CHAPTER 39 

 RICKETTSIA 



D EFiNiTioN . — Ricketts ia . 



Small, Gram-negative, bacterium-like organisms, usually less than half a micron 

 in diameter. More or less pleomorphic. Stain rather poorly with aniline dyes, but 

 well with Giemsa. Natural inhabitants of intestinal canal of arthropods; usually 

 occupy an intracellular position. Some species are parasitic in higher animals 

 and are pathogenic for man. The type species is Rickettsia proivazeki. 



Rickettsia is the name given to certain small bacteria-like bodies which are 

 found in the alimentary canal of insects and other arthropods, and which are 

 frequently associated with disease in man and animals. Definition of the group 

 presents several difficulties. Some workers, like Zinsser (1937), would auto- 

 matically exclude any organism such as R. melophagi that has been cultivated on 

 artificial media. Further, they would insist that, when growing in the animal 

 body or in tissue cultures, only such organisms as multiply intracellularly should 

 be regarded as true rickettsias. Most species appear to be unable to pass through 

 the ordinary bacterial filters, though there are exceptions, as with R. burneti. 

 Since, however, it is often diflS.cult to obtain a homogeneous suspension of the 

 organisms free from cellular material, too much weight should not be placed on 

 this characteristic. It seems clear that the rickettsiae occupy a position in between 

 the smallest bacteria, like Bartonella, and the filtrable viruses. The fact that 

 they can be resolved microscopically by visible light, and that they are held back 

 by membranes which allow most of the filtrable viruses to pass through, brings 

 them into line with the bacteria ; but their failure to grow on ordinary culture 

 media, and their predilection for intracellular multiplication, show that their 

 metabolic requirements are more akin to those of the filtrable viruses. 



We shall not attempt to define the genus too closely, nor shall we exclude 

 from the present chapter organisms, such as R. quintana, merely because they 

 appear to grow extracellularly. Mention will be made of R. conjunctivce, R. catiis, 

 R. hovis, and R. ovina, even though it is doubtful whether these organisms are 

 natural inhabitants of arthropods, or even whether they are living organisms 

 at all. 



The first-named species was described by da Rocha-Lima in 1916, who found 

 these bodies in lice taken from patients with typhus fever ; he proposed the name 

 of Rickettsia prowazekii in honour of Ricketts and of Prowazek, both of whom 

 died of typhus fever while investigating the disease. The second species was 

 described by Topfer (1916), also in 1916, in lice taken from patients suffering from 

 Wolhynian fever — better known as Trench fever ; this species has been given the 

 alternative names of Rickettsia quintana — on account of the 5-day febrile parox- 



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