1915. | T. SOUTHWELL : Indian Parasites of Fish. 319 
Healthy wild fish placed in hatcheries where the disease is endemic 
soon become affected. The first symptoms of disease of the 
thyroid is simple hyperplasia usually marked in living specimens 
by a redness of the throat (‘‘red floor’’) near the second gill 
arches, and caused by an increase in the blood supply to the 
thyroid, and to hyperaemia of the adjacent tissues. This condition 
of simple hyperplasia passes gradually into the stage of visible 
tumour. Various structural types of infiltrating tumour are known 
amongst Salmonoid fishes, including the alveolar, solid, tubular, 
papillar and mixed, and the investigations of workers in America 
show that the tumour is of a true malignant nature. 
The cause of the disease has not been definitely determined, 
but it is known that trout fed on animal proteid food, in an 
uncooked condition, are more heavily infected than those fed on 
cooked animal proteid food. The crowding together of fish in con- 
fined spaces, and other generally unsatisfactory hygienic conditions 
also favours the spread of the disease. 
General insanitary conditions alone are, however, insufficient 
to account for the phenomenon. A specific living organism is 
suspected, although no such organism has been isolated up to the 
present. In America it was found that scrapings from the inner 
surface of the wooden tanks in which diseased fish were kept, if 
suspended in water and administered to certain mammals. pro- 
duced in such animals a definite condition of goitre, and it was 
accordingly believed that this agent was the cause of the disease 
amongst the fish in the tank. 
It was further shown that by boiling the water the effective 
agent was destroyed. The disease does not appear to be directly 
transmissible from one individual to another. 
Some species of the Salmonidae are practically immune from 
the disease, and in other species spontaneous recovery frequently 
occurs. Especially is this the case if the diseased fish are removed 
to natural conditions and allowed to feed on natural food. More- 
over the disease is directly susceptible to treatment. At all times 
the normal thyroid contains traces of iodine. During hyperplasia 
the proportion of iodine appears to be reduced. Occasionally 
human goitre reacts favourably to treatment with iodine. In all 
stages’ of its growth the tumour in fish is favourably affected by 
solutions of iodine as well as by those of mercury and arsenic. It is 
thus possible, in a limited way, to treat these diseases in hatcheries 
and in limited water-areas. At the same time it will be obvious 
that the object of fish culturalists should be to prevent the disease 
rather than to effect its cure. 
A tumour, apparently of a similar nature, is recorded by 
Williamson (Fisheries Scotland, Scientific Investigations for 1911, 
Glasgow, 1913, page 23) in the following words. 
‘* Tumour tn the pharynx of a Salmon caught in the sea.”’ 
‘*It was found loose in the gill cavity after the fish had been 
killed by a blow on the head. ‘Two of the gills were found to be 
