FISH-BREEDING AND DISEASE 257 



The Myxosporidia have now been discussed briefly, 

 and, in closing, it would be well to sum up their role 

 as agents of disease. When large tumours contain- 

 ing parasites are present, the action of the organism 

 is obvious, but where there are no external manifesta- 

 tions, the results must be determined by other means. 

 One of the best guides to the condition of an animal 

 is its weight, and loss of weight coincides with the 

 presence of Myxosporidia. Digestive and excretory 

 troubles also are accompanied by emaciation. Loss 

 of fat is general. Some cases show marked mal- 

 formation of the bones and of the cartilage. Breath- 

 ing also may be impaired, and then anaemia results. 

 The worst effects are seen in the cases of diffuse 

 infiltration, when no organ is safe from the attacks 

 of the parasite. 



What, then, can be done to remedy this state of 

 affairs, and how can myxosporidian infection be 

 stamped out? Deep-sea fishes certainly number 

 many victims, but it is obviously impossible to apply 

 remedial measures to them. All that can be done 

 in that case is to burn any infected fish taken, and 

 endeavour to keep the main feeding-grounds of fishes 

 as clean as possible. 



Fresh-water and partly fresh-water fish, such as the 

 Salmonidae, are more easily subjected to experiment, 

 for the trade of fish-breeding has now reached a high 

 state of perfection, and hatcheries have sprung up in 

 many places. At such institutions two evils may arise 

 namely, overcrowding and inbreeding, and both are 

 conditions favouring myxosporidiasis. Overcrowd- 

 ing involves ingestion of spores from the contami- 

 17 



