A recent study by University of Maryland researchers examines the mechanisms underlying transmission of combined avian-human viruses and illustrates how virus outbreaks like that of the current swine flu come about. The study suggests that the ease with which a dangerous avian influenza virus can cause a human flu pandemic may be greater than previously thought.
In a study published last week in the Proceedings of the National Academy of Sciences, Associate Professor Daniel Perez from the University of Maryland showed that after an avian and human-like virus combine, the virus requires relatively few mutations to spread rapidly between mammals by respiratory droplets.
“This is similar to the method by which the current swine influenza strain likely formed,” said Perez, program director of the University of Maryland-based Prevention and Control of Avian Influenza Coordinated Agricultural Project, AICAP. “The virus formed when avian, swine, and human-like viruses combined in a pig to make a new virus. After mutating to be able to spread by respiratory droplets and infect humans, it is now spreading between humans by sneezing and coughing.”
Generally, avian flu viruses infect birds, and human viruses infect humans. Because their immune systems “remember” what the viruses look like from previous exposures, humans and birds tend to have some level of immunity to their respective viruses. Though avian flu viruses do sometimes infect humans and cause severe illness, these viruses do not transmit easily from human to human so the spread is rare.
A problem arises when an intermediary species that can host both avian and human-like viruses, such a pig, is infected with both types of virus. In cases like this, the viruses can combine in the host to make hybrid avian-human viruses. These viruses can infect humans but escape the immune response because their surface proteins are foreign to the immune system. While these viruses can cause serious illness, they are generally not passed easily between humans. However, Perez has shown that this type of virus can fairly easily mutate to spread quickly and potentially cause a human pandemic.
In his study, Perez used the avian H9N2 influenza virus, one that is on the list of candidates for human pandemic potential. Using reverse genetics, a technique whereby individual genes from viruses are separated, selected, and put back together, Perez and his team created a hybrid human-avian virus. Their research hybrid had internal human flu genes and surface avian flu genes from the H9N2 virus. While similar in origin to the swine flu virus, in that it involved a combination of avian and human influenza viruses, the lab virus came from a different strain of virus than the H1N1 virus now causing the swine flu outbreak.
Perez infected ferrets (considered a good model for human influenza transmission) with the virus he created, and allowed the virus to mutate in the species. Before long, healthy ferrets that shared air space but not physical space with the infected ferret had the virus, showing that the virus had mutated to spread by respiratory droplets.
When the genetic sequences of the mutant virus and original hybrid virus were compared, they found only two surface mutations responsible for supporting respiratory droplet transmission. Because so few mutations were necessary to make the hybrid H9N2 transmissible this way, they concluded that after an animal-human hybrid influenza virus forms in nature, a human pandemic of this virus is potentially just a few mutations away.
“We do not know if the mutations we saw in the lab are the same that have made the H1N1 swine flu transmissible by respiratory droplets,” Perez said. “We will be doing more research on the current swine flu strain to study its specific genetic mutations.”
Perez found that one of the two of the genetic mutations in his lab strain that enabled respiratory transmission between mammals was on the tip of the HA surface protein, one of the sites where human antibodies created in response to current vaccines would bind.
“Because the binding site of the mutant virus is different from the virus upon which the vaccine is modeled, it may mean that current vaccine stocks would not be as effective against the H9N2 mutant strain as previously anticipated,” said Perez. “We should keep this in mind when designing vaccines for an avian flu pandemic in humans.”
A virus vaccine is derived from the virus itself. The vaccine consists of virus components or killed viruses that mimic the presence of the virus without causing disease. These prime the body’s immune system to recognize and fight against the virus. The immune system produces antibodies against the vaccine that remain in the system until they are needed. If that virus, or in some cases a closely similar one is later introduced into the system, those antibodies attach to viral particles and remove them before they have time to replicate, preventing or lessening symptoms of the virus.
However, scientists cannot predict what the actual mutations will look like if and when they occur in nature, or even which strain of avian influenza will mutate to infect mammals, so it is difficult to anticipate what the vaccine needs to look like.
“This is just the tip of the iceberg,” said Perez. “Many more studies have to be done to see which combinations of mutations cause this type of transmission before we can design the appropriate vaccines.”
Perez will be talking this week with the NIH and the CDC to discuss his team’s role in researching the current swine flu virus strain. Perez will likely do studies related to vaccine development, virus transmission between humans and animals, and the pathogenesis of the virus.
[Lee Tune @ University of Maryland]