838 THE FILTRABLE VIRUSES 



together they form a hardly visible roughening of the surface of the agar : 

 when sufficiently far apart they may attain the size of a pin's head. These 

 colonies can be stained en masse and if they be transferred intact to a slide, 

 preparations can be obtained which stain with the ordinary aniline dyes and 

 are gram-positive. 



The inoculation of. cows with cultures obtained by Nocard and Roux's 

 method is followed by a typical attack of the experimental disease : occa- 

 sionally the animal dies : when it recovers it is immune to inoculation with 

 cultures or with the exudate from a case of pleuro-pneumonia. Pure cultures 

 are now used in place of the serous exudate for the purpose of Willens' vaccina- 

 tion (vide ante}. 



Filtration. If the exudate from a case of pleuro-pneumonia be filtered 

 through a Chamberland (F) or Berkefeld bougie, the filtrate does not produce 

 the disease in calves neither does it give cultures : but, on the other hand, 

 if the same exudate be diluted with 20-30 volumes of water the filtrate is 

 infective and grows on serum-broth ; under these conditions the organisms 

 pass through the filters (save Ohamberland B through which they never 



Filtration thus enables a pure culture of the organism to be obtained from 

 contaminated exudates, ordinary bacteria being held back by the filter. 



SECTION II. THE VIRUS OF FOOT AND MOUTH DISEASE. 



(Aphthous fever.) 



Foot and mouth disease infects cattle, sheep, goats, and pigs and is trans- 

 missible to man. 



Microscopical examination and cultural methods fail to reveal the presence 

 of any micro-organism in the lymph contained in unbroken vesicles, but this 

 lymph is nevertheless infective and on inoculation into cattle, pigs, sheep and 

 goats will reproduce the disease. 



Lceffler and Frosch have shown that the virus of foot and mouth disease 

 is an invisible organism, which easily passes through a Berkefeld bougie if 

 care be taken to dilute the serous fluid with 40-50 volumes of water but 

 is held back by closer filters such as Kitasato's bougie and Chamberland B. 



Vaccination. Serum therapy. Aphthous lymph loses its infectivity if 

 kept for some time (3-8 weeks), or if heated (12 hours at 37 C. or 30 

 minutes at 60-70 C.). By inoculating into the veins of an ox a mixture 

 of an old inactive lymph and fresh lymph attenuated by heating at 60 C. 

 for 5 minutes Loeffler has been able to produce an immunity to the disease, 

 but this immunity is only acquired slowly. By hyper-immunizing oxen with 

 increasing doses of an active lymph of constant virulence, Loeifler obtained 

 a serum which has some slight prophylactic properties. 



When a stable is found to be infected and infection of all the animals 

 in it has become inevitable, recourse may be had to an emergency inocula- 

 tion : all the animals are purposely infected in such a manner as to secure 

 that they shall suffer from the least severe form of the disease. For this 

 purpose the tongues and the inner surface of the lips of the healthy animals 

 are rubbed with a rough cloth soaked in virulent saliva. 



SECTION III. THE VIRUS OF HORSE-SICKNESS. 



Horse sickness is a fatal disease affecting horses in South Africa which can 

 be experimentally inoculated and though not spontaneously contagious is 

 apparently transmitted by biting insects. 



