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More H5N1 Bird Flu in Michigan
October 26, 2006
The USDA has reported detection of H5N1 in Michigan for the third time. The most recent report is in Mallard ducks in St Claire County. Like the sample reported for Tuscola County in Green-winged teals, viral isolation tests are ongoing. In August, H5N1 was isolated from Mute swans in Monroe County. Similarly, H5N1 has been isolated from resident wild mallards in Queen Anne’s County, MD as well as Mallard ducks in Crawford County, PA.
More recently, H5N1 isolation attempts have failed. H5N3 was isolated from Northern pintail ducks in Cascade County, MT and H6N2 was isolated from Green-winged teal in Fulton County, IL. No virus was isolated from H5N1 positive samples from Northern pintail ducks from Ottawa County, OH. In all reported incidents, sequence data indicated low path American strains in the H5N1 positive samples.
All detections listed in the table are from samples that are positive for both H5 and N1. Media reports have indicated H5 has also been detected in live markets in New Jersey as well as wild birds in northern California and Washington State.
The failure to isolate H5N1 from positive samples is cause for concern. The USDA site suggests the isolation failures were due to a lack of viable H5N1, but viability is dependent on isolation procedures and each test has a detection limit, and results indicate the sensitivity of the isolation procedure is lower than the PCR test. This detection limit may also be impacted by handling, shipping, and pooling of samples.
Dual infections may also limit isolation and dual infections are common in wild birds as indicated by the isolation of H5N3 and H6N2 from H5N1 positive samples.
An alternative approach for detecting high path H5N1 involves sequence analysis. Recombination is common between H5N1 and low path serotypes, including H5. Therefore release of the H5N1 from the earlier isolates would be useful.