332 BACTERIOLOGY OF THE EYE 



It must be admitted that the natural susceptibility towards infection by the 

 Pncumococcus varies, not only in the sense that strains exist which are pathogenic 

 for animals, but still have no action on man, 1 and vice versa, but also in the sense of 

 a variation in the susceptibility of man to infection. The variation in suscepti- 

 bility to the secretion of a pneumococcal conjunctivitis is evidence of this, and has 

 already been discussed. 



It is quite certain that the personal factor plays an important part in the 

 resistance of the cornea to pneumococcal infection i.e., the ulcus serpens. As an 

 illustration of this, the following case from my own experience will serve : In the 

 cornea of a man fifty years of age, having a dacryocystitis with Pneumococci in the 

 secretion, a subcentral ulcus serpens had developed after a superficial injury. The 

 ulcer would have been described clinically as unusually benign ; it increased very 

 slowly, and, untreated, in three weeks had only reached the extent of 4 millimetres 

 wide and 2 millimetres high, remaining quite superficial. The nasal margin was 

 slightly infiltrated, and formed a thin yellowish-grey curved line ; the rest of the 

 ulcerated area was clear, and appeared covered with epithelium. The hypopyon 

 was small. In spite of this, Pneumococci were taken from the margin of the ulcer 

 in large numbers, and they were found to have a high virulence and rapidly killed 

 susceptible animals when the original culture was used. 



Such cases are rare, and can be called chronic ulcus serpens. We can presume 

 that those Pneumococci which, immediately they were taken from the ulcer, 

 showed a high virulence for animals, had also a considerable pathogenicity for 

 man. 2 If this be true, then the relative mildness of the corneal process must be due 

 to a peculiar want of susceptibility. At least this is one of the factors which, along 

 with the virulence and number of the organisms and the nature of the injury, must 

 be taken into account to explain the variations in the intensity of the clinical 

 appearances of the ulcer. 



As the Pneumococci were able to retain their power to produce the disease for such 

 a long time, they must have had a pathogenicity for man. It is remarkable that 

 the struggle between the cocci and the cornea was not settled sooner in one or 

 the other way. It is obvious that susceptibility can vary in one and the same 

 person. 



The blood-serum of animals convalescent from a pneumococcal infection can 

 protect against an artificial infection ; a pneumococcal affection must therefore 

 leave a certain degree of immunity. 



In man the period of the immunity passes quickly by, and it is of very slight 

 degree, as is shown by the recurrences. Such recurrences after comparatively 

 short intervals are to be seen in pneumococcal conjunctivitis (Gifford), and 

 occasionally after ulcus serpens. A pure local pneumococcal immunity of the eye 

 either occurs to a very limited extent or not at all. 



The meaning of the results obtained on the artificial immunization of animals is 

 much more obvious. 



Frankel (the discoverer of the Diplococcus lanceolatus), Foa, Borclone and 

 Uffreduzzi, demonstrated the immunity of animals, who have suffered from a 

 pneumococcal infection. The experimental animals, in response to the first infec- 

 tion, formed protective substances by developing an active immunity. 



The artificial production of immunity can be obtained in several ways : Foa, 



1 The brothers Klemperer have inoculated themselves with doses which were fatal for 

 rabbits, without causing any damage. Homer and a doctor in the Wurtzburg clinic have 

 also demonstrated the same on themselves. 



- This probability can only be assumed when the virulence has been tested immediately 

 after the material has been obtained, and the .same is the case with regard to Streptococci. 

 After long cultivation with interpolated passages through animals, a strongly pathogenic 

 strain need not show a corresponding pathogenicity for man. 



