Page 28 - Veterinary Immunology, 10th Edition

an animal to control invading organisms (or at least minimize
VetBooks.ir damage), viruses are under even more potent selective pressure.

They must find a host or die. Viruses that cannot evade or
overcome the immune defenses will not survive and will be

eliminated. Fungi, like bacteria, are opportunistic invaders that can
take advantage of local circumstance to invade the host. They
commonly exploit situations where the host's immune system is
defective or suppressed in some way. Parasitic worms and

protozoan parasites, like viruses, must be able to survive within a
host or be eliminated. They have evolved numerous and complex
strategies to evade immune destruction.
An organism that can cause sufficient damage to result in disease

is said to be a pathogen. Remember, however, that only a small
proportion of the world's microorganisms are associated with
animals, and very few of these can overcome the body's defenses
and become pathogens. Pathogenic microorganisms vary greatly in

their ability to invade the body and cause damage. This ability is
termed virulence. Thus a highly virulent organism has a greater
ability to cause damage than an organism with low virulence. If a
bacterium can cause significant damage almost every time it

invades a healthy individual, even in low numbers, then it is
considered a primary pathogen. Examples of primary pathogens
include canine distemper virus; feline panleukopenia virus; and
Brucella abortus, the cause of contagious abortion in cattle. Other

pathogens may be of such low virulence that they will only cause
disease if administered in very high doses or if the immune
defenses of the body are impaired first. These are opportunistic
pathogens. Examples of opportunistic pathogens include bacteria

such as Mannheimia hemolytica and fungi such as Pneumocystis
jirovecii. These organisms rarely cause disease in healthy animals.
For many years, it was believed that the role of the immune
system was simply to ensure the complete exclusion of all invading

microbes by distinguishing between self and not-self and
eliminating foreign antigens. We now know, however, that this is
insufficient to ensure health. The immune system must also
determine the threat level posed by the microbes it encounters and
adjust its response accordingly. It must maintain tolerance to the

normal microbiota or food antigens while, at the same time, be

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