Date Awarded


Document Type


Degree Name

Master of Science (M.Sc.)


Virginia Institute of Marine Science


Andrew R. Wargo

Committee Member

Wolfgang K. Vogelbein

Committee Member

Troy D.. Tuckey


American eels are infected by the introduced parasitic nematode, Anguillicoloides crassus, which can cause significant damage to their swimbladders. Despite the high prevalence and severe damage caused by A. crassus, the population level effects on American eels are not well understood. The prevalence and swimbladder damage in young glass eels and elvers are relatively unstudied, despite the potential for this parasite to cause tissue damage. Additionally, the effects of environmental, temporal, and spatial variables have been debated in previous studies without consensus. Also, the potential for eels to recover from infection and tissue damage has been speculated but not definitively shown. Therefore this Master’s thesis sought to answer these questions through field and laboratory studies. Glass and elvers stage American eels were collected during the spring and summer of 2015 and dissected to enumerate infection intensity and estimate swimbladder condition using the swimbladder degenerative index (SDI). Data were then combined with a larger dataset for yellow eels and analyzed using zero-inflated and ordinal logistic regressions to determine the effects of season, site, and eel length on infection intensity and swimbladder condition. The relationship between infection intensity and swimbladder condition was evaluated. This dataset was then used to investigate if the force-of-infection (i.e. rate of uninfected eels becoming infected) varied by host age and if there was evidence of disease associated mortality. to investigate recovery from parasite-induced swimbladder damage, 270 wild caught (presumably infected), individually tagged yellow eels were held in a freshwater recirculating system and fed a parasite-free diet for six months. Each month eels were x-rayed, weighted, and measured. The length and area of the swimbladder of each individually tagged eel were measured on x-ray images for temporal comparisons. at the end of the experiment, all eels were euthanized and dissected to determine infection intensity, SDI, and dissected swimbladder length. The trends of average monthly length ratio index (LRI; length of swimbladder to eel total length) and area of swimbladders were determined, as well as the relationships to SDI and infection intensity. Our field study showed that glass eels have a very low prevalence compared to elvers and yellow eels. Infection intensities of elvers and yellow eels varied by season and site and increased with total length. Swimbladder damage also varied by season and increased with total length. Infection intensity and swimbladder damage were non-linearly related. Force-of-infection was highest for age 2 eels and also varied by season, with the highest values in the winter and lowest in the early spring. Parasite-associated mortality was observed, with infected eels having an annual survival rate of 0.76 that of uninfected eels. Results from our x-ray experiment showed that LRI and area increased slightly through time. SDI also increased slightly over the course of the experiment, and SDI, LRI, and swimbladder area all were in agreement of improvement in swimbladder condition, however full recovery was not observed. In conclusion, the health of American eels in the Chesapeake Bay is adversely impacted by A. crassus, though that effect varies by season, system, length of the eel and whether infection level is being measured by infection intensity or swimbladder condition. Also American eels may have the ability to recover from A. crassus infection, but more work is needed to determine if this occurs in the wild.




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