Avian flu: beyond H5N1
Will H5N1 avian flu seed the next human pandemic? Or could another form of bird flu, like H7N9 or H10N8, turn out to be the one that goes global?
The first known cases of human H5N1 infection occurred in Hong Kong in 1997. Mass chicken culling contained it for a while, but it reappeared in humans in 2003 and began its march across the globe.
By October 2014, H5N1 cases had been reported in 16 countries, mostly in the Far East and Asia, with 393 deaths.
The critical factor limiting its spread still seems to be the absence of human-to-human transmission – each new case is typically caught directly from birds (possibly, one or two people may have caught the virus from close family members).
Control centres on mass culling – hundreds of millions of birds have been killed to prevent the spread of the virus. When people are infected, anti-flu drugs are of some help – if given early – but cases of resistance have already been seen.
H5N1 vaccines have been developed for human use, but there is no guarantee any would be effective against a pandemic strain should it arise – a new pandemic-specific vaccine would need to be developed, using the technology platforms that already exist for making influenza vaccines.
Pre-exposure to an H5N1 vaccine might, however, boost responses to a newly developed vaccine. In addition, it may be possible to develop vaccines that provide broad protection across a range of strains.
A new threat?
While H5N1 remains a serious concern, a new avian flu virus – H7N9 – also has scientists worried. First seen in humans in 2013, it has been described by the World Health Organization (WHO) as “an unusually dangerous virus”. By October 2014 more than 450 human deaths from H7N9 had been confirmed, mostly in China.
H7N9 causes severe disease, but is not as deadly as H5N1 (it kills around one in five of those infected). However, it appears to have acquired some of the mutations associated with transmission between people. Although human-to-human spread has not yet been seen, it is further along the pathway than H5N1.
H7N9 appears to be sensitive to neuraminidase inhibitors, although resistance has already been seen in some infections. No vaccine is yet available for public use, although encouraging progress is being made with experimental vaccines.
H5N1 and H7N9 are not the only avian flu viruses that might pose threats to human health. H10N8 human infections have been identified (though this virus is considered less of a threat to humans) and multiple novel viruses have been found in birds and other animals.
Given the huge numbers of animals, such as poultry and pigs, that could transmit novel viruses to humans, vigilant surveillance in animals and humans is essential to detect emerging strains.Lead image:
Max Westby/Flickr CC BY NC
- Wikipedia: Human mortality from H5N1
- WHO: H5N1 avian influenza – timeline of major events [PDF]
- WHO: Influenza at the human–animal interface
- Evolution of drug resistance in multiple distinct lineages of H5N1 avian influenza (2009)
- Immunogenicity of avian influenza A/Anhui/01/2005(H5N1) vaccine with MF59 adjuvant: a randomized clinical trial (2014)
- Induction of broadly cross-reactive antibody responses to the influenza HA stem region following H5N1 vaccination in humans
- WHO: Frequently asked questions on human infection caused by the avian influenza A(H7N9) virus
- BBC: Q+A – H7N9 bird flu
- WHO: Human infections with avian influenza A(H7N9) virus – summary of surveillance and investigation findings (2 October 2014)
- WHO: Background and summary of human infection with avian influenza A(H7N9) virus – as of 31 January 2014
- Serological responses to an avian influenza A/H7N9 vaccine mixed at the point-of-use with MF59 adjuvant (2014)
- BBC: New H10N8 bird flu ‘not imminent global threat’