Avian Flu Diary: COVID-19: The Avian Vector Loophole

Credit ECDC

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Coronaviruses are divided into 4 distinct genera; Alphacoronaviruses, Betacoronaviruses, Gammacoronaviruses, and Deltacoronaviruses - and while both birds and mammals are susceptible to coronavirus infection - they each (at least, for the most part) stay in their own lane.

Birds are primarily infected by gammacoronaviruses, such as infectious bronchitis virus (AIBV) and occasionally by deltacoronaviruses, while mammals are primarily affected by alphacoronaviruses and betacoronaviruses.

But there are a few crossovers - particularly with Deltacoronaviruses - which have been detected mostly in birds and but occasionally in mammals (see Discovery of seven novel Mammalian and avian coronaviruses in the genus deltacoronavirus . . . . ).

Porcine deltacoronavirus (PDCoV), first identified in 2012, is one of those DCoV outliers we keep an eye on (see PNAS: Broad Receptor Engagement of PDCoV May Potentiate Its Cross-Species Transmissibility) due to its feared zoonotic potential (see also New pig virus found to be a potential threat to humans).

Which is why, when dealing with viruses, one never likes to say `never'.

Luckily SARS, MERS-CoV, and SARS-CoV-2 (COVID-19) are all Betacoronaviruses, and have shown no signs of infecting avian species, which would likely complicate this pandemic enormously.

But, according to the The Danish Veterinary and Food Administration - which has been investigating the rapid spread of COVID-19 between mink farms - there is a plausible way birds could still be carrying and transmitting the virus between farms.

The DVFA FAQ on Infection Routes For Mink first examines a number of possible ways the virus could have been spread, including :

Machine stations / veterinarians / temp agencies : The Danish Veterinary and Food Administration investigates several routes of infection, and is in contact with feed suppliers, machine stations, temp agencies and veterinarians who have their way on the farms. These data are compared with data from SSI on knowledge of infection with COVID-19 in groups of people associated with mink productions.
SSI and the University of Copenhagen have come to the conclusion that herd size and distance to the nearest infected mink farm are the two factors that are decisive for whether there is an increased risk of a mink farm becoming infected.
Relocation : Trade in mink has not been a contagion in the suspected or infected herds. Live mink are not usually traded in the middle of the season, and in the few places that have happened, it has been a long time before the infection in the herd.
Fences : The fences are generally intact on the Danish mink farms. Only very few farms have defective fences where mink can escape. The Danish Veterinary and Food Administration considers it too unlikely that escaped mink from infected farms play a role in the spread of COVID-19. To follow up on the risk that potential escaped mink from infected mink farms may have an impact on the spread of infection, the killing of wild mink has been intensified. Abandoned (and dead) mink are collected and sent regularly to the Danish Veterinary and Food Administration, which ensures that they are tested for COVID-19. So far, no COVID-19 has been found in mink killed / found outside mink farms.
Feed : The Danish Veterinary and Food Administration has tested feed samples for the entire period, without finding COVID-19. We do not immediately assess that feed is a source of infection for COVID-19 in mink. Read more about feed further down the page.
Pets : The Danish Veterinary and Food Administration has tested pets and other domestic animals. Some dogs have been infected, but only for a short period of time. The dogs have been isolated on the property. In addition, cats infected with COVID-19 have also been found. No spread to other domestic animals has been seen. ( Read more in the FAQ about pets and coronavirus )

In a separate section, the FAQ looks at Can birds transmit infection?

Yes, it is estimated that birds may play a role in the spread of COVID-19 between mink farms, both over short and long distances. This can be done by transporting the virus passively, either on the feet, beak or feathers or in the digestive tract.

Aarhus University has conducted a study for the Danish Veterinary and Food Administration on the potential role of birds in the spread of COVID-19 between mink farms. According to the study, it is considered likely that birds visiting a mink farm infected with COVID-19 will come into contact with viruses, for example via contact with feed residues or mink hair that has been in contact with infected mink.

Birds can transmit infection to mink by, for example, eating their food before the mink get it or while it is lying on top of the cage. In at least one case, it has been observed that rainwater dripping from a leaky roof over an enclosed hall has contained viruses. This virus could originate from the feet of birds that have been sitting on the roof.

Updated November 25th

Those with very long memories will recall this type of `mechanical transmission' (via the common house fly) was considered seriously during the rapid spread of H5N1 in Indonesia and East Asia during the last half of the first decade of the 21st century (see Cats and Dogs and Flies, Oh My!).

While scientists were able to successfully isolate the HPAI H5 virus either on/or in blowflies (see study below), actually proving they had spread the virus is another matter altogether. But it did suggest it was possible.

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.

We've seen other studies (which may be applicable) linking infected `dust' (dried feces, feathers, etc.) from poultry farms - often propelled by large exhaust fans - being carried several kilometres by the wind and infecting other nearby farms.

The science of all of this even has a name; aerobiology – the study of how bacteria, fungal spores, pollen and even viruses can be passively transported in the air.

In 2010 (see Viruses Blowin’ In The Wind?) we saw a report in the journal Environmental Health Perspectives, that suggested that it was possible for H5N1 (or any Influenza A virus) to be transported across long (hundreds of kilometers) distances in the air, although viability was unknown.
In 2012's Barnstorming Avian Flu Viruses? we looked at a study in the Journal of Infectious Diseases called Genetic data provide evidence for wind-mediated transmission of highly pathogenic avian influenza that found patterns that suggested farm-to-farm spread of the 2003 H7N7 in the Netherlands due to the prevailing wind.
Another study of the same outbreak, Modelling the Wind-Borne Spread of Highly Pathogenic Avian Influenza Virus between Farms (PloS One 2012), found that wind borne transmission could have accounted for up to 24% of the transmission over distances up to 25 km.

In the spring of 2015 during the North American H5Nx epizootic, the idea of farm-to-farm spread via infected dust was openly discussed by the USDA (see Bird Flu’s Airborne `Division’).

While not directly addressing this possibility of wind-borne spread of COVID-19, the ECDC's Rapid Risk Assessment on COVID-19 in mink (LINK) does note:

Mink are housed in adjoining cages made of wire netting, allowing free airflow and contact between animals in adjacent cages, which explains the rapid animal-to-animal transmission. Furthermore, the presence of viral RNA in inhalable dust collected from inside the farms indicates the possibility of workers having been exposed to the virus in mink excretions.

Following outbreaks of SARS-CoV-2 infection at mink farms in the Netherlands during 2020, the points below should be borne in mind.
Mink-to-mink transmission is very efficient
No link has been identified between infected farms.
A proportion of farm animals had clinical symptoms suggestive of acute respiratory illness for about four weeks, but some animals were still RT-PCR–positive for SARS-CoV-2 in throat swabs after clinical symptoms had disappeared. This may vary, as seen recently in Danish outbreaks, where clinical symptoms were observed for shorter periods.
The high diversity in the sequences from some mink farms can probably be explained by the existence of many generations of infected animals before an increase in mortality was observed.The current estimates are that the substitution rate of SARS-CoV-2 is one mutation per two weeks.
There was also a relatively high sequence diversity observed in farms, which still tested negative one week prior to the outbreak, implying that the virus evolves more quickly in the mink population. This can indicate that the virus might replicate more efficiently in mink or that it may have acquired mutations which make it more virulent in mink [5].

Whether carried by the movement of humans or machinery between farms, by the wind, or on the feet, feathers, or GI tract of of wild birds, COVID-19 has repeatedly demonstrated its ability to quickly spread among mink farms in numerous countries.