Figure 4

Relationship between viral RNA load and AIV-NP antibody levels for ducks. Each point represents an individual duck (n = 76), and colours indicate infection status. Individuals currently infected, shown in the top panel, were those that had detectable viral RNA loads by RT-qPCR. Samples with low RNA loads that provided expected amplification curves but that did not surpass the cycle threshold value are represented as having a Ct value of 45. All individuals that were negative for viral RNA are shown in the lower panel, with the antibody level coloured by the infection status in shades of blue. Recently infected individuals were classified as such if they were viral RNA-negative but had elevated antibody levels. For individuals sampled soon after the population-wide infection event at the start of the outbreak, these recently infected ducks are further separated as being infected one, three, or six months prior. Individuals that were seronegative and had low antibody levels (high S/N ratios) were classified as being naïve or that their antibodies had waned. An arrow denotes the expected immunological response shown by the five currently infected individuals with Ct values < 40, illustrating the observed trend of increasing antibody levels with decreasing viral RNA load. The vertical line at an S/N ratio of 0.5 represents the threshold value used to classify seropositivity, with a line at 0.7 also shown as the threshold occasionally used for this assay in other studies.