U.S. Department of the Interior

U.S. Geological Survey

Open-File Report 2007–1065

USGS Upper Midwest Environmental Sciences Center and National Wildlife Health Center,

in cooperation with the U.S. Fish and Wildlife Service, Upper Mississippi River National 

Wildlife and Fish Refuge

Finding the Exotic Faucet Snail (Bithynia tentaculata): 

Investigation of Waterbird Die-Offs on the Upper 

Mississippi River National Wildlife and Fish Refuge
Finding the Exotic Faucet Snail (Bithynia tentaculata): 

Investigation of Waterbird Die-Offs on the Upper 

Mississippi River National Wildlife and Fish Refuge

By Jennifer S. Sauer, Rebecca A. Cole, and James M. Nissen

USGS Upper Midwest Environmental Sciences Center and National Wildlife 

Health Center, in cooperation with the U.S. Fish and Wildlife Service, Upper 

Mississippi River National Wildlife and Fish Refuge

Open-File Report 2007–1065

U.S. Department of the Interior

U.S. Geological Survey
U.S. Department of the Interior

DIRK KEMPTHORNE, Secretary

U.S. Geological Survey

Mark D. Myers, Director

U.S. Geological Survey, Reston, Virginia: 2007

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Suggested citation:

Sauer, J.S., Cole, R.A., and Nissen, J.M., 2007, Finding the exotic faucet snail (Bithynia tentaculata): Investigation of 

waterbird die-offs on the Upper Mississippi River National Wildlife and Fish Refuge: U.S. Geological Survey Open-File 

Report 2007–1065, 3 p.

Cover photo:

Lesser scaup and redheads collected on Lake Onalaska (Navigation Pool 7), in the Upper Mississippi River System in 

November 2006. (Photograph by Calvin Gehri, U.S. Fish and Wildlife Service, Upper Mississippi River National Wildlife 

and Fish Refuge.)
Beginning in 2002, there have been major waterbird 

die-offs every spring and fall in Lake Onalaska (Navigation 

Pool 7 of the Upper Mississippi River) located near La Crosse, 

Wisconsin (fig. 1). This area is part of the Upper Mississippi 

River National Wildlife and Fish Refuge (UMR Refuge) and 

lies within the Mississippi Flyway, through which an estimated 

40 percent of the continent’s waterfowl migrate. Through the 

2006 spring migration, an estimated 22,000–26,000 birds died 

on the UMR Refuge, primarily lesser scaup (Aythya affinis) 

and American coots (Fulica americana). Other waterbirds 

affected include northern pintail (Anas acuta), American 

wigeon (A. americana), northern shoveler (A. clypeata), bluewinged teal (A. discors), mallard (A. platyrhynchos), American black duck (A. rubripes), gadwall (A. strepera), redhead 

(Aytha americana), ring-necked duck (A. collaris), bufflehead 

(Bucephala albeola), tundra swan (Cygnus columbianus), herring gull (Larus argentatus), and ruddy duck (Oxyura jamaicensis).

Wildlife pathologists at the U.S. Geological Survey’s 

National Wildlife Health Center (NWHC), Madison, Wisconsin, found that infection by two trematode parasites (Cyathocotyle bushiensis and Sphaeridiotrema globulus) was the cause 

of death. An exotic snail known as the faucet snail (Bithynia 

tentaculata; fig. 2) is the first and second intermediate host for 

the trematodes (Friend and Franson, 1999). The faucet snail, 

with a shell length that can reach 15 mm, is typically found in 

freshwater ponds and shallow lakes (Bensen and others, 2004). 

Waterbirds are at risk of consuming the trematodes inadvertently as they eat snails, which are important in their diets 

(Eldridge, 1990). Depending on how heavily snail populations 

are infected, some birds can receive a lethal dose in less than 

24 hours of feeding and die within 3–8 days. Although small 

in size, hundreds of trematodes, more than sufficient to cause 

death, were found in the lower intestines of most of the birds 

examined. One lesser scaup contained over 46,000 trematodes. Birds (scaup or coot) infected with C. bushiensis had 

typhlitis with mutlifocal lesions and sloughing of the mucosa 

(fig. 3). Waterbirds infected with S. globulus typically had 

hemorrhagic distended small intestines.

