Pullets had higher bursal and thymic weight indices and more antibody response to La Sota vaccination than broiler chickens (Gallus gallus domesticus)

This study investigated the immune responses to La Sota vaccination, used in protection of chickens against Newcastle disease, in light weight type breeds of chickens (pullets) and heavy weight type breeds of chickens (broilers) used in commercial poultry production. Seven‐week‐old 50 White Marshall broilers (Br) and 50 Isa Brown pullets (Pu) were randomly divided into four groups: vaccinated broilers chickens; (VBr), unvaccinated broiler chickens (UBr), and vaccinated pullet chickens (VPu) and unvaccinated pullet chickens (UPu). Chickens in groups VBr and VPu were vaccinated with La Sota vaccine, whereas groups UBr and UPu were not vaccinated. On day 0 post vaccination (PV), six chickens from group Br and Pu, and on day 4 PV, three chickens from each four groups were sacrificed and the bursa weight index (BWI), thymus weight index (TWI) and the splenic weight index (SWI) were obtained. The chickens were observed for clinical signs and lesions. Serum samples were collected from the chickens in all the groups on days 0, 7, 14, 21, 28 PV and assayed for haemagglutination inhibition (HI) antibodies. The BWI, TWI and SWI were 0.37 ± 0.05, 0.35 ± 0.17, 0.65 ± 0.26 for pullets and 0.11 ± 0.04, 0.13 ± 0.02, 0.36 ± 0.17 for broilers on day 0 PV. On day 4 PV there was no significant difference (p < .05) between the indices of the vaccinated and unvaccinated chickens. The geometrical mean antibody titres (GMT) of the pullets were 2 to 3 times higher than those of the broilers on days 7 to 28 PV. Vaccination did not produce clinical signs or lesions. The above observations show that naturally pullets produce higher antibodies than broilers because of their higher BWI.