Oct 18, 2007 (CIDRAP News) – A study on the effects of the H5N1 avian influenza virus on small land birds suggests it 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.

The results of the study, conducted at St Jude Children´s Research Hospital in Memphis, were published early online by Emerging Infectious Diseases.

Scientists have fleshed out some H5N1 patterns in waterfowl species, which have been shown to shed the virus for prolonged periods and are thought to play some role—along with the poultry business—in the geographic spread among the world´s poultry populations. However, less is known about small terrestrial birds, which also intermingle with waterfowl and poultry.

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.

The birds were monitored each day for death and illness, and oropharyngeal and cloacal swabs were collected on days 2, 4, 6, 8, and 11 for sparrows and starlings and on days 3, 5, and 7 for pigeons. At the end of the 14-day period, the investigators collected serum samples from the inoculated and contact birds for hemagglutanin-inhibition testing.

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.

Virus titers showed sparrows and starlings shed similar amounts of the virus, but titers from cloacal swabs of sparrows were higher than those from the starlings.

The researchers found no evidence of spread of the virus among the sparrows and pigeons. Only one starling showed evidence of transmission, involving one of the Hong Kong strains.

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.

A key question is whether smaller wild birds can be intermediate hosts or long-term reservoirs for the H5N1 virus, the researchers point out. The results in sparrows suggest that they could potentially infect poultry and mammals, but, given the high death rate, wouldn´t likely serve as a reservoir for prolonged viral shedding. Starlings, because they survived and shed the virus longer, could act as an intermediate host, but transmission evidence in the study was limited, the report says. The authors suggest that the role of pigeons in spreading the virus may be minor because they shed only small amounts of the virus and didn´t transmit the disease to other pigeons.

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.