Will ducks be the source of an H5N1 pandemic?

[Laidback Al]

Will ducks be the source of an H5N1 pandemic?

Most confirmed human H5N1 cases have been attributed to cross species transmission from sick or died chickens (e.g. the most recent case in Indonesia, an 8 year old girl that died). However, infections from domestic ducks seem to present a more serious pandemic threat based new lines of evidence.

In 2009, Jeong-Ki Kim and colleagues published an article entitled “Ducks: The “Trojan Horses” of H5N1 influenza” (full article). The authors speculate that wild ducks, as the main reservoir of HPAI, could be “silent spreaders” of H5N1 and could easily make a transmission leap to other species including humans.

They state in part

Another recurring characteristic of recent H5N1 HPAI viruses in ducks is that virus titers are frequently higher in oropharyngeal swabs than in cloacal swabs [30,34]. Pharyngeal excretion of H5N1 HPAI viruses has been suggested to originate from the lung and/or air sac, as only these tissues have shown immunohistochemical evidence of virus replication. Preferential pharyngeal excretion suggests that pharyngeal swabs, as well as the customary cloacal swabs, should be taken when conducting surveillance of avian influenza viruses in wild ducks [34]. Otherwise, the prevalence of H5N1 HPAI may be underestimated. Additional studies of the role and rates of respiratory transmission of H5N1 HPAI viruses in ducks are needed, especially as they relate to cloacal excretion.””
While all duck species tested to date are susceptible to lethal infection with H5N1 HPAI viruses, some species, including the mallard and Pintail ducks, are less susceptible and many survive. It is in these relatively resistant species that the H5N1 HPAI viruses could be maintained. However, surveillance studies to date in these species have detected no H5N1 HPAI viruses in breeding or juvenile birds. Experimental studies show that some mallard ducks continue to shed virus for up to 17 days, allowing the development of humoral immunity and subsequent selection of antigenic variants within the same bird [23]. If this occurs, it could be argued that a limited number of ducks would be sufficient to maintain the virus in nature. Continued surveillance is needed to determine whether H5N1 HPAI viruses are maintained in nature by a small number of naturally resistant ducks that are long-term virus shedders.

Shifts in receptor binding

The authors of the article cited above suggest that a shift in receptor binding has been taking place in wild and possibly domestic ducks. A PHD dissertation from 2010 seems to confirm that ducks are more likely to transmit H5N1 through there respiratory system rather than through their excrement (FT link).

The shift of receptor binding from lower to upper airways for H5N1 poses an increased risk for airborne transmission as demonstrated by Fouchier’s recent H5N1 transmission study. So, asymptomatic ducks shedding the H5N1 virus from the oropharynx should be a concern.

Fouchier notes that the laboratory created, aerosolized H5N1 virus

has a changed receptor specificity. So rather than preferring the receptors that are present in the lower arrays, this particular virus – this airborne virus now prefers to bind to virus receptors present in upper airways. And that's why it can be transmitted. . . .
http://www.eurekalert.org/h5n1/H5N1Transcript.pdf

Asymptomatic ducks

Confirming that ducks can be asymptomatic carriers of H5N1 is a 2012 article noting that clinically healthy ducks in Indonesia are carrying H5 antibodies (link). Similarly, another study from Vietnam published in 2012 shows that about 3.3% of the pooled samples from live ducks tested positive for H5N1 with up 6% of the ducks from Mekong delta testing positive (FT link).

Increases in H5N1 infections in ducks

Since the beginning of 2012 numerous confirmed and suspected outbreaks of H5N1 among ducks have been reported in Vietnam, China, and elsewhere. Whether these outbreaks represent an uptick in H5N1 infections in ducks is not clear.

Vietnam - Nghe An province, 1,300 ducks destroyed following H5N1 positive test Jan 2012

Indonesia: Hundreds of ducks die in Lumajang, E. Java-poisoning suspected Jan 2012

South Korea starts investigation mass bird deaths - no H5N1 found Jan 2012

India - Bird flu resurfaces in Tripura 6.2k birds culled Jan 2012

Vietnam - Haiphong: discovered new outbreak of influenza A H5N1 in ducks Feb 2012

H5N1-infected poultry appeared in Quang Tri Feb 2012

Vietnam Ha Tinh: outbreak of avian influenza H5N1 Feb 2012

Vietnam: Quang Nam renewed outbreak of avian flu in ducks Feb 2012

China - 1000s ducks dead - suspected cholera or H5N1 bird flu in Zhuzhou, Hunan province Feb 2012

Vietnam - Detection of influenza A/H5N1 virus in ducks in Hanoi Mar 2012

Ducks infected with influenza A/H5N1 in Quang Ngai Mar 2012

China - H5N1 bird flu outbreak reported in ducks, 17,127 culled in Xinhui district, Jingmen, Guangdong province June 2012

But what is clear is that H5N1 infected ducks pose a pandemic threat. A recent article reviewed potential risk factors for H5N1 (abstract link). Although the authors do not apparently single out ducks, they identified domestic waterfowl and the presence of a water source as two of the main statistical associations with HPAI H5N1 outbreaks across all regions studied.

Recent Human Case in China

Perhaps the most evident example of the potential for H5N1 infections from ducks is the 2 year old child in Guangdong Province in China. The child most likely contracted the virus from an infected, but asymptomatic, duck that was slaughtered in a local wet market in the Haizhu District. The child was apparently present when the duck was slaughtered (FT link).

Conclusion

All of this information taken together suggests that ducks pose a special risk for harboring and transmitting H5N1. Not only are asymptomatic domesticated ducks a vector for H5N1 transmission but they could also be the mixing vessel for an aerosolized strain transmissible among humans. There needs to be increased surveillance in both wild and domestic duck populations for H5N1 with a particular emphasis on wet markets. It now seems that an H5N1 pandemic strain could make the jump to humans from diseased ducks just as likely as from infected chickens.

 

[JJackson]

Re: Will ducks be the source of an H5N1 pandemic?

Absolutely agree. HP H5N1 is not going away and at present chickens die in short order so are obviously sick and, in endemic areas, given a wide birth. Ducks can be asymptomatic and are therefore dangerous as they do not advertise the fact they are sick.
As you point out a secondary factor, of unknown significance, is that the ducks can develop extended quasi-species infection allowing considerable scope for recombination and reassortment which we discussed in more detail in the 'are we quacking up the wrong tree' thread.

The shift of receptor binding from lower to upper airways for H5N1 poses an increased risk for airborne transmission as demonstrated by Fouchier’s recent H5N1 transmission study. So, asymptomatic ducks shedding the H5N1 virus from the oropharynx should be a concern.

I wondered about this. While I know that in humans the URT is a2,6 heavy and the LRT is a2,3 heavy - relatively - is this also true for ducks? I know the 'normal' avian transmission route is via the gut by a2,3 but are duck respiratory tract epithelial cells mainly a2,3 or a2,6?