Page 1459 - Small Animal Internal Medicine, 6th Edition

CHAPTER 91 Laboratory Diagnosis of Infectious Diseases 1431

media for culture procedures or inoculated into laboratory airway washings) stored at 20° C for 1 to 2 hours, 4° C for
animals, if indicated, before further handling. 24 hours, or 4° C for 72 hours if placed in transport medium.
VetBooks.ir to remove excess blood and then lightly touching the tissue lected aseptically into a syringe and the needle covered with
Anaerobes can be successfully cultured from fluid col-
Gently blotting the cut edge of the tissue on a paper towel
multiple times to a microscope slide make tissue impressions
media within 10 minutes of collection. Because of time limi-
for cytologic examination. Tissue specimens can then be a rubber stopper if the material is to be placed on culture
frozen, placed into 10% buffered formalin solution, or placed tations, transport media is generally required for samples
into glutaraldehyde-containing solutions. Frozen specimens from animals with suspected anaerobic infections. These
are generally superior for immunohistochemical staining media will support the growth of most anaerobes for 48
and molecular diagnostic procedures. Routine histopatho- hours if stored at 4° C.
logic evaluation is performed on formalin-fixed tissues. Samples for blood culture should be collected aseptically
Special stains can be used to maximize the identification of from a large vein after surgical preparation of the skin. In
some infectious agents. The clinician should alert the histo- general, three 5-mL samples are collected over a 24-hour
pathology laboratory to the infectious agents most suspected period in stable patients or at 1- to 3-hour intervals in septic
to allow for appropriate stain selection. Glutaraldehyde- patients. Unclotted whole blood is placed directly into trans-
containing fixatives are superior to other fixatives for elec- port media that will support the growth of aerobic and
tron microscopic examination of tissues; this technique can anaerobic bacteria, and it is incubated at 20° C for 24 hours.
be more sensitive than other procedures for demonstration Culture for Bartonella spp. from blood of dogs or cats is
of viral particles. Molecular diagnostic assays like fluores- generally performed on whole blood samples collected asep-
cence in situ hybridization (FISH) are now being used to tically and placed in an EDTA-containing tube. In dogs, the
identify nucleic acids of infectious agents within tissues (see combination of culture and PCR performed on 3 mL of
Molecular Diagnostics, p. 1432). blood in EDTA collected up to 3 days weekly may be required
to detect Bartonella spp. infections (see Chapter 94).
CULTURE TECHNIQUES Culture of feces for Salmonella spp. or Campylobacter spp.
Bacteria, fungi, viruses, and some protozoans can be cul- and performance of antimicrobial sensitivity testing is occa-
tured. In general, a positive culture can be used to establish sionally indicated in small animal practice, particularly if the
a definitive diagnosis. Aerobic bacterial culture can be com- animal has signs of sepsis or an outbreak is suspected.
bined with antimicrobial susceptibility testing to determine Approximately 2 to 3 g of fresh feces should be submitted to
optimal drug therapy. Successful culture depends on collect- the laboratory immediately for optimal results; however, Sal-
ing the optimal materials without contamination, transport- monella and Campylobacter are usually viable in refrigerated
ing the materials to the laboratory as quickly as possible in fecal specimens for 3 to 7 days. To increase the likelihood of
the most appropriate medium to minimize organism death achieving positive culture results, a transport medium should
or overgrowth of nonpathogens, and using the most appro- be used if a delay is expected. The laboratory should be noti-
priate culture materials. fied of the suspected pathogen so that appropriate culture
Culture results of body systems with normal bacterial and media can be used.
fungal flora, including the skin, ears, mouth, nasal cavity, Mycoplasma and Ureaplasma cultures are most com-
trachea, feces, and vagina, are the most difficult to interpret. monly performed on airway washings, synovial fluid, exu-
Finding positive culture results and inflammatory cells cyto- dates from chronic draining tracts in cats, urine from animals
logically suggests the organism is inducing disease. Culture with chronic urinary tract disease, and the vagina of females
of a single agent, particularly if the organism is relatively with genital tract disease. Samples should be transported to
resistant to antimicrobials, is more consistent with a disease- the laboratory in Amies medium or modified Stuart bacterial
inducing infection than if multiple, antibiotic-susceptible transport medium. Mycoplasma spp. culture should be spe-
bacteria are cultured. Materials for routine aerobic bacterial cifically requested.
culture can be placed on sterile swabs if the swabs remain Mycobacterium spp. grow slowly, and culture is often
moist and are placed on appropriate culture media within 3 limited by overgrowth of other bacteria. Special medium is
hours of collection. If a delay of greater than 3 hours is required; therefore the laboratory should be specifically
expected, swabs containing transport medium should be instructed to culture for Mycobacterium spp. Tissue samples
used. These swabs should be refrigerated or frozen to inhibit or exudates from animals with suspected Mycobacterium
bacterial growth if cultures are not to be started within 4 spp. infection should be refrigerated immediately after col-
hours; some bacteria will grow more rapidly than others, lection and transported to the laboratory as soon as possible.
potentially masking fastidious organisms. Most aerobes will Exudates should be placed in transport media.
survive at 4° C (routine refrigeration temperature) in tissue Cutaneous fungal agents can be cultured in the small
or on media-containing swabs for 48 hours. Solid-phase animal office by using routinely available culture media.
transport media that will support the growth of most aerobes, Materials from dogs or cats with suspected systemic fungal
anaerobes, Mycoplasma spp., and fungi for several days if infection can be transported to the laboratory as described
refrigerated are also routinely available. Routine aerobic for bacteria, and the laboratory can be told specifically that
culture is generally successful on fluid samples (e.g., urine, fungal culture is necessary. The yeast phase of the systemic