PATHOGENICITY OF R. PROWAZEKI 935 



the most certain way of establishing that an attack of fever in the guinea-pig is really due 

 to the typhus virus is to inject lice intra-rectally with a suspension of the guinea-pig's 

 platelets, and observe the development of rickettsiae in the excreta. Infected guinea-pigs, 

 it may be noted, do not give a positive Weil-FeUx reaction, though natural agglutinins 

 to Proteus OX 19 are sometimes present in a titre of 1/25 or less. 



The typhus virus can be passed from man to monkeys, from monkeys to guinea- 

 pigs, and from guinea-pigs to monkeys. 



Pathogenicity of R. mooseri for Animals. — The ynurine typhus virus gives rise 

 in guinea-pigs to a disease differing in certain respects from that caused by the classical 

 louse-borne virus (Pinkerton 1929, 1931, Zinsser and Castaneda 1930). After intra- 

 peritoneal inoculation with the murine type the temperature rises rather earHer, about 

 the 4th to 6th day, though the actual height reached may be less than with the louse- 

 borne type. The scrotal and testicular reaction caused by the murine type, first described 

 by Neill (1917) when investigating Mexican typhus, is much more intense, and micro- 

 scopical examination reveals the presence of large numbers of rickettsiae — sometimes 

 known as Mooser (1928) bodies — in the tunica vaginalis. On the other hand, nodular 

 lesions in the brain are more frequent in louse-borne than in murine type infections. 

 B. mooseri is further distinguished from R. prowazeki by its ability to give rise after intra- 

 peritoneal inoculation to a febrile disease in rats, accompanied by proliferation of rickettsiae 

 in the scrotal sac ; R. prowazeki causes a completely inapparent infection in these animals. 

 Moreover, R. mooseri causes a heavy infection of the lung after intranasal inoculation 

 into rats or mice (see Castaneda 1939), whereas R. prowazeki usually grows much less 

 abundantly. Incidentally, it may be noted that, when growing in the lungs of rats or 

 rabbits, the murine virus may give rise to intracellular inclusion bodies of the morula 

 type, consisting of colonies of rickettsiae (Begg et al. 1944). Cross-immunity tests indicate 

 that the two types of virus are very closely related (Mooser and Dummer 1930, Nicolle 

 and Laigret 1932, Zinsser and Castaneda 1934). More recent work — mainly unpubUshed 

 — suggests, however, that they are not antigenically identical. 



Pathogenicity of R. rickettsi for Animals. — The disease produced in guinea-pigs by 

 inoculation of Rocky Mountain spotted fever virus is similar to that caused by the typhus 

 virus, but is much more severe. After intraperitoneal inoculation with the Western 

 type, the incubation period is usually only 2 to 4 days. The temperature rises rapidly to 

 about 106° F., and death usually occurs within a week. From the 3rd or 4th day of the 

 fever swelhngs and haemorrhages of the scrotum and ears occur, which may go on to necrosis. 

 Post-mortem examination shows a considerable enlargement of the spleen, and frequentlv 

 a marked scrotal reaction with rickettsial bodies in the tunica vaginalis. The Eastern 

 type is said to be less virulent, but both viruses produce a characteristic rash in the monkey 

 (Badger 1933c). 



Rabbits can be infected with the Rocky Mountain virus ; they develop a febrile disease ; 

 rabbits inoculated with the typhus virus do not react at all. It is interesting to note, as 

 indicating the closeness of the relationship between the two viruses, that rabbits experi- 

 mentally infected with the Rocky Mountain virus may develop agglutinins to Proteus X 19 

 and give a positive Weil-FeUx reaction ; rabbits inoculated with the typhus virus likewise 

 develop agglutinins — usually to a rather higher titre (Munter 1928). But inoculation of 

 a rabbit with typhus is said not to protect it against subsequent inoculation with Rocky 

 Mountain virus ; indicating that though both viruses closely resemble each other anti- 

 genically, they are distinguishable by their virulence and by their immunizing properties. 

 Experiments on guinea-pigs, however, indicate that inoculation with either virus provides 

 a certain amount of protection against subsequent inoculation with the other (Breinl 1928). 

 White tnice and rats are said to develop a symptomless infection after intraperitoneal 

 inoculation with Rocky Mountain spotted fever virus (Fukuda 1929), but after intranasal 

 inoculation they develop pulmonary lesions in which considerable numbers of rickettsiae 

 are present (Durand and Giroud 1940, Durand and Sparrow 1940). 



