Download Diagnosing Disease Problems In Livestock

ALABAMA COOPERATIVE EXTENSION SERVICE, AUBURN UNIVERSITY, ALABAMA 36849-5612
Diagnosing Disease Problems In Livestock
James G. Floyd, Jr.
Extension Veterinarian
W hat killed her, Doc?'' is a question farmers often
ask veterinarians. It is not unusual for livestock
producers to encounter disease and death in their
animals and herds. Sometimes death losses occur suddenly and unexpectedly. Howeyer, diseases such as
mastitis in dairy cows, chronic pneumonia in stocker
calves, or diarrhea in piglets may cause economically
important production losses without sudden death.
Often, diseases like some pneumonias in finishing hogs are caused by more than one factor. In these
cases, the solution to the problem lies not just with
finding the particular microbe (virus, bacteria, or
mycoplasma) that caused the disease. Identifying the
other conditions that may have made the pigs more
susceptible to the disease in the first place is equally
important. The challenge is finding a solution that is
both practical and cost-effective.
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What Is A Disease?
The most useful definition of disease in livestock
is stated in terms of productivity:
Disease
=
Production Goals - Actual Production
Consider this example: your production goal is to market hogs in 170 days or less, but your actual number
of days to market is 190. By definition, your swine
farm is suffering from "disease:' This disease may not
be caused by a single factor, such as an infection. It
could be the result of a particular type of pneumonia
in the finishing barn combined with less-thanadequate nutrition or poor genetics for fast, lean
growth.
To define disease in terms of production goals, a
producer must document actual production. Keeping
accurate and current records is essential to this
process. For example, in a dairy operation with mastitis problems, the producer needs information about
milk production and somatic cell count to determine
the effect of the disease on production. Without good
production records, such as those available from the
Dairy Herd Improvement Association (DHIA), the
problem quickly becomes a guessing game.
Losses from disease or death should not be accepted as a natural part of a livestock operation.
Resources are available to help determine the cause
or causes of disease and the appropriate action to prevent further losses. The producer's two main resources
are a practicing veterinarian and the State Veterinary Diagnostic Laboratory. Used together, they
can become valuable assets for any livestock
operation.
The Veterinarian As A Resource
The best source of information on livestock health
and disease is the practicing veterinarian. He or she
is the local professional best qualified to offer an unbiased opinion about livestock health and how to
maintain it.
Establish a good veterinarian-client relationship
with a veterinarian who is knowledgeable about your
type of livestock and who will be able to work with
you to solve any problems that may arise. A veterinarian can assist a livestock owner by designing a good
preventive medicine program that minimizes the effects of disease on production. With a beef herd, for
example, this may involve designing a good vaccination and parasite-control program, evaluating bulls for
breeding soundness before the breeding season, and
checking cows for pregnancy after the breeding season. The veterinarian can also help keep a herd free
from diseases of regulatory importance, such as by
vaccinating replacement heifers for brucellosis.
If you work with a veterinarian on your herd's
health-maintenance program, he or she will also be
available to help in cases of disease outbreak. A good
veterinarian-client relationship will allow the
veterinarian to have a head start on helping you. In
addition, such a relationship is necessary for the
veterinarian to legally prescribe and use certain drugs
on your farm.
A veterinarian is also an invaluable resource in
working with the veterinary diagnostic laboratory on
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any problems your herd may be having. For an accurate diagnosis, it may be necessary to send samples from animals on your farm to the lab for testing.
Only a veterinary professional can interpret the test
results effectively. All the available medical technology used by laboratories is useless to a livestock owner
unless the results can be translated into a plan of action or treatment for the farm. Knowledge of your
farm, its animals, and your production system is an
essential part of your veterinarian's ability to help solve
disease problems.
The Veterinary Diagnostic Laboratory As
A Resource
There are various veterinary diagnostic laboratories that can serve the livestock producer. The State
Veterinary Diagnostic Laboratory offers a variety of
services. Some private laboratories are also available
throughout the country. Work with your veterinarian to use the diagnostic services available in your area.
A written history of the problem must accompany
any animals, tissues, or samples submitted to the diagnostic laboratory. This will aid the laboratory diagnostician in piecing together different parts of the
puzzle. The history often determines the type of tests
required. Therefore, a good history is essential for
reaching a good diagnosis.
Post-mortem examination ("necropsy" or
"autopsy"). Particularly with death losses in livestock,
a dead animal offers an important diagnostic opportunity. In some cases, several post-mortem examinations may be needed to establish a good diagnosis.
Your veterinarian can work with you on submitting
the appropriate samples. There is no guarantee that
a necropsy will always provide a definitive answer
to a problem. However, there is a 100-percent chance
that the laboratory will not come up with an answer
if the dead animal is not necropsied.
An important part of any necropsy is the examination of the animal's body and organs, which is sometimes called "gross pathology:' Some diseases produce
characteristic changes in certain organs. For example,
blackleg in calves, caused by a Qostridium bacterium,
can be rapidly fatal, causing characteristic discoloration in the muscles and gas formation under the skin.
Usually the veterinarian will not rely on gross
pathology alone to make a diagnosis. The veterinarian will submit samples of various tissues to the
laboratory for further testing. This will invariably include histopathology, the microscopic examination of
preserved tissues by a pathologist The pathologist will
examine thinly sliced and stained tissue sections for
microscopic changes that will help point toward a diagnosis.
If your veterinarian is not available, you may
deliver a dead or sick animal to the State Veterinary
Diagnostic Laboratory. Be sure to take a good history
of the problem and ask that a report of the laboratory results be sent to your veterinarian.
