Seasonal Dynamics of Two Mortality-Related Trematodes Using an Introduced Snail

Abstract

Seasonal dynamics of 2 trematode species, Cyathocotyle bushiensis and Sphaeridiotrema globulus, were assessed in relation to life history traits of the parasites and their hosts, as well as abundance of host species and abundance of infective stages. Both of these trematodes are associated with recurrent mortality of migrating waterbirds on the Upper Mississippi River National Wildlife and Fish Refuge. An invasive snail species, Bithynia tentaculata, serves as intermediate host for both trematode species. In total, 2,970 snails were collected at 2 study sites. Prevalence and mean abundance of the 2 trematode species varied among dates and was attributed to several factors, including migration patterns of definitive hosts, snail population dynamics, and seasonal changes in temperature. The surge of new infections of both parasites seems to be due to avian hosts foraging at this site during spring migration. The high prevalence and abundance of metacercariae among the snail population promote mortality among molluscivorous birds by increasing the probability of ingestion of a lethal dose. Additionally, mortality of non-molluscivorous birds can be explained by accidental ingestion of a couple of highly infected snails resulting in a lethal dose.

Spatio-temporal variation in prevalence and intensity of trematodes responsible for waterfowl die-offs in faucet snail-infested waterbodies of Minnesota, USA

“…Parasite prevalence and intensity of infection might thus decline in adult snails after the demands of summer reproduction. Consistent with this, Herrmann and Sorensen (2009) reported a reduction in the adult snail population after reproduction, which may result in reduced parasite prevalence and intensity after reproduction, but could not determine whether it occurred through movement or mortality.…”

“…Likewise, if snail mortality rates increase with infection intensity, then snails with more intense infections would be lost from the population faster than those with less intense infections. Differences in overwinter mortality rates were reported in Central Europe (Richter, 2001), but were not found in the Midwestern USA (Herrmann and Sorensen, 2009). Richter (2001) reported that almost all of the heavily parasitized older females died between May and July.…”

“…The abundance of snail hosts where trematode eggs are deposited is expected to be an important predictor of parasite prevalence and intensity in snails, because higher snail abundance should improve trematode success infecting hosts. However, in the summer, the addition of young, uninfected snails to the population reduces parasite prevalence in snails (Ménard and Scott, 1987b, Richter, 2001, Herrmann and Sorensen, 2009). The average infection intensity in a sample would be expected to decline temporarily as well, as young snails acquire infections of initially low intensity.…”

“…The average infection intensity in a sample would be expected to decline temporarily as well, as young snails acquire infections of initially low intensity. Metacercarial abundance has been documented to decline in summer (Ménard and Scott, 1987b, Herrmann and Sorensen, 2009), and has been attributed to this new cohort of uninfected snails.…”

“…However, waterfowl defecation of parasite eggs should increase both prevalence and intensity of sporocyst and rediae infection in snails, and eventually (i.e., 3–4 months later after eggs hatch and cercariae begin shedding) metacercarial infections would be expected to increase. Furthermore, because warmer water temperatures favor faster maturation of eggs and a high proportion of eggs fail to hatch in cold water in the fall (Ménard and Scott, 1987a, McKindsey and McLaughlin, 1993), the influence of waterfowl defecating eggs would be expected to be greater after spring migration than after fall migration because conditions are better for parasite egg hatching in the spring and summer than in the fall (Herrmann and Sorensen, 2009). …”