Ducks: the "Trojan horses" of H5N1 influenza

Abstract

Wild ducks are the main reservoir of influenza A viruses that can be transmitted to domestic poultry and mammals, including humans. Of the 16 hemagglutinin (HA) subtypes of influenza A viruses, only the H5 and H7 subtypes cause highly pathogenic (HP) influenza in the natural hosts. Several duck species are naturally resistant to HP Asian H5N1 influenza viruses. These duck species can shed and spread virus from both the respiratory and intestinal tracts while showing few or no disease signs. While the HP Asian H5N1 viruses are 100% lethal for chickens and other gallinaceous poultry, the absence of disease signs in some duck species has led to the concept that ducks are the "Trojan horses" of H5N1 in their surreptitious spread of virus. An important unresolved issue is whether the HP H5N1 viruses are maintained in the wild duck population of the world. Here, we review the ecology and pathobiology of ducks infected with influenza A viruses and ducks' role in the maintenance and spread of HP H5N1 viruses. We also identify the key questions about the role of ducks that must be resolved in order to understand the emergence and control of pandemic influenza. It is generally accepted that wild duck species can spread HP H5N1 viruses, but there is insufficient evidence to show that ducks maintain these viruses and transfer them from one generation to the next.

Figure 1

Overview of the annual movement and behavior of migratory ducks and their role in interspecies transmission. During spring and fall migration, the ducks rest and feed for a few days to weeks at numerous stopover sites (wetlands, lakes, or ponds) along the migration route. The length of stay and the aquatic habitat allows the transmission of influenza viruses to and from the domestic duck populations. Domestic ducks that become infected are likely to maintain the virus locally and increase the probability of its spread to other species. In the diagram, solid arrows indicate confirmed routes of transmission of LPAI and/or HPAI viruses between species. The dashed line represents a probable but unconfirmed route of transmission. The graphs indicate the average prevalence of low‐pathogenic avian influenza in North American and European duck populations during 3 stages of the annual migration. ¹⁰ , ¹⁶

Figure 2

General breeding areas and fall migration patterns of wild ducks (white) and their relation to reports of H5N1 HPAI viruses. Most wild duck migrations occur in the northern hemisphere, where ducks generally fly north to breed during the summer and return south to spend the winter. With a few exceptions, ducks in the southern hemisphere are largely sedentary and rarely travel long distances. H5N1 HPAI viruses have spread westward from Southeast Asia into Europe and Africa. Circles indicate the number of human deaths attributed to H5N1 HPAI in the indicated countries (0 indicates infection but no mortality). Red marks reports of H5N1 HPAI in poultry; green indicates reports of H5N1 HPAI only in wild birds. Data were obtained from the World Health Organization (http://gamapserver.who.int/mapLibrary/app/searchResults.aspx) and Wikipedia (http://en.wikipedia.org/wiki/Anatidae).