The current study reports an outbreak of a high mortality within an 86 days old chicken flock. The birds showed severe respiratory manifestations associated with obstruction of the upper tracheal tube with diphtheritic lesions. Collected tracheal samples were examined using direct electron microscopy (EM), polymerase chain reaction (PCR), virus isolation in embryonated chicken eggs (ECE) and histopathological examination. Electron microscopy findings revealed intracytoplasmic dumbbell-shaped FPV particles within fat inclusions.  Histolopathologically, the tracheal sections revealed fibrinous tracheitis, associated with presence of intracytoplasmic eosinophilic inclusion bodies (Bollinger bodies). Virus isolation showed generalized thickening with white opaque pock lesions on the Chorioallantoic membranes (CAMs) of the inoculated eggs. PCR results were positive for Fowl Pox virus (FPV) and Infectious Laryngotracheitis virus (ILTV). In conclusion, dual infection of both FPV and ILTV viruses should be taken in consideration in fibrinous tracheitis lesions. Moreover, PCR was sensitive in the detection of ILTV virus than histopathology and electron microscopy.

Key words: 4b gene, Co-infection, Egypt, FPV, ILTV