Has the 2009 H1N1 flu pandemic increased the risk that the H5N1 avian flu virus could evolve to create a human pandemic?
That’s a possibility raised by the work of Yoshihiro Kawaoka of the University of Wisconsin, Madison, the main conclusions of which — but not the details — are revealed in a Comment article in Nature today. His team created a virus that has the H5 haemagglutinin (HA) surface protein from the H5N1 virus, with all the remaining genes coming from the 2009 pandemic H1N1 virus. The resulting virus proved to be highly transmissible in ferrets, and is therefore likely to have the same behaviour in other mammals, including humans.
What’s intriguing is that before the 2009 pandemic, several research groups had tried the same experiment, using the garden-variety seasonal H1N1 flu, but without success. The difference is that the 2009 pandemic H1N1 virus, which is a triple reassortant of pig, avian and human viruses, contains the triple reassortant gene (TRIG) cassette, which is believed to make it far easier for a flu virus to swap genes with those from other species. This suggests that H5N1 may find it much easier to reassort with pandemic 2009 H1N1 virus circulating in the wild to create a pandemic virus, whereas it had coexisted with seasonal flu since 1997 without evolving into a pandemic strain, explains Bruno Lina, a virologist and flu researcher at the CNRS, France’s basic-research agency, who works at the University of Claude Bernard Lyon-1.
The study by Kawaoka’s team, which has been accepted for publication by Nature, is one of two studies that have succeeded in creating H5N1 strains capable of transmitting between ferrets. The other, by a team led by Ron Fouchier of Erasmus Medical Center in Rotterdam, the Netherlands, has been submitted to Science. The papers have been at the centre of controversy since 20 December, when the United States government — acting on advice from the US National Science Advisory Board for Biosecurity (NSABB) — asked both journals to publish only the main conclusions of the two flu studies, but not to reveal details. Insights from the research might help to improve pandemic preparedness in the future, but some are concerned that the publication of such work would amplify the risk of an accidental, or intentional, release of the virus that could spark a human pandemic. Flu researchers working on such studies last week declared a 60-day voluntary pause to allow governments and other bodies “time to find the best solutions for opportunities and challenges that stem from the work” (see ‘Pause on avian flu transmission studies‘).
Kawaoka and Fouchier succeeded in creating the transmissible virus in completely different ways. Fouchier used mutation, taking a H5N1 virus and then mutating it until it became transmissible. He initially introduced three mutations, using a technique called reverse genetics, but the resulting virus was not transmissible, so he then took that virus and passaged it through multiple ferrets, a procedure that is known to allow viruses to adapt to their host. The result was a virus with just five mutations, which were enough to make it highly transmissible.
Kawaoka instead used reassortment. He took an HA protein from H5N1 and inserted it into a virus made of up genes from the pandemic 2009 H1N1. The flu virus has eight genes. Two code for the surface proteins HA and neuraminidase (NA), and six code for internal proteins. The eight genes are on separate segments, which means that when two different flu viruses infect the same host, they can swap genes and create new viruses in a process known as reassortment. An H1N1 human and H5N1 avian virus, for example, could generate a new virus that has most of the genes from the human virus, making it transmissible in humans, but an avian haemagglutinin and/or neuraminidase. A largely human virus carrying an H5, to which humans have no previous exposure of immunity, could cause a pandemic if it retained the transmissibility of the human virus, and the lethality of H5N1.
Fouchier’s virus was lethal in ferrets, whereas Kawaoka’s was “no more pathogenic than the pandemic 2009 virus”, and killed none of the animals. A reassortant that occurred in the the wild might have different pathogenicities. But two independent groups have now shown that H5N1 can transmit in ferrets, and so such human-transmissible viruses could potentially arise naturally in avian and other animal populations. What controls the exchange of genes between viruses is poorly understood, says Lina, who himself failed in the past to create highly transmissible reassortants of H5N1 and seasonal H1N1. Triple-reassortant viruses that have this TRIG cassette, of six highly conserved internal genes, seem capable of capturing various HA and NA genes from multiple species, he says. “The pandemic 2009 H1N1 virus has a flexibility of function which makes it capable of associating at the molecular level with virus and gene segments from pig, bird and humans.”
The 2009 pandemic H1N1 is circulating in humans in countries such as Indonesia, China and Egypt, where H5N1 cases in human continue to occur. Co-infection of a person with both viruses would give them opportunities to reassort. Pandemic H1N1 also infects pigs, from which it originally emerged, which could provide further opportunities for reassortment. This emphasizes the need for better surveillance to detect human cases of H5N1 infection.
Monitoring of human cases could also help to prevent flu viruses acquiring human transmissibility. There has been some evidence of limited human-to-human transmission of H5N1 in clusters of human cases, and a virus that passes along even a small chain of human hosts has opportunities to adapt to its host, just as H5N1 did in Fouchier’s ferrets.
But as a news article in this week’s edition of Nature shows (see ‘Caution urged for mutant flu work‘), surveillance of H5N1 in birds worldwide is patchy, particularly in poorer countries, where the virus is prevalent. It is also largely geared towards simply detecting and monitoring outbreaks, and few of the viral samples collected are ever sequenced, with just 160 H5N1 isolates submitted to the GenBank database last year. Moreover, if H5N1 surveillance in birds is poor, the situation is far worse in pigs, where there is virtually no systematic surveillance, even in richer countries. H5N1 infections in pigs are uncommon and cause only mild illness, creating little economic incentive to monitor them — GenBank contains partial sequences from just 24 pig H5N1 isolates in total.
Read all Nature‘s coverage of the issue at the mutant flu special.