430 PATHOGENIC BACTERIA.

tents, and Kitt, who discovered them in this position,
points out that the infection of swine probably takes
place by the entrance, along with the food, of the fecal
matter of diseased animals into the alimentary apparatus
of others.

Pasteur, Chamberland, Roux, and others have worked
upon a protective vaccination based upon the attenuation
of the virulence of the organism by passing it through
rabbits. Two vaccinations are said to be necessary to
produce immunity. The vaccinated animals, however,
may be a source of infection to others, and should always
be isolated. Klemperer in 1892 found that the blood-
serum of immunized rabbits would save infected mice
into which it was injected.

Ivorenz in 1894 found an antitoxic substance in the
blood of rabbits immunized to the disease. The effect
of its injection into other animals is, however, only a
temporary immunity. Later1 he found it possible to
protect hogs against the disease by injecting them first
with a serum obtained from a hog immunized in the
ordinary manner described by Pasteur, afterward with
a feeble culture of the bacillus, and finally with viru-
lent cultures. The strength of the serum should be
determined by injecting varying quantities of it into
mice infected with definite amounts of a culture of
known virulence. The immunity prodticed by Lorenz
lasted for a year.

1 CentralbLf. Bakt. u. Parasitenk.^ Jan., 1896, p. 168.