DISEASE RESISTANT AND SUSCEPTIBLE POPULATIONS 

 OF BROOK TROUT (SALVELINUS FONTDMALIS) 



By 



S. F. Snieszko-' 



1/ 



It has been observed that brook trout 

 received from some hatcheries are much more 

 resistant to fish furunculosis caused by 

 Aeromonas salmonicida and to ulcer disease 

 caused by Hemophilus piscium than those ob- 

 tained from other sources (Wolf, 1954). It will 

 suffice to cite several references to support 

 the assumption that resistance to diseases is a 

 genetic characteristic, the manifestation of 

 which can to som e degree be modified by ecol- 

 ogy. Gowen (1952) stated: Through efforts of 

 workers in the field it is now recognized that 

 agents of diseases and their hosts each are or- 

 ganized and get their characteristics from the 

 development of an inheritance, made up of dis- 

 tinct genes . By specifying the gene for a 

 particular disease reaction it has been possible 

 to show that genes act as protectors from a 

 given disease, as independent of the disease re- 

 action, and as causative agents in the disease 

 syndrome. A clinically manifest disease only 

 results when the proper combination of the geno- 

 type of the victim, the genotype of the pathogen, 

 where one is necessary, are properly synchro- 

 nized with the environment. This thesis has 

 been further developed by Dubos (1954) and 

 Burnet (1953) . Therefore, on the basis of the 

 information just given above and the rationale 

 presented in Marr's article in this collection 

 (The Problem of Defining and Recognizing Sub- 

 populations of Fishes) one should feel justified 

 in classifying as populations, or at least sub- 

 populations, strains of brook trout which are 

 kept at separate hatcheries and which differ 

 from each other by significant differences in 

 susceptibility to some infectious diseases. 



Furunculosis (McGraw, 1952) and ulcer 

 disease (Snieszko, 1952) are two most destruc- 

 tive bacterial diseases of salmonid fishes. The 



1/ Director, Microbiological Laboratory, 

 U.S. Fish and Wildlife Service, 

 Leetown(P.O. Kearneysville), W. Va. 



outbreaks of these diseases among brook trout 

 frequently result in total loss of the infected 

 lots. Because an effective therapeutic control 

 of furunculosis was unknown until 1946 (Gutsell) 

 and of ulcer disease until 1952 (Snieszko, Grif- 

 fin and Friddle), Embody and Hayford (1925) 

 and Davis (1946) carried out selective breeding 

 of brooK trout strains resistant to furunculosis. 

 The results were very encouraging. The strain 

 bred by Embody and Hayford is still maintained 

 at the HacKettstown, N.J. hatchery. Recently 

 Wolf (1954) secured fingerling brook trout from 

 11 hatcheries and reared them under identical 

 conditions . They were exposed simultaneously 

 to infection with the agents of furunculosis and 

 ulcer disease. At the time the results were 

 prepared for publication, two strains of trout 

 were found to be particularly resistant; one from 

 Hackettstown and the other from the State hatch- 

 ery at Bellefonte, Pa . 



Wolf's work was conducted at the Rome 

 hatchery in northern New York State . In order 

 to reduce the possibility that ecological condi- 

 tions contributed to the resistance of these two 

 strains of brook trout to ulcer disease and fur- 

 unculosis, similar experiments were repeated 

 in 1954 and 1955 at the Federal experimental 

 fish hatchery at Leetown, W.Va. Brook trout 

 eggs were obtained from the Federal hatchery 

 at Berlin, N. H., and from the State hatcheries 

 at Bellefonte, Pa., HacKettstown, N.J., and 

 Beaver Creek, Md. Fingerling trout were re- 

 ceived from Erwin, Tenn. During incubation 

 and after hatching the fish were maintained under 

 identical conditions of water and nutrition until 

 they were at least half a year old. 



The first experiment was started when 

 ulcer disease appeared in the Berlin trout. 

 Some of the diseased Berlin trout were added to 

 trout which originated from the Bellefonte hatch- 

 ery. Mortalities were recorded and bacteriolog- 

 ical examination carried out for 243 days. The 



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