#18,965
Even after more than 2 decades of outbreaks linked to farms, live markets, and wild birds, there remains a good deal we don't know about how HPAI H5 (and other novel flu types) spread; often defeating ramped up biosecurity measures.
While many were quick to blame contaminated milking equipment for the spread of H5N1 among dairy herds in 2024, four months ago, in Preprint: Surveillance on California Dairy Farms Reveals Multiple Sources of H5N1 Transmission, we saw evidence that Bird flu is ‘everywhere’ on dairy farms.Following that report, UNMC's Global Center for Health Security - quoting Dr. Richard Webby as saying “It’s a ridiculously contaminated environment” - published Bird Flu on Dairy Farms May Be Airborne After All.
These airborne concerns go far beyond just`exhaled' breath from infected cattle in milking parlors, or `milk spray', as massive quantities of contaminated milk and manure must be safely handled and disposed of (along with farm wastewater); none of which are trivial tasks.We've seen numerous environmental persistence studies showing that - under the right conditions - HPAI H5 can survive for days, weeks, or even months outside of a living host.
- In 2012's EID Journal: Persistence Of H5N1 In Soil, we looked at several studies that found H5N1 could remain viable on various surfaces, and in different types of soil, for up to 13 days (depending upon temperature, relative humidity, and UV exposure).
- In 2017, researchers showed that - when refrigerated - H5N1 infected poultry could remain infectious for months (see Appl Environ Microbiol: Survival of HPAI H5N1 In Infected Poultry Tissues).
- In 2020 we looked at a study from researchers at the USGS (see Proc. Royal Society B: Influenza A Viruses Remain Viable For Months In Northern Wetlands - USGS), which found long-term (up to 7 months) survival of influenza A viruses in wetlands in both Alaska and Minnesota.
How long avian flu viruses may remain viable, and how far they might be carried (by personnel, vehicles, peridomestic mammals, birds, flies, or even the wind), continues to be poorly understood.
While flies weren't shown to be infected with the virus, they could ingest (and subsequently regurgitate or defecate) infected material, or potentially spread it mechanically by their feet or body, thereby spreading the disease.Very early on in this blog (2007) - in Cats and Dogs and Flies, Oh My! - we looked at a 2006 study by Kyoko Sawabe et al. that found that at least 2 types of flies could carry the H5N1 virus.
The authors wrote:We've revisited this idea a number of times since, including 2023's preprint (later published in Sci Repts: Blowflies As Potential Vectors Of Avian Influenza) that tested blowflies for HPAI at the national wildlife reserve in Izumi City, Kagoshima Prefecture, which is the overwintering home for thousands of endangered Hooded Cranes.
In December 2022, 648 Calliphora nigribarbis were collected. Influenza virus RT-PCR testing identified 14 virus-positive samples (2.2% prevalence), with the highest occurrence observed near the crane colony (14.9%). Subtyping revealed the presence of H5N1 and HxN1 in some samples. Subsequent collections in December 2023 identified one HPAI virus-positive specimen from 608 collected flies in total, underscoring the potential involvement of blowflies in HPAI transmission.
And last July, in H5N1 in California: The Return of the Fly, after Raj Rajnarayanan @RajlabN - Associate Dean of Research and Associate Professor, NYITCOM at Arkansas State University - uploaded to X/Twitter a quick analysis of H5N1 sequences sampled from a HouseFly uploaded to @GISAID from California (2.3.4.4b B3.13 Collected Oct 2024).Our observations suggest C. nigribarbis may acquire the HPAI virus from deceased wild birds directly or from fecal materials from infected birds, highlighting the need to add blowflies as a target of HPAI vector control.
Today we've a report, published this week in Scientific Reports, which documents the detection of HPAI H5N1 RNA in flies collected from several California dairy farms in 2024.
While they state that `. . . infectious virus was not detected in this study', based on the following, it appears they did not attempt to isolate or culture the virus.
The successful amplification and sequencing of full-length HPAI H5N1 genomic DNA from fly samples collected from positive dairy operations, along with the incidence of relatively low Ct values in some pools, together suggest the presence of intact viral genomes at the time of fly collection.
However, the detection of intact genomes does not necessarily equate to the presence of infectious virus, which is a limitation of our approach that could be improved if virus culture precedes molecular characterization.
