126 



JOHN S. BARLOW 



concerning the total number of cases in a population at a particular time), 

 incidence data (i.e. the number of new cases appearing in a population per 

 year), and statistics concerning hospital admissions for the disease. 



Since multiple sclerosis is a chronic disease (the average case has a dura- 

 tion of approximately 20 years after onset of symptoms) , it appears that the 

 most reliable indication of its distribution may be obtained from prevalence 

 data. Table I shows results from a series of surveys in the United States and 



TABLE I 



Multiple Sclerosis Prevalence Ratios for the White Population 

 IN Selected Communities in the United States and Canada" 



" Data from Kurland el at., 1957. 



* Values in parentheses are corrected values on "clinical review" of the reported cases. 



Canada. In the present study, emphasis is laid on relative rather than abso- 

 lute frequencies of occurrence of the disease in diflferent areas, and the table 

 indicates the prevalence in each city relative to that for Winnipeg. Relative 



1.0 



0.8 



0.6 





0.2 - 



OL- 



20 30 40 50 30 40 50 60 



Geographic latitude °N Geomagnetic latitude °N 



Fig. 2. Relative prevalence of multiple sclerosis in selected communities in the 

 United States and Canada (see Table I). 



