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Lesson Twelve- Diagnosis of Diseases
Assignment:
• Read Chapter 18 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:
• List the major methods used to diagnose disease and cite examples of disease
diagnosis with each testing method.
• Discuss the clinical significance of disease diagnosis.
Diagnosis of Diseases
The Merck Veterinary Manual explains helpful steps for disease diagnosis and
documentation:
As with any disease, diagnosis is based on history, clinical signs, lesions, laboratory
examinations, and, in some cases, biological procedures. Circumstantial evidence is
valuable and should be noted, but does not replace a thorough clinical and
postmortem examination. Histories from animal owners may stress obvious factors
and omit subtle, important details. "Sudden death" is often actually "tardy
observation."
Pertinent data and samples should be submitted to the diagnostic laboratory. A
complete history of the animal is necessary for developing a laboratory
investigation and may be valuable in case of legal action. Information should be
detailed. For example, a notation of central nervous system signs is insufficient;
most animals exhibit some type of central nervous system signs prior to death.
Exact actions and signs should be described. Examples of pertinent information
include the following:
 number of animals exposed/sick/dead, age, weight, and a chronology of
morbidity and mortality;
 clinical signs and course of the disease;
 any prior disease conditions;
 lesions observed at [death] with careful examination of [stomach contents];
 response to treatment (medication should be listed to avoid analytic
confusion);
 related events such as feed change, water source, other medications, feed
additives, pesticide applications;
 description of facilities (a drawing may be helpful), access to refuse,
machinery, etc; and

recent past locations
and when moved. The
diagnostic laboratory
should be contacted if
there are questions
regarding the
appropriate sample,
amount, or container
("Diagnosis").
According to the article
"Collection and Submission of
Laboratory Samples" from The
Merck Veterinary Manual,
Each veterinary diagnostic laboratory offers a unique set of diagnostic tests that is
subject to frequent changes as better tests become available. The protocols for
sample collection and submission are also subject to change. The practitioner and
diagnostic laboratory staff must maintain good communication in order to complete
their diagnostic efforts efficiently and provide optimal service to the animal owner.
Practitioners must be specific and clear in their test requests. The laboratory staff
can provide guidance when there are questions regarding sample collection and
handling, as well as offer assistance in interpretation of test results. Most diagnostic
laboratories publish user guidelines with preferred protocols for sample collection
and submission.
Regardless of the type of submission, a detailed case history should be included with
the samples to assist laboratory personnel in determining a diagnosis. The
information should include: owner, species, breed, sex, age, animal identification,
clinical signs, gross appearance (including size and location) of the lesion(s),
previous treatment (if any), time of recurrence from any previous treatment, and
morbidity/mortality in the group. If a zoonotic disease is suspected, this should also
be clearly indicated on the submission sheet to alert lab personnel. The submission
form should be placed in a waterproof bag to protect it from any fluids that might be
present in the packaged materials. Waterproof markers should be used when
labeling specimen bags and containers.
Clinical Biochemistry
According to The Merck Veterinary Manual,
Clinical biochemistry refers to the analysis of the blood plasma (or serum) for a
wide variety of substances—substrates, enzymes, hormones, etc—and their use in
the diagnosis and monitoring of disease. Analysis of other body fluids, such as urine,
is also included. One test is very seldom specific to one clinical condition. Thus,
rather than six tests that merely confirm or deny six possibilities, a well-chosen
group of six tests can provide information pointing to a wide variety of different
conditions by a process of pattern recognition. Biochemistry tests should be
accompanied by full hematology, as evaluation of both together is essential for
optimal recognition of many of the most characteristic disease patterns ("Clinical
Biochemistry").
Hematology refers to the study of the numbers and morphology of the cellular
elements of the blood—the red cells (erythrocytes), white cells (leukocytes), and
platelets (thrombocytes) and the use of these results in the diagnosis and
monitoring of disease ("Clinical Hematology").
Clinical Microbiology
Authors of the article "Clinical Microbiology" in The Merck Veterinary Manual offer
the following information on specimen collection, materials, and practices:
In-house microbiology can be a valuable asset to practitioners, providing quick
results with minimal investment. Expensive equipment and materials are not
usually necessary for the recovery of common aerobic or anaerobic bacterial
pathogens … . While microbiologic media is not difficult to prepare, it may be more
convenient to purchase from a scientific supply house. Most bacteria will grow
readily on standard media when incubated aerobically. Basic equipment should
include an incubator, refrigerator, Bunsen burner or portable gas torch, and
microscope with low, high, and oil immersion objective lenses. Materials should
consist of inoculating loops, prepared microbiologic media, microscope slides,
Gram-stain reagents, 3% hydrogen peroxide, oxidase reagent, microbial
identification systems, and a current veterinary microbiology textbook.
