Thymus Gland in the Harbor Seal (Phoca vitulina): Relationship to Age and Disease
IAAAM Archive
Murray L. Johnson; Steven J. Jeffries; Larry Cargol; M.J. Wicks
University of Puget Sound, Tacoma, WA


The thymus glands of over 70 wild-collected harbor seals were examined to determine what correlation existed between age, sex, season, presence of disease, and other conditions. Collection was by fire-arm. Complete evaluations including gross and histopathology examinations, blood counts and chemistries were done. Aging was done by tooth section. The presence of disease was determined by the presence of gross or microscopic evidence of parasites and other abnormalities, and this correlated with the weight of the thymus glands. Thymus glands were found to be large in comparison with usual mammalian norms. A high degree of parasitism, especially heartworms, was found. Lipogranulomatosis of the liver was very common. Despite some positive correlations, the reason for persistently enlarged thymus was not satisfactorily explained.


Samples were collected from Gray's Harbour, Washington, in a study supported by the Marine Mammal Commission and Fisheries Department. Gross heartworm in 66 animals:

  • Less than 2 years age - 52% had gross heartworm
  • 2-4 years age - 81% had gross heartworm
  • 5-9 years age - 31%
  • Over 10 years age - 25%

The presence of heartworm does not appear to be related to the season.

Microfilaria were detected in some specimens.

The vector is unknown but may be the sucking louse which is more common in subadults.

Relation of thymus to parasites:

Increased numbers of lymphocytes, mitotic figures and lymphoblasts were found in the thymus glands of parasitized animals. The hyperplastic thymus noted in these animals may be a physiological defense against heartworm. The antigenicity of heartworm may affect the thymus gland just as grafted organs stimulate the thymus to produce killer cells which reject the graft.

20 of 28 essentially normal animals had liver lipogranulomas.

Comment by Murray Dailey: To detect microfilaria in lice, squash the lice, put them in a gauze bag and immerse in MS 222 medium. The microfilariae will come out immediately.

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Murray L. Johnson

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