RESUMEN
A sero-epidemiological study was carried out on 5660 sera collected, between 2006 and 2008, from different flocks in different regions of the country. The ELISA results showed low levels of antibodies indicating vaccination failures. 45 to 69% of the flocks showed positive levels of antibodies and only 5 to 15% of these were protected. The pathogenicity studies of the Tunisian field isolates TN20/00 and TN335/01 demonstrated high clinical and lesion scores indicating the pathogenic effect of the two isolates. The challenge experiments conducted to evaluate the cross-protection between the H120 vaccine and the field isolates showed low protection rate, especially against the TN20/00 virus. The overall results allowed the determination of the pathogenic nature of the field isolates and a vaccination program based on the use of the only Massachusetts H120 strain did not reduce tracheal and kidney lesions. To better control the disease, adapting the vaccination program by using vaccine allowing better protection against variant strains, is recommended.