Page 1455 - Small Animal Internal Medicine, 6th Edition

PART FOURTEEN Infectious Diseases

Michael R. Lappin

VetBooks.ir CHAPTER 91

Laboratory Diagnosis of

Infectious Diseases

Clinical syndromes induced by infectious agents are DEMONSTRATION OF THE ORGANISM
common in small animal practice. A combination of sig-
nalment, history, and physical examination findings is used FECAL EXAMINATION
to develop a list of differential diagnoses ranking the most Examination of feces can be used to help diagnose parasitic
likely infectious agents involved. For example, young, unvac- diseases of the gastrointestinal (see Chapter 27) and respira-
cinated cats with conjunctivitis are generally infected by her- tory tracts (see Chapter 20). The techniques used most fre-
pesvirus type 1, Chlamydia felis, or Mycoplasma felis; if a quently include direct and saline smear, stained smear, fecal
dendritic ulcer is present, herpesvirus type 1 is most likely. flotation, and Baermann technique; each procedure can
Results of a complete blood count (CBC), serum biochemi- easily be performed in a small animal practice.
cal panel, urinalysis, radiographs, or ultrasonography can
also suggest infectious diseases. For example, a dog with Direct Smear
polyuria, polydipsia, neutrophilic leukocytosis, azotemia, Fresh, liquid feces or feces that contain large quantities of
pyuria, and an irregularly marginated kidney on radio- mucus should be microscopically examined immediately for
graphic examination likely has pyelonephritis. After making the presence of protozoal trophozoites, including those of
a tentative diagnosis, the clinician then must determine Giardia spp. in cases with small-bowel diarrhea and Trit-
whether to “test or treat.” Empiric treatment is sometimes richomonas foetus (T. blagburni proposed) in cases with
adequate in simple, first-time infections of dogs or cats large-bowel diarrhea. A direct saline smear can be made to
without life-threatening disease (see Chapter 92). However, potentiate observation of these motile organisms. A 2 mm ×
having a definitive diagnosis is usually preferred so that 2 mm × 2 mm quantity of fresh feces is mixed thoroughly
treatment, prevention, prognosis, and zoonotic issues can be with 1 drop of 0.9% NaCl or water. The surface of the feces
addressed optimally. or mucus coating the feces should be used because the tro-
Documenting that the infectious agent is still present, phozoites are most common in these areas. After application
using cytology, culture, antigen assays, and molecular diag- of a coverslip, the smear is evaluated for motile organisms
nostic assays is the best way to make a definitive diagnosis. by examining it under ×100 magnification (i.e., using the 10×
Antibody detection is commonly used to aid in the diag- objective in most microscopes).
nosis of specific infectious diseases but can be inferior to
organism demonstration for three reasons: (1) Antibodies Stained Smear
can persist long after an infectious disease has resolved, (2) A thin smear of feces from all dogs and cats with diarrhea
positive antibody test results do not confirm clinical disease can be made. Material should be collected by rectal swab, if
induced by the infectious agent, and (3) in peracute infec- possible, to increase the chances of finding white blood cells
tions, results of serum antibody tests can be negative if the and some organisms like Histoplasma capsulatum. A cotton
humoral immune responses have not had time to develop. swab is gently introduced 3 to 4 cm through the anus into
This chapter discusses the common organism demon- the terminal rectum, directed to the wall of the rectum, and
stration and antibody detection techniques used in small gently rotated several times. Placing 1 drop of 0.9% NaCl on
animal practice. the cotton swab will facilitate passage through the anus and

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