Natural concurrent infections associated with infectious laryngotracheitis in layer chickens

Layer chickens with severe respiratory disease belonging to farms with chickens infected by gallid herpesvirus (GaHV-1) were evaluated. Samples of nasal turbinates/sinuses and trachea from 31 chickens were subjected to histopathological analysis, which showed that 22.6% of chickens had lesions suggestive of co-infection, most frequently by GaHV-1 or Mycoplasma spp. PCR analysis showed the presence of at least two respiratory pathogens in 61.2% of chickens. Partial sequencing was performed for identification of the intergenic spacer region (IGSR) and cytadhesin 2 (mgc2) gene of the Mycoplasma gallisepticum genome. Analysis of IGSR revealed the involvement of two field and one vaccine strains. However, analysis of mgc2 from the same samples revealed five different strains of M. gallisepticum, four of which were field strains and one of which was a vaccine strain. These results demonstrate that combined molecular and histopathological analysis is a reliable and conclusive method for diagnosis of respiratory diseases in chickens since some etiologic agents can remain latent or maintain chickens as carriers. The results further confirm the importance and feasibility of the molecular characterization of M. gallisepticum genes to differentiate between vaccine and field strains involved in cases of respiratory disease in chickens.