The Canada Goose and Parasites

Canada Geese Unwittingly Spread Cryptosporidium sp. and Giardia sp.

Aug 10, 2007

Canada Geese, Branta canadensis, are great migrators that have adapted to life with humans. Studies show that they help spread human parasitic diseases to new places.

The Canada Goose, Branta canadensis, numbers in the millions and is now resident in many parts of Europe as well as North America. The species has adapted well to human communities, visiting wetlands and parks and sometimes staying year round. Large and attractive birds, they are not universally loved—droppings can be problem wherever the geese congregate and they sometimes become aggressive toward people.

There is another side to the problem of goose droppings: they’re not just unsightly and disgusting to walk on, they often contain parasitic forms which, when they contaminate surface waters, can cause outbreaks of diarrheal parasitic disease in people. The best known of these parasites are Giardia lamblia and Cryptosporidium parvum.

Geese pick up parasites while feeding

Geese like to nibble on the grass in parks, which is why they are often attracted to urban green spaces. They also visit agricultural lands and enjoy the young grass shoots in cattle pastures, the grain left on the ground at livestock feeding stations, and kernels left on the field after a crop has been harvested. They’ll even pick undigested corn and wheat kernels out of livestock droppings.

Geese pick up Cryptosporidium parvum oocysts from farm animals, particularly cattle: young calves often suffer from cryptosporidiosis, and older cattle can carry the parasite. Cow pats and damp soil in cow pastures can contain millions of oocysts. When oocysts are washed off agricultural lands after heavy rain, they contaminate rivers and the wetlands downstream, another place where geese congregate and feed. Geese ingest cryptosporidium oocysts and Giardia lamblia cysts when they swallow contaminated water.

The feces of wild animals also contaminate rivers and other surface waters—water loving mammals, particularly beavers, have often been implicated when water supplies became contaminated with Giardia lamblia. Human sewage is a frequent source of contamination as well.

Canada Geese don’t suffer from cryptosporidiosis

When a Canada Goose swallows millions of Cryptosporidium parvum oocysts, nothing bad happens to the bird—the parasite is not capable of causing illness in birds and simply passes through, returning to the environment in the bird’s droppings. Evidence suggests that if these oocysts are subsequently ingested by a human, they are still capable of causing severe diarrheal parasitic disease—cryptosporidiosis. Migrating Canada geese can potentially pick up oocysts in one locality and subsequently contaminate a municipal water supply far away.

The story for giardia is not as clear as that for cryptosporidium. Giardia sp. cysts passed in bird droppings may have simply passed through the bird, or may be the result of infection in the bird’s intestine. Again, however, evidence points to at least some of the cysts being capable of causing giardiasis in humans.

Don’t blame the Canada Goose

All of this is not to point an accusing finger at the Canada Goose--other water birds are probably spreading the parasites in the same way though other species have not been studied as extensively. The fact remains, however, that there would not be so many parasitic cysts and oocysts for birds to pick up if humans did not concentrate large numbers of livestock together, allow runoff from pasturelands to enter rivers and streams, discharge sewage into rivers, and contaminate the environment with our own wastes. Humans have done far more to disseminate Cryptosporidium parvum and Giardia lamblia around the globe than any species of wildlife ever could. The solution lies in targeting the practices of people, not the natural behavior of geese and other wild species.