Mute swan deaths in St. Catharines caused by enteric parasite
Photo: APHOTOMARINE
On 10 November, 2012 14 mute swans were found dead in Martindale Pond, which lies just south of Lake Ontario within the city of St. Catharines. The swans were collected by the Lincoln County Humane Society and transferred to the enforcement branch of the Canadian Wildlife Service. The CWS submitted the swans to the CCWHC lab in Guelph for necropsy examination.
At post-mortem, findings were similar in all birds examined. The birds were in good body condition, with ample amounts of fat and were well-muscled. All, however, were extremely pale. There was severe hemorrhage into the intestinal tract which contained both fresh blood and large clots of blood. Mixed within this were large numbers of a tiny, pinhead-sized parasite called Sphaeridiotrema globulus.
S. globulus is a trematode (fluke) parasite with a complicated life cycle that involves snails as an intermediate host. When the birds consume the snails they acquire the parasite. The fluke burrows into the intestinal wall, causing ulcers and blood loss. It is recognized as a cause of localized die-offs of waterfowl that encounter the parasite. Here in Ontario, it has been seen previously in a number of species of waterfowl, including mute swans, tundra swans and various species of diving ducks. In the spring of 2008, the Canadian Cooperative Wildlife Health Centre published a report on the deaths of mute swans and a white-winged scoter in Ontario which occurred in the fall of 2007.
News of the 2012 mortalities was first published in the St. Catharines Standard.
So one wonders what in the environment of the mute swans is compromising their ability to deal with these parasites, or what is encouraging unusually high density or infectivity of the Sphaeridiotrema fluke. Is the density of birds higher, or the water quality/quantity different from normal?
There is experimental evidence that mute swans are more susceptible to this parasite than are some other species. Also, experimentally, there is some evidence that birds may develop some resistance to the parasite if exposed early in life.