Effect Measure

Hard on the heels of my semi-facetious prediction that bird flu would return to Germany because Germany had declared itself bird flu free, the Swiss announced an infected wild duck on the shores of Lake Sempach. Since this duck didn’t have a passport on him I am sure he never strayed over the nearby border with Germany. We don’t know what kind of duck this was [see update, below], a question that is of surprising interest in light of a new paper.

Bird flu is avian influenza, i.e., an infection of birds by the influenza virus. The role of wild migratory birds versus human caused movement of poultry in the spread of bird flu is a hotly debated and controversial topic. If a bird is infected with highly pathogenic avian influenza virus, subtype H5N1, is it well enough to fly long distances? This is a potentially testable proposition, but there are a lot of different kinds of birds and it turns out even closely related bird species may act very differently. Wild waterbirds, such as ducks, have been prominently mentioned as a possible vector for bird flu spread but information about how severely they are debilitated by infection with H5N1 is confusing and sometimes contradictory. A team in The Netherlands recently undertook to infect six different common species of ducks to find some preliminary answers. They chose duck species that were abundant, favored freshwater habitats and had migration patterns spanning Asia, Europe and Africa.

Ducks are part of a large family of birds and come in two main varieties, diving ducks and dabbling ducks. Diving ducks forage for food underwater. They are good swimmers but not such good flyers, slower and heavier than the dabbling ducks. Dabblers feed on the surface or near surface or on land. The Dutch team infected 2 species of diving ducks (tufted and pochard) and 4 dabbling ducks (mallard, common teal, Eurasian wigeon and the gadwall) with highly pathogenic avian influenza virus (A/turkey/Turkey/1/2005 (H5N1)). The idea was to see which ducks would be infected, would excrete sufficient virus and be healthy enough to fly long distances. Here are the results:

By experimentally infecting wild ducks, we found that tufted ducks, Eurasian pochards, and mallards excreted significantly more virus than common teals, Eurasian wigeons, and gadwalls; yet only tufted ducks and, to a lesser degree, pochards became ill or died. These findings suggest that some wild duck species, particularly mallards, can potentially be long-distance vectors of highly pathogenic avian influenza virus (H5N1) and that others, particularly tufted ducks, are more likely to act as sentinels. (Keawcharoen et al., Emerging Infectious Diseases)

Even though low doses were used, all the species were readily infected. Both species of dabbling diving ducks (tufted and pochard) became clinically sick, while the dabbling ducks remained apparently well (NB: Redheads, a North American duck has been shown to be readily infected, not susceptible to illness and is a diving duck.) Ducks either excreted a lot of virus through their mouth or not. The high excreters were tufted ducks, pochards and mallards, although even within a species there was considerable variation in excretion. Not all ducks shed virus to the same degree. Cloacal (“rectal”) excretion was low, suggesting that the common practice of sampling wild birds for virus with cloacal swabs might not be efficient. This also contrasts with low pathogenicity avian flu viruses which are mainly shed through the intestines. Chickens and other poultry have multisystem failure associated with active viral infection of the lining of the blood vessels and heart. This apparently doesn’t occur in ducks.

The mallard and possibly some pochard are thus the birds most likely to spread the virus over distance, while the pochard and tufted duck are more likely to be found dead, sentinels of local infection. This means that active surveillance of live birds, as the Swiss were doing at Lake Sempach, should concentrate on mallards, while passive surveillance, analyzing dead birds, should look for tufted ducks and pochards. In either case, the virus should be looked for in pharyngeal swabs and “internal organs such as brain, trachea, lung, pancreas, kindey , and spleen.”

The mallard emerges from this as a potential vector to be reckoned with. It is estimated that in Western Eurasia there are some 9 million mallard ducks flying around. Time, perhaps, to check their cargo manifests.

Update, 0830 EDST: We learn via CIDRAP News that the duck was a European pochard. This is consistent with the findings of the Dutch paper, although the diving pochards did become ill to varying degrees. This duck was alive and said to be asymptomatic when virus was obtained (it would be interesting to know the nature of the specimen) and any necropsy findings if they are available. The pochard sheds H5N1 through its pharynx (nose and throat) in good quantity so the discovery of this infected animal has to be of concern.

Comments

  1. #1 JJackson
    March 28, 2008

    “Chickens and other poultry have multisystem failure associated with active viral infection of the lining of the blood vessels and heart. This apparently doesn’t occur in ducks.”

    Why?

    My understanding was that lack of organ specificity in HP AI infections was a function of the polybasic HA cleavage site being amenable to cleavage by a wide range of host proteases. If that is so why do ducks not suffer in the same way as poultry, pigs and us?
    Am I wrong or do ducks have unusually proteases. I had assumed that as they (proteases) are such an important part of all RNA/DNA based life forms their genetic lineages would be ancient and therefore very similar in species that differentiated in the reasonably recent past i.e. a lot closer in ducks and chickens than ducks and ferrets.

    Obviously my reasoning has some flaws which I would be grateful if someone could help me find.

    JJ

  2. #2 revere
    March 28, 2008

    JJ: You have it correct. I don’t think they know why the tissue tropisms are different in different species of birds.

  3. #3 Steve
    March 29, 2008

    Both species of dabbling ducks (tufted and pochard) became clinically sick, while the dabbling ducks remained apparently well…
    Should read:
    Both species of diving ducks (tufted and pochard) became clinically sick, while the dabbling ducks remained apparently well…

  4. #4 revere
    March 29, 2008

    Steve: Yikes. Thanks. Will correct.

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