Faucet snails are native to and well distributed throughout Europe (Bank, 2004). The snail was introduced into the 

Great Lakes in the early 1870s (Mills and others, 1993). In 

Wisconsin, faucet snails were only found in Lake Michigan 

and the Wolf River drainage until 2002 when they were found 

in Lake Onalaska (NWHC, unpublished data). In 2004, led by 

the NWHC in collaboration with the U.S. Geological Survey’s 

Upper Midwest Environmental Sciences Center in La Crosse, 

Finding the Exotic Faucet Snail (Bithynia tentaculata): 

Investigation of Waterbird Die-Offs on the Upper 

Mississippi River National Wildlife and Fish Refuge

By Jennifer S. Sauer, Rebecca A. Cole, and James M. Nissen

Figure 1. Areas sampled for the faucet snail (Bithynia 

tentaculata).

Open-File Report 2007–1065 1
Wisconsin, UMR Refuge, and Minnesota State University–

Mankato, we began a study of the waterfowl die-off and distribution of faucet snails in Lake Onalaska (a backwater of the 

Upper Mississippi River). Abundance of faucet snails in Lake 

Onalaska was highest on submersed rock surrounding islands, 

but they also were found in sediment and on aquatic vegetation. About 40 percent of the snails sampled in 2004 were 

infected with Sphaeridiotrema globulus and/or Cyathocotyle 

bushiensis. Infected snails were found throughout the sampling period (April through September). In the 2005 sampling 

season in Lake Onalaska, between 0.5 and 1,131 snails were 

collected per person hour at 19 sites on submersed rock. More 

than 70 percent of the snails were infected by the trematodes. 

The majority (77 percent) of infected snails in 2005 had 

10 or fewer trematodes (moderate level) in them but occasionally a snail might host over 100 trematodes (highly infected). 

Experimental work has shown that the minimum lethal dose of 

Sphaeridiotrema globulus in 12-week-old mallards is somewhere between 43 and 66 worms (Mucha and Huffman, 1991). 

The same study showed that only about 24–29 percent of the 

larval parasites in a snail will actually mature and cause disease. Therefore, if snail infections are prevalent with a high 

level of larval stages of parasites, waterbirds could pick up a 

lethal dose of trematodes from eating a few snails within 2–3 

days. Birds can be infected with both parasites, and waterbird 

susceptibility to infection appears to be species and age specific (Hoeve and Scott, 1988). Infection from both parasites 

may decrease the number of parasites needed to induce death 

in waterbirds. 

Because Lake Onalaska is a major spring and fall stopover area for waterfowl in the Mississippi Flyway, concerns 

were raised that the snail and trematodes may be spreading 

to other waterfowl stop-over areas on the river. In 2004, field 

crews from the Minnesota, Wisconsin, and Iowa Departments 

of Natural Resources collected snails during routine sampling 

under the Long Term Resource Monitoring Program (www.

umesc.usgs.gov/ltrmp.html) in Navigation Pools 4, 8, and 13 

(fig. 1). None of the snails were identified as faucet snails. 

However, these samples were only taken on soft-substrates, 

which are not the preferred substrate habitat of faucet snails.

In 2005, we conducted additional sampling in Navigation 

Pools 8 and 9 focusing on known waterfowl feeding areas that 

contained submersed rocks or aquatic vegetation—habitat 

preferred by faucet snails. From 1 to 84 snails per site were 

collected in 4 of 15 sites in Pool 8 and 2 of 5 sites in Pool 9. 

Infected snails were found in both pools. Waterbird die-offs 

were highest in Lake Onalaska in fall 2005 (estimated 5,300–

7,000), but several dead waterbirds also were found in lower 

Pools 8 and 9. These birds may have flown downriver to these 

areas after ingesting infected snails from Lake Onalaska, but 

our data indicate they could have died from consuming snails 

locally. 

Exploratory sampling for faucet snails was conducted 

in summer 2006 in Pools 4–9 (excluding Pool 5a which is 

located between Pools 5 and 6), 11, and 13 (fig. 1). Numbers 

were from 2 to 109 per site with specimen sizes from 2.1 to 

13.1 mm. Infected snails were found in all the sampled pools 

except Pool 6. To our knowledge, the 2005 and 2006 data are 

the first records of faucet snails and associated trematodes 

beyond those found in Pool 7, Lake Onalaska. Fall 2006 

waterbird mortality numbers in Lake Onalaska (Pool 7) were 

estimated at 3,700–5,000 birds. Mortality increased in lower 

Pools 8 and 9 with estimated mortality at 3,700–5,000 and 

500–1,000; respectively (J. Nissen and T. Yager, pers. comm.). 

