Kyushu University: The Return Of The Fly

#17,842

One of the mysteries surrounding the spread of HPAI is exactly how it manages to thwart farm biosecurity so easily, and make its way into hen houses, mink barns, or occasionally swine herds.  Quite often, when one farm in a region is affected, others in close proximity are struck as well. 

Early on, it was assumed cross-farm contamination - from vehicles, personnel, or products - was to blame.  Farm quarantines, and decontamination stations, became part of the biosecurity  playbook. 

But often, despite their best efforts, we'd still see the virus spread from farm to farm.  Migratory birds - once thought unlikely carriers of the virus (see 2014's Bird Flu Spread: The Flyway Or The Highway?) - became increasingly suspect.

But even flocks kept indoors, and protected by several layers of biosecurity, continue to succumb to the virus. 

Small peridomestic animals, like rats, voles, shrews, and even cats and dogs are suspected of being capable of picking up the virus from the outside environment and bringing it inside (see Taking HPAI To The Bank (Vole) and Experimental Infection of Peridomestic Animals With Avian H7N9). 

And even dust (dried feces, feathers, etc.) from poultry farms and live bird markets have been found to contain viable virus (see EID Journal: HPAI (H5N8) Clade 2.3.4.4b Virus in Dust Samples from Poultry Farms, France, 2021), and some studies have suggested it may be carried for several (perhaps dozens) kilometers by the wind. 
 

During the 2014-15 avian epizootic in the United States, airborne spread between farms miles apart was considered possible, although never conclusively proven see APHIS: Partial Epidemiology Report On HPAI H5 In The US) .

One recurring entry to this rogue's gallery of potential vectors is the common housefly, a pest which is frequently found in abundance in and around poultry farms and live bird markets.

While it wasn't its first mention in this blog, nearly 17 years ago in Cats and Dogs and Flies, Oh My!, we looked at a 2006 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. 

They wrote:

The H5 influenza A virus genes were detected from the intestinal organs, crop, and gut of the two blow fly species, Calliphora nigribarbis and Aldrichina grahami, by reverse transcription-polymerase chain reaction for the matrix protein (M) and hemagglutinin (HA) genes. The HA gene encoding multiple basic amino acids at the HA cleavage site indicated that this virus is a highly pathogenic strain. . . . . Our results suggest it is possible that blow flies could become a mechanical transmitter of H5N1 influenza virus.

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]. 

We suggest that mechanical transmission by flies may also be involved in the outbreak and pandemic of infectious diseases other than HPAI. However, although there are high densities of a variety of fly species during all seasons in Southeast Asia, their ability to transmit viruses has not been evaluated. The prevalence of H5N1 avian influenza is still a public health problem for birds and humans. Therefore, field and laboratory studies on mechanical transmission of pathogens by flies would be very important for controlling H5N1 avian influenza outbreaks, at least in epidemic Southeast Asian countries.

We're not talking about `infected' flies. but rather contaminated flies.  But it seems likely that they may be a contributor to the spread of the virus.  

All of which brings us to a new study, called Relationship between avian influenza virus and blowfly found in Izumi city, Kagoshima, a presentation of which is summarized on the University's website:

Ryusuke Fujita, Takuji Tate, Kosuke Nagata, Masato Hino, Masahiro Saiki, Relationship between avian influenza virus and giant blowflies observed in Izumi City, Kagoshima Prefecture, 75th Annual Meeting of the Japanese Society of Hygienic Zoology, 2023.04, 2022–2023 winter season will be nationwide The country experienced the largest outbreak of highly pathogenic avian influenza (HPAI) on record. Izumi City, Kagoshima Prefecture, is the world's largest wintering site for cranes, with more than 10,000 cranes arriving each year, and is also a major poultry industry base, with the highest production of chicken eggs in Japan in 2019. The damage caused by HPAI is serious, with more than 10% of chickens dying from HPAI, and HPAI occurring frequently in neighboring poultry farms.  Regarding HPAI, it has been previously suggested that flies such as the giant blowfly are involved in its spread. Therefore, we collected flies that feed in winter, such as the giant blowfly, in the area, and investigated the influenza that was taken into the flies' bodies. Overall, 2% of the 651 flies tested were positive for the virus. Looking at the distribution by region, the virus positivity rate was highest at 14.7% around the crane migration area, and the positive rate was 4.8% and 1.7%, respectively, in the lower reaches of two rivers in the city where waterfowl were seen. showed that.  On the other hand, no virus was detected in areas several kilometers inland from these areas. From these results, it was inferred that the HPAI nuclear power plants in the region are crane colonies and waterfowl habitat rivers, and that poultry farms within 2-3 km of these sites could be a risk area for virus transmission by flies.   This study is getting a lot of attention in the Japanese press, with Dr. Ryusuke Fujita quoted as saying, ``Until now, countermeasures have been taken based on the assumption that small animals and people will bring the virus. There was no improvement, and when we suspected it was a fly, a virus was detected. We will conduct a more detailed investigation, take measures to prevent fly intrusion, and verify their effectiveness." Along with many of the other vectors mentioned above, it seems increasingly likely that flies carry some of the blame for the spread of avian influenza on farms. How much, is anyone's guess.   Viruses haven't endured for hundreds of millions of years on this planet by being a one-trick-pony.  While we may not make a huge dent in HPAI by controlling flies, given the damage it does, anything that can help reduce its spread is probably worth pursuing.