Specimen Collection
Although it is not always easy to obtain optimal specimens when working with
animals, certain practices can ensure the best possible specimen under the
circumstances. Application of the following principles should result in acceptable
specimens that produce high-quality microbiology results:
• Specimens must be obtained aseptically, [in order to prevent outside
contamination or infection], from a site that is representative of the disease
process.
• Swabs are the most common specimen collected, but they are generally not the
specimens of choice, as they may become contaminated during the collection
process, and they provide a small sample volume. Swabs are most useful for
obtaining specimens from skin pustules, ears, conjunctiva, deep within
draining tracts or wounds, soft tissue infections, or the reproductive tract.
• A sufficient quantity of material should be collected to permit adequate
examination.
• Specimens must be collected at the proper time in the disease process and prior to
the initiation of antimicrobial therapy to maximize pathogen recovery.
• If cultures are not immediately initiated after collection, specimens should be
refrigerated.
Processing Specimens
Microbiologic testing should include both direct, microscopic examination and
culture of the specimen. Gram-stained smears should be examined using oil
immersion in order to determine the correct reaction. Generally, both solid (agar)
and liquid (broth) media should be inoculated. Solid media permit colony isolation
[…] and selection or differentiation of normal flora from potential pathogens, while
broth media allow for the recovery of small numbers of organisms.
Clinical specimens should be inoculated onto both general purpose and selective
media to maximize bacterial recovery. [Microbiologic media] should be stored in the
refrigerator, but allowed to warm to room temperature prior to inoculation.
Transfer of specimens to plated media depends on the type of specimen. Liquid
specimens are inoculated by use of a sterile syringe or pipette. Swabs are generally
plated directly by rolling the swab over an area ~2 cm in diameter. Feces are
inoculated by dipping a swab into the specimen. Surgical biopsy specimens may be
touched directly to the agar surface … .
After inoculation, plates and tubes should be labeled and placed in an incubator set
at 35—37°C. Plates are incubated with their lids down to prevent condensed water
from dropping down from the lid onto the agar surface, which can result in
confluent bacterial growth, [or the mixing of bacterial colonies].
Cytology
Cytology is the study of the structure, function, growth, and multiplication of cells.
Cytology is a useful clinical tool for the investigation of disease processes and was
originally developed mainly for use in the practice environment. It should be
considered as a guide only and rarely provides a diagnosis. It should not be used as
a substitute for more accurate diagnostic methods such as histopathology
("Cytology: Overview").
Urinalysis
Urinalysis is an important laboratory test that can be readily performed in
veterinary practice, and is considered part of a minimum database. It is useful in
documenting various types of urinary tract diseases and may provide information
about other systemic diseases, such as liver failure and hemolysis. Urine may be
collected by cystocentesis, urethral catheterization, or voiding and should be
evaluated within thirty minutes. If this is not possible, then it may be refrigerated
for up to 24 hours; however, this may alter results of some tests, such as pH, protein,
and sediment examination (crystals, casts, and cells) ("Urinalysis: Overview").
Normal urine is typically transparent and yellow or amber on visual inspection. The
intensity of color is in part related to the volume of urine collected and
concentration of urine produced; therefore, it should be interpreted in context of
urine specific gravity (USG). Significant disease may exist when urine is normal in
color. Abnormal urine color may be caused by presence of various pigments, but it
does not provide specific information. Interpretation of semiquantitative reagent
strips, which are colorimetric tests, requires knowledge of urine color because
discolored urine may result in a false-positive result … .
Normal urine has a slight odor of ammonia; however, the odor is dependent on
urine concentration. Some species, such as cats and goats, have pungent urine odor
because of urine composition. Bacterial infection may result in a strong odor …
("Urine Appearance").
Interval Parasite Diagnosis
Diagnosis of internal parasites in small animals is typically done by examination of
feces for parasite eggs. Fecal samples should be fresh, preferably collected from the
rectum using a fecal loop. Specimens submitted to a diagnostic laboratory should be
fixed in 10 percent formaldehyde solution or sent chilled … .
Routine examinations should be done by both direct fecal smear and fecal flotation.
Direct smears are prepared by mixing feces (an amount of feces fitting on one-half
the tip of an applicator stick) with 1 drop each of saline and iodine stain. Flotation
methods concentrate diagnostic stages and provide a cleaner final preparation,
using about 2 g feces. Sugar and sodium nitrate are commonly used flotation media
("Internal Parasite Diagnosis in Small Animals").
