September 30, 2013 at 8:12 pm #455MikeKeymaster
By BETSY BLOOM / La Crosse Tribune
ONALASKA, Wis. — Hundreds of waterfowl again are turning up dead on
Lake Onalaska, the victims of an internal parasite thought to come
from eating river snails.
Since March 22, about 850 dead coots and ducks, mostly lesser scaup,
have been counted on the lake, which also is Pool 7 of the
Mississippi River, said Jim Nissen, La Crosse district manager for
the Upper Mississippi River National Wildlife and Fish Refuge.
Some of the dead birds were sent to the U.S. Geological Survey’s
National Wildlife Health Center in Madison. As before, they were
found to have hundreds of trematodes, a very small fluke worm, in
their digestive tract.
The trematodes use snails as an intermediate host, then are ingested
by the birds when they eat the snails. One type feeds off blood and
in high numbers can cause anemia or internal bleeding. Another can
affect the bird’s water and electrolyte balance, leaving it
vulnerable to other infections.
The birds apparently can ingest a lethal dose of worms within 24
hours, dying three to eight days after feeding on the snails. Most
have been lesser scaup and coots, but Friday a refuge worker saw a
dead ring-necked duck and ruddy duck, Nissen said.
Similar die-offs were seen on Lake Onalaska during migration in fall
2002 and spring and fall 2003, when more than 3,000 carcasses were
found. Wildlife officials estimate more than twice that number
probably died but their bodies were eaten or buried by river sand and
Though only a small fraction of the population, the number of
trematode deaths is discouraging, especially since most of the birds
otherwise appeared to be healthy, Nissen said. “This time of year, as
the birds are working to get to their nests, you hate to be picking
up these dead females,” he said.
If last year was an indication, the spring toll should not be as
high, Nissen said. But unlike last spring, more coots have been dying
this year. The numbers do appear to be tapering off, he said, as the
flocks continue to move north.
Eagles, gulls and crows have been seen eating the dead birds, but are
not thought to be at risk of picking up the parasites.
Any sick or dead birds found on the river should be reported to the
La Crosse District office of the U.S. Fish and Wildlife Service at
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