308 THE STREPTOCOCCUS-PNEUMOCOCCUS GROUP 
would produce variously a rapidly fatal septicemia, erysipelas, arth- 
ritis, endocarditis or peritonitis. Marmorek has shown that the 
virulence of streptococci for animals may be greatly increased by 
repeated passage ; after a series of passages an incredibly small amount 
of culture, even one one-hundred-millionth of a cubic centimeter of 
a forty-eight-hour broth culture introduced intraperitoneally may 
cause death within two days. Streptococci which are virulent for 
man frequently exhibit but little virulence for animals; it is essential, 
therefore, that large amounts of material be injected into experi- 
mental animals to obtain infection. Rabbits are more susceptible 
than other laboratory animals, particularly for human strains.^ Sub- 
cutaneous injections of morbid material into rabbits result variously, 
depending upon the virulence of the strain for this animal; a localized 
abscess may form or an erysipelatoid inflammation may occur, which 
is usually somewhat localized, but may develop into a widespread 
Fig. 40.— Streptococci in. liver, section stained by Gram's method. X 100. (Kolle and 
Hetsch.) 
cellulitis. Intraperitoneal injections are usually followed by rapidly 
fatal peritonitis. Death may occur within twenty-four hours. Intra- 
venous injections may cause a rapidly fatal generalized septicemia, 
or, if the strain is less virulent and death does not occur during the 
first three to four days, the serous surfaces may be violently inflamed. 
Less virulent strains which do not cause acute death usually lead 
to endocardial or joint involvement. Mice are nearly as susceptible 
to streptococcus infection as rabbits. Guinea-pigs are less suscept- 
ible; subcutaneous inoculations usually lead to abscess formation, 
which soon heals, but intraperitoneal injections may result in peri- 
tonitis and death. Horses are quite susceptible to infection with 
streptococci, particularly with Streptococcus equi (Streptococcus 
coryzte contagiosa^ equorum), which causes equine distemper or 
strangles. The udders of milch cattle occasionally become infected 
1 Davis: Jour. Infec. Dis., 1916, 19, 236. 
