The occasional influenza pandemics and the seasonal influenza epidemics have destroyed millions of lives since
the last century. It is therefore necessary to understand the virus replication patterns as this provides essential
information on the virus infectivity, pathogenicity and spread patterns. This study aimed to investigate the replication
of avian influenza A virus H5N1 (A/Chicken/Malaysia/5858/2004) in MDCK cells. In this study, the TCID50 (50% tissue
culture infectious dose) of AIV H5N1 was first determined. The MDCK cells were then infected with AIV H5N1 at TCID50
for 0-48 h. The CPE (cytopathic effect) was observed and cell death was determined hourly. The virus-infected cells
and media were subsequently collected for gene analysis. The results showed that the TCID50 of AIV H5N1 was 10-9
dilution. The CPE percentage showed a strong and positive correlation with the infection period (r = 1.0, n = 9, p <
0.01). The amount of a highly conserved influenza viral gene, M2 gene amplified from infected media (r = 0.471, n =
9, p= > 0.05) and infected cell (r = 0.73, n = 9, p < 0.05) were also positively correlated with the infection period. In
conclusion, although CPE started to be observed in the early time points of infection, however, the M2 gene was only
amplified from the infected media and cells after 48 h and 24 h, respectively. This signifies that AIV H5N1 used in this
study is pathogenic and it is able to cause severe cytopathology to host cells even at low virus load.