Comparison study on pathogenicity of H9N2 avian influenza virus isolates

To study the pathogenicity of H9N2 subtype avian influenza virus,25 isolates from China during 1998-2008 were compared and analyzed by crossing-hemagglutinin inhibition test(HI),EID50 test,ELD50 test,ICPI test,IVPI test and challenge infection test,consulting the way for virulence evaluation of NDV.The results revealed that the pathogenicity among 25 H9N2 subtype AIVs was different.8 strains of them with different pathogenicity were chosen and evaluated with ICPI,IVPI and challenge infection test in 8-week-old SPF chicken.It showed that the ICPI and IVPI of most strains were zero.The peak of shedding virus was from the 5th to 6th day after challenging.Among them,A/CK/HN/A3/98(H9N2)(3#) and A/CK/HN/02(H9N2)(12#) indicated high pathogenicity,not only had the highest EID50,10-8.8/0.2 mL and 10-8.8/0.2 mL,the highest ELD50,10-7.9/0.2 mL and 10-8.7/0.2 mL,respectively;but also had the shortest MDT,66.5h and 69.9 h.And the ICPI as 0.238 and 0.437;IVPI 0.34 and 0.51.In the challenge infection test the two strains shed much more virus and lasted longer(from 2 to 9) than others.According to these results,all the 25 strains can be mainly divided into three types:high,normal,and low pathogenicity,which revealed that the strains of different years and different places had different pathogenicity and their pathogenicity did not increase year by year and presented polymorphism.The strains of higher pathogenicity may result in more mortality in chicken.