Move and Mix
My wife and I will be moving to a new house. While packing my share of boxes, I was thinking of our new neighbors, one of whom raises free-range chickens. During a break, I picked up a recent medical journal and was reminded that “change is one of the constants with viruses.” The journal article described attempts of researchers to anticipate how the H5N1 influenza virus may evolve.[1]
Anticipation of our own migration and delicious chicken dishes were quickly displaced by thoughts about spring migration along major bird flyways. Could bird migration and virus evolution be related?
Mix and match?
Mild forms of bird flu are present in many geographic areas. They pose little risk to humans. But what happens as H5N1 spreads across continents and infects birds that already are infected with local influenza strains? In such dual infections, entire segments of virus genes can find their way from one strain to the other.
This mixing up of gene segments is called recombination, and it sometimes can rather suddenly change a mild strain into a deadly one. As the distribution of H5N1 gets wider, the chance of recombination increases.
Recombination recently occurred in the H7N3 virus, another subtype of bird flu. This virus changed very suddenly from a mild form to a form that was deadly for poultry, in both British Columbia, Canada (Hirst et al. 2004)[2] and in Chile (Suarez et al. 2004)[3]. The recombined virus also infected two Canadian poultry workers. Fortunately, they only experienced mild disease. (Tweed et al. 2004)[4]
Great Britain and the Netherlands
Recent press reports suggest that the H7N3 subtype has made an appearance in the United Kingdom. Officials there quickly suggested that this is a disease of birds and not humans; has no relationship to the highly pathogenic H5N1 strain; causes only very mild disease; is not easily transmitted from human to human; and requires close contact with dead or dying poultry.[5] All true, but there is more.
H7N1 is genetically quite similar to H7N7 and H7N2 bird flu viruses, which have also infected humans. In a large outbreak of H7N7 among poultry in the Netherlands, humans experienced no or mostly benign effects, but the virus did kill a veterinarian, caused substantial influenza-like illness in seven people, and jumped readily from person to person. (Fouchier et al. 2004)[6] Subsequent antibody tests suggest that at least 1000 people were infected, far more than originally thought. Those who developed symptoms passed on the infection to 59% of their household contacts. (Enserink 2004)[7]
Right now, there are H5N1 and H7N3 viruses in the United Kingdom. The chances may be small -- we don’t really know -- but it is conceivable that a bird could be infected with a H7N7 or N7N3 virus and also with H5N1. What if one of the millions of recombinations produces a virus that has the killing power of H5N1 and transmits as easily as an H7 subtype?
West Africa
Four West African countries[8] have reported the highly pathogenic H5N1 strain in birds. [For a map of West Africa click here][9] Given the extreme poverty and lack of resources for detection and control of influenza outbreaks, it would not be a surprise if H5N1 had spread to the rest of West Africa. This is a region where migrating wild birds from Eurasia mix with African species. There is plenty of opportunity for virus strains to recombine, especially in the floodplains of the Niger River in Nigeria and Mali, and around Lake Chad. (Olsen et al. 2006)[10]
Why worry about a faraway continent? Well, birds will transport recombinant virus strains that may emerge from West Africa. One route leads north along the East Atlantic Flyway to Canada where it meets up with the Atlantic Americas Flyway, which covers a wide swath of eastern America.
Higher summer temperatures in Africa and in the northern hemisphere are slowing the rate of new bird flu outbreaks, but at the same time, the H5N1 virus may be adapting to higher temperatures. According to Robert Webster, one of the prime influenza researchers, current H5N1 strains from Indonesia and Thailand can survive up to seven days at 37°C, while a 1997 strain of H5N1 from Hong Kong was able to survive for only two days at that temperature.[11]
What to make of H5N1’s moves?
It’s a “one hand versus the other hand” story. On the one hand, you would think that the H5N1 virus would be less likely to cause major outbreaks among humans the longer it spreads without learning to efficiently move from person to person. On the other hand, the longer it is around, the wider it is distributed, the more opportunities the virus has to recombine with influenza viruses that have the ability to move quickly from person to person. We’ll just have to keep watching these moves.
Source: Olsen et al. 2006
[1] Hampton T.
Avian flu risk to humans probed: viral adaptation to human cells would aid spread.
JAMA. 2006 Apr 26;295(16):1885-6.
[2] Hirst M, Astell CR, Griffith M, Coughlin SM, Moksa M, Zeng Tet al.
Novel avian influenza H7N3 strain outbreak, British Columbia.
Emerg Infect Dis. 2004 Dec;10(12):2192-5.
[3] Suarez DL, Senne DA, Banks J, Brown IH, Essen SC, Lee CW, et al.
Recombination resulting in virulence shift in avian influenza outbreak, Chile.
Emerg Infect Dis. 2004 Apr;10(4):693-9
[4] Tweed SA, Skowronski DM, David ST, Larder A, Petric M, et al.
Human illness from avian influenza H7N3, British Columbia.
Emerg Infect Dis. 2004 Dec;10(12):2196-9.
[5] http://news.bbc.co.uk/1/hi/uk/4956224.stm
[6] Fouchier RA, Schneeberger PM, Rozendaal FW, Broekman JM, et al.
Avian influenza A virus (H7N7) associated with human conjunctivitis and a fatal case of acute respiratory distress syndrome.
Proc Natl Acad Sci U S A. 2004 Feb 3;101(5):1356-61.
[8] Nigeria, Niger, Burkina Faso, Ivory Coast
[9] http://www.reliefweb.int/rw/fullMaps_Af.nsf/luFullMap/48599768B40A83918525713A005BB26C/$File/ocha_REF_afr300605.pdf?OpenElement
[10] Olsen B, Munster VJ, Wallensten A, Waldenstrom J, Osterhaus AD, Fouchier RA.
Global patterns of influenza a virus in wild birds.
Science. 2006 Apr 21;312(5772):384-8. Review.
[11] http://www.bloomberg.com/apps/news?pid=10000087&sid=aRIMUwJ1hyGg&refer=top_world_news
I find I very hard to locate trustworthy information regarding the "bird flu." Thank you for this site.
My husband and I have discussed this and he believes the government and other sourses are "trying to create a panic." I am not so convinced of that. Would it be prudent to stock up as the government has advised? Or is my husband correct in thinking it is a way to boost buying and spending in a tight fisted economy?
Posted by: H. Wright | May 11, 2006 at 05:51 AM