M41K-Beau(S) is attenuated in vivo.

At 8 days of age, four groups of (A-C) 12 or (D-F) 15 SPF RIR chicks were inoculated via the intraocular and intranasal route with 10⁵ PFU or equivalent CD₅₀ dose of either (A-C) M41K-Beau(S), M41-K or M41-CK or (D-F) with M41K-4/91(S), M41-K or 4/91. Mock infected birds were inoculated with PBS. Clinical signs were observed from either day 2 or 3 to day 7 post-infection. (A, D) The average number of snicks per bird per minute was calculated for each group based on counts from two persons. (B, E) The percentage of birds exhibiting rales in each group was calculated. (C, F) Ciliary activity in the trachea was measured on days 4 and 6 post-infection. Trachea were extracted from three or five randomly selected birds on each day and sectioned into 10 x 1 mm rings. Each ring was analysed under a light microscope and assigned a score between 1 and 4 depending on the proportion of cilia beating. 1 = 25% beating, 2 = 50%, 3 = 75%, 4 = 100%. Each data point represents the average percentage ciliary activity of each bird. Error bars represent SD. Statistical differences were assessed using a Two-Way ANOVA followed by Tukey’s test for multiple comparisons and are highlighted by ***(p<0.0005) and **** (p<0.0001).