Egg sampling as a possible alternative to blood sampling when monitoring the exposure of yellow-legged gulls (Larus michahellis) to avian influenza viruses.

We explored whether antibody detection in egg yolks could serve as an alternative to antibody detection in plasma samples when monitoring yellow-legged gulls (Larus michahellis) for exposure to avian influenza viruses (AIVs). We tested female plasma and eggs for anti-AIV antibodies and used the data we obtained to check whether the two sample types yielded the same antibody status (positive or negative) and to compare the antibody prevalence estimated from the blood data with that estimated from the yolk data. Our results showed that sampling one egg per clutch, regardless of that egg's position in the laying sequence, is sufficient to provide an unbiased estimate of antibody prevalence across clutches. The results also showed that almost 25% of the clutches laid by positive females contained only antibody-negative eggs, which suggests that yolk samples might underestimate female antibody prevalence. However, this result may stem from differences in the methods used to assess plasma versus yolk antibody status. Further research is needed to clarify this issue; while the number of false negatives could be reduced by adapting antibody detection techniques, it may be that they are an unavoidable consequence of natural avian maternal transfer dynamics.