Lesson 10: Principles of Infectious
Disease and Disease Prevention
Assignment:
• Read Chapters 14 and 15 in the textbook.
• Read and study the lesson discussion.
• Complete the Check Your Understanding activity.
Objectives: After you have completed this lesson, you will be able to:
• Describe Koch's postulates.
• List the important distinguishing features and give examples of major disease
agents and discuss the resulting disease.
• Relate textbook and discussion material to common presentations.
• Name the basic components of disease prevention.
• Describe the types of vaccines available and their roles in disease prevention.
• Link clinical significance of the academic material studied in this chapter to
veterinary practice.
Principles of Infectious Diseases
Diseases are a major concern for any animal owner. Diseases can be divided into
two categories—infectious and noninfectious. There is little to nothing that can be
done for animals who are inflicted with noninfectious diseases. Most of these are
genetic in nature and can only be managed, not prevented. It is the infectious
diseases that owners must be most cognizant of, and that is what we will focus on in
this discussion.
Definitions
In order to have a firm understanding of the principles of infectious disease, it is
critical that you understand the basic and fundamental terminology. Dr. John R.
Stevenson, Associate Professor in the Department of Microbiology at Miami
University, offers clarification of key terms you will encounter as you study animal
health. The following information is adapted from his instructional notes on
infectious diseases.
Most infectious diseases are caused by pathogens, which are microorganisms that
are capable of causing a disease. In order for a pathogen to be effective, it must find
a host. A host is an organism that provides nutrients and the needed environment
for another organism. Remember that there are some beneficial relationships
between microorganisms and a host, but in the case of infectious diseases, the
relationship is beneficial only to the pathogen. One of the most common pathogens
is the parasite, which is an organism that lives at the expense of (and may even
harm) its host. A parasite is generally smaller than the host, and it is metabolically
dependent upon it. If the parasite and pathogen are able to get through all defenses
of the host, they will cause a disease. A disease is defined as an upset in the
homeostasis of the host resulting in the generation of observable changes.
Observable changes in the animal can appear as signs or symptoms. Signs are
objective evidence of damage to the host, such as a fever, rash, or vomiting. A
symptom is subjective evidence of damage to the host, such as a headache or
anorexia.
If the pathogen is able to disturb the homeostasis of the host, it is referred to as an
infectious disease. An infectious disease is a disease that causes detrimental
changes in the health of the host that occur as a result of damage caused by the
parasite. When the animal is diagnosed with an infectious disease, the virulence of
the pathogen will be measured. Virulence is a measure of the pathogenicity, which
is the ability to cause a disease. The pathogen may be found to be virulent, in which
case the microorganism can readily cause disease. Remember: only a small number
of the microorganisms are required to initiate and sustain an infection. If the
pathogen is attenuated, the microorganisms have a reduced ability to cause disease.
Better yet, if the pathogen is deemed avirulent, the microorganism will not cause a
disease. An opportunistic pathogen is one that may or may not cause disease. This
type of pathogen will generally colonize, but does not infect the host when found
within the host.
Koch's Postulates
To cause a disease, a pathogen must do the
following:
• Contact the host in some way, which
means it must be transmissible.
• Colonize the host, meaning it must
adhere to and grow or multiply on
host surfaces.
• Infect the host by proliferating in host
cells or tissues.
• Evade the host defense system by
avoiding contact that will damage
the pathogen.
• Damage the host tissue by physical
(mechanical) or chemical means.
Virulence Factors
Virulence factors are responsible for the virulence of a microorganism because
they influence its ability to cause disease by affecting its invasiveness and/or
its toxigenicity. There are several key concepts to become familiar with when
discussing the virulence factors of a microorganism. Adhesions enable parasites to
attach to host cells or tissues, which is a critical part of the invasion process.
Invasins enable parasites to enter and/or move through host cells or tissues,
while evasins allow parasites to escape from host defenses. Microorganisms with
high virulence factors have toxins, which are poisonous substances that allow parasites
to damage host cells.
Epidemiology Principles
Epidemiology is the study of the occurrence of disease in an animal population, especially
the cause (etiology) and the transmission of the disease. Again, students must become
familiar with several key terms in order to gain a basic understanding of disease
prevention.
An epidemic is often a term used in reference to human disease, but it has implications for
the animal world as well. An epidemic literally means "upon the people" and commonly
refers to an unusually high incidence of a disease in a community or population at one time.
In 1952 an outbreak of foot-and-mouth disease in Saskatchewan was brought under
control. The disease affects cloven-hoofed animals. All infected and exposed animals were
destroyed and buried, and strict quarantine measures were imposed to prevent any escape
of the highly contagious virus beyond the buffer zone. The meticulous disinfection of farms,
vehicles and all potential carriers was completed. Finally, before the quarantine was lifted,
the animals were tested to make sure that all danger of contagion had passed. View this
video clip on the precautions taken to contain the epidemic.
The following terms relate specifically to epidemics. Prevalence is the proportion or
percentage of diseased individuals in a population at one time, while the incidence indicates
the number of diseased individuals in a population at one time.
