Sample titration and processing NW, CW, and RS samples were serially titrated in eggs, starting at a 1:10 dilution (NW, RS; limit of detection, 10 1.5 EID 50 /ml) or 1:2 dilution (CW; limit of detection, 10 0.8 EID 50 /ml). Virus infectivity for all samples was calculated by the method of Reed and Muench [54] . At the time samples were thawed for virus titration, RNA was extracted from CW samples using a QIAamp Viral RNA kit (Qiagen). Real-time RT-PCR was performed with a QuantiTect SYBR Green RT-PCR kit (Qiagen) using an influenza A virus M1 gene primer set to determine viral load [32] . Viral RNA copy numbers were extrapolated using a standard curve based on samples of known virus as previously described [32, 57] . Baseline levels were determined by collecting CW samples from uninfected ferrets.