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79 Laboratory Diagnosis of Infectious Diseases 845

Commercial biochemistry test strips enable the identifi- accessibility to abscesses), concentration of antibiotic in
VetBooks.ir cation of cultured bacteria and automated commercial the affected tissue, and inhibition of antibiotics by
body proteins.
identification systems are often used in large bacteriol-
ogy laboratories for rapid bacteria identification.
Some bacteria such as Anaplasma platys and hemo-
trophic mycoplasms (e.g., Mycoplasma hemofelis, Fungal Infections
Candidatus Mycoplasma hemominutum) are uncultura-
ble. The detection of these bacteria relies mainly on Fungal agents have a remarkably varied spectrum of clin-
molecular biology techniques such as PCR amplifying ical picture: primary skin disease, single to multiple sub-
DNA sequences which are specific for the suspected cutaneous masses, mass lesions within body cavities,
bacteria. Other bacteria such as Rickettsia spp., E. canis, disseminated disease with multiple organ dysfunction,
and A. phagocytophilum are strictly intracellular and can and disseminated disease that presents as single‐organ
only be grown in appropriate cell culture lines, restrict- dysfunction or as a cause of sudden death.
ing their isolation to research laboratories with exper- Clinicopathologic findings are not specific but mild ane-
tise. Molecular techniques can also be useful for the mia of chronic disease, leukocytosis, hypergammaglobu-
identification of bacteria that have been propagated in linemia, hypoalbuminemia, and hypercalcemia can be
culture and require classification to the species and present in some of these diseases. Primary (i.e.,
strain levels. These can be sometimes be achieved by Blastomyces, Coccidioides, Cryptococcus, Histoplasma)
multigenic sequencing and characterization, with phylo- and opportunistic pathogens, mainly saprophytic soil
genetic analysis. fungi (i.e., Aspergillus, Basidiobolus, Cladosporium,
Providing the bacteriology laboratory with a good clin- Conidiobolus), can be responsible for these diseases.
ical history, physical examination and initial blood test Opportunistic fungi can cause disease by trauma and
work‐up and cytology findings can be very helpful in the often present as a single subcutaneous mass. When
selection of different isolation techniques. Sampling the dissemination occurs, it is often associated with
correct organ sites is imperative and should be based on decreased immunocompetence.
clinical judgment. Common body fluids and anatomic
sites sampled for bacteriology from dogs and cats include Microscopic Examination
urine, blood, feces, bronchoalveolar lavage, skin and ear,
abdominal and pleural fluids, CSF, and cornea. Suspicion of fungal diseases should be made when mac-
rophagic or macrophagic‐neutrophilic inflammation is
Interpretation of Culture Results noted in any tissue by cytologic or histopathologic
The results of culture should be interpreted cautiously, specimens. Bacterial inflammatory lesions (Figure 79.3)
considering that the bacterium isolated, when culture is that are unresponsive to appropriate medical therapy
successful, may not actually be the cause of infection, or should be investigated further to rule out underlying
that in some other cases, culture is negative despite bac- mycotic disease due to the fact that bacterial and fungal
terial infection. This can be due to several reasons co‐infections do occur.
including technical difficulties in transporting the cul- Cytological evaluation of any tissue or body cavity
ture with infectious agent, use of incorrect isolation fluid specimen is very useful for identification of fun-
technique or culture site, intermittent shedding of gal disease in dogs and cats. Cytology is a more rapid
bacteria to the blood or other body fluid, and previous and simple technique for the detection of some fungal
antibiotic treatments. microorganisms than histopathology. Histopathology
commonly requires special staining such as periodic
Antimicrobial Susceptibility Tests acid–Shiff (PAS) or Gomori methenamine silver
Determination of minimum inhibitory concentrations (GMS) to detect fungal infections. In addition, some
(MIC) for isolated bacteria can be helpful in deciding laboratories offer immunohistochemistry for fungal
which antibiotic should be used for treatment of suscep- organisms within tissues submitted for routine histo-
tible infections. This is also important in the detection of pathology. Yeasts, different types of spores, and
antimicrobial resistance, if present. The standard tech- hyphae can be observed in cytologic specimens
niques for antibiotic susceptibility testing involve either (Figure 79.4).
gel diffusion or dilution methods. Although they can Cytologic features of most fungal pathogens are
provide an idea of the antibiotics to which the isolated described elsewhere. Routine Romanowsky staining ade-
bacteria is susceptible in vitro, this does not always con- quately stains most fungal organisms within cytologic
form with the actual susceptibility due to in vivo factors preparations. Organisms that do not stain well appear as
such as accessibility to the infected tissue (e.g., decreased negatively stained elements (Figure 79.5).