As this report points out:
Prior bioassays determined that infectious HPAI H5N1 survives in the digestive tract (including crop) of house flies for up to 72 h [18] and blow flies for up to 24 h [24], although viral RNA can be detected for much longer (for example, up to 4 days in house flies and 14 days in blow flies).
Due to its length, I've only posted the abstract and a brief excerpt. Follow the link to read it in its entirety. I'll have a brief postscript after you return.
(SNIP)
Detection of H5N1 highly pathogenic avian influenza virus RNA in filth flies collected from California farms in 2024
Dana Nayduch, Stacey L.P. Scroggs, Phillip Shults, Luke A. Brendel, Lindsey M. Reister-Hendricks, Caitlin Taylor, Edward Bird, Brina Lopez & Edith S. Marshall
Scientific Reports , Article number: (2025) Cite this article
Abstract
The emergence of highly pathogenic avian influenza (HPAI) H5N1 clade 2.3.4.4b in U.S. dairy cattle highlights the urgent need to understand transmission dynamics within and among farms. House flies (Musca domestica) and blow flies (Calliphoridae), ubiquitous in agricultural settings, are suspected mechanical vectors of numerous pathogens, including viruses.
We investigated the presence of H5N1 viral RNA in filth flies collected opportunistically from four H5N1-positive farms (three dairy and one poultry) in California during the 2024 outbreak. H5N1 RNA was detected via qRT-PCR in fly pools from all four locations, with the lowest Ct values (highest viral RNA) in house flies collected near milking parlors and dead animals. Whole-genome sequencing confirmed H5N1 viral genomes in flies from dairy farms, demonstrating high similarity (99.92–99.95%) to the B3.13 lineage circulating in the region and grouping closely with farm-associated viral sequences from milk.Although infectious virus was not detected in this study, our findings suggest that filth flies acquire HPAI H5N1 RNA from their environment, supporting their potential role as sentinels and/or mechanical vectors. These results underscore the critical importance of fly control, targeted surveillance, and integrated pest management strategies in agricultural settings to enhance biosecurity and potentially mitigate HPAI H5N1 transmission.
Our data suggest that both the milking parlor and dead animals may be potentially potent sources of virus for flies on the two dairy operations for which we had these metadata, as indicated by low Ct values (i.e., high viral RNA template), a high prevalence of positive fly pools collected from these locations, and the close relatedness of genomes of fly-associated and milk-associated virus from the same farm and nearby farms in the same county (e.g., Fig. 1).Together, these findings support local acquisition of HPAI H5N1 by house flies from both milk and dead animals (carcasses, aborted fetus), which concurs with the trophic behaviors of filth flies.
Female filth flies readily seek out and feed on proteinaceous substances like milk (e.g., house flies; [15]) and both species visit dead or decaying materials for oviposition [28,29], often ingesting the substrate during this process. Interestingly, milk seems to extend the stability of HPAI H5N1 in wastewater and on structural surfaces [30,31]. Milk also has been shown to increase persistence, perhaps by decreasing degradation rates, of SARS CoV-2 ingested by house flies [32].
While there are probably multiple pathways that HPAI utilizes to spread within and between farms, it seems increasingly likely that flies are at least part of the equation.
In addition to flies, over the years we've looked at a number of other `less obvious' ways the virus may be spreading.
- There is growing evidence that HPAI may be spread via `contaminated dust particles' - carried by the wind - from one farm to the next (see Preprint: Genetic & Meteorological Data Supporting Windborne Transmission of HPAI H5N1).
- The role of peridomestic mammals - particularly in and around farms - in the spread of HPAI is only partially understood, and remains under-researched (see Emer. Microbe & Inf.: HPAI Virus H5N1 clade 2.3.4.4b in Wild Rats in Egypt during 2023).
- While contaminated milking machines have been blamed for the cow-to-cow spread of HPAI, a recent preprint (Dairy Cows Infected with Influenza A(H5N1) Reveals Low Infectious Dose and Transmission Barriers) was unable to duplicate transmission in a controlled lab setting, leading researchers to warn `. . . Other agent, host, and environmental cofactors that might contribute to transmission cannot be ruled out and must be explored . . . ).
Unless and until we get a better handle on how HPAI is spreading in the wild - and among and between farms - our ability to slow or contain these outbreaks will remain limited.
