Migration and the spread of disease
Migratory birds can play a role in the spread of infectious diseases. Their ability to travel over long distances and through a variety of habitats exposes them to a wide range of microorganisms.
In most cases they have developed natural immunities and the risks posed by exposure to disease is limited. In a few rare cases, however, the birds are exposed to diseases for which they have no immunity. This can be caused by the introduction of new pathogens within their ranges or mutation of an existing pathogen.
The introduction of West Nile Virus into North America is a recent example of a disease that was not particularly dangerous to birds or people within its historic range. Once it moved to North America, where it was not previously known, birds and mammals without a natural immunity were placed at risk. Thousands of birds have died from West Nile Virus and the long-term effects are still unknown. While the disease spread rapidly across North America, its spread was slowed by cold winter weather, which temporarily eliminates mosquito populations. There is great concern on the long-term affects of West Nile Virus as migratory birds carry the disease to their tropical winter homes. There is no known natural resistance to West Nile Virus in Central and South American bird species. The situation might be significantly worse than in North America as mosquitoes have a year round presence in the tropics.
Avian Flu (bird flu, avian influenza)
Avian flu is an infection caused by avian influenza viruses. These viruses occur naturally among birds. While present in wild birds on a worldwide basis, the viruses do not usually cause illnesses. The avian flu virus can be very contagious between birds and bird species. Various strains can make some domesticated birds, including chickens, ducks, and turkeys, ill or even cause death.
Reports of human infection by the avian flu virus have been monitored since 1997. In most cases it appears the spread of the virus was the result of direct contact with infected poultry or contaminated surfaces. To date (February, 2006) there have been no indications of sustained human-to-human spread of the disease.
Because the avian flu virus may change and become more infectious among humans, close monitoring of both domestic and wild birds is being conducted on a worldwide basis. This includes the monitoring of migratory species such as ducks and geese.
The situation regarding avian flu changes rapidly. The following links can be used to obtain current information.
The Cornell Lab of Ornithology's Bird Flu web site
National Public Radio--Avian Flu
From the UK--Bird migration tracker to help fight avian flu
From Project Feeder Watch--What North American bird watchers should know about the "bird flu"
West Nile Virus
In the case of West Nile Virus, the birds serve as a carrier of the disease but do not typically spread the disease directly. There are no known cases of West Nile Virus in humans that are the result of direct contact between birds and humans. Instead, mosquitoes spread the disease by first biting a bird infected with West Nile Virus, then biting a human or other mammal.
Current information on West Nile Virus is available from the following resources.
Center for Disease Control - West Nile Virus
National Audubon - West Nile Virus
Migratory Birds and Spread of West Nile Virus in the Western Hemisphere