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Veterinary Technology
Chapter 14
Principles of Infectious Disease
Objectives
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Identify the two categories of disease and
determine the causes of each.
Describe Koch’s postulates
List the important distinguishing features
and give examples of major disease agents
and discuss the resulting diseases.
Relate text material to common
presentations
Examine the primary and secondary
defenses the body uses to resist disease.
Recognize the two categories of immunity
and compare the types in each category.
Terms
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acquired immunity
antigen
colostrum
endospores
endotoxins
exotoxins
infectious disease
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natural immunity
noninfectious
disease
pathogen
phagocytized
prokaryotes
vaccine
Vocabulary
SID: Medicine to be given once daily
 BID: Medicine to be given twice daily
 TID: Medicine to be given three times daily
 BAR: Bright, Alert and Responsive
 UTI: Urinary Tract Infection
 BP: Blood Pressure
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Introduction
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A disease is a
change that occurs
in the body and
prevents normal
function.
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Read page 199 –
“A Day in the life of
a Vet.”
Mastitis?
Introduction
The two categories of disease:
 1.infectious disease is a disease caused
by an interaction between two organisms,
usually a pathogen and a host.
 Agents of infectious diseases are divided
into four major classes: bacteria, viruses,
fungi, and parasites.
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A pathogen is an agent capable of producing
disease.
 Bacteria and viruses are the most common
pathogens.
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Transmission methods of an
infectious, contagious disease (not all
infectious diseases are contagious):
Through a vector: insects, mites, ticks,
mosquitoes
 Direct: feces, discharges
 Air
 Soil
 Food
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Introduction
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2. A noninfectious disease is a
disease not caused by a pathogen
and not communicable from one
animal to another.
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It may be caused by hereditary factors
or by the environment in which an
animal lives.
Koch’s Postulates
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Dr. Robet Koch German physician
Prounnounced
(KoK)
His investigations
led to the
development of
certain foundation
principles about
infectious disease.
Anthrax
Koch’s Postulates
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A set of principles that help lead to
define an infectious disease:
1. The infectious agent should be
detectable in sick animals but not
healthy animals.
 2. It should be possible to isolate and
culture the organism.
 3. Organisms taken from the culture and
introduced into a healthy animal should
cause the same disease.
 4. the same organism should be isolated
from this second animal as well.
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Disease Agents
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Infectious Diseases are divided into
four classes:
bacteria
 Viruses
 Fungi
 Parasites
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Microbiology: the study of
microorganisms
Bacteria as a Disease Agent
Bacteria are prokaryotic!
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Eukaryotic
Prokaryotic
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Bacteria as a Disease Agent
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The 2 most important factors in
classifying bacteria are by stain and
by shape
Gram-positive will stain blue because
they absorb the crystal violet stain
 Gram-negative will remain pink colored
because they do not pick up the crystal
violet stain.
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Bacteria as a Disease Agent
Bacteria are also classified by shape:
 page 204
 Staphylococci
 Streptococci
 Bacilli
 Spirochetes
 Mycoplasma – smallest
of all bacteria, don’t
have cell wall
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Bacteria as a Disease Agent
In addition to the main chromosome, in
bacteria they also contain a plasmid.
 Plasmid = small fragment of DNA
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Can reproduce independently of the bacteria
Resistance to antibiotics
is a growing concern
both in humans and
veterinarian field.
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Bacteria as a Disease Agent
3 Methods for bacteria to transfer genetic
material:
 1. Conjugation= direct contact b/t 2 bact.
 2. Transformation= a fragment of DNA is
released by one bacteria, and engulfed by
another.
 3. Transduction = requires the
transmission of the DNA by a virus
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Viruses as a Disease Agent
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Viruses have no cell walls, organelles, or
enzymes and are not considered living
organisms.
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They cannot reproduce without living host cells.
A virus is classified as either RNA or DNA, according
to the type of nucleic acid found in its core.
Other distinguishing features include amount of
nucleic acid, capsid shape, type of host infected,
and whether the virus needs a vector to carry it
between hosts.
Viruses as a Disease Agent
Viruses cause harm to animals by attaching
to and invading host cells, replicating, and
then destroying the host cells.
