The Dying Swan

You can add Austria, Germany and Iran to the list of countries where swans are dying of H5N1 infection. They join reports of dead swans in Italy and Greece and elsewhere. The birds seem very susceptible and because of their large size their carcasses are especially visible. The virus is penetrating rapidly into the heart of the EU.

The German swans are in the north on the island of Rügen. The German Agriculture Minister has ordered domestic birds indoors starting next week with the hope this will protect them from any infected migratory birds. In Iran, 135 swans died on the Caspian littoral. In Austria two wild swans succumbed to the disease at Mellach, near Graz. Restrictions on trading poultry within a 3 kilometer zone are in effect. The virus had already been found in Croatia and Slovenia, to Austria's south as well as nearby Italy. Bulgaria climbed aboard as well.

It is by now generally assumed the virus is traveling via wild migratory birds, but a piece today in The Scientist and another in Nature suggests there is still some question. Migratory birds were certainly the thought behind our recent post where we called the Nigerian cases another stop on "the Qinghai Express." But scientists within the UN's Food and Agriculture Organiazation are now saying the disease was almost certainly stewing away in the country for weeks before its official detection, something echoed by local residents.

Nigerian authorities notified the World Organization for Animal Health about the outbreak last Monday after tens of thousands of chickens in battery cages died in the northern state of Kaduna. It was confirmed as H5N1 on Wednesday, and in following days, officials reported worrying outbreaks in two neighboring states. This week (February 13), the country's health minister Eyitayo Lambo reportedly said that officials were examining suspected H5N1 cases in a further 5 states. If confirmed this would mean that 8 of Nigeria's 36 states were affected.

The expanding crisis comes amidst concerns that the virus had been circulating in the country for a long time before it was first reported. Juan Lubroth, head of FAO's Emergency Prevention System livestock component, told The Scientist that the disease had likely been killing domestic poultry before it hit commercial farmers. "We have suspicions that it had been going on for several weeks," he said. "We were all a little bit amazed that it took several weeks to get samples sent outside the country to confirm it." (Stephen Pinnock in The Scientist)

That still doesn't prove it wasn't wild birds, but the bird conservation community once again is putting forth arguments to the contrary. The chief suspect vectors were two species of duck (Garganey and Northern Pintail), but Alex Kaat of Wetlands International says the initial outbreak was not in a wetlands where the ducks roost, but in a dry region.

"There might have been a chain of infections from wild birds across Africa, but again we don't think so," he said. If this had been the case, it would have been more likely to see outbreaks in regions such as the Senegal coast, Lake Chad, or the Inner Niger Delta in Mali, he said.

"So, we don't think there is any connection between migratory birds and the outbreak in Nigeria," Kaat said. "We think that it is more likely that some poultry trade brought the virus."

The same theme appears in a story by the senior correspondent for Nature, Declan Butler, in this week's issue. While a recent study clearly showed migratory birds did carry the virus and could remain healthy enough to travel the requisite distances, the route of transmission may be from poultry to wild birds, not the reverse:

The same study, which analysed more than 13,000 virus samples, also found that trade and poultry movements were by far the biggest factor driving both the repeated spread of the virus outwards from its cradle in southern China and its genetic diversity. "The link [to migratory birds] seems to remain circumstantial and speculative," says Nick Davidson, deputy secretary-general of the Swiss-based Ramsar Convention on Wetlands.

"No clear evidence is available," agrees Salim Javed, an ornithologist at the Environmental Research and Wildlife Development Agency in Abu Dhabi. He adds that if migratory birds had played a major role he would have expected widespread outbreaks across India, the Middle East and Africa during the autumn migration. This did not happen.

Outbreaks may have occurred in other African countries but not been reported, says Juan Lubroth, a senior official at the United Nations' Food and Agriculture Organization. "There may be points in between that we don't know about, that might connect the dots," he says. Indeed, international agencies are tracking rumours of outbreaks in several African countries, including Mali, Egypt, Malawi and Libya. But Lubroth agrees that "it is too easy to point at wild birds". In many African countries there is scant surveillance of travellers carrying eggs and other poultry products, he points out. (Declan Butler in Nature; subscription only)

Another (and in our view more likely) possibility is that both explanations are correct. The virus makes the occasional long jump via migratory birds but primarily spreads locally via the poultry trade. This kind of spread would be similar to a "small world" network topology. Determining exactly what topology the virus spreads on is an important task. While the usual random graph topology is clearly inadequate, other connection modes, such as spread on a "scale free" network, are additional likely candidates. Knowledge of network topologies could direct us to finding the few highly connected nodes in the network as a control measure.

At this point it isn't clear how the dying swans of the EU are related (transmission-wise) to the dying ostriches and chickens in Nigeria.

Except that they were all killed by H5N1.