A pandemic is a term that literally means "all the people." There is current concern over the
potential for the bird flu to become a pandemic. An endemic means "in the people" and is a
disease that is constantly present, but usually at a low incidence in a population. Many of
you have heard of or used the term outbreak, which is an appearance of several cases of a
disease, usually in a short period of time, in an area previously experiencing no cases or
only sporadic cases of a disease.
Once an illness has reached a certain stage—an epidemic of anthrax in a buffalo herd, for
example—there are certain terms that are used to distinguish its effects. Morbidity is the
number of animals in a herd that are expected to come down with a disease in an epidemic
situation. The mortality rate is the percentage of animals that will die when symptoms
begin to appear. Every disease that has been researched and studied has a morbidity and
mortality rate. Some diseases, such as rabies, has an extremely low morbidity rate but an
extremely high mortality rate. The mortality rate of rabies is 100%.
Many diseases, such as anthrax, can stay alive a long time in the environment. A reservoir is
a term that is used to describe a site in which infectious agents remain viable (alive) and
from which infection of individuals may occur. It is at these reservoirs that a microorganism
can become a carrier, which is an infected individual not showing obvious signs or
symptoms of clinical disease. Even though there are no obvious signs of the disease, the
carrier is still shedding the etiological agent for a long period of time. A vector is a living
organism which transmits infectious agents. Ticks, fleas, flies, and mosquitoes are common
vectors. A fomite is a non-living object that transmits infectious agents and can include
almost any object that is shared among animals. Fomites that are food- or water-based are
called vehicles.
In some instances, diseases can be transmitted from animals to humans. These are referred
to as zoonoses. Plague, Lyme disease, and Rocky Mountain spotted fever are examples of
zoonotic diseases. All animal owners should be aware of the concept of zoonoses.
Veterinarians are trained in this as they must keep themselves safe from zoonotic diseases.
Transmission of Infectious Diseases
There are several stages of transmission of pathogens and microorganisms. The first
stage of transmission is escape from the old host. Remember that a majority of
pathogens have a life cycle that includes being excreted from the animal through
feces. This part of the life cycle is crucial for the microorganism's species survival.
Then, the microorganism will travel to a new host. This is usually done by the new
host animal eating around the fecal material of the old host. This is the main way
microorganisms are transmitted from host to host. The final stage of transmission is
entry into the new host. There are two modes of transmission for pathogens. They
are direct and indirect. Direct transmission includes transmission via close but not
intimate contact, such as touching. The second direct transmission is through
intimate contact, including sexual contact. Indirect contact is transmission via
vectors and vehicles or fomites. Fomites are basically almost anything an infected
animal can touch, upon which can be left a residue of contagious pathogen.
Types of Epidemics
There are two basic types of epidemics—common source and propagated.
Common source epidemics include infection or intoxication of many animals from a
single contaminated source and are characterized by a rapid onset, a sharp peak,
and rapid decline in incidence. A propagated epidemic is one that begins with an
introduction of an infected animal into a
susceptible population leading to the
transfer of the etiological agent to
others, who then transfer it to many
others. This type of epidemic is
characterized by slow onset, a blunted
peak, and slow decline of incidence.
When new pathogens are introduced
into susceptible populations, there is
often a rapid onset of epidemics that
continue until sensitive individuals in
the population are selected against or
many individuals in the population
develop immunity (herd immunity)
and survive the disease.
Acquiring Infections in the Vet Office
Vet offices are good places to acquire infections and other pathogens due to the
presence of many patients with infectious diseases. An infection that is the result of
a medical procedure is referred to as an iatrogenic infection. Injections and
surgeries are common causes of iatrogenic infections. Transmission of
microorganisms can also occur between patients if proper sanitization is not used.
Veterinary hospitals should be aware of crowding which leads to crosscontamination of patients by direct and indirect means. Another area of concern in
the vet hospital is that of the immunocompromised. Some animals are more
susceptible to pathogens because their immune systems are weakened or not fully
functioning. Animals can become immunocompromised due to disease, medical
treatment, age, and stress. This means that veterinarians must be aware of their
patients and take extra care if the animal is immunocompromised. A final area of
concern in veterinary hospitals is that of antibiotic-resistant strains of an
infection. This leads to problems in the vet clinic if a strain of infection is present
that is resistant to antibiotic treatments.
Health Measures for the Control of Epidemics
Several strategies are employed to break the "chain of transmission." The first
strategy is immunization of the species. Humans do this all the time, and the classic
example is the flu vaccine. Booster shots are frequently required because the aim of
this immunization is not full immunization as the flu virus is continually changing
and adapting. In animal health, quarantine is a common practice to prevent the
transmission of infectious diseases. This is a simple concept which keeps all infected
animals isolated from others to prevent future spread of the pathogen. The
minimum time required for a quarantine period is equal to the longest period of
communicability of the disease. In other words, animals should be isolated long
enough to know that the pathogen has no chance of being transmitted to other
animals. The elimination of animal reservoirs is also a common strategy for
controlling a pathogen. This includes immunizing animals that act as reservoirs of
diseases. At times, eradication of animals that act as reservoirs may also be
required. In the case of the bird flu epidemic, millions of birds were destroyed in
Asia to prevent further spread of the disease.
