BIRD STUDIES CANADA:
Botulism Outbreak on Lakes Erie and Huron in 1999

Reprinted by Permission: Joanne C. Williams, State Coordinator MLPA/MLW

Michigan Wildlife Diseases Manual: Botulism

We have had many inquiries concerning the large number of loons found dead along the shores of Lakes Huron and Erie in the summer and fall of 1999.

Bird Studies Canada was contacted, and Russ Weeber, Aquatic Surveys Coordinator, sent us the following summary provided by the Canadian Cooperative Wildlife Health Centre, Ontario regional lab in Guelph, Ontario.

Significant numbers of several species of mainly fish-eating birds have died over a five-week period extending from late September to early November on southeastern Lake Huron (between Kettle Point and Grand Bend, approximately 50-80 km NE of Sarnia, Ontario), and along the north west shore of Lake Erie (between Point Pelee and Rondeau, approximately 60-80 km SE of Windsor, Ontario).

On Lake Erie, minor mortality was noted throughout September, until an episode of large scale mortality was recognized, beginning 26 October, that lasted for at least one week. It is likely, based upon the appearance of further carcasses on the beach during November, that there has been at least one subsequent, smaller episode. About 90% of birds found sick or dead have been red-breasted mergansers, although common loons, homed grebes, diving ducks and several species of gulls have also been affected. Mortality is likely of the order of 1000 birds, with 1-2 carcasses per 10 metres of beach in some localities.

On Lake Huron, where beached carcasses were observed beginning about 14 October, with a notable increase on 23-24 October, approximately 90% of the birds involved were common loons. Fewer red-breasted mergansers, homed grebes, red-throated loons, some diving ducks, ring-billed gulls and Bonaparte’s gulls also were reported. The number of birds affected has not been determined, but based on carcass counts, at least 700 loons died.

Type E botulism has been confirmed in red-breasted mergansers that died in late September on Lake Erie, and in common loons, red-mergansers and ring-billed gulls from the October outbreak. It has also been confirmed in common loons, red-throated loons, red-breasted mergansers and ring-billed gulls from Lake Huron.

It seems likely that cases of Type E botulism first occurred this summer on Lake Erie in late July or early August. At that time, large numbers of gulls, and later shorebirds, were found dead on beaches on the Canadian side of the lake, between Point Pelee and Long Point. Ring— billed gulls and Sanderlings from Point Pelee National Park were tested at that time for botulism, but it was not possible to confirm botulism in these birds. although no other disease was recognized, and the clinical signs reported and the fact that some birds were released following supportive care are compatible with botulism. A positive botulism test in a great black-backed gull found dead in August has pushed back the confirmed start of the episode into late summer at least. The US National Wildlife Health Center in Madison, Wisconsin has confirmed Type E botulism in gulls found dead during the summer at Presqu’ile State Park in Pennsylvania, indicating that the disease was occurring throughout the central basin of Lake Erie in late summer.

Type E botulism is associated with fish, and the majority of birds involved in these outbreaks are fish-eaters or scavengers. Even diving ducks, which are primarily mollusk eaters, had fish in their digestive tracts at the time of death. Large rafts of migrating loons, mergansers and diving ducks were resting and feeding offshore in the affected areas at the time of the outbreak. Episodes of Type E botulism are periodically recurrent in fish-eating birds, especially loons, on the Great Lakes in late fall botulism among common loons (Gavia immer) in Michigan’s Upper Peninsula. Journal of Wildlife Disease 24:471476, 19881. We diagnosed botulism as the cause of a similar small die-off of common loons in the Kettle-Point-Grand Bend area in fall 1998.

What is unclear is the circumstances under which fish become toxic. No unusual fish mortality been detected in association with the episodes on Lakes Erie & Huron over the period of the major outbreak, and loons, mergansers and grebes would not be expected to consume dead fish. However, there were reports of large-scale fish mortality earlier in the summer prior to the appearance of bird carcasses.

Testing of samples collected from all species involved is ongoing. Plans for future work include retesting samples form the earlier summer incident on Lake Erie in an attempt to confirm that botulism was present at that lime. The reported clinical signs of the birds, the lack of lesions and the circumstances under which mortality occurred all suggest that the late July-early August die-off of gulls and shorebirds was due to botulism acquired from eating dead fish or invertebrates which scavenged fish carcasses. The fact that botulism was confirmed from gulls on the US side of the lake which died at this lime strengthens this suspicion. It seems reasonable to believe that botulism was present in the area, and it was the arrival of large numbers of fish-eating birds that caused the large increase in mortality.

Other follow-up work includes the identification of the fish species being consumed. Most birds had the bones of small fish in their gizzards. It is hoped that it will prove possible to identify these fish by examination of otoliths. There were a number of reports of dead sturgeon found washed up on shore during this same time period. One sturgeon, from southern Lake Huron, has also tested positive for Type E botulism.

There has been considerable speculation concerning the possibilities of changes of climate, water temperatures, water levels in the lake, floral and faunal shifts etc. in causing this epidemic, which, while not unprecedented, is certainly unusual. The epidemiological factors involved in outbreaks of Type E botulism are not understood, and it is a problem that deserves further investigation. It is probably worth mentioning that it may be an underreported or often unrecognized disease. There were a smaller number (dozens, rather than hundreds) of loons found dead on Lake Huron beaches at approximately the same time last year. Botulism was confirmed in these birds, but the strain was not successfully typed. In 1994, botulism was diagnosed in herring gulls and ring-billed gulls from Goderich, also in the same area, during late November and early December. Typing of the strain was not attempted in that case. Mortality in gull species, particularly on a small scale or within a restricted locality, may go undetected or may not raise the same level of concern as a similar event in a species such as the common loon. Loons are present within the region only during a relatively restricted time period during migration, whereas gulls are potentially exposed over a much longer period of lime.

The reporting of this event, the investigation and diagnosis, and the communication of results, involved personnel from a wide range of public and private agencies, including birders, wildlife rehabilitators, staff of the Ministry of Natural Resou:rces, Ontario Provincial Parks, Ontario Ministry of Health, Parks Canada, Canadian Wildlife Service, Health Canada, Bird Studies Canada, the Animal Health laboratory of the University of Guelph and the Ontario regional laboratory of the CCWHC.

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