H5N1, a known causative pathogen behind avian influenza or bird flu, also spreads through houseflies and blowflies. Raj Rajnarayanan posted on X on 18 July 2025 a quick analysis of H5N1 sequences sampled from a housefly uploaded to the Global Initiative on Sharing All Influenza Data or GISAID from California. This causes a bit of discussion online. It demonstrated that the virus responsible for the devastating avian influenza can be spread through houseflies.
Houseflies and Blowflies as Vectors of H5N1 Avian Influenza Virus: Evidence Linking H5N1 Transmission to Insects
Studies on Blowflies and Houseflies
The aforementioned is not a new phenomenon. Kyoko Sawabe et al. analyzed samples of blowflies collected in the vicinity of a poultry farm in Kyoto where an outbreak of highly pathogenic avian influenza occurred in 2004. Results revealed that these insects do not get infected by the virus, but can be effective spreaders through mechanical transmission via their feet or bodies, or through ingestion and subsequent regurgitation or defecation of contaminated materials.
Note that the findings above were published in 2006. Sawabe and his colleagues published another paper in 2011. It presented blowflies as one of the possible candidates for transmission during the 2004 avian influenza outbreak in Kyoto. It further recommended that field and laboratory studies should focus their attention on mechanical transmission via blowflies to control avian influenza outbreaks in high-risk areas with high densities of different fly species.
Researchers Ryosuke Fujita et al reiterated the aforesaid in their 2024 paper. 14 percent of samples of flies collected in 2022 were positive for H5N1 and other subtypes. Another round of samples was collected in 2023. One out of 608 flies was positive for highly pathogenic avian influenza. This indicated virus persistence in fly populations. The same paper also noted that flies acquire the virus via feeding on infected animals or contact with contaminated fecal matter.
It is important to note that houseflies are as capable of spreading the H5N1 virus. An experiment carried out by S. Wanaratana, S. Panyim, and S. Pakpinyo used lab-reared houseflies that were exposed to the virus. Results proved that these insects can harbor the H5N1 virus after feeding on contaminated materials. The virus did not replicate inside these flies. These houseflies can harbor infectious virus particles in and on their bodies and transfer them to the environment.
Key Implications and Takeaways
The discovery that houseflies and blowflies can harbor and transmit the H5N1 avian influenza virus has significant implications for disease control strategies. Note that bird flu transmission has been traditionally associated with direct contact between infected birds, contaminated materials, and humans or animals. However, based on evidence from studies, current contamination protocols may be underestimating a crucial transmission route involving insect vectors.
Biosecurity measurements in farms also tend to focus primarily on human movement, exclusion of wild birds, and surface disinfection. However, houseflies and blowflies, which are common in such environments, may readily bypass these measures and cause either localized outbreaks or further disease transmission outside the containment area. Evidence suggests that insect control should be considered a critical element of biosecurity measures in farm operations.
Blowflies in the wild underscore another critical concern. These insect vectors can act as bridges between outbreaks in the wild and domestic animal populations. Infected flies could potentially introduce the virus to new geographic regions or species. This further complicates surveillance and containment efforts. This overlooked pathway also expands the known ecology of avian influenza and emphasizes the role of environmental vectors in spreading pathogens and diseases.
Public health policies and veterinary guidelines may need urgent revision to incorporate this new risk factor. Fly control approaches like insect-proof barriers, sanitation, and targeted insecticide use should become standard in high-risk zones. Surveillance systems could include entomological sampling to detect early signs of viral circulation. Recognizing insects as vectors of the H5N1 virus can result in a more integrated and multi-species approach to managing outbreaks.
FURTHER READINGS AND REFERENCES
- Fujita, R., Tachi, T., Hino, M., Nagata, K., Saiki, M., Inumaru, M., Higa, Y., Itokawa, K., Uemura, N., Matsumura, R., Kai, I., Sawabe, K., Kobayashi, M., Isawa, H., Kusakabe, T., Matsuo, K., and Kasai, S. 2024. “Blowflies are Potential Vectors for Avian Influenza Virus at Enzootic Area in Japan.” Scientific Reports. 14(1). DOI: 1038/s41598-024-61026-1
- Rajnarayanan, R. 18 July 2025. “Quick Analysis of Seqs Samples from Housefly Uploaded to GISAID from California.” X. Available online
- Sawabe, K., Hoshino, K., Isawa, H., Sasaki, T., Kim, K. S., Hayashi, T., Tsuda, Y., Kurahashi, H., and Kobayashi, M. 2011. “Blow Flies Were One of the Possible Candidates for Transmission of Highly Pathogenic H5N1 Avian Influenza Virus During the 2004 Outbreaks in Japan.” Influenza Research and Treatment. 201: 1-8. DOI: 1155/2011/652652
- Sawabe, K., Hoshino, K., Isawa, H., Sasaki, T., Hayashi, T., Tsuda, Y., Kurahashi, H., Tanabayashi, K., Hotta, A., Saito, T., Yamada, A., and Kobayashi, M. 2006. “Detection and Isolation of Highly Pathogenic H5N1 Avian Influenza A Viruses from Blow Flies Collected in the Vicinity of an Infected Poultry Farm in Kyoto, Japan, 2004.” The American Journal of Tropical Medicine and Hygiene. 75(2): 327-332. DOI: 4269/ajtmh.2006.75.327
- Wanaratana, S., Panyim, S., and Pakpinyo, S. “The Potential of House Flies to Act as a Vector of Avian Influenza Subtype H5N1 Under Experimental Conditions.” Medical and Veterinary Entomology. 25(1): 58-63. DOI: 1111/j.1365-2915.2010.00928.x