To compare the pathogenicity of co-infection of MS and IBV to SPF chickens was observed and compared,144 SPF chickens were randomly divided into six groups,negative control,MS-only infection,IBV M41-only infection,M41 and MS co-infection,QX-only infection,QX and MS co-infection,chickens was infected via left and right eye inoculation with MS(106 CCU50)and IBV(105 EID50),respectively,with the control group receiving KM2 culture medium and sterile allantoic fluid.Clinical symptoms were monitored daily,and at 7,14,21,and 28 days post-infection,six chickens from each group were randomly selected for necropsy to assess air sacculitis and score air sac lesions.Pathogens were re-isolated from the trachea and identified by PCR and RT-PCR for MS and IBV respectively.Tracheal mucosal thickness and histopathological lesions were determined after 10%formaldehyde fixation.Transient respiratory symptoms were observed in all infected groups except the control and MS-only infected chickens by 4dpi.Air sacculitis was observed in MS-only infected group at 21 and 28dpi,while IBV M41 and QX infected group showed it only at 7dpi or 14dpi,with incidence rates below 50%.Air sacculitis was occurred by 28dpi in SPF chickens co-infected with MS and QX,and the incidence rate was 100%(6/6)at 14 and 21dpi,whereas air sacculitis was by 21dpi in the MS and M41 co-infection group and the highest rate was on 14dpi(5/6).Average air-sac lesion scores were were significantly higher in QX and MS co-infection group at 14,21 and 28dpi compared to MS-or IBV-infected groups alone(P<0.001),as seen in the M41 and MS group on 14dpi(P<0.001).Re-isolation results showed that MS antigen was re-isolated by 28dpi in both single and co-infected groups,while IBV re-isolation occurred only by 7dpi.Pathological lesion results showed that co-infection groups underwent long term of tracheal mucosal thickness and lesion scores compared to single-infected groups.At 14dpi,the average tracheal mucosal thickness in chickens co-infected with M41 and MS were significantly lower than in those co-infected with QX and MS(P<0.05),and a difference that was highly significant at 21dpi(P<0.001).Additionally,the positive rates of tracheal lesions were 100%at 7,14,21 and 28dpi in co-infected groups except for M41 and MS group at 7dpi,with tracheal lesion scores of co-infected groups significantly higher than those in single-infected groups(P<0.01 or P<0.001).These results suggest that IBV may enhance pathogenicity in 28-day-old SPF chickens when co-infected with MS,with the incidence of air sacculitis and tracheal pathology dependent on the M41 or QX strain.
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