First Advisor

Deborah Duffield

Term of Graduation

Spring 2022

Date of Publication


Document Type


Degree Name

Master of Science (M.S.) in Biology






Harbor seal -- Parasites -- Oregon, Harbor seal -- Parasites -- Washington (State), Parasitology, Acanthocephala



Physical Description

1 online resource (ix, 87 pages)


Parasites have the capability to infect virtually every living organism on the planet and have adapted to infiltrate every trophic level. Many species have complex indirect life cycles and rely upon hosts at different levels of the food web for growth and reproduction. In the marine environment, having a high level of parasite diversity is thought to indicate a more stable ecosystem than an environment with low parasite diversity. As one of the top predators in their environment and because of their amphibious behaviors, pinnipeds (seals and sea lions) are exposed to a wide variety of parasites, making them ideal for parasite research. One of the most common and widely distributed pinnipeds is the harbor seal, Phoca vitulina. While parasitic infections are common in harbor seals, they are often overlooked unless they have a direct impact on human health or the fisheries industry.

Although there have been recent studies conducted on the parasites of Pacific harbor seals, P. vitulina richardii, along the coasts of California, Washington, and Alaska, there have been no reports for Oregon since the 1970's. Earlier studies in Oregon looked at parasite presence and diversity, but lacked any in-depth analyses on parasite prevalence with host characteristics like sex, age, health status, season, or over time. The Northern Oregon/Southern Washington Marine Mammal Stranding Program (NOSWSP; Portland State University, Department of Biology) responds to stranded marine mammals from Tillamook, OR through Long Beach, WA. These are routinely necropsied and all are examined for parasites. Pacific harbor seals are one of the most commonly stranded pinnipeds in the NOSWSP area. We examined and collected parasites from 53 stranded Pacific harbor seals between the years of 2018-2019.

Parasites were collected from the heart, lungs, stomach, and intestines of each seal and found in 51 of the 53 processed seals (96% overall parasite prevalence). Nematodes were found in 43 seals (81% prevalence) and in each organ examined (heart, lungs, stomach, and intestines). The nematodes from the stomach (72% prevalence) were all from the family Anisakidae. Nematodes from the heart (21% prevalence) were from the family Onchocercidae, and strongly suspected to be Acanthocheilonema spirocauda. Nematodes from the lungs (28% prevalence) were from the order Strongylida with the possibility of being either Parafilaroides sps. or Otostrongylus circumlitus. Cestodes were found only in the intestines and in a total of 4 seals (8% prevalence) and were most likely from the family Diphyllobothriidae. Acanthocephalans, all from the genus Corynosoma, were also found in the intestines of 50 seals (94% prevalence) and were the most frequent parasites.

The aim of this work was to: 1) update the diversity of parasites in stranded Pacific harbor seals along the coast of Northern Oregon and Southern Washington, and 2) evaluate potential correlations between parasite diversity, prevalence, and intensity with host sex, age, health status, season, and stranding year.

Parasites in the lungs were found to have significantly higher rates of prevalence in yearlings when compared to other age classes (p < 0.001), and had a significant relationship to season (p < 0.01) with winter having the highest prevalence (100%). No other parasite had any significant findings with host sex, age, health status, season, or year. However, we did observe that compromised seal hosts had a higher prevalence of parasites in the heart (31%) than healthy hosts (8%) and that the stomach and intestines had consistently high parasite prevalence regardless of sex, age, health status, season, or year suggesting that the intermediate hosts for these parasites are present year-round.

The intestines of 50 seals were used for parasite intensity analyses against host sex, age, health status, season, and year. The acanthocephalan, Corynosoma sp., was the most dominant parasite in the intestines with a prevalence of 96% (48/50) and an intensity range of 0-851 parasites per host. While Corynosoma was the dominant genus and used for all analyses, C. strumosum was also found in this study. Our findings indicate that there is a significant relationship between Corynosoma sp. intensity and host age, with the pups having significantly lower intensities than the subadults and adults (p < 0.001). This was also supported by a strong positive correlation between Corynosoma sp. intensity and host body length (p < 0.001). No other variables were found to have statistically significant relationships with parasite intensity. The distribution of Corynosoma sp. along the intestinal tract was evaluated and we found that the colon had statistically lower intensity than any other section of the intestines (p < 0.001), and that the second section (10m anterior to the colon) had significantly higher intensities than the intestinal section which was the closest to the stomach (p < 0.05). This suggests Corynosoma sp., while inhabiting the length of the small intestine, may prefer the microhabitat found in the posterior section of the intestines.

This study demonstrated that the parasites of stranded Pacific harbor seals are common and consistently present in our area. Efforts should be made to continue monitoring their diversity and prevalence, using them as bioindicators to assess any potential changes to the marine ecosystem. As a result of this work, a parasitology CURE was developed and implemented at Westfield State University in Westfield, Massachusetts. Undergraduate students used the Corynosoma sp. specimens collected in this study to conduct a variety of research projects including trophic web analyses, heavy metal analyses, and confirmed species identification of Corynosoma strumosum through DNA sequencing. This was a very exciting opportunity to involve undergraduates in real research experiences, proving that these parasites are a rich resource of experimental data.


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