A study conducted at St Jude Children’s Research Hospital in Memphis, on the effects of the H5N1 avian influenza virus on small land birds, suggests that the virus is often lethal in sparrows but has lesser effects on starlings and pigeons and does not readily spread to other birds of the same species.
However, the researchers say their findings also suggest that sparrows and starlings could potentially spread the virus to poultry and mammals.
More importantly the study concludes that compared to earlier reports on the susceptibility of sparrows, starlings and pigeons to a 1997 Hong Kong H5N1 virus, the bird species are more susceptible to the more recent H5N1 isolates used in the study.
The recent influenza [H5N1] virus is therefore showing increased virulence in a mrange of species.
To gauge how the H5N1 virus behaves in small birds, the researchers inoculated sparrows, starlings, and pigeons with four different strains that were isolated from birds. Two of the strains had previously been shown to infect waterfowl in Thailand, and two were recently isolated during wild-bird surveillance in Hong Kong.
The sparrows and starlings used in the study were captured in the wild, while 6-week old Carneux pigeons were bought from supply houses.
At the start of the study, the authors obtained cloacal swabs from the birds to rule out existing influenza A infections.
After the birds were inoculated with the H5N1 strains, researchers placed them in cages with uninfected birds of the same species for 14 days to gauge virus transmission.
The ratio of infected to uninfected birds was 1:1 for sparrows and starlings and 2:3 for pigeons.
Death rates were highest for the sparrows: 66% to 100% of them died, depending on the H5N1 strain they received.
High viral loads were detected in dead sparrows’ brain and lung tissues. However, none of the starlings or pigeons died.
Regular testing after inoculation showed that all of the sparrows and starlings were infected, but infection in pigeons depended on the strain of the virus.
One of the Hong Kong strains infected both sparrows and starlings, as well as all of the inoculated pigeons, though the authors found the viruses replicated relatively poorly in the pigeons.
The authors concluded that that the birds varied in their susceptibility to the H5N1 viruses but that transmission to the contact birds was infrequent.
Compared to earlier reports on the susceptibility of sparrows, starlings, and pigeons to a 1997 Hong Kong H5N1 virus, the current study suggests that the bird species are more susceptible to the more recent H5N1 isolates used in the study.
“Although drawing conclusions on the basis of a single 1997 isolate is inappropriate, these data are consistent with studies that have demonstrated increased virulence or host range for recent influenza [H5N1] viruses in mammalian species,” the authors write.
The researchers conclude that terrestrial wild bird species vary considerably in their susceptibility to H5N1 virus strains, and some species, such as sparrows, could suffer substantial losses during H5N1 outbreaks.
Also, they write that mutations in circulating H5N1 viruses could enhance the role of sparrows and starlings as intermediate hosts.
Full text of the report here: http://www.cdc.gov/eid/content/13/11/pdfs/07-0114.pdf
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