When a post-mortem examination is conducted,
it is important that the animals be as fresh as possible. As more time elapses after death, the post-mortem
exam becomes less effective in establishing a cause
of death. The bodies of small animals, such as piglets,
should be put on ice for transport to the laboratory.
In some cases it may be necessary to euthanize,
or humanely put to sleep, a live representative animal
from a herd experiencing a disease problem. This will
help to ensure that the animal's tissues are fresh. In
addition, a post-mortem examination on an animal
that has just become ill and has not been treated for
disease is often needed. This is important in cases of
infections, when the laboratory will try to grow (''culture" J and isolate the organism causing the disease
and determine which antibiotics will be most effective in treating other animals. If the animal has been
treated, isolating the agent causing the infection will
be more difficult.
Your veterinarian can work with you to obtain the
proper samples for submission to the laboratory. For
example, in cases of abortions in cattle, the aborted
fetus alone is not enough. You also need to submit
samples of the fetal membranes, or placenta. Your
veterinarian may recommend collecting blood samples from several animals in the herd, some affected
and some unaffecteed, taken at intervals of 2 to 3
weeks, to provide all the pieces of the puzzle. The
failure to include all of these may greatly decrease
your chances of success. Perhaps this is why few cases
of abortion in cattle, perhaps less than 10 to 20 percent, are definitively diagnosed.
Microbiology. Microbial organisms, such as bacteria, viruses, fungi, or mycoplasmas, are often the
infectious agents that cause disease in livestock. The
veterinary diagnostic laboratory can take various samples from sick or dead animals and attempt to grow
the organism on a culture medium. In some cases,
the laboratory can also determine which drugs the
organism will be susceptible to by conducting a sensitivity test.
Samples for culture and sensitivity-testing can be
taken during the necropsy from organs such as the
lungs or intestines or from swabs from live animals.
Toxicology. When poisoning is suspected, your
veterinarian can submit samples of the stomach contents for analysis. Other samples, such as the liver,
kidney, and brain, may also be examined for the evidence of poisonous substances. If a certain feedstuff
is suspected as a source of poisoning, it may also be
tested.
These tests are sometimes quite difficult and expensive to conduct, and the laboratory may test only
for the poisons that are most likely based on the history submitted with the sample. This is another reason for giving as accurate a history of the animal and
the herd as possible.
Blood Tests. Perhaps no other diagnostic area is
more misunderstood than "blood tests:' Blood from
animals can be used to test for many things, including:
• Concentrations of certain chemicals in organs
such as the liver.
• The presence of toxic substances such as nitrate.
• The presence of bacteria.
• The relative numbers of various types of red and
white blood cells, also known as a "complete blood
count:'
• Concentrations of antibodies to various infectious agents, also known as serology.
Serology is often used to determine whether an
animal has been exposed to an infectious agent, such
as certain viruses or bacteria. If an animal is exposed
to such a microbe, the animal may produce antibodies against it. The concentration of antibodies is often
called the "titer:' The presence of a high titer in the
serum, the liquid portion of the blood, may suggest
that the animal was recently exposed to the agent and
may provide evidence that it was a cause of disease.
It is important to remember that a titer indicates previous exposure to an organism, not necessarily that the
animal is infected with the organism or that the organism was the cause of the disease.
With some diseases, such as brucellosis (Bang's)
in cattle or pseudorabies in swine, the mere presence
of a titer demonstrates that the animal is infected with
the organism. Previous vaccination against those diseases will also cause a titer to be present. Titers of
anitbodies against these diseases are particularly important because they are regulated by state and federal laws, and, in cases of positive titers, the
government may require the herd to be quarantined
or individual animals destroyed.
With most diseases, interpreting serology reports
is difficult and requires careful professional judgement. For example, routine vaccinations for infectious
agents, such as leptospirosis in cattle, will cause an
increase in the leptospirosis titer in the blood.
For a serological analysis, it is best to submit samples from several animals, including some that did not
show signs of disease. The concentrations of antibodies
in these different animals may help to establish a diagnosis. Submit blood samples from these animals as
soon as possible after the disease outbreak (the "acute''
sample) and then repeat the samples from the same
animals 2 to 3 weeks later (the "convalescent" sample). If the convalescent titer is significantly higher
than the acute titer, there is evidence that the infectious agent caused the condition.
In any case, serology is frequently difficult to interpret, especially when it is the only test being used
to diagnose disease. Serology is usually most helpful
in conjunction with other diagnostic methods, providing an important part of the whole picture.
Other 'Thsts. Many other diagnostic tests can be
run, such as hormone concentrations in blood, fluorescent antibody (FA) tests for antibodies in tissues,
and fecal flotation tests for the presence of worm eggs
or larvae in the manure. As with other diagnostic tests,
the veterinarian and the diagnostic laboratory should
work together to submit the appropriate samples for
testing. When the testing is complete, the veterinarian is responsible for interpreting the results and
providing suggestions for solving the underlying problem on the farm.
Review
Diagnosis of disease in livestock can be difficult,
but by using available professional resources you will
have a better chance of diagnosing and solving the
disease problem. Your practicing veterinarian and a
competent veterinary diagnostic laboratory, working
together, are indispensable assets. Both stand ready
to help when you are faced with disease problems.
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E
The
.6.1Aiabama
i7Cooperative
Extension Service
AUBURN UNIVERSITY
For more information, call your county Extension office. look in your telephone directory under your county's
name to find the number.
Issued in furtherance of Cooperative Extension work in agriculture and home economics, Acts of May 8 and
June 30, 1914, in cooperation with the U.S. Department of Agriculture. The Alabama Cooperative Extension
Service, Auburn University, Ann E. Thompson, Director, offers educational programs and materials to all people without regard to race, color, national origin, sex, age, or handicap and is an equal opportunity employer.
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