The threat of a pandemic or epidemic of influenza, particularly from a mutated avian influenza virus, still looms large for
people as well as dogs and cats. People and cats have already become infected and died of a form of the avian flu. A single
vaccine that could be administered to birds, pets, and people to prevent influenza sounds like science fiction. But some researchers at the University of Maryland may have discovered
the key to creating just such a wonder.
The researchers took the central genes of the avian influenza virus H9N2, which is capable of infecting many different species,
including poultry, mice, and pigs, and modified the genes to make the virus less virulent. They hypothesized that since this
strain has such a broad host range, using an attenuated form as a backbone for vaccination may be effective, without modification,
in many species as well. They first tested this H9N2 backbone in birds, and it successfully prevented illness. Their next
step, as published in the November 2008 issue of the Journal of General Virology, was to use this same backbone in mice, which if protective, would suggest efficacy in many species, including cats, dogs,
In this study, the researchers intranasally inoculated 5-week-old female mice with differing concentrations of one of two
recombinant influenza viruses that also contained the attenuated H9N2 backbone. The viruses they used were the H1N1 (the culprit
in the Spanish flu of 1918) and H5N1 (a bird flu virus). After inoculation, the mice were monitored for three weeks, looking
for any clinical signs of flu, changes in body weight, and death. Three weeks after inoculation, the mice were challenged
with the corresponding wildtype H1N1 or H5N1 virus intranasally.
To evaluate each vaccine’s effect on virus replication, the researchers also looked for virus titers through homogenized lung
tissue obtained from three euthanized mice in each test group three to six days after challenge.
The results? For both viruses, those mice receiving the genetically modified versions of the viruses and subsequently challenged
with the corresponding wildtype virus did not develop clinical signs of flu, and they all survived. They were able to clear
infection in three days in most cases. In contrast, all control mice died by 8 to 10 days after challenge.
Mice who received the H1N1 attenuated vaccine had little evidence of virus in their lungs three days after challenge in contrast
to controls. But even though the mice that received the attenuated H5N1 vaccine were protected from clinical infection, a
significant amount of virus was isolated from their lungs. The researchers decided to test whether a booster inoculation of
the H5N1 attenuated virus three weeks after the first inoculation and subsequently challenged three weeks later would reduce
virus replication. The mice that received the booster had much less virus in their lungs by six days after challenge. So the
researchers concluded that a booster vaccine will lead to faster virus clearance.
Another part of the study tested cross-protective immunity of the H9N2 backbone. Mice were immunized with a heterologous subtype
virus, H7N2, and then challenged with one of the two wildtype viruses used in the H1N1 and H5N1 experiments. The mice had
some weight loss after challenge, but all survived, suggesting that the H9N2 backbone provides cross protective immunity against
the two lethal virus challenges. In the case of the H5N1 challenge, the researchers again tried an attenuated H7N2 booster
vaccine three weeks after inoculation in some mice and then challenged with wildtype virus three weeks later. This time, significant
virus titers were found in the lungs of the mice that received the H5N1 attenuated vaccine regardless of whether they received
a booster. So the booster did not stop virus replication in the lungs. The researchers surmised that the H9N2 backbone does
protect mice from challenge but doesn’t necessarily stop virus replication early in the infection.
Overall, the researchers concluded that the H9N2 backbone is attenuated in mice, no matter what virus is present. The H9N2
backbone was as effective in preventing the flu in these mice as it was in birds in the previous study. The implications for
this are great. If a vaccine is developed with this backbone for birds, the same vaccine can be used in mice and potentially
in other mammals, including dogs, cats, and people. And we would be on our way to a universal influenza vaccine.
Source: Hickman D, Hossain MJ, Song H, et al. An avian live attenuated master backbone for potential use in epidemic and pandemic
influenza vaccines. J Gen Virol 2008;89:2682-2690.