Low-pathogenic avian influenza LPAI viruses can pass through the upper digestive tract of ducks and replicate in the lower intestinal tract without causing clinical manifestations of disease. 3, 6 Further evidence that the intestinal tract is the target organ of LPAI viruses in ducks includes the replication of virus in the lower intestinal tract, but not in the lungs, after direct inoculation into the crop and rectum 6 and high fecal virus titers after intravenous inoculation. 3 The specific site of LPAI virus replication is believed to be the crypts of Lieberkühn in the large intestine. 3 The other potential target organ for LPAI viruses in ducks is the respiratory tract. Lungs of mallard ducks intranasally inoculated with LPAI viruses showed mild pneumonia, and lymphocyte and macrophage infiltration within 2 days. Immunostaining for viral nucleoprotein revealed intermittent staining of respiratory epithelial cells but no viral replication in the lung tissue. 24 This evidence indicates that the respiratory tract and not the lung tissue itself is the primary target of infection.