Over the past few months several outbreaks of avian influenza, or bird flu, have killed birds in Europe and East Africa. The most recent outbreak occurred in Uganda and is thought to have spread across continents via wild migratory birds.
Bird flu is an infectious disease of birds. It can cause a wide spectrum of symptoms in birds, ranging from mild illness to respiratory failure and death.
Avian influenza comes in different forms. As with the human flu, most kinds are of low concern and not harmful to people. A much smaller number of avian influenza viruses are highly pathogenic. This means they can kill birds rapidly and may pose a threat to people if they change or mutate in ways that allow them to infect humans.
Outbreaks of highly pathogenic avian influenza viruses have occurred in Asia, Europe, and Africa. These viruses are mainly associated with domestic poultry and their movements by people. Several different lines of evidence suggest that although there’s some potential for avian influenza outbreaks in southern Africa, the risk to people is low.
Most experts agree that natural reservoir hosts for avian influenza viruses are ducks and geese. Reservoir hosts can maintain populations of a pathogen without becoming sick themselves, typically because they have evolved with the pathogen and have resistance to it. It’s unclear how readily highly pathogenic avian influenza viruses are moved by wild birds, although there’s evidence that ducks infected with the highly pathogenic H5N2 virus can move substantial distances.
Between 2007 and 2009, my collaborators and I collected samples from nearly 5,000 wild birds to provide the first comprehensive picture of avian influenza in southern Africa. We found that there was a generally low prevalence of avian influenza in wild birds, with mainly low pathogenic viruses circulating year-round in the sub-region.
Low risk region
Southern Africa is a low risk region for highly pathogenic avian influenza transmission by wild birds. This is because it’s further south than the ends of the migration routes of ducks from Europe to East and West Africa. It’s far away from Europe and Asia, and the Eurasian flyway, which are the main potential sources of avian influenza viruses entering Africa.
Southern Africa’s warmer climate also means that flu viruses will persist less long in water bodies, because viruses die faster under warmer conditions. The main transmission pathway for African birds is therefore by direct contact, rather than by sharing viruses through water.
None of the duck species found in southern Africa is a proven long-distance migrant, with the possible exception of the Knob-billed Duck. It has been recorded moving from southern Africa as far as West Central Africa on a few occasions.
Direct introduction of a virus from Europe, or transmission from a location in East or Central Africa, would therefore require several different infection and dispersal events to act as “stepping stones” for the virus to move south. Each additional step reduces the likelihood of successful transmission of a pathogen along a chain. It only needs one weak link – a bird that either dies or gets better before transmitting the virus – for the chain to fail.
We identified Red-billed Teal and Egyptian Geese as the highest risk species for avian influenza transmission in southern Africa. Both undertake moult migrations of distances up to about 1,500km. These movements are not along flyways, but rather, radiate out from large bodies of water in southern Africa like the spokes of a bicycle wheel. Each individual bird returns once a year to moult its flight feathers. The large aggregations formed at this time, in water bodies that are shared by hundreds of moulting birds, are potential transmission hotspots.
Our data however indicates that even in these large aggregations, only around 2-3% of birds are typically infected with avian influenza. Highly pathogenic viruses are a tiny proportion of the total numbers of avian influenza viruses detected.
Carrying the infection
Ducks are not the only reservoir hosts of avian influenza. Other birds have also been found to carry it, increasing the risk of transmission.
Of particular interest are the long distance migrants, such as terns and ruffs. They could, in theory, carry an infection from Europe to southern Africa without the need for “stepping stone” infections. In fact, the first known case of highly pathogenic avian influenza in South Africa was from a tern in 1961. Although these species are difficult to capture in large numbers, we found little evidence of avian influenza infection in the individuals that we sampled. This may be due to the fact that terns are aerial foragers that spend little time in direct contact with water where the virus could be transmitted. For the same reason, the chances of terns spreading avian influenza virus to poultry or people are remote.
These factors don’t rule out the potential for infection via the movements of domestic poultry. This is the most common route by which avian influenza is spread. In southern Africa, however, mixing between people and domestic poultry is less intensive than in Asia, backyard poultry densities are much lower, and the commercial poultry industry is well regulated. Highly pathogenic avian influenza at present appears to be primarily a potential problem for high-density captive populations of ostriches and chickens.
Despite all these hurdles, South Africa has experienced several outbreaks of highly pathogenic avian influenza in its population of captive ostriches and a few related outbreaks in its domestic chicken populations.
The strains of virus identified in these outbreaks have killed ostriches but haven’t been capable of infecting people; captive ostriches appear to be relatively susceptible to avian influenza. It’s been suggested that avian influenza was introduced to captive ostrich flocks via contact with drinking water by Egyptian Geese. Phylogenetic (evolutionary) data suggest shared ancestry between influenza viruses in southern Africa and Eurasia.
Although there is some evidence for occasional transmission of avian influenza between African and Eurasian birds, such transmission events appear to be infrequent and the likelihood of highly pathogenic avian influenza outbreaks in southern Africa remains low.