To the FWGNA group:
Freshwater snails have suffered a spate of bad press in the upper
Midwest recently. Late last week our friend Henry Fieldseth sent
us an article from the Minneapolis Star Tribune
(6Nov07, pasted below) attributing the death of thousands of waterfowl in a
local lake to infections by trematode worms, with the "banded mystery
snail" (Viviparus georgianus)
indicted as a co-conspirator. This is not the first mass murder
rap to be pinned on a digenean trematode or an invasive gastropod
henchman, nor will it be the last, I fear.
Massive late-summer mortalities of waterfowl attributable to
trematodiasis have been reported at least since the 1960s, when
thousands of ducks were apparently killed by mixed infections of the
digeneans Cyathocotyle bushiensis and Sphaeridiotrema globulus
in the St. Lawrence River south of Quebec. Hoeve & Scott (1)
linked these deaths to ingestion by the ducks of the invasive European
gastropod Bithynia tentaculata. In 1997, tens of thousands of coots were killed in Wisconsin's Shawano Lake by a third digenean worm, the European Leyogonimus polyoon, again carried by Bithynia (USGS-NWHC Fact Sheet 6/2001, PDF).
And since 2002, all three worms have been implicated in massive
waterfowl mortalities around Lake Onalaska, a backwater of the upper
Mississippi River near La Crosse (USGS-NWHC Bulletin 07-01, PDF).
All three of these digenean trematodes, the introduced Leyogonimus as well as the natives Cyathocotyle and Sphaeridiotrema,
display Type III life cycles – entering their ultimate
(vertebrate) host by ingestion of an infected meal (2). The
vertebrate host sheds eggs through its feces, which hatch into ciliated
miricidia and penetrate the first intermediate host, a freshwater
gastropod. (The photo of Bithynia tentaculata at left was taken by Lars Peters.) The miricidium develops numerous sack-like rediae in
its first intermediate host, each of which gives rise to many
cercariae. Then the cercariae swim out of the snail to find a
second intermediate host, which may be selected from among a wide
variety of aquatic animals – a second snail or an aquatic insect,
for example. (Typically substantial specificity is demonstrated
for the first intermediate host, but not for the second.) The
definitive vertebrate host (a duck or coot in this case) becomes
infected with mature worms when it ingests the second intermediate host.
Trematodes do not typically kill their definitive hosts. The
massive mortalities we have seen scattered through diverse waterfowl
populations in the upper Midwest are most likely attributable to
unusually high rates of snail ingestion as well as to double or exotic
infections. So since the ducks (with their digenean parasites)
disperse readily on continent-level scales, federal and state agencies
have signaled their intent to focus their efforts to control the
trematodiasis by controlling the snails (USGS-NWHC report 2007-1065, PDF).
Thus last week's discovery of the massive waterfowl mortality attributable to Cyathocotyle and Sphaeridiotrema
in Minnesota's Lake Winnibigoshish is especially bad news. For if
the initial reports hold up, here the first intermediate host of the
worm is not Bithynia, but rather Viviparus georgianus (Minnesota DNR 9Nov07). V. georgianus is a native of the American southeast, and is much more widely distributed throughout the United States than Bithynia. And if the problem trematodes can infect Viviparus, they would also seem likely to be able to exploit Bellamya (Cipangopaludina) and Campeloma, and in so doing expand their potential ranges nationwide.
I am often asked about the potentially negative effects of
artificially-introduced freshwater gastropod populations. My
usual response has been that such effects are difficult to
establish. Very few studies have ever demonstrated community or
ecosystem effects attributable to exotic freshwater gastropods –
they seem to invade environments that are already disturbed and compete
rarely (if ever) with native populations.
But here splashed across our morning newspapers
we read that "carcasses dropped by eagles hung in the trees on the
island, and feathers littered the shore" (LaCrosse Tribune 28Oct07). This is a dramatic
example of unfortunate environmental consequences directly attributable to the spread of exotic freshwater gastropods.
And at this point I don't think that snail control will solve the
problem. And I really wish I could think of any other alternative.
We'll keep in touch,
Rob
Notes
(1) Hoeve, J. and M. E. Scott (1988) Ecological studies on
Cyathocotyle bushiensis (Digenea) and Sphaeridiotrema globulus
(Digenea), possible pathogens of dabbling ducks in southern
Quebec. J. Wild. Diseases 24: 407 - 421.
(2) See my Chapter 6 of Dillon (2000) for a more complete review of
trematode life cycles, from the standpoint of the gastropod.
Although most of that chapter (perhaps unsurprisingly) focuses on Type
I Fasciola and Type II Schistosoma, there is a bit about Type III Echinostoma as well.
-------------------------
Nov. 6: Parasite has killed thousands of scaup
The deadly organism has infected snails at Lake Winnibigoshish
and has been picked up by the ducks, who dive below water to feed. The
DNR is concerned the parasite might spread to other lakes.
By Doug Smith, Star Tribune
Last update: November 08, 2007 – 4:49 PM
A parasite has killed thousands of ducks on Lake Winnibigoshish in
northern Minnesota, and could kill many more before the fall migration
is over.
"We picked up 1,000 dead scaup [also known as bluebills] on Saturday,"
said Steve Cordts, Minnesota Department of Natural Resources waterfowl
specialist.
He saw many other scaup still alive but unable to fly, or to fly far.
"You could boat right up to them," Cordts said. Perhaps 3,000 ducks,
mostly scaup, and some coots have died in the past week. "I'm
sure there's more dead birds," Cordts said Tuesday.
The ducks apparently are dying from trematodes, a tiny 1-millimeter
intestinal parasite or fluke that has infected snails in the lake.
Scaup -- a duck that dives below water to feed -- eat the snails, then
are infected. "They essentially bleed to death," Cordts said.
The parasite was confirmed in scaup and coots sent to the National
Wildlife Health Center in Madison, Wis. Similar die-offs caused
by trematodes have occurred spring and fall since 2002 on the
Mississippi River near La Crosse, Wis., killing about 40,000 ducks and
coots since then. Die-offs again are occurring there this fall.
Cordts said he's not sure how the trematodes made it to Winnie. They
apparently have infected a snail called the banded mystery snail, which
was first found on Winnie about eight years ago. They are infecting the
faucet snail on the Mississippi.
Officials aren't sure how many ducks might eventually die on Winnie, or
what impact, if any, it will have on the scaup population. But Cordts
is concerned that the snails and parasites might spread to other
Minnesota waters. Other duck species also could eat the snails
and become infected, he said.
There is concern because the continental scaup population has been
declining since 1984 and hit an all-time low last year at about 3
million. Hunters annually kill about 300,000. Minnesota hunters killed
about 20,000 last year. Lake Winnibigoshish is a major scaup
resting area during migration. "We could have 20,000 scaup show up on
Winnie right now," Cordts said. "If that happens, they'd pretty much
all be at risk."
Cordts plans to check the lake again today, but he won't collect any
more dead ducks. Instead, carcasses will be left to decompose or be
eaten by scavengers. The parasite apparently is not a threat to other
species, including humans, but Cordts said hunters shouldn't eat sick
waterfowl.
Doug Smith • dsmith@startribune.com
© 2007 Star Tribune. All rights reserved.
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