The potential of avian H1N1 influenza A viruses to replicate and cause disease in mammalian models

Abstract

H1N1 viruses in which all gene segments are of avian origin are the most frequent cause of influenza pandemics in humans; therefore, we examined the disease-causing potential of 31 avian H1N1 isolates of American lineage in DBA/2J mice. Thirty of 31 isolates were very virulent, causing respiratory tract infection; 22 of 31 resulted in fecal shedding; and 10 of 31 were as pathogenic as the pandemic 2009 H1N1 viruses. Preliminary studies in BALB/cJ mice and ferrets showed that 1 of 4 isolates tested was more pathogenic than the pandemic 2009 H1N1 viruses in BALB/cJ mice, and 1 of 2 strains transmitted both by direct and respiratory-droplet contact in ferrets. Preliminary studies of other avian subtypes (H2, H3, H4, H6, H10, H12) in DBA/2J mice showed lower pathogenicity than the avian H1N1 viruses. These findings suggest that avian H1N1 influenza viruses are unique among influenza A viruses in their potential to infect mammals.

Figure 1. Comparison of the mean pathogenicity scores of each pathogenicity index (PI) category.

Each avian H1N1 virus and the 2009 pandemic strains were assigned to a PI category based on their total pathogenicity score in DBA/2J mice. The PI-4 category includes the most pathogenic isolates; PI-3, moderately pathogenic isolates; PI-2, low pathogenic isolates; and PI-1, least pathogenic isolates. (*P<0.05).

Figure 2. Percent weight loss and survival in BALB/cJ mice infected with 10⁶ EID₅₀ of selected H1N1 isolates.

(A) Daily weight loss was calculated as the percentage of weight relative to that at 0 dpi, and mean weight loss for each group is shown. (B) Mortality was recorded based on actual death or euthanasia at 25% weight loss, according to our protocol. For each virus isolate, 5 mice were infected.

Figure 3. Hematoxylin & eosin staining of influenza-infected lungs from DBA/2J mice.

(A) In the uninfected lung, the bronchiole is lined with epithelium. (B) The infected lung shows severe bronchiole epithelial necrosis. (C) In the uninfected lung, the alveoli lumens are free of cells and have thin septal walls. (D) The infected lung shows focal alveolitis with neutrophils and mononuclear cells in the alveoli lumens, and the alveolar septal walls are thickened. Magnification: 40×(A–B) or 80×(C–D).

Figure 4. Transmissibility of A/shorebird/DE/300/2009 in ferrets.

Viral titers were detected in (A) nasal washes and (B) rectal swabs of donor ferrets (red), direct-contact ferrets (DC, green) and respiratory-droplet–contact ferrets (RDC, blue) 14 dpi. Data from each animal are shown. Viral titers are given in terms of EID₅₀.