Common Diseases of Farm Animals | Page 3

becomes heavy and appears rough if the animal is exposed to
severe cold. A rough, staring coat is very common in horses affected by
disease. The outer layer of the skin becomes thickened when subject to
pressure or friction from the harness. This change in structure is purely
protective and normal. In disease the deviation from normal must be
more permanent in character than it is in the examples mentioned above,
and in some way prove injurious to the body functions.
CLASSIFICATION.--We may divide diseases into three classes:
_non-specific, specific_ and parasitic.
_Non-specific diseases_ have no constant cause. A variety of causes
may produce the same disease. For example, acute indigestion may be
caused by a change of diet, watering the animal after feeding grain, by
exhaustion and intestinal worms. Usually, but one of the animals in the
stable or herd is affected. If several are affected, it is because all have
been subject to the same condition, and not because the disease has
spread from one animal to another.
_Specific Diseases._--The terms infectious and contagious are used in
speaking of specific diseases. Much confusion exists in the popular use
of these terms. A contagious disease is one that may be transmitted by
personal contact, as, for example, influenza, glanders and hog-cholera.
As these diseases may be produced by indirect contact with the
diseased animal as well as by direct, they are also infectious. There are

a few germ diseases that are not spread by the healthy animals coming
in direct contact with the diseased animal, as, for example, black leg
and southern cattle fever. These are purely infectious diseases.
Infection is a more comprehensive term than contagion, as it may be
used in alluding to all germ diseases, while the use of the term
contagion is rightly limited to such diseases as are produced principally
through individual contact.
Parasitic diseases are very common among domestic animals. This
class of disease is caused by insects and worms, as for example, lice,
mites, ticks, flies, and round and flat worms that live at the expense of
their hosts. They may invade any of the organs of the body, but most
commonly inhabit the digestive tract and skin. Some of the parasitic
insects, mosquitoes, flies and ticks, act as secondary hosts for certain
animal microorganisms that they transmit to healthy individuals
through the punctures or the bites that they are capable of producing in
the skin.
CAUSES.--For convenience we may divide the causes of disease into
the predisposing or indirect, and the exciting or direct.
The predisposing causes are such factors as tend to render the body
more susceptible to disease or favor the presence of the exciting cause.
For example, an animal that is narrow chested and lacking in the
development of the vital organs lodged in the thoracic cavity, when
exposed to the same condition as the other members of the herd, may
contract disease while the animals having better conformation do not
(Fig. 1). Hogs confined in well-drained yards and pastures that are free
from filth, and fed in pens and on feeding floors that are clean, do not
become hosts for large numbers of parasites. Hogs confined in filthy
pens are frequently so badly infested with lice and intestinal worms that
their health and thriftiness are seriously interfered with. In the first case
mentioned the predisposition to disease is in the individual, and in the
second case it is in the surroundings (Fig. 2).
[Illustration: FIG. 1.--Side and posterior view of bull showing
conformation favorable to the development of disease.]

The exciting causes are the immediate causes of the particular disease.
Exciting causes usually operate through the environment. With the
exception of the special disease-producing germs, the most common
exciting causes are faulty food and faulty methods of feeding. The
following predisposing causes of disease may be mentioned:
Age is an important factor in the production of disease. Young and
immature animals are more prone to attacks of infectious diseases than
are old and mature animals. Hog-cholera usually affects the young hogs
in the herd first, while scours, suppurative joint disease and infectious
sore mouth are diseases that occur during the first few days or few
weeks of the animal's life. Lung and intestinal parasites are more
commonly found in the young, growing animals. Old animals are prone
to fractures of bones and degenerative changes of the body tissues. As a
general rule, the young are more subject to acute diseases and the old to
chronic diseases.
[Illustration: FIG. 2.--Insanitary yards.]
The surroundings or environments are important predisposing factors.
A dark, crowded, poorly ventilated stable lowers the animal's vitality,
and renders it more susceptible to the disease. A few rods difference in
the location of stables and yards may make a marked difference in the
health of the herd. A