Experimental infections of broilers with avian Metapneumovirus subtype A and B.

Avian Metapneumovirus (aMPV) affects both turkeys and chickens. The virus is associated with swollen head syndrome (SHS) in broilers. Most of the studies regarding aMPV have been done in turkeys. Not much is known about the pathogenesis and immune response to aMPV in broilers. Therefore, our objectives were to study the pathogenesis and immune responses of broilers experimentally infected with aMPV subtype A and B. Three groups of 16-day-old commercial broilers were inoculated oculonasally with 10 ciliostatic dose50 (CD50) of turkey isolates aMPV subtype A or aMPV subtype B. Control birds were inoculated with virus free trachea organ culture (TOC) supernatant. Clinical signs started to appear at 4 days post infection (dpi), and reached peak levels at 6-dpi. At 15 and 17 dpi subtype A and B-infected broilers were free of respiratory signs, respectively. Subtype B-infected broilers showed significantly more severe clinical signs than subtype Ainfected ones comparing the clinical score index (P < 0.05). The distribution of aMPV in different tissues was investigated by nested RT-PCR. The viral genome was detected in aMPV subtype A infected chickens at 3 and 6-dpi in the upper respiratory tract tissues such as nasal turbinate, Harderian gland and trachea. In subtype B infected chickens the viral genome was detected not only in the upper respiratory tract tissues but also in the lung, spleen and bursa cloacalis. Virus neutralizing antibodies in tracheal washes of infected broilers started to appear at 3 dpi, reached peak levels at 14 dpi and then declined gradually. Serum virus neutralizing antibody titres reached peak levels at 11 dpi, and persisted throughout the experimental period up to 24 dpi. The effect of aMPV-infection on T cell activity was determined. Splenic T cells were stimulated with concanavalin A (ConA), a T cell mitogen, and the ability of T cells to release IFN gamma was detected by ELISA. At 3 and 6 dpi, aMPV-infected birds showed a significant enhanced T cell response following ConA-stimulation in comparison to non-infected broilers (P < 0.05). Overall, our study demonstrates that broilers are susceptible to infection with turkey isolates aMPV subtype A and B, and the upper respiratory tract was the main target tissue for aMPV in broilers. aMPV B was detected in other systemic tissues such as lung, spleen and bursa cloacalis. Both subtypes stimulated not only systemic immunity but also local antibody production in the respiratory tract of broilers.