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BULLETIN OE THE BUREAU OF FISHERIES. 
X. In the hyperplastic thyroids of three puppies and one adult dog which were 
given pond mud and water, or water from fish-trough scrapings, minute nematode worms 
were found immediately beneath the capsule or in the substance of the thyroid. The 
worms were surrounded by connective tissue tubercles. In two instances only remains 
of small nematode worms were found in the thyroid region of brook trout with carcinoma 
of the thyroid undergoing regression. If these worms have any etiological significance 
it must be merely as carriers of a causative agent. 
CONCLUSIONS. 
1 . The disease known as gill disease, thyroid tumor, endemic goiter, or carcinoma of 
the thyroid in the Salmonidae, is a malignant neoplasm. 
2. The disease occurs in fish living under conditions of freedom in populated areas. 
3. When introduced into fish-breeding establishments it becomes endemic with 
occasional epidemic outbreaks. 
4. Normal fish taken from the wilderness may be made to acquire the disease when 
placed in fish-breeding establishments where the disease is endemic. 
5. The feeding of uncooked animal proteid favors and the feeding of cooked animal 
proteid retards the disease as compared with the uncooked. Feeding alone is not an 
efficient cause. It must be combined with an agent transmitted probably through the 
water or food, or both. 
6. By scraping the inner surface of water-soaked wooden troughs in which the disease 
is endemic, an agent may be secured which from its action upon the mammalian thyroid 
when administered through drinking water is no doubt the cause of the disease in the 
fish confined in these troughs. 
7. The agent is destroyed by boiling. 
8. Fish in all stages of the disease are favorably affected in the direction of cure by 
the addition to the water supply in suitable concentration of mercury, arsenic, or iodine. 
9. The effect of mercury, arsenic, and iodine in carcinoma of the thyroid in fish 
and the subsequent positive experiments with metals in mammalian cancer are probably 
the expression of a therapeutic relation of these elements to carcinoma. 
10. Certain species of the Salmonidae have an almost complete natural resistance to 
the disease. 
11. Certain lots of fish of susceptible species show a high degree of immunity to 
the disease. 
12. Spontaneous recovery occurs in a considerable percentage of individuals. 
13. Removal from ponds in which the disease is endemic to natural conditions, 
or a change to more natural food, increases the percentage of spontaneous recoveries. 
14. Spontaneous recovery appears to confer a degree of immunity against recurrence. 
15. The percentage of spontaneous recoveries in the early stages of the disease 
appears to be higher than in the later stages of the disease. 
16. The incidence of the disease increases with the age of the fish, at least up to 
five years. 