Fresh fecal samples should be collected from livestock from pasture or, preferably,
per rectum using plastic gloves (which may be inverted to act as a receptacle after
sample collection). A representative number of herd samples should be collected
from a minimum of ten animals to account for the typically high individual variation
in numbers of eggs shed. Samples can be combined after thorough mixing to enable
examination of a single herd composite sample ("Internal Parasite Diagnosis in
Livestock").
Laboratory Testing
The article "Diagnostic Procedures for the Private Practice Laboratory" offers
helpful notes on lab tests:
Numerous laboratory tests can be done in a private practice laboratory. Use of a
commercial laboratory versus in-house testing should be evaluated to determine
whether in-house testing is practical and economical. Because the availability of
diagnostic laboratories and their reporting intervals may be problematic (for
example, nights and holidays), performing some diagnostic screening tests in-house
is often desirable. However, because the people performing these tests often have
minimal technical training, quality control procedures must be rigorous. The time
and care that must be devoted to quality control issues may preclude in-house
testing in many practices. Errors may occur not only in testing procedures but also
in sample collection and handling and in recording results.
Tests can be done using either manual or automated methods. Manual methods tend
to be time-consuming and are more subject to human error. Automated and
semiautomated systems
are available but are
considerably more
expensive. Factors to be
considered include
instrument and reagent
costs (including materials
for calibration and quality
control), availability of
personnel training,
technical support, and
instrument maintenance
and service. Although
service contracts can cost
up to 10 percent of the
purchase price of an
instrument, they are often
cost-effective due to the
expense of instrument
repair.
Summary
Each time veterinarians see a new patient, they must gather a complete history
so that a more accurate diagnosis can be made. By following a step-by-step
procedure after the history is obtained, veterinarians work to eliminate
potential causes of the disease or disorder. This systematic approach allows
veterinarians to make their diagnosis in an orderly, confident manner.
Sources Cited:
Kahn, Cynthia, ed. "Clinical Biochemistry." The Merck Veterinary Manual. Ninth Edition.
New Jersey: Merck and Co. Inc. 2006. 2 Jan. 2007
<http://merckvetmanual.com/mvm/index.jsp?cfile=htm/bc/150202.htm>.
—. "Clinical Hematology: Overview." The Merck Veterinary Manual. Ninth Edition. New
Jersey: Merck and Co. Inc. 2006. 2 Jan. 2007
<http://www.merckvetmanual.com/mvm/index.jsp?cfile=htm/bc/150212.htm>.
—. "Clinical Microbiology." The Merck Veterinary Manual. Ninth Edition. New Jersey:
Merck and Co. Inc. 2006. 2 Jan. 2007
<http://www.merckvetmanual.com/mvm/index.jsp?cfile=htm/bc/150204.htm>.
—. "Collection and Submission of Laboratory Samples: Introduction." The Merck
Veterinary Manual. Ninth Edition. New Jersey: Merck and Co. Inc. 2006. 2 Jan. 2007
<http://www.merckvetmanual.com/mvm/index.jsp?cfile=htm/bc/150100.htm>.
—. "Cytology: Overview." The Merck Veterinary Manual. Ninth Edition. New Jersey: Merck
and Co. Inc. 2006. 2 Jan. 2007
<http://www.merckvetmanual.com/mvm/index.jsp?cfile=htm/bc/150205.htm>.
—. "Diagnostic Procedures for the Private Practice Laboratory." The Merck Veterinary
Manual. Ninth Edition. New Jersey: Merck and Co. Inc. 2006. 2 Jan. 2007
<http://www.merckvetmanual.com/mvm/index.jsp?cfile=htm/bc/150200.htm>.
—. "Internal Parasite Diagnosis in Livestock." The Merck Veterinary Manual. Ninth Edition.
New Jersey: Merck and Co. Inc. 2006. 2 Jan. 2007
<http://www.merckvetmanual.com/mvm/index.jsp?cfile=htm/bc/150223.htm>.
—. "Internal Parasite Diagnosis in Small Animals." The Merck Veterinary Manual. Ninth
Edition. New Jersey: Merck and Co. Inc. 2006. 2 Jan. 2007
<http://www.merckvetmanual.com/mvm/index.jsp?cfile=htm/bc/150222.htm>.
—. "Urinalysis: Overview." The Merck Veterinary Manual. Ninth Edition. New Jersey:
Merck and Co. Inc. 2006. 2 Jan. 2007
<http://www.merckvetmanual.com/mvm/index.jsp?cfile=htm/bc/150217.htm>.
—. "Urine Appearance." The Merck Veterinary Manual. Ninth Edition. New Jersey: Merck
and Co. Inc. 2006. 2 Jan. 2007
<http://www.merckvetmanual.com/mvm/index.jsp?cfile=htm/bc/150218.htm>.