Domestic ducks and H5N1 influenza epidemic, Thailand

Abstract

In addition to causing 12 human deaths and 17 cases of human infection, the 2004 outbreak of H5N1 influenza virus in Thailand resulted in the death or slaughter of 60 million domestic fowl and the disruption of poultry production and trade. After domestic ducks were recognized as silent carriers of H5N1 influenza virus, government teams went into every village to cull flocks in which virus was detected; these team efforts markedly reduced H5N1 infection. Here we examine the pathobiology and epidemiology of H5N1 influenza virus in the 4 systems of duck raising used in Thailand in 2004. No influenza viruses were detected in ducks raised in "closed" houses with high biosecurity. However, H5N1 influenza virus was prevalent among ducks raised in "open" houses, free-ranging (grazing) ducks, and backyard ducks.

Figure 1

Duck-raising systems in Thailand. A) Closed system with high biosecurity, an evaporative cooling system, and strict entrance control. B) Open system but with netting to prevent entrance of passerine birds. Biosecurity was not strictly enforced. This system is no longer approved for the raising of poultry. C) "Grazing duck raising." Biosecurity is never practiced in this system. D) Backyard Muscovy ducks raised for a family; no biosecurity is practiced in this system.

Figure 2

Example of grazing-duck movement. A single flock of ducks was moved 3 times by truck in 1 season in 2004. The size of the flock is 3,000–10,000. The time spent at each site depends on the availability of rice fields at the site: an acre of rice could support 3,000 ducks for 1 to 2 days. The duck owners have agreements with the landowners regarding the time of harvest and the acreage available. One flock could spend as long as 1 month at a single site before being moved to the next.

Figure 3

A) A White Cherry Valley duck (Anas platyrhynchos), infected with HPAI H5N1 displays nervous signs, convulsions. B) Histopathologic features of the lung of an HPAI H5N1–infected white Cherry Valley duck; infiltration of inflammatory cells in the lung parenchyma (magnification ×100).

Figure 4

Immunohistochemistry of an HPAI H5N1–infected white Cherry Valley duck (Anas platyrhynchos). The viral antigen is detected in myocardial cells and lymphoid cells (arrow) (A) and renal tubular cells (B) (magnification ×100). The primary antibody used for immunohistochemistry in this study was a mouse anti–avian influenza H5 antibody (Magellan Biotechnology, Chunan, Taiwan).