Not all birds are equal – a new paper debunks the notion of passerines as influenza A virus reservoirs

Influenza A viruses are elusive, just like the Scarlet Pimpernel - scientist seek them everywhere!

Influenza A viruses are elusive, just like the Scarlet Pimpernel – scientist seek them everywhere!

By Jonas Waldenström

In each scientific field there are findings that stand out as peculiar; odd findings that are not widely replicated. Still, as they are part of the scientific record, you need to relate to them in your own work, even cite them at times. For the influenza A virus field, one such oddity has been the detections of virus in passerines. A bird is a bird, you might say – so if ducks and other waterfowl are loaded with these viruses, why cannot other birds be infected?

However, birds cannot (and should not) be lumped together in a big pile just because they have feathers. Among the world’s 10,000 or so species there are both physiological and ecological differences – not to neglect millions of years of evolution. Thus, there are likely differences both related to exposure (geographical distribution, habitat preferences, behaviors, diet, etc.) and to susceptibility or pathogenesis (distribution of receptors and perceptive cell types, physiology of the gastrointestinal tract, immune responses, etc.) that govern how readily different bird species are infected. On top of this, the very methods we use to detect virus have their issues. It is not uncommon to have lab contaminations, especially of PCR-products, that can make the very sensitive RRT-PCRs say ‘bing’, when they should say ‘bong’.

This week, Morgan Slusher et al. in Georgia, US, published a comprehensive review of influenza A virus in passerines. Not only did they critically evaluate all articles reporting findings, they also conducted a large prospective study where they sampled and screened wild birds.

So what did they find? First of all, the review (in total 60 papers published up till 2012) revealed that the majority of virus findings in passerines were associated with outbreaks in domestic birds, or were from birds in periurban settings. Only few cases were described from wild birds in more natural settings. Furthermore, the authors identified a general lack of confirmatory proof, e.g. if samples were positive in a PCR screening there was no subsequent isolation (or sequence) of virus from those samples. Some papers were even pinpointed as potentially flawed, due to non-validated screening methods (nested PCRs that are prone to yield false positives) or to potential lab contaminants of viruses (where the same subtype was isolated in many samples collected from several locations, but processed in the same lab).

Second, the prospective screening of samples, both by RRT-PCR, isolation attempts, and an antibody-based ELISA, yielded very few positive signals. Actually, none of the birds tested by RRT-PCR (547 samples) or virus isolation (900) were positive, and only 3 out 3,358 tested with the ELISA method gave a signal for past infections.

The conclusions, at least to me, is that terrestrial passerines should not be considered as reservoir hosts. This is not the same as saying that they are never infected, but that in terms of influenza A virus epidemiology and evolution they are accidental hosts, often caused by spillover infections from infected poultry in connection to outbreaks. I think this is similar to what most influenza A virus ecologists thought already, but it is extremely important that a study such as this was published – again, because it becomes part of the scientific literature, and not just opinions of the individual researcher.

On a general note, I think this exemplifies how one needs to distinguish between different types of hosts. As most pathogens can infect multiple hosts, but with varying proficiency, a mere positive finding in a species should not be implied as that species is a functional host, or a reservoir. Most spillovers are dead-end infections, or result in short stuttered transmission chains. They should of course be studied – not the least because a pathogen may evolve better transmissibility in the new hosts – but some level of caution in language use is needed, as we otherwise give the wrong information about host range and epidemiology.

So, at last, let me paraphrase the Scarlet Pimpernel:

We seek it here, we seek it there,
Those Scientists seek AIV everywhere!
Is it in sparrows? Is it in trogons?
Where are those damn elusive AIV virions!

A Red-headed Trogon - not exactly a passerine, but it was the only bird to rhyme (although not great) with virion. Photo by JJ Harrison  [CC-BY-SA-3.0, via Wikimedia Commons].

A Red-headed Trogon – not exactly a passerine, but it was the only bird to rhyme (although not great) with virion. Photo by JJ Harrison [CC-BY-SA-3.0, via Wikimedia Commons].

Link to the paper:

Slusher, M.J., Wilcox, B.R., Page Lutrell, M., Poulson, R.L., Brown, J.D., Yabsley, M.J. and Stallknecht, D.E. 2014. Are passerine birds reservoirs for influenza A viruses? Journal of Wildlife Diseases, ahead of print.

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2 thoughts on “Not all birds are equal – a new paper debunks the notion of passerines as influenza A virus reservoirs

  1. because transmission is waterborne and more effective in mallards but
    respiratory in passerines,and poultry ?!
    Inbetween come Chinese domestic ducks as mixing vessel

    • In order to be a reservoir host a species needs to be capable of sustaining transmission in absence of other hosts. The serological evidence so far is not compatible with widespread transmission in passerines, but more studies looking at this would be good (and there is plenty of blood collected in different avian biology studies that could be screened). Passerines may still function, in certain settings, as possible bridge species – and possibly viruses adapted for transmission in poultry may be better suited to infect passerines. More work to be done, for sure. But for general LPAI epidemiology, they are more the victims of rare spillovers than driving any virus dynamics.

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