RICKETTSIA BODIES f)43 
or at best a very faint tinge. They do not retain the Gram stain. 
The polychrome methylene blue dyes, especially the Giemsa stain, 
color them rose-red. Adventitious bacteria that may be simultane- 
ously present are stained deep purple. 
Motility.— True motility has not been observed. Under the dark- 
field illumination they exhibit Brownian movement. 
Growth in Artificial Media. This has not been definitely obtained in 
spite of numerous attempts by various observers, although Kuczynski^ 
reports its cultivation. 
Resistance to Physical and Chemical Agents.— Rickettsia prowazeki 
appears to be very susceptible to moderate heat and to drying.-"'' 
Freezing at 0° C. for twenty-four hours in defibrinated blood does not 
destroy its infectivity for experimental animals. 
Transmission —Typhus may be transmitted to man by the human 
louse, Pediculus humanus. It is possible that the virus may pass from 
the first infected generation of lice to the second generation through 
the egg.-* 
The exact mechanism of transmission from the infected louse to 
man is still unknown: it may be by direct implantation of the virus at 
the time the infected louse bites; or somewhat more probably, the virus 
may be deposited upon the skin with infected feces and the prospective 
victim himself rubs the virus into the tissues by scratching. 
Maxcy,'^ has recently cast doubt upon the louse-transmission theory 
of mild typhus (Brill's disease) although not denying the louse trans- 
mission in areas where the disease occurs in severe form. He suggests 
as an hypothesis that a reservoir for mild typhus exists other than in 
man, and that this reservoir is in rodents, probably rats and mice, from 
which the mild type is occasionally transmitted to man. Thus far, 
however, the only positive transmission experiments have been either 
through blood from actual cases, or lice that have been allowed to 
bite actual cases. 
Pathogenesis.— Typhus, as indicated above, occurs both in severe 
form in man, esj)ecially in parts of Europe, Central and South America, 
and in mild form, known as Brill's disease, in the eastern and southern 
parts of the United States. Monkeys and guinea-pigs are susceptible 
to infection, the former more so than the latter. 
Immunity.— Animal experimentation seems to indicate that recovery 
from typhus indisposes to subsequent infection. Advantage was taken 
of this observation to show that typhus, Brill's disease and tabardillo 
mutually confer immunity on monkeys; that is, an animal reco^'eret^ 
> Med. Klin., 1920, 16, 706, 733, 759. 
- GoldberKcr and Anderson: Public Health Reports, 1912, 27, No. 22. Ciavifio and 
Gerard; Pub. d. Institute Bacteriologico Nacional, Mexico, 1910, May 10, June 10, 
August 23. 
^ See Wolbach, Todd and Palfrej': Loc. cit. Da Rocha-Lima: Arch. f. Schiff.s. ti. 
Tropen. Hyg., 1916, 20, 17; Berl. klin. Wchnschr., 1916, 531, 567. Arkwright: Proc. 
Roy. Soc. Med., Sect. Med., London, 1920, 13, 87. 
* Ibid, 5 Public Health Reports, 1926, 41. 2967. 
