Comparison of cloacal and oropharyngeal samples for the detection of avian influenza virus in wild birds.

This study was conducted to compare oropharyngeal (OP) and cloacal samples of wild birds (n = 137) for the detection and isolation of avian influenza virus (AIV). A total of 39 (28.5%) cloacal and 85 (62.0%) OP samples were positive for AIV by real-time reverse transcription-PCR (RRT-PCR). The AIV nucleic acid was detected in both cloacal and OP samples from 27 (19.7%) birds, in cloacal samples only from 12 (8.8%) birds, and in OP samples only from 58 (42.3%) birds. Thus, a total of 97 (70.8%) birds were AIV positive by RRT-PCR. The cycle threshold values for the cloacal samples ranged from 16.6 to 36.9 (mean 31.5), and those for OP samples ranged from 18 to 38.9 (mean 34.9). Of the cloacal samples, 12 were positive for H5 subtype influenza virus by RRT-PCR, with one being low pathogenic H5N1. In contrast, five of the OP samples were H5 positive, but none was H5N1. None of the cloacal or OP samples was H7 positive. Eight cloacal samples yielded AIV on inoculation in embryonated chicken eggs, while only one isolate was obtained from OP samples. Thus, from testing of 137 birds, only nine (6.6%) AIV isolates were obtained. The isolates from cloacal samples were subtyped as H6N1 (n = 5), H3N8 (n = 2), and H4N8 (n = 1), and the isolate from OP sample was subtyped as H6N1. No virus was isolated from the corresponding cloacal sample of the bird whose OP sample yielded AIV on virus isolation. These results suggest that surveillance programs for detection of AIV by RRT-PCR may include both sample types (cloacal and OP) to obtain a better picture of AIV prevalence, and OP samples may yield additional isolates of AIV when tested in conjunction with cloacal samples.