The ecologist’s view of bird flu

Are migratory birds responsible for the spread of bird flu? Should we kill them all? Lucienne Niekoop and Froukje Rienks from the Netherlands Institute of Ecology argue for a more scientific approach.

In mid-2003, the H5N1 bird flu outbreak began in south-east Asia. We do not yet fully understand how this virus spreads, but to prevent a pandemic, it is important to take a multidisciplinary approach. Ecologists from the Netherlands Institute of Ecology^w1 are co-operating closely with virologists at the Erasmus Medical Centre in Rotterdam^w2, the Netherlands, to understand the role of migratory birds in the transmission of bird flu.

First, let us go back to bird flu basics. Viruses, small pieces of RNA or DNA covered in a protein coat, occur in many forms. Flu is caused by the influenza virus, which has three types: A, B, and C, all of which can infect humans. Bird flu (avian influenza) is caused by influenza A viruses, for which birds form the reservoir.

There are several subtypes of influenza A. The severe virus that is currently spreading has the codename H5N1, derived from the membrane proteins on the virus particle. H5 stands for the haemagglutinin type 5 membrane protein (there are 16 types in total). Neuraminidase is the full name of the membrane protein N, of which there are nine variants. Different combinations of the H and N membrane protein types result in many virus subtypes. Subtypes may be harmless, in which case they are known as low pathogenic avian influenza (LPAI) viruses. Harmful or lethal subtypes are classed as highly pathogenic avian influenza (HPAI) viruses.

Usually, influenza viruses do not cross the species barrier, which means that different species of organisms cannot pass the infection from one to another. However, the highly pathogenic bird flu H5N1 has been shown to jump from birds to humans, although not easily. In the recent outbreak, there have been at least 256 human cases, of whom more than half died (as of 16 October 2006), according to the World Health Organization (WHO)^w3. Nonetheless, the WHO states that this is a small number compared with the huge number of birds affected.

It is likely that all species of birds can become infected with avian influenza. Affected birds secrete the virus in high concentrations in their faeces, and the main known infection route is oral intake of these contagious faeces. Waterfowl like ducks, geese and swans seem to be among the most infected species of birds and form a natural reservoir of all influenza viruses, usually the low pathogenic forms.

Jan van Gils, an ecologist at the Netherlands Institute of Ecology, explains: “Notably water-foraging birds appear to be sensitive. This is probably because they forage in contaminated water.”

Colleague Marcel Klaassen continues: “There are four potential reasons for the higher sensitivity of waterfowl. First, many of them travel over long distances as migratory birds, so their chances of visiting an infection hotspot are higher. Second, they prefer to live in wet areas. Outside the host, the virus is capable of surviving for long periods, especially in fresh water at low temperatures: after one month in ice, the virus is still virulent. Furthermore, waterfowl are highly social birds, which can increase the possibility of exchanging the virus.”

Klaassen finishes by pointing out the role of their food source: “They produce large amounts of faeces due to their vegetarian diet. As such a diet has low nutritional value, herbivores eat large amounts of food per day and produce a lot of dung. For example, geese defecate every five minutes.”

Ecological research is necessary for us to understand how the bird flu virus is distributed in a bird population, the location of infection, the transmission and the effects influenza viruses have on their migratory host. To this end, ecologists at the Netherlands Institute of Ecology study migratory water birds, such as the Bewick’s swan. The scientists collect faeces and take swabs or cloacal samples, observe the birds’ behaviour and study the food intake and diet of this small swan. The faeces and swabs are then sent to virologists at the Erasmus Medical Centre, who analyse them for pathogens like influenza viruses. By doing this, the researchers aim to make an inventory of which hosts carry which influenza subtype.

So far, migratory water birds have received the most attention as health hazards. They are considered by many to be the main or even the only virus vectors infecting people and poultry, but solid scientific evidence is lacking. Ecologist Marcel Klaassen says, “There is a lot of hysteria about the role migratory birds play. Perhaps because this distracts the attention from the real core of the problem: the very densely kept poultry, constituting a virus’s paradise. And due to trade, poultry is transported very frequently and extensively. In these ways, the flu virus can spread easily.”

Furthermore, virologists from the Erasmus Medical Centre found that LPAI infections naturally present in wild migratory birds are stable, unlike the rapidly mutating LPAI viruses found in poultry. In summary, migratory birds are involved in the spread of bird flu, as victims but not necessarily as offenders.