96 Foot-and- Mouth Disease. 



Liquid which can thus he proved to contain what is con- 

 veniently called the virus of the disease may show no bacteria 

 or other solid particles when examined under the highest 

 powers of a good microscope, and when it is taken from a 

 freshly formed vesicle with the precautions necessary to pre- 

 vent the admission of accidental bacteria from the mouth no 

 bacteria can be cultivated from it. Finally, when the diluted 

 liquid from a mouth vesicle is passed through a filter proved to 

 be quite efficient for arresting the smallest visible bacteria it is 

 still capable of infecting an ox or other animal susceptible to 

 foot-and-mouth disease — a result which can only be explained 

 by suppbsing that although the pores of the filter are small 

 enough to stop minute but visible bacteria they are large 

 enough to permit the microbes of foot-and-mouth disease to 

 pass through them. 



Such is the evidence upon which it has been generally 

 accepted by bacteriologists that, just as in other contagious 

 diseases, the cause of foot-and-mouth disease is a living microbe, 

 but that it is too small to be recognisable with the eye. And 

 it ought to be observed that in all probability failure to detect 

 this microbe is not a mere temporary disability likely to be 

 overcome by improvements in miicroscopes, for there are 

 physical limitations to the minuteness of particles that can be 

 made visible to the human eye. 



Although it is thus possible to account satisfactorily for the 

 fact that the individual foot-and-mouth disease microbes have 

 not been seen, it is not so easy to understand why all attempts 

 to grow these microbes outside the body have failed. It is 

 obvious that, however minute individual living things may be, 

 if they can be induced to multiply on the surface of some solid 

 substance outside the body they must eventually form a visible 

 growth. For example, a single anthrax bacillus in the course of 

 twelve hours will build up a mass of growth plainly visible to 

 the naked eye, and although a longer time is often required the 

 result is the same with nearly all the known visible bacteria. 

 It is a curious fact, however, that it has not yet proved possible 

 to induce any of the so-called invisible organisms to multiply 

 outside the body, a fact which has led some authorities to think 

 that these minute living things may not be bacteria (which 

 belong to the vegetable kingdom) but protozoa, or animal 

 parasites, similar to but smaller than those which are the cause 

 of human malaria and redwater in cattle, for these, although 

 visible, have resisted all attempts to cultivate them under 

 artificial conditions. There is, however, no disease known 

 to be caused by protozoa which is so intensely contagious as 

 foot-and-mouth disease, and upon the whole it appears to be 

 probable that the invisible microbe of the latter is a bacterium. 



