Avian influenza: more is now known about the role of migratory birds

20/08/2010 - Article

Were migratory birds behind the galloping spread of avian influenza that had Asia, Europe and Africa running scared in 2005-2006? A study part-conducted by CIRAD has come up with some answers.

When avian influenza spread from Asia to Europe and Africa in 2005-2006, the media pointed the finger at migratory birds. However, to date, there was no real proof of their role in the epizootic. A study published by CIRAD researchers and their peers has come up with some answers. It shows that migratory birds are indeed capable of transport the H5N1 virus over hundreds if not thousands of kilometres. However, on an individual scale, the probability of that happening is slight.

Four days without symptoms

The study, conducted under the FAO avian influenza programme, began with a review of the experimental data for infection in wild Anatidae (ducks, geese and swans). Most of these species can contract and excrete the virus without showing any symptoms for several days (four, on average). During that time, the birds are contagious while apparently still being able to migrate.

The largest data set

The second phase consisted in gathering precise data on the movements of these birds. They were obtained through telemetric monitoring by satellite, and to date constitute the largest data set on wild Anatidae migration ever compiled. The birds were fitted with transmitters capable of sending a geolocation signal to be picked up by satellite. "We could not have conducted the study a few years ago, since transmitters were much too big then to be fitted to birds the size of a duck " says Nicolas Gaidet, a CIRAD ecologist and co-author of the study. Satellite telemetry has the dual advantage of tracking birds in completely inaccessible zones and over thousands of kilometres. The scientists were thus able to calculate the frequency and extent of movements. They discovered that some species can cover up to 2900 km in just four days. This suggests that wild Anatidae could have been responsible for the spread of the virus in 2005-2006. But "the individual would have had to be infected at just the time when it was beginning to migrate ", Nicolas Gaidet adds. Hence the third stage of the study: calculating the probability of a bird being infected during the migratory period, hence of transporting the virus over long distances.

Between reality and probability

The result was that in fact, there are just 5 to 15 days a year during which a bird may spread the virus over a distance of 500 km. This small probability is notably due to the fact that Anatidae migrate in stages. While they are indeed capable of flying hundreds or thousands of kilometres, this can only last for three or four days, after which they have to rest. The birds therefore fly in several relatively long stages. As a result, either the spread of the virus stops there and then, or it is relayed. This relay transmission phenomenon will probably have to be studied in future, but in the meantime, the data gathered and the model developed by the scientists are valuable tools for epidemiologists in assessing the risks linked to avian influenza and other diseases carried by wild Anatidae, such as West Vile virus, salmonella or Lyme disease.

Avian influenza is still around

Avian influenza is still around in several developing countries. This Summer, a woman died of the H5N1 virus in Egypt, and another death may well be confirmed in Indonesia. The virus is endemic in Egypt and several Southeast Asian countries, where it is disrupting poultry production and compromising the livelihoods of small-scale producers. Moreover, this endemicity is a threat to neighbouring countries and a virus source that could evolve into other forms more dangerous to humans.

History of a pandemic

- 1997: first cases of H5N1 avian influenza in humans in Hong Kong. Eighteen people infected, six deaths
- As of late 2003: several Southeast Asian countries report cases simultaneously
- Summer 2005: the virus spreads westwards, as far as central Asia
- Autumn 2005: first cases in eastern Europe
- Early 2006: first cases in Africa and France
- Today: twelve countries still affected by avian influenza.