506 BULLETIN OF 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. 



1 6. The incidence of the disease increases with the age of the fish, at least up to 

 five years. 



