MORPHOLOGY OF RICKETTSIA 929 



ysms characteristic of this disease — and R. wolhynica ; it appears to be closely 

 related to, if not identical with, R. ■pediculi, which was found by da Rocha-Lima 

 in the apparently normal human body louse. A third species, which was first 

 described by Ricketts as far back as 1909 in Rocky Mountain spotted fever, has 

 been called Dermacentroxenus rickettsi by Wolbach ; as this organism appears 

 to belong to the Rickettsia group, we shall refer to it as Rickettsia rickettsi. 

 Sellards in 1923 claimed to have cultivated a fourth species, R. nifponica, from 

 animals experimentally infected with tsutsugamushi fever in Japan. This species 

 has been given the alternative names oiR. tsutsugamusJii, R. akamushi, and R. orien- 

 talis ; but, in spiteof Philip's (1943) argument that the name 7?. nipponica is ruled 

 out because of the reported cultivation of this organism on artificial media, there 

 still seems insufficient reason for discarding it. If every organism on which a 

 fallacious observation has been made must be 

 re-named, bacterial taxonomy will become even 

 more perplexing than it is already. 



Cowdry (1925) described a fifth species, R. 

 ruminantium, as the cause of heart-water of 

 sheep, goats and cattle in South Africa. A 



murine type of typhus virus, sometimes referred • !^^ *?. "•'■•. ^' ^ .-V -• 



to as R. mooseri, was discovered as the result of . _ , ^ ^ 



epidemiological and bacteriological studies ''','*• • .• k 

 undertaken by American and Mexican' workers '* ^ r - ., 



(for references see p. 1845). More recently, the . ;,. m 



observations of Derrick (1937) and Burnet and . ^ , ^'^ 



Freeman (1937) in Australia and of Davis and ■ ' * 



Cox (1938) in the United States have revealed 



a rickettsia, R. burneti IR. diaporica), as the ^ ^^, ,,.,,,. , i • 



., . , ■' 1*1G. 221. — Rickettsia melopnagi. 



cause of a febrile disease known as U lever. ^, . ^ 



. . . . , Smear preparation irom gut oi sheep- 



Besides these six or seven species, which j^ed. Giemsa. (x 1000). 



have been found in association with disease of 



mammals, over forty other, apparently non-pathogenic, species of Rickettsia have 

 been described in various insects and other arthropods. One or two of these 

 will receive brief mention at the end of this chapter, together with a few patho- 

 genic organisms whose relationship to the Rickettsia group is still in doubt. 



As the rickettsise have been found both in blood-sucking and in non-blood- 

 sucking insects it seems probable that they are primarily inhabitants of the 

 alimentary canal of insects, and that infection of insects occurs by contamination 

 with infected excreta (Hindle 1921). Some species are found not only in the lumen 

 of the ahmentary canal, but also in the epithelial cells lining the canal. A further 

 invasion of the body may occur, leading to infection of the salivary glands and other 

 tissues. Most species appear to be transmitted hereditarily to successive gener- 

 ations through infection of the eggs. There is a marked host specificity. Some 

 species appear to have reached a perfect equilibrium with their insect host, and 

 sometimes with the animal on which the insect is a parasite, such as in the system 

 comprised by the murine typhus virus, the rat ilea, and the rat. Others, however, 

 which by some workers (see Zinsser 1937, Burnet 1942) are supposed to have 

 developed later in the evolutionary period, are pathogenic for their insect hosts, 

 killing a high proportion of them — as, for instance, the classical typhus rickettsia 

 and the louse. A few species appear to have become adapted to an alternate 



P.B. H H ■ 



