Aerosol Transmission of an Avian Influenza H9N2 Virus with a Tropism for the Respiratory Tract of Chickens

A low pathogenic avian influenza virus (LPAI H9N2) was administered to 3-wk-old chickens by aerosol exposure, intranasal inoculation, and by oral inoculation. Tests for virus were by in ovo assay and by real-time reverse-transcriptase PCR. The aerosol dosage was determined by aerosolizing virus into a chamber when it was empty and when it contained chickens. Air was collected and the amount of virus inhaled was estimated to be about 18% of the total body uptake. In transmission studies, tests for virus were conducted on oropharyngeal or cloacal swabs. The 50% infectious dose (ID50) for aerosolized virus was about 2 log 10 and 5 log 10 lower than by nasal or oral inoculation, respectively. The recovery rate was higher from swabs of the oropharyngeal region than from the cloacal region (P < 0.05). For horizontal transmission studies, uninfected chickens were held in isolators with seeders that had been inoculated intranasally with the H9N2 virus. Chickens exposed by indirect contact were separated by screens from the seeders. In another isolator those directly exposed were intermingled with the seeders. During the 10-day test period, none of the chickens developed symptoms of disease, but infection was detected as early as 4 and 7 days in the indirectly and directly exposed groups, respectively. These findings suggested that aerosol transmission of viruses similar to LPAI H9N2 could efficiently occur, at least over short distances.