Protection and virus shedding of falcons vaccinated against highly pathogenic avian influenza A virus (H5N1)

Abstract

Because fatal infections with highly pathogenic avian influenza A (HPAI) virus subtype H5N1 have been reported in birds of prey, we sought to determine detailed information about the birds' susceptibility and protection after vaccination. Ten falcons vaccinated with an inactivated influenza virus (H5N2) vaccine seroconverted. We then challenged 5 vaccinated and 5 nonvaccinated falcons with HPAI (H5N1). All vaccinated birds survived; all unvaccinated birds died within 5 days. For the nonvaccinated birds, histopathologic examination showed tissue degeneration and necrosis, immunohistochemical techniques showed influenza virus antigen in affected tissues, and these birds shed high levels of infectious virus from the oropharynx and cloaca. Vaccinated birds showed no influenza virus antigen in tissues and shed virus at lower titers from the oropharynx only. Vaccination could protect these valuable birds and, through reduced virus shedding, reduce risk for transmission to other avian species and humans.

Figure 1

Titers (log₂) of hemagglutination-inhibiting antibodies of 5 vaccinated Gyr-Saker hybrid falcons before and 11 days after challenge with 10^6.0 50% egg infectious doses of the highly pathogenic avian influenza strain A/Cygnus cygnus/Germany/R65/06 (H5N1), tested against antigen of the challenge virus and the low pathogenicity avian influenza vaccine strain A/duck/Potsdam/1402/86 (H5N2). Open circle, individual outlier.

Figure 2

Immunohistochemical demonstration of influenza A virus antigen (red, see arrows) in numerous splenic macrophages of a falcon after challenge with 10^6.0 50% egg infectious doses of the highly pathogenic avian influenza strain A/Cygnus cygnus/Germany/R65/06 (H5N1). Avidin-biotin-peroxidase complex method. Bar = 25 μm.

Figure 3

Detection of viral RNA by real-time reverse transcription–PCR (RT-PCR) from oropharyngeal (A) and cloacal (B) swabs of 5 vaccinated and 5 nonvaccinated falcons after challenge with 10^6.0 50% egg infectious doses of the highly pathogenic avian influenza virus strain A/Cygnus cygnus/Germany/R65/06 (H5N1). Y axis shows cycle-of-threshold (Ct) values of real-time RT-PCRs detecting an M gene fragment in individual swab samples of each animal. Asterisks represent extreme values; open circles show individual outliers; black bars within boxes indicate medians.