CARCINOMA OF THE THYROID IN SARMONOID FISHES. 5O5 
the fish. Macroscopically visible growths have not been seen in fish under about five 
months of age. Anemia and cachexia, sometimes extreme in degree, are a frequent 
but not constant accompaniment of the disease. Immunity is strikingly exhibited not 
only among species, as the Scotch sea trout, but with given lots of a susceptible species. 
Recovery or regression occurs when affected fish are removed from domestication to 
wild conditions and also in fish in ponds in which the disease was acquired. 
VII. Feeding of fish tumors, or of human cancer, to brook trout has not during a 
period of several months produced the slightest evidence of the disease attributable to 
this feeding. The intimate association of susceptible trout with trout tumor material 
in standing water, or with tumor fish in circulating unchanged water, has brought only 
negative results. The fish tumor has not yet been successfully transplanted, but 
implants have grown slightly and were alive at the end of three months. The tumor 
extract is highly toxic to trout when injected into the thyroid region or subcutaneously. 
Wild brook trout brought from the wilderness and confined in cement tanks and 
fed raw heart or raw liver have developed microscopic evidence of the disease by the 
end of the first year, and visible carcinoma between the first and second year. The 
feeding of cooked liver retarded the process. Spontaneous regression occurred in a 
high percentage of the meat-fed fish by the end of the second year. Similar trout fed 
upon marine fish, vegetable food, or a combination of mussels and live maggots retained 
their normal thyroids. 
VIII. Either of the elements iodine, mercury, or arsenic dissolved as salts in the 
water in which the fish are living interrupts the progress of the disease and restores the 
thyroid epithelium to a condition approximating the normal. Recognizable effects are 
produced within a few days. Visible tumors are markedly affected and may be much 
reduced in size. Iodine and mercury are effective even when diluted by many millions 
of parts of water. Iodine is effective when introduced into the digestive tract as well 
as through the medium of the water. Negative results were obtained with thymol by 
both these methods of administration. 
IX. The administration to dogs of mud and water from fish ponds in which thyroid 
carcinoma was endemic gave suggestive evidence that the water and mud contained an 
agent capable of producing marked changes in the thyroid. Scrapings from the inside 
of old wooden fish troughs in which thyroid carcinoma was constantly produced gave 
positive results. Four dogs were given for six months water to drink in which these 
scrapings were immersed. All developed marked thyroid hyperplasia and three of them 
enlarged thyroids. The thyroids of the three control animals remained of normal size. 
Two of them were normal in structure while one showed slight evidence of hyperplasia, 
probably referable to a previous experiment. 
Rats given for six months mud and water which had been taken from ponds in which 
thyroid carcinoma was prevalent and transported several hundred miles gave negative 
results. Rats given for four months water from the fish trough scrapings, also trans- 
ported as above, produced results similar to those obtained with the dogs but less marked 
in degree. 
