FOOT-AND-MOUTH DISEASE 681 



on the basis of insufficient proof, that infection may take place through 

 the air, without actual contact. 



Rarely the disease may be transmitted to man. Such infection, 

 when it does take place, occurs usually among the milkers and attend- 

 ants in dairies, and is transmitted by direct contact. The disease in 

 man is usually very mild. Mohler states that the disease may be trans- 

 mitted to man by the milk of infected animals. He l adds that in the 

 United States the disease has been practically eradicated. 



The causative agent of foot-and-mouth disease is unknown. A num- 

 ber of organisms have been cultivated from the vesicles and mucous 

 membranes of afflicted animals, but none of these could be shown to 

 have etiological significance. Loeffler and Frosch, 2 have demonstrated 

 that the virus contained in the vesicles may pass through the pores of a 

 filter. The virus is easily destroyed by heating to 60 C. and by com- 

 plete desiccation. 



One attack of foot-and-mouth disease protects against subsequent 

 attacks. This immunity in most cases lasts for years, though rare cases 

 of recurrence within a single year have been reported. Loeffler has ac- 

 tively immunized horses and cattle with graded doses of virus obtained 

 from vesicles and with the sera of such animals has produced passive 

 immunity in normal subjects. 



FILTRABLE VIRUS 



Recent investigations into the causation of disease have revealed 

 that a considerable number of infections may be caused by organisms 

 too small to be held back by filters through which even the smallest 

 bacteria cannot pass. The earliest observations of such "filtrable 

 virus" are probably those of Frosch (1898) in foot-and-mouth disease 

 and of Beijerinck in the mosaic disease of tobacco. Since then similar 

 investigations have shown that a large number of diseases are probably 

 caused by such minute organisms; their investigation, long delayed 

 by the belief in their invisibility by even the most powerful microscopic 

 aid, and by our inability to cultivate them, has taken new impetus from 

 the discovery of and the cultivation of minute globoid bodies from the 

 virus of poliomyelitis by Flexner and Noguchi (see below). The follow- 

 ing tabulation is based largely on the comprehensive summary published 

 by Wolbach. 3 



1 Mohler, Bull. No. 41, U. S. Pub. Health and Mar. Hosp. Serv., Wash., 1908. 



2 Loeffler und Frosch, Cent. f. Bakt., 1, 1908. 



3 Wolbach, Jour, of Med. Res., xxvii, 1912. 



