GEOGRAPHIC DISTRIBUTION OF MULTIPLE SCLEROSIS 145 



for such a phenomenon would still have to be small compared with its 

 latitude effect.) Such a nonlinear relationship between inciting agent and 

 disease can obtain, for example, if the susceptibility of individuals in the 

 population is distributed in a normal (Gaussian) manner. A normal distribu- 

 tion for susceptibility and the concomitant nonlinear relationship between 

 per cent incidence and dose rate at low doses appears to obtain for the 

 occurrence of tumors in mice following exposure to ultraviolet light, and it 

 has been suggested that a similar relationship may occur for the appearance 

 of cancer in man following exposure to ionizing radiation (Blum, 1959a,b). 



Duration of Exposure to Inciting Agent 



Whatever is the cause of multiple sclerosis, a most interesting question 

 arises in connection with the incubation period for the disease. Acheson et 

 al. (1960) have pointed out that if the inciting or protective agents are 

 prolonged in their effect, then the important factor in the history of indi- 

 vidual cases will be that of place of residence over an extended period; 

 alternatively, only the place of residence early in life might be the important 

 factor. These workers suggest that a long incubation period may be impli- 

 cated by their own findings as well as by the observation of Dean (1949) 

 that multiple sclerosis is almost unknown in persons of European stock born 

 and raised in the Union of South Africa, whereas it is more frequently 

 described in persons born in Europe who have emigrated to South Africa. 

 An analogous observation in Israel by Rozansky (1952) is being verified 

 by Alter (1960). 



The present approach does not help in elucidation of this question, for 

 it is conceivable that radiation could have either a single-shot effect, perhaps 

 analogous in animals to the graying of hair produced by heavy cosmic ray 

 nuclei at very high altitudes (Chase and Post, 1956), or alternatively it 

 might have a cumulative effect, as does the carcinogenic property of ultra- 

 violet radiation. 



Possible Experimental Approaches 



Despite the difficulties of establishing more than a correlative (and there- 

 fore possibly fortuitous) relationship between cosmic radiation and multiple 

 sclerosis, it is perhaps in order to consider briefly some possible experimental 

 approaches to the problem, particularly if additional carefully collected epi- 

 demiologic data substantiate the results so far obtained. These experimental 

 approaches difTer somewhat according to whether it is the place of residence 

 in adult life, or in early life, that is established as being the important factor 

 in the geography of the disease. 



