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THE AMERICAN SOCIETY OF ANIMAL PRODUCTION
SOME GENETIC
ASPECTS
OF DISEASE
47
CONTROL
BY E. ROBERTS
University of Illinois
I t is a m a t t e r of c o m m o n o b s e r v a t i o n that variation is a universal
characteristic of animals. S o m e v a r i a t i o n s are due to e n v i r o n m e n t ,
while others are due to heredity. T h o s e for which the e n v i r o n m e n t
is responsible are of no value ill the p e r m a n e n t i m p r o v e m e n t of
a n y population of animals, while those which are genetic m a k e possible the f o r m a t i o n of new t y p e s and breeds. Of course, in a certain sense, e v e r y c h a r a c t e r p r e s e n t in an individual is the p r o d u c t of
b o t h e n v i r o n m e n t a l and genetic f a c t o r s ; b u t there is no d o u b t t h a t
the differences which are to be found in a g r o u p of individuals hav•ng the s a m e e n v i r o n m e n t are genetic and p e r m a n e n t and not env i r o n m e n t a l and t e m p o r a l ; and that the genetic factors which are
responsible for such differences are a p a r t of the h e r e d i t a r y s t r e a m
flowing on from g e n e r a t i o n to generation.
A c c o m p l i s h m e n t s in Genetics W o r k
Up to the present time m o s t of the w o r k t h a t has been accomplished in the field of genetics has been with m o r p h o l o g i c a l and
color characters. This is natural since these c h a r a c t e r s are visible
and therefore can be followed w i t h o u t g r e a t difficulty t h r o u g h successive generations.
B u t for the m o s t part, the c h a r a c t e r s t h a t are of g r e a t e s t economic value a m o n g our domestic animals are physiological, such as
yield of milk and b u t t e r fat, quality of meat, intelligence, and so
forth. O f course such c h a r a c t e r s as these m a y be in part m o r p h o logical and in p a r t physiological, and p r e s u m a b l y are due to the
interaction of a large n u m b e r o f genetic factors. H o w e v e r , the
f u n c t i o n i n g or the physiological activities of animals should be
considered as in the s a m e class as are o t h e r h e r e d i t a r y characters.
Diseases and Heredity
For s o m e t i m e it w a s t h o u g h t that certain diseases were heredit a r y , t h a t children, for example, inherited tuberculosis or syphilis
from their p a r e n t s ; but in so far as we know, there is no disease
which is of an infectious nature that is hereditary. In o t h e r words,
there is no disease due to a living o r g a n i s m which can be classed
a m o n g h e r e d i t a r y characters.
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THE AMERICAN SOCIETY OF ANIMAL PRODUCTION
Before any character can be hereditary, the factors or genes
which are responsible for it must be an integral part of the germ
cell and more specifically of the c h r o m o s o m e s in the nucleus of
the cell. O r g a n i s m s producing such diseases as pebrine in the
silk worm, avain tuberculosis, and syphilis, m a y be carried in the
eggs, but they are not an integral part of the egg structure and
are included accidentally or by chance with the other materials
of the egg.
T h e fact that an individual is born with a certain disease is no
proof that such a disease is hereditary, but only points to the fact
that infection may occur before birth during the period of embryonic development as well as after birth. This is the explanation
of congenital cases of infectious disease. Likewise an individual
m a y possess a degree of i m m u n i t y acquired prenatally, which
should not be confused with hereditary immunity.
In the case of non-infectious diseases a different situation exists.
By non-infectious disease is meant a disease which is not due to a
living organism in so far as is known, but due rather to abnormal
structure or functioning of certain of the body tissues. A m o n g
such diseases are H u n t i n g t o n ' s chorea, ichthyosis, certain types of
insanity, and feeble mindedness, hypotrichosis, or hairlessness in
man and other animals, haemeophi!ia, and m a n y others.
All of these are hereditary and have been found appearing more
or less consistently in successive generations of given families. A
large a m o u n t of work on the inheritance of resistance and susceptibility to transplantable t u m o r s has been done by Slye, T y z z e r ,
Little, Strong, Loeb and others, in which the hereditary nature of
these characters in mice has been shown.