This disease is an added stress to declining waterbird 

populations. For example, scaup (greater and lesser) were 

37 percent below their long-term average in 2006 as recorded 

by the Waterfowl Breeding Population and Habitat Survey 

Figure 2. Faucet snail (Bithynia tentaculata). Figure 3. Gross lesions of Cyathocotyle bushiensis in ceca of 

lesser scaup (Aythya affinis).

2 Finding the Exotic Faucet Snail (Bithynia tentaculata)
(Wilkins and Otto, 2006). Beyond the waterbird deaths, other 

effects of the die-offs include (1) UMR Refuge staff time spent 

on monitoring waterfowl reference points, dead bird pickup, 

disposal, and outreach, and (2) health concerns by waterfowlers hunting in areas of heavy mortality and increased difficulty 

of retrieval of hunter-killed birds. Hunting dogs are having difficulty retrieving hunter-killed ducks and are retrieving coots 

and other waterbirds that died from trematodiasis.

Waterbird die-offs are becoming a UMR Refuge-wide 

problem. Knowing the distribution of infected faucet snails 

on the UMR Refuge will help managers prepare for potential waterbird die-offs. Continued monitoring and focused 

research of faucet snails is needed to help understand the 

transmission dynamics and track the distribution of the snail. 

Information obtained through research and monitoring, including the identification of the origin of infections in snails and 

birds and the role various environmental factors have on this 

process, should help guide managers to develop effective 

mitigation and control measures. 

References Cited

Bank, R.A., 2004, Fauna Europaea: Bithyniidae: Fauna 

Europaea version 1.2, accessed March 14, 2007, at 

http://www.faunaeur.org

Benson, A.J., Jacono, C.C., Fuller, P.L., McKercher, E.R., and 

Richerson, M.M., 2004, Summary report of nonindigenous 

aquatic species in U.S. Fish and Wildlife Service 

Region 5: U.S. Department of the Interior, U.S. Fish and 

Wildlife Service, Arlington, Virginia. 145 p.

Eldridge, J., 1990, Aquatic invertebrates important for 

waterfowl production: Fish and Wildlife Leaflet 13.3.3, 

Waterfowl Management Handbook, U.S. Department of the 

Interior. 8 p.

Friend, M., and Franson, J.C., 1999, Field manual of wildlife 

diseases general field procedures and diseases of birds: U.S. 

Geological Survey Information and Technology Report 

1999–0001. 426 p.

Hoeve, J., and Scott, M.E., 1988, Ecological studies on Cyathocotyle bushiensis (Digenea) and Sphaeridiotrema globulus

(Digenea), possible pathogens of dabbling ducks in southern Quebec: Journal of Wildlife Diseases, v. 24, i. 3, p. 

407–421.

Mills, E.L., Leach, J.H., Carlton, J.T., and Seacor, C.L., 1993, 

Exotic species in the Great Lakes: a history of biotic crises 

and anthropogenic introductions: Journal of Great Lakes 

Research v. 19, i. 1, p. 1–54.

Mucha, K., and Huffman, J.E., 1991, Inflammatory cell 

stimulation and wound healing in Sphaeridiotrema globulus 

experimentally infected mallard ducks (Anas platyrhynchos): Journal of Wildlife Diseases, v. 27, p. 428–434.

Wilkins, K.A., and Otto, M.C., 2006, Preliminary report: 

Trends in duck breeding populations, 1955–2006: U.S. 

Fish and Wildlife Service, Division of Migratory Bird Management. 19 p.

For additional information:

Jennifer S. Sauer

USGS Upper Midwest Environmental Sciences Center

2630 Fanta Reed Road

La Crosse, Wisconsin 54601

jsauer@usgs.gov

Dr. Rebecca A. Cole

USGS National Wildlife Health Center

6006 Schroeder Road

Madison, Wisconsin 53711-6223

Rebecca_cole@usgs.gov

James M. Nissen

USFWS Upper Mississippi River National Wildlife and Fish 

Refuge

555 Lester Avenue

La Crosse, Wisconsin 54650

james_nissen@fws.gov

Open-File Report 2007–1065 3