matched those present in the birds that initially fell ill on each farm, indicating that the duck farm likely served as the source of the chicken outbreak.
Despite being nearly 5 miles apart, there were no discernible physical links between the duck farm and the chicken facilities, and investigators ruled out any human connections between them. Moreover, there were no significant bodies of water near the chicken farm to suggest wild birds as carriers of the flu virus. Upon examining the weather conditions when the chickens became infected, researchers noted a consistent westward breeze, extensive cloud cover hindering UV light exposure, and cool temperatures ideal for virus survival and transportation.
Dr. Kamil Sedlak, from the State Veterinary Institute in Prague and the study’s senior author, proposed windborne transmission as the most plausible explanation. The study, published as a preprint by Czech researchers, suggests that virus particles may have traveled via duck dander or similar means to cover long distances. Although air sampling was not conducted near the farms, previous studies around poultry houses have detected high concentrations of avian influenza viruses. The slow onset of illness in the chickens may be attributed to lower virus exposure levels from the air, with birds near air intake vents being the first affected.
Dr. Montse Torremorell, a Veterinary Population Medicine professor at the University of Minnesota, commended the study for its thorough investigation based on farm knowledge. Drawing from her air sampling experience during a previous avian influenza outbreak, Torremorell highlighted the significance of where the virus lands, along with the amount dispersed.
Those plumes of infectious aerosols have the potential to land on clothing, equipment, and vehicles, effectively transporting the virus from one location to another. Dr. Torremorell emphasizes the importance of considering airborne spread when implementing protective measures for livestock and flocks on farms. She suggests that airborne transmission should not be dismissed as a factor in virus containment efforts. However, discussing airborne spread can be challenging because individuals may feel powerless in the face of such transmission. This sense of helplessness can lead to neglecting essential biosecurity practices that are crucial in preventing virus spread.
Drawing from lessons learned during the Covid-19 pandemic, it is evident that a multifaceted approach is necessary to effectively protect against viral transmission. While current measures such as limiting farm access and wearing personal protective equipment are vital, additional steps like air filtration in barns could further safeguard animals and farmworkers from diseases like bird flu. Dr. Michael Osterholm, from the Center for Infectious Disease Research and Policy at the University of Minnesota, underscores the role of windborne spread in the transmission of H5N1.
He notes the increased risk of wind-driven H5N1 transmission due to infected aquatic birds such as ducks and geese contaminating lakes. The virus carried by wind from these water sources can potentially reach nearby farms, contributing to the spread of the disease. Osterholm warns that the current situation presents unprecedented challenges, as windborne spread may explain instances of infection where direct contact with sick animals cannot be identified. Notably, the case of three veterinarians testing positive for H5N1 antibodies without known animal exposure highlights the potential risks associated with airborne transmission to humans.
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