BACILLUS PESTIS 235 



occurs in man occasionally, as it frequently occurs in rats, both 

 naturally and experimentally. 



Latent Plague. It was Hunter, of Hongkong, who undertook to 

 explain the seasonal appearance of plague among animals and man 

 by attempting to show the existence of a latent form of plague in 

 which small numbers of bacilli were present in the blood, but for the 

 time being no multiplication nor development of symptoms of disease. 

 From such latent cases outbreaks of epidemics might originate. 

 Such conditions, of course, prevail in Texas fever and trypanosomiasis 

 of cattle. Herzog and Hare, investigating this question in the Philip- 

 pine Islands, have shown that latency in plague does not occur. 



Morphology and Staining Properties. The plague bacillus was dis- 

 covered simultaneously and independently in 1894 by Kitasato and 

 Yersin. 



It is relatively variable in morphology, a fact which it is important 

 to remember in connection with the bacteriologic diagnosis of the 

 disease. In postmortem smears prepared from a recent non-suppur- 

 ating primary bubo, a pneumonic focus, the spleen, and occasionally 

 from other internal organs of both man and rats, numerous plague 

 bacilli are generally found. The simplest staining method for 

 plague bacilli in smears and in pus, necrotic material, etc., is dilute 

 carbol-fuchsin (1 part of the original stain to 5 to 10 aqua destillata) 

 for twenty to forty seconds and then washing freely in water. 



In smears made from the organs, the plague organism appears as 

 a rather short, plump bacillus, being 1.5 to 1.75 micra long and 

 0.5 to 0.75 micron thick; generally the proportion of width to length 

 is as one to two. Individuals considerably longer than 1.75 micra 

 are occasionally seen. The bacilli are generally single, occasionally 

 diplobacilli, and very rarely in short chains. In smears which have 

 been fixed in absolute alcohol and properly dyed the bacilli are not 

 uniformly colored, but show a distinct polar staining. Frequently 

 the entire periphery is stained and only the centre left uncolored. 

 Other forms, differing in certain respects from the above description 

 and not representing the most characteristic type, are so frequently 

 found in smears that the student must thoroughly familiarize himself 

 with them. These are elliptical, egg-shaped, or almost spherical 

 forms, which show only a very narrow peripheral staining, or do not 

 stain at all, giving the appearance of empty shells, which in all prob- 

 ability is the case, since they are most commonly found in older buboes. 

 Various involution forms are also frequently seen in smears from cases 

 which have succumbed but a few days after sickness. They appear 

 like yeast cells, and are either quite hazy and indistinct or club-shaped 

 and irregular in outline. In cover-glass preparations from pure cul- 

 tures the bacilli are not so characteristic. Plague bacilli from pure 

 cultures, particularly from the water of condensation of agar tubes, or 

 from bouillon, frequently show shorter or longer chains, in which 

 dividing lines between the individual bacilli are so indistinct as to 



