366 POLIOMYELITIS [CH. 



previously to 1907, for large numbers of emigrants had been 

 arriving for many years before that date, and also why 

 the infection should have suddenly appeared in many other 

 parts of the world. Since this date, however, the disease has 

 taken on fresh activities in its original home, and in 1911, 

 Sweden was visited by a very severe epidemic. 



Distribution. Poliomyelitis is especially prevalent in Scan- 

 dinavia and Northern Europe but, in addition, epidemics have 

 occurred in England, France, Germany, Italy, Austria, and 

 Spain ; in America large numbers of cases have been recorded 

 from New York and Boston in the east, to San Francisco in 

 the west, and a large outbreak occurred in Cuba in 1909. The 

 disease has also been recorded from various parts of Australia. 



Causal agent. Although the most varied methods of 

 staining and cultivation have been employed, no parasite 

 has ever been detected in patients infected with poliomyelitis, 

 but the scarcity of polymorphonuclear leucocytes in the altered 

 cerebro-spinal fluid and spinal cord and the large increase in 

 the number of mononuclears suggest that the parasite is 

 protozoal in nature. The filterable nature of the virus has 

 since been demonstrated, for if the spinal cord of a recently 

 paralysed monkey is made into an emulsion with either 

 distilled water, or normal saline, and passed through a Berkefeld 

 filter, the resulting filtrate is still infective. Moreover, its 

 activity is very considerable, for one thousandth of a cubic 

 centimetre of a filtered 2-5 per cent, suspension of the spinal 

 cord of an infected monkey is sufficient to produce infection and 

 paralysis when injected into another monkey. The resistance 

 of the virus is extraordinary, for in dust, especially within 

 protein matter, it remains virulent for months. In diffuse day- 

 light it survives indefinitely and resists the action of pure 

 glycerine and 0-5 per cent, phenol for many months. 



The virus may be preserved by passage through monkeys, 

 as these animals are easily infected by the intra-cerebral 

 injection of an emulsion of the brain or spinal cord of an animal 

 suffering from poliomyelitis. The susceptibility of monkeys 

 has enabled Flexner, in conjunction respectively with Lewis 

 and Clark, and also Landsteiner and Levaditi, to make some 



