Influenza A virus (H5N1) infection in cats causes systemic disease with potential novel routes of virus spread within and between hosts

Abstract

The ongoing outbreak of avian influenza A virus (subtype H5N1) infection in Asia is of great concern because of the high human case fatality rate and the threat of a new influenza pandemic. Case reports in humans and felids suggest that this virus may have a different tissue tropism from other influenza viruses, which are normally restricted to the respiratory tract in mammals. To study its pathogenesis in a mammalian host, domestic cats were inoculated with H5N1 virus intratracheally (n = 3), by feeding on virus-infected chicks (n = 3), or by horizontal transmission (n = 2) and examined by virological and pathological assays. In all cats, virus replicated not only in the respiratory tract but also in multiple extra-respiratory tissues. Virus antigen expression in these tissues was associated with severe necrosis and inflammation 7 days after inoculation. In cats fed on virus-infected chicks only, virus-associated ganglioneuritis also occurred in the submucosal and myenteric plexi of the small intestine, suggesting direct infection from the intestinal lumen. All cats excreted virus not only via the respiratory tract but also via the digestive tract. This study in cats demonstrates that H5N1 virus infection causes systemic disease and spreads by potentially novel routes within and between mammalian hosts.

Figure 1-6703

Infectious virus titers in pharyngeal, nasal, and rectal swabs obtained from cats infected with influenza A virus (H5N1) by the intratracheal route (top, filled symbols), by horizontal transmission (top, open symbols), or by feeding on virus-infected chicks (bottom, filled symbols) at various time points after infection. No virus was isolated from any swabs of negative control cats (data not shown).

Figure 2-6703

Infectious virus titers in organ tissue homogenates obtained from cats infected with influenza A virus (H5N1) by the intratracheal route (cats 1 to 3), by horizontal transmission (cats 4 and 5), or by feeding on virus-infected chicks (cats 6 to 8) 7 days after infection. Organs that were tested included stomach (A), liver (B), kidney (C), heart (D), cerebrum (E), cerebellum (F), brain stem (G), olfactory bulb (H), nasal concha (I), lung (J), trachea (K), jejunum (L), tonsil (M), eyelid (N), tracheo-broncheal lymph node (O), mesenteric lymph node (P), and spleen (Q).

Figure 3-6703

Cats infected with influenza A virus (H5N1) (cats 1 to 8) have lesions associated with virus replication in multiple tissues. Necrotizing and inflammatory changes are seen in multiple tissues, except the spleen, of cats infected with H5N1 virus (first column). See the text for a more detailed description of these lesions. The asterisk indicates protein exudate in the Bowman’s capsule of a renal glomerulus. The two arrowheads point to the inflamed myenteric plexus in the intestinal wall. Serial sections of these tissues (second column) show that these lesions are closely associated with the expression of influenza virus antigen. Tissues of negative control cats (cats 9 and 10) show neither lesions (third column) nor influenza virus antigen expression (fourth column). Tissues were stained either with H&E or by immunohistochemistry using a monoclonal antibody against the nucleoprotein of influenza A virus as a primary antibody.