Intestinal Trematodes Sphaeridiotrema globulus AND S. PSEUDOGLOBULUS

causative agent (classification, morphology) Sphaeridiotrema is a genus containing at least two species: S. globulus and S. pseudoglobulus (Yamaguti 1971) in the Family Psilostomatidae (App.

host range and distribution Many species of ducks, geese, and swans are parasit­ized by these species (Yamaguti 1971, Huffman 2005), and both trematodes cause epizootics in waterfowl. Sphaeridiotrema globulus is distrib­uted throughout wild and domestic waterfowl of North America (Cole and Friend 1999) and Europe (Gylstorff and Grimm 1987), as well as Australia (Campbell and Jackson 1977). In North America, S. globulus occurs in the eastern seaboard and Midwest (McLaughlin et al. 1993) as well as the western United States (Berntzen and Macy 1969); S. pseudoglobulus occurs primarily in Quebec, Canada (McLaughlin et al. 1993).

life cycles and variations The life cycle of each includes a snail for both first and sec­ond intermediate host as well as a vertebrate definitive host (Huffman 2005). At 28°C, eggs hatch ¿¿ to 13 days after being shed; mira- cidia keep their mobility for at least 6 hours (Dimitrov et al. 2001). A number of snail spe­cies are used as intermediate hosts (Berntzen and Macy 1969, Huffman and Roscoe 1986), and the same snail host can be used for both the first and second intermediate host. For example, the rediae of a snail develop into cercariae, which escape and then often encyst on the shell of the same individual snail (Yamaguti 1971).

reservoirs and transmission Eggs of S. pseudoglobulus had a hatching success of 70% or more after 28 weeks under natural overwin­tering conditions in a lake; mean hatch time decreased from 18 to 11 days with increased storage due to slow embryonation of the eggs (McKindsey and McLaughlin 1993). The para­sites are transmitted to the definitive host when the host preys upon infected snails or ingests empty snail shells still containing viable meta- cercariae (Lepitzki and Bunn 1994).

clinical effects and identification Sphaeridiotrema globulus is a blood-sucking fluke that parasitizes the lower small intestine. Severe hemorrhage from damaged submucosal capillaries provides blood meals for the parasite, with a resulting anemia in the host (Huffman and Roscoe 1989).

Death from the parasite generally can occur in 1 to 2 weeks. As few as six parasites caused mortality in a mute swan (Cygnus olor), whereas at least 53 were required to kill a Canada goose and 79 to kill a mallard (Huffman and Roscoe 1989). For birds recovering from infections, lesion resolution was evident 8 to 10 days post­inoculation (Mucha and Huffman 1991).

Waterfowl infected with S. globulus do not experience weight loss. Ballooning of the jejunum and ileum occurs and the mucosa is hyperemic, with trematodes located singly or in groups in ulcers that are circumscribed by hemorrhage (Roscoe and Huffman 1982, 1983; Huffman and Roscoe 1989). Clinical effects reported for Sphaeridiotrema pseudoglobulus are very similar to those of S. globulus, includ­ing hemorrhagic diarrhea, ataxia, and droop­ing wings. These clinical signs are observed

5- 6 days after infection and often occur

6- 12 hours before death (Wobeser 1997)

Sphaeridiotrema spp. can be identified by their small size, characteristic eggs, and cercar- iae (Yamaguti 1971); S.

population effects There have been a number of recurrent epizootics among wild­fowl in North America. Some, presumably from S. globulus, have included periodic losses of several hundred American coots (Fulica americana) in Wisconsin (Trainer and Fischer 1963), wild mute swans (Cygnus olor) in New Jersey (Roscoe and Huffman 1982), and lesser scaups (Aythya affinis) in the Washington, D.C., area (Price 1929, 1934). Other recurrent epizo­otics, presumably caused by S. pseudoglobulus, have been reported among a variety of dabbling ducks in Quebec, Canada (Hoeve and Scott 1988, Wobeser 1997)

In Europe, S. globulus causes occasional epi­zootics among wild birds, especially in late win­ter (Gylstorff and Grimm 1987). However, there has been no evidence of persistent mortality among wild waterfowl (Wetzel and Rieck 1972).

special problems Besides the recurrent epizootics noted in the preceding section, there are no other special problems noted with either of these species. On occasion, there may be syn­ergistic effects when more than one intestinal parasite infects the same bird population in a particular habitat. For example, Cyathocotyle bushiensis, another pathogenic fluke (Gagnon et al. 1993) of the cecum in waterfowl and coots in England and parts of North America (Gibson et al. 1972, Cole and Friend 1999), uses Bithynia tentaculata, the same snail intermedi­ate host used by S. pseudoglobulus (Lepitzki et al. 1994). Both S. pseudoglobulus and C. bush­iensis can cause mortality at a given site (Hoeve and Scott 1988), with the potential for greater overall mortality among migratory birds arriv­ing to that site than would be produced by either parasite alone.

control and host immunity There are no specific control methods identified for these parasites. On heavily infected sites, snail con­trol could be a consideration for high-risk host populations, but this has not yet been reported.

Host immunity may be effective in some cases. Both mast cells and eosinophil numbers respond to infection by S. globulus (Mucha and Huffman 1991). Immunity to S. globulus can develop both in Pekin ducks (Macy 1973) and mallards (Huffman and Roscoe 1986) when they are infected with smaller doses of meta- cercariae; in contrast, birds exposed to high doses in one exposure are unlikely to survive infection. Thus, movement of migratory birds from uninfected sites into heavily infected areas can carry considerable risk for suscep­tible hosts.