This is why there are such strict regulations regarding animals and animal products
being imported from foreign countries. We do not want to end up with a pathogen
that we have no way of controlling. It is important also if you travel to foreign
countries to make sure you are properly immunized so that you do not become ill
from a foreign pathogen, or become a carrier of any pathogens.
Disease Prevention
The following information has been
adapted from an article originally written
by Dr. Ernest Mochankana, titled
"Importance of Pet Vaccination."
Animals, like people, can be protected
from some diseases by vaccination.
Although this discussion provides basic
information about vaccines for an animal,
a veterinarian is always the best source
for advice regarding an animal's
vaccination needs.
What are Vaccines?
Vaccines are health products that trigger
protective immune responses in animals
and prepare them to fight future infections from disease-causing agents. Vaccines
can lessen the severity of future diseases, and certain vaccines can prevent infection
altogether. Currently, there are a variety of vaccines available for use by
veterinarians.
Is it Important to Vaccinate?
Absolutely. Animals should be vaccinated to protect them from many highly
contagious and deadly diseases. Experts agree that widespread use of vaccines
within the last century has prevented death and disease in millions of animals. Even
though some formerly common diseases have now become uncommon, vaccination
is still highly recommended because these serious disease agents continue to be
present in the environment.
Does the Vaccination Ensure Protection?
For most animals, vaccination is effective and will prevent future disease.
Occasionally, a vaccinated animal may not develop adequate immunity and,
although rare, it is possible for these animals to become ill. It is important to
remember that although breakdowns in protection do occur, most successfully
vaccinated animals never show signs of disease, making vaccination an important
part of the animal's preventative health care.
Are There Risks to Vaccinations?
Although most animals respond well to vaccines, like any medical procedure, a vaccination
carries some risk. The most common adverse responses are mild and short-term, including
fever, sluggishness, and reduced appetite. Animals may also experience temporary pain or subtle
swelling at the site of the vaccination. Although most adverse responses will resolve within a
day or two, excessive pain, swelling, or listlessness should be discussed with a veterinarian.
Serious adverse responses occur rarely. A veterinarian should be contacted immediately if an
animal has repeated vomiting, diarrhea, whole body itching, difficulty breathing, swelling of
the face or legs, or collapses. These signs may indicate an allergic reaction. In very rare instances,
death can occur. Visit with a veterinarian about the latest information on vaccine safety,
including rare adverse responses that may develop weeks or months after the vaccination.
Remember that while vaccination is not without risk, failure to vaccinate leaves your animal
vulnerable to fatal illnesses that are preventable.
Why Do Young Animals Require a Series of Vaccinations?
Very young animals are highly susceptible to infectious diseases. This is especially
true as the natural immunity provided in their mother's milk gradually wears off.
This concept was discussed in Lesson Eight. To keep gaps in protection as narrow as
possible and to provide optimal protection against disease for the first few months
of life, a series of vaccinations are scheduled, usually three to four weeks apart. For
most young animals, the final vaccination in the series is administered when they
are twelve to sixteen weeks old.
Which Vaccines Should a Young Animal Receive?
A veterinarian should discuss an animal's lifestyle, access to other animals, and
travel to other geographical locations, since these factors affect the animal's risk of
exposure to disease. Not all animals should be vaccinated with all vaccines just
because these vaccines are available. "Core" vaccines are recommended for most
animals in a particular area. "Non-core" vaccines are reserved for animals with
unique needs. A veterinarian will consider the animal's particulars, the disease at
hand, and the application of available vaccinations to customize a vaccine
recommendation for the animal.
How Often Should an Animal be Vaccinated?
A veterinarian should tailor a vaccination schedule to suit an animal's needs. For
many years, a set of annual vaccinations was considered normal and necessary for
animals. Veterinarians have since learned more about diseases and animals'
immune systems, and there is increasing evidence that immunity triggered by some
vaccines provide protection beyond one year. The immunity triggered by other
vaccines may fail to protect for a full year. More than one successful vaccination
schedule is possible. A veterinarian should always be ready to discuss what is best
for an animal.
A final thought …
Many factors are taken into consideration when establishing an animal's vaccination
plan. A veterinarian will tailor a program of vaccinations to help the animal
maintain a lifetime of infectious disease prevention.
Summary
Koch's postulates are very important principles in regard to infectious diseases.
Knowing and understanding these postulates and the four major disease agents is
vital for veterinarians, as this knowledge helps them identify, prevent, and treat
infectious diseases. Additionally, tailoring vaccination programs to each specific
animal also helps prevent the spread of diseases.
Sources Cited:
Mochankana, Dr. Ernest. "Importance of Pet Vaccination." Botswana College of Agriculture. 2007. 2
Jan. 2007 <http://www.bca.bw/NewsEvents/Vaccination.htm>.
Stevenson, John. "Principles of Infectious Diseases and Epidemiology." Department of Microbiology,
Miami University. 19 Aug. 2004. 1 Jan. 2007
<http://www.cas.muohio.edu/~stevenjr/mbi111/disprinepid111.html>.