Very Virulent Plus Strains of MDV Induce an Acute Form of Transient Paralysis in Both Susceptible and Resistant Chicken Lines

Marek's disease (MD) is a lymphoproliferative disease of domestic chickens caused by a highly cell-associated alpha herpesvirus, Marek's disease virus (MDV). Clinical signs of MD include depression, crippling, weight loss, and transient paralysis (TP). TP is a disease of the central nervous system that affects MD-susceptible chickens 8-11 days post-infection (dpi), normally resulting in recovery 1-3 d after the onset of clinical signs. In this study we inoculated chickens from lines 7(2) (MD-susceptible) and 6(3) (MD-resistant) with a very virulent plus strain of MDV at 2wk of age, and collected brain samples from birds with and without TP at 5, 11, and 21 dpi for gene expression profiling and histological analysis. Data revealed that chickens inoculated with MDV had higher levels of IL-6, IL-10, IL-18, IFN-alpha, IFN-beta, IFN-gamma, MHC I, and CD18 in their brains at 11 dpi compared to the uninfected control birds. In addition, the expression levels of IL-6, IL-10, IFN-alpha, IFN-beta, and IFN-gamma were significantly higher in the brains of the birds showing clinical signs of TP than in asymptomatic chickens. Comparative analysis between the two chicken lines showed that the expression levels of IL-6, IL-10, IFN-beta, IFN-gamma, IL-18, CD18, and MHC I were significantly higher in the brains of the birds from line 6(3) with TP than those of line 7(2) exhibiting neurological disorders. A differential expression pattern was observed for some of the tested genes at different time points post-inoculation. Histological analysis showed lymphocytic meningitis, perivascular cuffing, and neuronal degeneration within the brains of birds from both susceptible and resistant lines exhibiting TP at 11 dpi. Vaccination prevented development of TP and other MD-associated clinical symptoms. These observations are suggestive of an underlying immunological mechanism for viral-induced neurological dysfunction, and the differential responses of the two chicken lines to MDV infection.