H e r e d i t a r y Factors and R e s i s t a n c e to D i s e a s e
All of this offers little direct help in the application of genetics
to the problem of controlling infectious diseases. H o w e v e r , even
t h o u g h an infectious disease is not in itself hereditary, the possibility of the existence of hereditary factors which are responsible
for resistance and susceptibility of individuals to such a disease,
should be recognized. W h e n the different genera of animals are
considered, it is found that one genus m a y be entirely immune to
a given infectious disease while a n o t h e r is v e r y susceptible; or a
given species of a certain genus m a y be resistant while another
species of the same genus is susceptible.
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49
In J a p a n the native cattle are r e p o r t e d to be highly resistant to
tuberculosis while i m p o r t e d cattle are susceptible. T h e B r a h m a
cattle are r e s i s t a n t to T e x a s fever, and for this reason are used in
crossing with our native cattle as a m e a n s of c o m b a t t i n g this disease. H u m a n t u b e r c u l o s i s a m o n g the w h i t e s is not nearly so fatal
as a m o n g the Indians. N u m e r o u s illustrations of such a n a t u r e
could be given, s h o w i n g the differences to be found a m o n g g r o u p s
of a n i m a l s in reference to resistance and susceptibility. N o t only
a m o n g g e n e r a and species are these differences found, but also
a m o n g individuals b e l o n g i n g to the s a m e species or to the s a m e
breed or strain of animals.
Evidences of Inheritance of Resistance to D i s e a s e
T h e w o r k of the D e p a r t m e n t of A n i m a l H u s b a n d r y of the Univ e r s i t y of Illinois on the inheritance of resistance in p o u l t r y to
bacillary w h i t e diarrhea has clearly d e m o n s t r a t e d t h a t it is possible b y m e t h o d s of b r e e d i n g to establish strains which are m u c h
m o r e r e s i s t a n t t h a n are other strains. D u r i n g the s p r i n g of 1926
chicks f r o m r e s i s t a n t stock and control stock w e r e inoculated w i t h
pure cultures of Salmonella pullorum, the causative o r g a n i s m of
bacillary white diarrhea.
Fifty-five p e r c e n t of the chicks f r o m
selected stock lived while only ten percent of the control stock survived. T h i s result would be obtained b y m e r e chance not m o r e
t h a n once in a billion times. W h e n we consider t h a t similar results
w e r e obtained in 1925, there s e e m s to be no r o o m for d o u b t t h a t
there are genetic factors involved which are responsible for the
differences b e t w e e n resistant and susceptible individuals.
E v i d e n c e f r o m general o b s e r v a t i o n , which is s o m e t i m e s valuable,
is not lacking for a s u p p o r t of the idea t h a t individual resistance
and susceptibility m a y be hereditary. P h y s i c i a n s r e p o r t t h a t the
whites are not nearly so susceptible to s m a l l p o x as t h e y once w e r e
and t h a t t h e y are also m u c h m o r e r e s i s t a n t to tuberculosis t h a n are
the Indians. I t is well k n o w n t h a t the Orientals are m u c h less
affected b y venereal disease t h a n are the people of the w h i t e races,
and in turn, the w h i t e people are not so m u c h affected as are the
tribes of Africa after these diseases h a v e been i n t r o d u c e d b y the
w h i t e man.
I t m a y be claimed t h a t the explanation for these differences lies
in a g r a d u a l acquisition of i m m u n i t y in s o m e f o r m or other. T h o s e
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who claim that the use of serums and vaccines may create an
hereditary resistance which is transmitted through successive generations must furnish evidence that induced or acquired characters
are hereditary, a subject upon which many investigations have
been made without yet having afforded sufficient evidence for its
support.
It is not to be disputed that individuals may receive mild infections at some time during the life cycle, presumably at an early
age, wh ich may establish an immunity. But this fact certainly
would not account for the resistance that our chickens evidence
when they are inoculated with the disease organisms 24 hours
after hatching, unless the immunity c o m e s from infected mother
hens. It is interesting to note that in tests which Dr. Card and
the writer made with inoculated offspring from non-infected and
infected hens, as determined by agglutination and intradermic tests,
the survival among the chicks from infected hens was greater tkan
among chicks from non-infected hens. In other tests chicks from
both types of hens were kept under normal conditions and not
inoculated. None of them died during the first three weeks after
inoculation, which is well beyond the period of appreciable loss
from bacillary white diarrhea. But the acquisition of prehatching
immunity could in no way account for the great difference between
the resistance of our selected and unselected stock without assuming the inheritance of a cumulative resistance due to repeated infections, an assumption unsupported by evidence.
