Recent Type E botulism outbreaks among fish in the Great
To illustrate by example the dynamic and sensitive nature
Lakes were in southern Lake Huron (1998-99), western and
of the Lake Erie ecosystem, after 1990, water clarity
central Lake Erie (1999-2000), eastern Lake Erie (2000-02),
improved in the western basin, but blooms of blue green
and Lake Ontario (2002). For eastern Lake Erie, a number
algae occurred. Summer phosphorus concentrations in the
of fish species have been affected as well as the benthic
waters of the central and eastern basins appeared to
species, mudpuppy. In terms of both numbers of birds and
decrease between 1990 and 1995, but then increase again.
species affected, the U.S. Geological Survey found that the
Such changes are not unidirectional but, rather, may reflect
largest outbreak of avian botulism occurred in eastern Lake
changes caused by the rapid development of, and subse-
Erie also in 2002, coincident with the outbreak among fish.
quent fluctuations in the dreissenid mussel population.
Environment Canada has investigated botulism events
along the north shore of eastern Lake Erie.
A long-established measure of the trophic status of Lake
Erie is hypolimnetic oxygen depletion, which can result in
The state of New York has investigated the number and the
formation of an anoxic dead zone. Although one should
timing of fish killed. Type E botulism has been present for
be concerned about changes in this important measure, the
a long time, but what is different now to trigger the massive
WQB was advised not to be alarmed by low dissolved
outbreak? As part of their investigation for cause-effect
oxygen levels. Because of its natural characteristics, the
relationships, the state has investigated recent ecosystem
central basin of Lake Erie has always experienced occa-
changes in eastern Lake Erie. These include declining
sional anoxic problems, the severity of which is determined
productivity, increased water clarity (changes in fish
by the thickness of the hypolimnion. Weather -- tempera-
distribution), and presence of alien invasive species (the
ture and storms -- determine the thickness. The size and
dreissenids zebra and quagga mussels and round gobies).
persistence of the dead zone cannot be used to gauge the
There appears to be a correlation of Type E botulism
success or failure of phosphorus load reduction policies
outbreaks with the presence and abundance of round
and programs introduced in the 1970s. Those programs
gobies. Environment Canada has reported that naturally
contributed to the elimination of anoxic conditions in
occurring but rapid changes in water temperature can
shoreline areas and in the western basin of Lake Erie, and
cause fish die-offs which, with other factors, may lead to
ameliorated the problem in the central basin. However,
conditions conducive for an outbreak. The Type E botu-
management of phosphorus loadings to achieve a state
lism outbreaks are, however, most likely unrelated to the
where anoxia never occurs in Lake Erie may not be a
central basin hypolimnion anoxia issue.
The need for research on how the toxin moves through the
The WQB notes that anoxia has also been documented in
food chain and why some species are affected so much,
smaller inland lakes.
while other similar species are affected so little, is key to
developing management and policy advice. Investigators
recognize the critical roles played by the host for the
Botulism Type E Outbreak -- Findings
bacterium, agents, and the environment. They are looking
for clues to the source of the toxin and its transfer among
The bacterium Clostridium botulinum has been recog-
gobies, zebra mussels, fish, fowl and maggots. Studies are
nized as a major cause of mortality since the early 1900s.
under way to answer key questions:
The bacterium is classified into seven types by the charac-
teristics of the neurotoxin produced. Death is caused by
Where are the anoxic conditions that promote
ingestion of the toxin. Both Type C and Type E botulism
germination of the spores and subsequent production
are found naturally in anaerobic environments. Type C is
of the toxin?
found in waterfowl worldwide, and Type E is found in fish-
What is the key link to fish -- dreissenids or other
eating birds in the Great Lakes. Spores can remain in the
ecosystem for years and are quite resistant to temperature
What is the role of bottom sediments?
extremes and drying.
Is the bacterium a strain native to the Great Lakes or an
alien invasive species itself?
In the absence of oxygen, with a suitable nutrient source,
and under favorable temperature and acidity conditions,
Studies are also under way to determine the risk posed by
spores can germinate and vegetative growth of bacterial
Type E botulism to human health. A potential pathway is
cells can occur. Botulism toxin is produced only during
consumption of fish, but risk can be minimized when fish
vegetative growth. The etiology of botulism is well
are properly prepared. Fish which display symptoms of
characterized, as are clinical signs of, and diagnostic tests
botulism should, of course, be avoided entirely.