Periplaneta americana - Credit Wikipedia
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The notion that insects could act as (likely mechanical) vectors of influenza viruses is not exactly new; nearly 20 years ago we looked at a study (see 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 by Kyoko Sawabe et al.) that found that at least 2 types of flies could carry the H5N1 virus.While these flies weren't believed 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.
Four years later Dr. Sawabe and his team would publish (Blow Flies Were One of the Possible Candidates for Transmission of Highly Pathogenic H5N1 Avian Influenza Virus during the 2004 Outbreaks in Japan) where they conclude:
We have suggested here that blow flies are likely candidates for mechanical transmission of HPAI because of their ecological and physiological characteristics as reviewed here. In fact, blow flies have already been recognized as important vectors for mechanical transmission of several serious infectious diseases, that is, poxvirus [28], rabbit hemorrhagic disease [29], and paratuberculosis [30]. Recently, it has been reported that the H5N1 viral gene was detected in house flies [31] and engorged mosquitoes [32].
While the `flies as potential vectors of H5N1' would come up occasionally, in 2024 we looked at a new study (Blowflies are potential vector for avian influenza virus at enzootic area in Japan) which warned:
` . . . 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.'
Six months ago we looked at two more studies; Nature Sci Rpts: Detection of H5N1 HPAI virus RNA in filth flies collected from California farms in 2024, along with a fascinating study on HPAI carriage by arachnids (see Preprint: Detection and Isolation of H5N1 clade 2.3.4.4b HPAI Virus from Ticks (Ornithodoros maritimus).
While none of these studies provide smoking guns on the actual spread of a viable influenza virus, they lay out a plausible mechanism for transmission.
All of which brings us to a new study - this time on cockroaches - which finds similar evidence of influenza virus carriage, although the evidence presented isn't quite as robust (or compelling) as the more extensive research on flies.
Once again, all testing was done by RT-PCR (targeting the Matrix Gene of influenza A & B), which cannot tell us if the virus was viable (infectious) or its subtype.
First the link, abstract, and some excerpts from today's study, after which I'll return with a bit more.
Preliminary molecular detection of influenza RNA in synanthropic cockroaches from shiraz, Iran
Mohsen Kalantari a 1, Mozaffar Vahedi a 1, Marzieh Jamalidoust b, Maryam Motevasel c, Amin Hosseinpour
https://doi.org/10.1016/j.parepi.2026.e00498Get rights and content
Under a Creative Commons license
HighlightsAbstract
- First molecular detection of Influenza virus RNA in Iranian cockroaches.
- Viral RNA found on cockroach exteriors and inside their digestive tracts.
- Both Influenza A and B RNAs were detected.
- Low detection rate (1.86%) of influenza virus RNA in cockroaches suggests mechanical carriage may be sporadic
- It needs more studies to confirm the presence of viable and infectious of influenza virus in cockroaches.
Cockroaches are recognized as significant mechanical vectors for a wide spectrum of pathogens, posing a considerable public health risk. Their habitation in unsanitary environments and promiscuous feeding habits allow them to acquire and disseminate bacterial agents, viruses—including poliovirus and influenza—and protozoan parasites such as Microsporidia and Giardia, as well as the eggs of parasitic worms. This study investigates the potential of cockroaches to mechanically carry influenza viruses, a subject that remains underexplored despite the significant global burden of influenza.
During the seasonal peak of influenza activity, a total of 322 cockroaches were collected from various high-risk locations in Shiraz, Iran, including hospital premises, university dormitories, and academic faculties. The sampling targeted two predominant species, Blattella germanica and Periplaneta americana, captured from diverse microhabitats such as kitchens, rooms, and sewers.
Using a highly sensitive real-time PCR methodology, which targeted the conserved matrix genes of influenza A and B viruses, both external body surface washes and internal digestive tract samples were analyzed. The results confirmed the presence of influenza A virus RNA in four external surface samples of Blattella germanica and in two internal digestive tract samples of Periplaneta americana.
Notably, the majority of positive samples (5 out of 6) were for influenza type A, with one sample positive for influenza type B. The overall detection rate was low (1.86%, 6/322), these findings demonstrate that cockroaches in urban environments can indeed harbor influenza virus RNA, either externally on their bodies or internally within their digestive systems. This molecular detection highlights the presence of viral RNA, suggesting possible mechanical carriage.
However, the detection of RNA alone does not confirm the presence of viable, infectious virus, which is a key limitation of this study. Given their intimate association with human dwellings and food sources, this molecular evidence indicates potential but unconfirmed route for mechanical carriage that warrants further investigation. Crucially, our study design cannot assess transmission risk. Consequently, the findings underscore the necessity for additional comprehensive studies to assess viral viability and to elucidate the potential, yet unproven, role of cockroaches in the epidemiology of influenza.
(SNIP)
5. Conclusion
This preliminary study provides the first molecular evidence of influenza A virus RNA associated with cockroaches in Iran. The detection of viral RNA on external surfaces and within the digestive tract provides molecular evidence of environmental contamination and ingestion, respectively. The detection of viral RNA on external surfaces and within the gut suggests environmental contamination and passive ingestion, respectively. However, the low prevalence rate (1.86%) indicates that such detections may be infrequent under the studied conditions.
The central limitation is that our PCR-based method cannot confirm viral viability or infectiousness, and no such assessments were performed. Therefore, the detection of RNA does not constitute evidence of transmission risk. The critical question of whether cockroaches can carry and transmit viable influenza virus remains unanswered, as our PCR-based method cannot confirm viability nor, without sequencing, provide definitive proof of target origin. Our findings should not be interpreted as demonstrating a transmission pathway but rather as identifying the presence of viral genetic material in an environmental context that justifies further investigation under a One Health framework. Future studies must include viral culture and infectivity assays to move from molecular detection to a substantive understanding of any potential epidemiological role.
While evidence for transmission remains elusive, it is worth pursuing.
We've also looked at a number of other `less obvious' ways the HPAI H5 virus may be spreading.
- We've seen 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).
