Are Migratory Birds at Increased Risk of Disease?

Bird populations are susceptible to a great many diseases. Interactions within the food web can bring them into contact with disease-carrying bacteria and viruses. Disease patterns are often created by exchanges that occur when birds migrate. Change within ecosystems can affect the pattern of diseases among birds.

These patterns change over time. Avian cholera has become a concern in the eastern Arctic. Botulism has emerged in the past decade to cause “die-offs” among waterbirds in Ontario. And avian influenza has come under global scrutiny since the spread of the virus known as Asian H5N1 from Asia to Europe and Africa.

Broad collaboration is imperative to monitor and share knowledge on disease and migratory birds. Environment Canada works with many agencies providing scientific support to manage diseases that impact wildlife conservation, including those that could harm people or domestic animals.

Avian Influenza on Long-Term Watch

Canada’s national inter-agency survey for influenza in wild birds began in 2005. The impetus for the survey stemmed from a major outbreak of influenza in the Canadian poultry industry that highlighted the need to better understand how these viruses function. Subsequently, a different strain of avian influenza (Asian H5N1) emerged as a potential threat to human health. The objective of the survey is to identify strains of North American influenza viruses found in wild birds and provide early detection of important disease-causing strains. Survey results help to understand the ecology of influenza viruses among wild birds.

The Canadian Cooperative Wildlife Health Centre coordinates the survey, which involves Environment Canada, veterinary colleges and diagnostic laboratories, plus other interested agencies. There is also a large American survey. The Canadian survey sampled wild ducks extensively in 2005, 2006 and 2007 because ducks are the natural reservoir of the gene pool of these viruses. Many other bird species were included on a smaller scale.

In 2007, scientists safely trapped, sampled and released more than 6,000 live wild birds and over 3,000 wild birds found dead. Just over 1,300 of the birds were infected with North American strains of avian influenza, none of which are pathogenic (disease-causing) for people or domestic birds. The survey now focuses on wild birds of any species found dead.

Continued vigilance will require substantial national and regional support and funding. The concern that foreign strains of highly pathogenic avian influenza may arrive in the Americas has not declined.

Botulism Deaths Continue on Great Lakes

Type E botulism emerged as an annual epidemic on the Great Lakes in the late 1990s. Outbreaks have occurred on Lake Ontario, Lake Erie, Lake Michigan and Lake Huron. In 2007, the range of the disease expanded to the Bruce Peninsula and Georgian Bay.

These outbreaks appear directly related to ecological changes in the lakes caused by invasive species: zebra and quagga mussels and the round goby. The sediment in the mussel beds support the bacterium that causes botulism, which enters the food web when mussels are eaten by bottom-dwelling fish such as the round goby. In addition, the mussels filter water and make it clearer which promotes the growth of algae and increases the water temperature. As the algae decays, it can cause anaerobic (oxygen-poor) conditions which, along with increased water temperatures, favour botulism growth.

Surveys conducted since 2004 reveal that local populations of Herring gulls and Caspian terns have declined by 25% and Great black-backed gulls have declined by 90%. In eastern Lake Ontario, more than 7,000 dead birds were counted during surveys at six bird colony sites. A subsample of these birds was submitted to the Canadian Cooperative Wildlife Health Centre for necropsy which found predominantly Type E botulism exposure. This constitutes compelling evidence that botulism is affecting fish-eating waterbirds at a significant level.

Environment Canada coordinates with provincial and American counterparts to share information such as numbers of birds found and the results of analyses. Ongoing studies are seeking to determine the routes of exposure (e.g., hunting for fresh food versus scavenging botulism-exposed carcasses) for waterbirds. A new study by Environment Canada scientists will attempt to trace the food habits of stricken birds using specific chemical and biochemical markers. Collaboration will be sought with the Canadian Cooperative Wildlife Health Centre to provide diagnostic support.

Avian Cholera May Be Expanding

This naturally occurring disease mainly affects waterfowl and is not a risk to people. It is an important disease in waterfowl populations, most notably Arctic-nesting geese. Avian cholera was found in the eastern Arctic in 2004 in eider ducks. And in 2007, an outbreak occurred on the open ocean off the Canadian Atlantic coast.

In the eastern Arctic, cholera outbreaks have occurred each year. A small number of eiders were found dead in the Southampton Island colony (Nunavut) the first year. Nearly 3,500 birds, a significant portion of the colony, were found dead from this disease in 2006.

Scientists monitor other eider colonies at Kimmirut, Cape Dorset, Coral Harbour and Rankin Inlet. They have also added new monitoring sites (Igloolik and Sanikiluaq) and connected with northern-based scientists from Greenland and the United States. They have also engaged Inuit hunters and trappers to watch for signs of the disease when on the land.

The geographic expansion of the disease raises the question of whether a changing climate allows some organisms to survive further north and infect species at increased rates. It is not clear whether the disease is "over-wintering" in the area or whether the migrating birds carry it there. Researchers are taking samples from the environment before, during and after outbreaks to try to determine the source of disease. Also, eiders are marked with leg bands each year and these birds, when possible, are re-captured for sampling and safely released.

A large-scale “die-off” of seabirds from avian cholera took place in the north Atlantic in the winter of 2007. The outbreak covered a large area that included the Hibernia Oil Field off Newfoundland, where the first dying gulls were discovered, to the Avalon Peninsula of Newfoundland and coast of Labrador, to Nova Scotia’s south-western shore. The birds discovered were mainly Great Black-backed gulls, Glaucous gulls, Icelandic gulls and Black-legged kittiwakes. The challenging environment of the open ocean made it difficult to determine the extent of the disease range and number of deaths.

As part of an ongoing study of avian cholera in Canada, researchers are seeking to understand the epidemiology of the disease in different bird populations including DNA typing and an investigation of any potential connections among outbreaks.

Why does Environment Canada track disease in birds?

Birds are excellent monitors of disturbance or contaminants in ecosystems. Birds are valued as indicators of ecosystem integrity and can be seen as an “umbrella”: if you have healthy birds, you generally have other healthy living organisms under them in the same ecosystem. Also, birds are barometers of change and may provide unique connections between wildlife habitat and factors such as climate change. Because migrants range widely, their health must be watched for early warnings of emerging risk.

Local bird populations must be protected and well managed, for their intrinsic value and for their value within global biodiversity. International collaboration is imperative to share knowledge and resources in tracking diseases that may have global significance for wildlife, domestic animals and human health.

Table adapted from "Avian disease by ecozone", prepared by Dr. Ted Leighton of the Canadian Cooperative Wildlife Health Centre.