1006 
ANIMAL DISEASES 
ously in the sterile isolator system with sterile 
water, food, bedding, and air. The diet now rec- 
ommended for germfree rats and mice is L485,'' 
which has been tested and monitored with satis- 
factory results. The rats and mice are free of 
demonstrable parasites, protozoa, fungi, and 
bacteria. A standard bacteriological monitoring 
system has been developed which combines ac- 
curacy with simplicity.^ 
"Vertically-transmitted" viruses (as de- 
scribed later) have been demonstrated in the 
mice, but not yet in the rats. Tests for viral 
flora have involved examinations of target or- 
gans (thymus) for virus-like particles by elec- 
tron microscopy, serological surveys, examina- 
tions of tissue cultures derived from fetuses and 
of tumors for cytopathogenic effects, viral in- 
clusions, and susceptibilities to known viral 
agents. 1°-^- It is significant that so-called 
"horizontally"-transmitted viruses have not 
been detected in germfree mice,^- nor have 
they been detected in germfree rats. Under 
such circumstances, the rats and mice reach ma- 
turity, reproduce, and grow old. New strains of 
germfree mice and rats are usually foster- 
nursed on germfree mothers up to weaning age, 
and thereafter they are self-sustaining. The 
germfree technology has been so simplified that 
short-term experiments can be implemented in 
most laboratories with mice and rats in equip- 
ment acquired from commercial sources. 
Germfree rats and mice have some unique an- 
atomical and physiological characteristics 
which distinguish them from conventional 
counterpart animals. The intestines and cecums 
are thin-walled and dilated, and the lamina pro- 
priae are small, thin, and contain relatively few 
cells. The turnover rate of mucosal cells in the 
intestine is lower than that in conventional 
animals. The Peyer's patches are usually 
small aggregations of lymphocytes, and free of 
germinal zones. The lymph nodes and spleens 
are small, contain rare germinal zones, and few 
plasma cells. The immunological mechanisms in 
germfree animals are dormant, but functionally 
intact; and this is reflected in the low levels of 
serum globulins. Following exposures to anti- 
gens, they enlarge, and germinal zones and 
plasma cells appear in them. White blood cell 
counts and blood hematocrit levels are lower in 
germfree rats and mice than in conventional 
counterpart animals.^^' The characteristic pat- 
tern of enlargement and involution of the thy- 
mus glands in germfree and. in conventional 
rats and mice are undistinguishable. In general, 
germfree animals are comparable to conven- 
tional animals in reproduction patterns, growth 
rates, and appearance; and whatever differ- 
ences they manifest are related to the effects of 
variable microbial flora and other non-viable 
environmental factors to which the conven- 
tional animals had been exposed. 
DISEASE IN GERMFREE MICE AND RATS 
Most germfree mice and rats are free of 
spontaneous diseases up to ages 6 and 12 
months, respectively. Except for mouse strains 
SJL/J, NZB, Haas, and AKR, the longevities of 
germfree mice and rats have been extended sig- 
nificantly through their unique insulated resi- 
dence. The patterns of spontaneous diseases in 
germfree mice differ from those in germfree 
rats. Germfree mice carry congenitally-trans- 
mitted viruses which are life-limiting in some 
strains (AKR and Haas) ; and in other strains 
(NZB and SJL/J) fatal diseases develop spon- 
taneously for which etiological agents have not 
been identified.^*'-!'' many cases, the differ- 
ences between the diseases in conventional and 
in germfree rodents are attributed to environ- 
mental factors which the germfree system has 
been designed to exclude, but this cannot be ap- 
plied to mice with congenitally-acquired dis- 
eases (viral or genetic) . 
SPONTANEOUS DISEASES IN GERMFREE 
MICE 
Thus far, three virus-related and two genetic- 
related diseases have been observed in germ- 
free mice : 
Viral Diseases 
Leukemogenic viruses have been observed in 
all strains of mice (conventional and germ- 
free) ;-" however, among the seven strains 
of germfree mice at Lobund Laboratory, only 
the AKR strain develops lymphatic leukemia 
