Experimental Infection of Mice with HK97 and CkHK97

The purpose of this experiment was to establish the pathogenicity of A/HK/156/97 (HK97), and A/Ck/HK/258/97 (CkHK97) in mice. The HK97 virus was isolated from respiratory specimens of humans infected with the H5N1 virus. The CkHK97 virus is the avian virus isolated from chickens. The RNA of the human isolate was identical to that of the avian virus, indicating that the isolates had not undergone genetic reassortment with human viruses.

Methods

The mice were inoculated intranasally with egg infectious doses of allantoic fluid the viruses were cultivated in the allantoic cavities of embryonated eggs of either HK97 or CkHK97. The mice were monitored daily for clinical signs of infection including lethargy, huddling and ruffled fur, weight loss and mortality. On day 5 postinfection, three mice from each group were sacrificed and the concentrations of virus in their lungs and brains were quantitated by titration. The infectious-virus units in 1 LD lethal dose of virus ₅₀ of both viruses was also determined via plating.

Results

• Both HK97 and CkHK97 grew to high titers in lungs
• Both HK97 and CkHK97 grew to moderate titers in brain
• Mice infected with HK97 exhibited 100% mortality by day 12
• Mice infected with CkHK97 exhibited 100% mortality by day 7
• 1 LD₅₀ of HK97 virus contained 10^1.0 plaque forming units (PFU)
• 1 LD₅₀ of CkHK97 virus contained 10^3.3 PFU

Questions to Ponder

• Are these viruses pathogenic to mice? Yes correct! Both viruses resulted in death of all mice No Incorrect: Both viruses resulted in the death of all mice, therefore, the virus is pathogenic in mice
• What does the viral replication in the brain tissue suggest? answer The fact that the virus can replicate in brain tissue without adaptation suggests that these viruses are neurotropic
• What does the number of PFU's of 1 LD₅₀ of each virus indicate? answer Fewer PFU's indicates fewer infectious-virus units in 1 LD. HK97 exhibited fewer PFU's, therefore, it must be more virulent than CkHK97. It takes less of HK97 to infect and cause death than it does of CkHK97.

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