636 FILTERABLE VIRUSES— RICKETTSIA 
animals chiefly. The characteristic eruptions, which are vesicular 
at first and filled with a clear fluid, soon become grayish, and the 
epidermis sloughs off, leaving a raw reddened surface. The eruption 
usually appears at three distinct sites— the mucous membrane of the 
mouth, the teats, and interdigital spaces. The incubation period is 
from one to six days, and little or no immunity to subsequent attacks 
is conferred on an animal by successful recovery. 
The milk of infected cows contains the virus, and the disease is 
transmissible to man, particularly young children, through raw or 
imperfectly pasteurized milk, and possibly from butter and cheese 
made from infected milk. The disease is mild, as a rule, in older 
children, but it may be severe or fatal for infants. 
The virus belongs to the group of filterable viruses and, in its purest 
state, is found in the contents of the vesicles. Early in the disease the 
virus also circulates in the blood stream, Loffler and Frosch,^ who 
discovered the filterable nature of the virus, found that the vesicular 
fluid, filtered through unglazed porcelain filters, retained its infectious- 
ness for some time, provided the fluid be kept cool and in the dark. 
Contagious Pleuropneumonia of Cattle.— This disease was the first 
to be described in which the virus passes through unglazed porcelain 
filters, although the filtration of the virus was not attempted at that 
time. Nocard and Roux^ examined the exudate from the lungs of 
diseased cattle microscopically with negative results. They suspended 
it in broth, enclosed in collodion capsules, in the peritoneal cavities 
of guinea-pigs. After two to four weeks the medium became turbid, 
while controls remained clear. Examination of the fluid under a 
magnification of 2000 diameters revealed very minute, highly retrac- 
tile spots which exhibited Brownian movement. They claim to have 
cultivated the virus in a peptone-serum medium and to have obtained 
minute colonies (0.5 mm. diameter) on peptone-serum agar. Later 
the virus was shown to pass through Berkefeld filters and the coarser 
grades of porcelain filters, but not the finer grades. 
The disease is confined to cattle; man is immune so far as is known. 
BACTERIOPHAGE. 
One of the remarkable developments of the last few years in the 
domain of the filterable group is the discovery of a group of phenomena, 
apparently related, which are called collectivel.y bacteriophagy. The 
first observations w^ere those of Twort,^ who, studying, and attempt- 
ing to cultivate a filterable virus from vaccine lymph, noticed that 
certain colonies of a micrococcus he obtained, on an agar plate, exhib- 
ited thin areas, which soon became transparent. He was unable to 
obtain growth from these transparent areas where bacteria had been, 
1 Centralbl. f. Bakteriol., I Abt., 1897, 23, 371. 
2 Ann. Inst. Pasteur, 1898, 12, 240. 
3 Lancet, 1915, ii, 1241. 
