The Distribution and Biology of the Invasive Eyefluke of Waterfowl (Philophthalmus gralli) in Texas




Leach, Jeremiah

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Philophthalmus gralli is an invasive trematode that primarily infects the conjunctival sac of waterfowl. Since its introduction in 1969, there has been little effort to determine the impacts of this parasite on the waterfowl of Texas. The purposes of this research were to explore how P. gralli may be impacting the eyes of Texas waterfowl, to study the COI and 18S genes for variation that may shed light on the invasion history of P. gralli in Texas, and to experimentally investigate the infection dynamics of the parasite to better understand its ecology. First, I investigated the current geographic distribution and infection status of the parasite in the eyes 1,362 hunter-harvested waterfowl wintering in Texas during the fall 2016 through winter 2018 hunting seasons, and rated the pathologies observed. I simultaneously studied COI and 18S sequences of some recovered worms for suggestions of invasion history. Then I investigated the substrate preferences and the behavior of the cercarial stage of P. gralli using multiple runs of a two-factor (substrate X depth) experimental design which included six substrate types, all placed at three depths within each of three replicate cylindrical glass aquaria. Lastly, I studied interactions between the intramolluscan larvae of P. gralli and those of co-infecting trematodes in the intermediate snail host. This experiment involved exposing wild-caught intermediate-host snails to miracidia of P. gralli. Experimental snails were divided into three groups: those shedding either of two invasive heterophyid cercariae, and those not shedding any cercariae. Each of the three groups was further subdivided into treatment and control subgroups, and snails in each treatment subgroup were exposed to 5-10 P. gralli miracidia. After incubating for 120 d, the snails were crushed and examined for infection status. The distributional survey revealed P. gralli infections in ducks from every sampled location except the Panhandle counties, with a mean intensity of 5.34, prevalences ranging from 0% to 55%, and typical pathologies represented by mild irritation to conjunctivitis. The sequence analysis revealed an unexpectedly high variation, suggesting that the invasion history includes multiple introductions from several sources. The preferred substrate and depth for cercarial encystation were water stargrass at the surface. Hours of cercarial observations revealed two very different and novel behaviors not previously attributed to P. gralli. The coinfection study produced no experimental infections of P. gralli, but retrospective analysis revealed what may be a marked preference of specific P. gralli genotypes for specific genotypes of the snail host. This invasive parasite has now spread over much of Texas, and the pathologies revealed in the study suggest that the parasite is most likely impacting overwintering success of infected birds. The preference of water stargrass for encystment substrate by P. gralli cercariae indicates that many species of aquatic birds are at risk of infection. The infection dynamics are much more complicated than expected, and indicate that interactions between snail and parasite genotypes should be studied experimentally.



Parasite, Ducks


Leach, J. (2019). The distribution and biology of the invasive eyefluke of waterfowl (Philophthalmus gralli) in Texas</i> (Unpublished thesis). Texas State University, San Marcos, Texas.


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