 Because they are not affected by antibiotics
and because very few
antiviral drugs are oral,
viral diseases are very
difficult to eliminate from
the body.
 Some antiviral drugs are
available.
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Fungi as a Disease Agent
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Like bacteria, most species of fungi are
harmless, whereas a few cause disease.
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Fungi have eukaryotic cells and cell walls but do
not contain chlorophyll.
Most fungi reproduce by spores.
Fungi can enter the body through the skin, eyes,
respiratory system, or digestive system.
Fungus as a Disease Agnet
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Ringworm is one common example of a fungal
infection that directly invades an animal through
its skin and hair follicles.
Fungi as a Disease Agent
Fungi can grow on old or poorly stored feed
and produce mycotoxins, which are harmful
to the animals that eat the feed.
 Producers must be careful to
harvest grain, such as corn
or wheat, only when its
moisture content is low
enough to prevent the
growth of fungi while the grain is in storage.
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Parasites as a Disease Agent
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Many parasites can cause disease-like
disorders, whereas others merely serve as
carriers of diseases.
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Parasites use host animals as sources of nutrients
and protection.
Parasites can be unicellular organisms visible only
under a microscope, or they can be multicellular
organisms visible with the naked eye.
Parasites can be internal or external
Parasites as a Disease Agent
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Internal parasitic worms:
 Flat-segmented
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Mammal = tapeworm
 Flat,
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non-segmented (flukes)
Invade liver, lungs or GI tract
 Roundworm
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(cestodes)
(nematode)
Dogs = heartworm
Parasites as a Disease Agent
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External Parasites
 All
are arthropods – class or organisms that
include insects and spiders.
 Ticks, fleas, mites, lice, mosquitoes and biting
flies.
 Botfly = lays eggs on the legs of horses and
cattle, when the animal licks the larvae move
into the mouth and GI tract.
Parasites as a Disease Agent
The two most common internal parasites:
 Flea
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 Blood
sucking = cause anemia, skin irritation, etc
 Eggs develop off animal, in environment
 Shampoo (Dawn), insecticide dip, collars, etc.
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Ticks
 Also
blood sucking = transmit Lyme disease
 Keep the tick once you remove it!
 Frontline does not kill the tick, only repel it
 Types of ticks on page 214
Antibiotics (page 213)
Products produced by one microorganism
that kill or slow the growth of another
microorganism.
 Can kill bacteria, not viruses
 Bacteria can become resistant
 Penicillin = first antibiotic discovered by Dr.
Alexander Fleming. 1929 in London.
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It was still almost 10 years before it was mass
produced!
 World War II
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Clinical Practice
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The immune system is activated when an
antigen is introduced into the body.
An antigen is any foreign molecule capable of
stimulating an immune response.
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Generally, antigens are relatively large and complex
so that they can be recognized by the immune
system.
A. Most diseases are warded off by the body’s
primary defenses before they can cause
serious infections.
Immunity
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Immunity occurs when an animal has built up
a resistance to a particular pathogen and is no
longer susceptible to the disease.
Two types:
1.Natural immunity is determined by an
animal’s individual genetic code.
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It does not depend upon prior exposure to a
disease pathogen.
Immunity
Some diseases affect only one or a few
species.
 Zoonotic: diseases that can be
transmitted between animals and
humans.
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For example, bovine spongiform
encephalopathy (BSE),
commonly known as
mad-cow disease, affects
only cattle and humans.
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Immunity
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An animal’s breed may also play a role
in immunity status.
Pure breed vs. “mutt” ??
 In this closed breeding system, the
offspring do not receive the benefit of
immunity that could be acquired through
crossbreeding.
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Immunity
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2.Acquired immunity is either gained from
exposure to a mild form of a disease or passed
from mother to newborn.
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A vaccine or exposure to a mild natural infection is
a way to provide active immunity.
A vaccine is a killed or weakened form of a
pathogen.
Exposure allows an animal’s immune system to
prepare a response and develop a memory for the
disease.
Colostrum: an antibody- rich milk, from its mother
soon after birth.
Review
What are the two categories of disease,
and what are the causes of each?
 What are the primary and secondary
defenses the body uses to resist
disease?
 What are the two categories of
immunity, and how do the types in each
category compare?
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