Among plants a great many varieties have been produced that
are resistant to certain diseases. While the structure of animals is
much different from that of plants, yet, theoretically, there would
seem to be no reason why if disease resistant strains of plants may
be established we may not also establish resistant strains among
animals.
Heredity of Resistance and Methods of Disease Control
If resistance and susceptibility to disease are hereditary then
some very serious complications may foIlow the present methods
of controlling disease. Under natural conditions the struggle for
existence is always acute. T he more dense population becomes,
whether in animals or in man, the more acute becomes the struggle.
Assuming that disease organisms are present in a country, the
process of natural selection, if resistance and susceptibility are
THE AMERICAN SOCIETY OF ANIMAL PRODUCTION
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hereditary, would gradually eliminate the more susceptible individuals and leave those which are more resistant. W e have no
experimental evidence upon the subject, but if the theory is correct, we should expect to find in a country like China, where animals have been domesticated for thousands of years, where the
population is dense, and where preventive measures are not in
general use, a greater resistance to disease than in a new country,
where the period of domestication has been short, where the population is sparse, and where preventive measures are generally used.
Th e same kind of reasoning may be applied to any set of hereditary factors, one of which is favorable and the other unfavorable
in the struggle for existence. For example, if efficiency in the
utilization of food materials is hereditary, then should we not
expect to find among the people in a country where the margin of
food supply is always limited, with famines occurring at rather
frequent intervals and extending over fairly long periods of time,
that those individuals who are able to utilize their food more efficiently are the one who have the balance of power in the struggle,
and consequently would be the ones most likely to survive? If
this is true, we should expect to find the maintenance requirements
of Chinese lower than those of Americans. Recently Benedict and
others have reported that the basal metabolism of Chinese is about
10% lower than that of Americans which they investigated, which
may be suggestive that the above line of reasoning may be correct.
If resistance and susceptibility are hereditary our methods of
disease control tend to protect the susceptible individuals and in
this way gradually lower the total resistance of any given animal
population. This is of little importance so long as a people can
afford the expensive measures which are being used at the present
time in the control and eradication of disease. W h e n the populati.on becomes more dense, both in man and in our.domestic animals, the wealth of the country may not permit such expensive
methods of disease control as are now being carried on. If such
conditions should prevail, then in time of financial depression one
would naturally expect violer~t outbreaks of disease much more extensive in their destruction than any which we have yet witnessed.
This would hold true for human populations as well as for populations of domestic animals, for if resistance and susceptibility to
disease are hereditary, the methods we are using to control human
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THE AMERICAN SOCIETY OF ANIMAL PRODUCTION
disease are conserving the more susceptible along with the less
susceptible and tending thereby to decrease the total resistance of
the people. Of course there probably is little danger from this
within our generation or the next, but trouble will come sooner
or later if attention is not given to the help which genetics as well
as pathology may offer in the control of disease.
This does not mean that one would advocate decreasing the
energy spent in present methods of controlling diseases which
afflict animals and man, even though biologically we are doing the
thing which will weaken both the human race and our animal populations; but the question is whether we should not increase investigation along other lines that offer possibilities of further help in
the solution of this problem? Do not present methods of disease
control bear the same relation to the permanent improvement of
animals that legislation bears to the permanent improvement of
social conditions? Morals cannot be legislated into a people, but
legislation can hold in check certain undesirable qualities until the
people may acquire the desirable. So the present methods of disease control may hold disease in check without permanently improving the genetic constitution of any population of animals.
Summary
If the reasoning set forth in this paper is sound, then it is clear
that the approach to the problem of disease control should among
animals, at least, be more than one of simply eradication by means
of vaccines, serums, and similar methods. The science o5 genetics
may show the way to more permanent and less expensive control
by producing individuals resistant to disease.