Page 1458 - Small Animal Internal Medicine, 6th Edition

1430 PART XIV Infectious Diseases

or macrophages should always be considered. Some organ- microscope slide to clear debris. The slide is then heated, but
isms such as Mycoplasma are rarely documented cytologi- not boiled, and examined for dermatophytes. All cats with
VetBooks.ir cally, whereas other organisms require special stains for chronic, draining skin lesions should have imprints of the
lesions made and stained with Wright-Giemsa stain followed
optimal visualization. For some, bacteria culture has never
been successful. For example, the hemoplasmas of dogs and
cats (previously called Haemobartonella felis and Haemobar- by microscopic examination for the characteristic round,
oval, or cigar-shaped yeast phase of Sporothrix schenckii
tonella canis) can be detected on the surface of red blood within the cytoplasm of mononuclear cells (see Fig. 99.3).
cells (RBCs) but have never been successfully cultured. Until Periodic acid–Schiff stain is superior to Wright-Giemsa stain
the advent of molecular diagnostic techniques (see p. 1432), for the demonstration of fungi.
documentation of infection was based on cytology alone;
Wright-Giemsa stain is the best stain to use in practice for Cutaneous Parasitic Diseases
these organisms. However, falsely negative results based on Cheyletiella spp., Demodex spp., Sarcoptes scabiei, Notoedres
cytology are common, and therefore molecular techniques cati, and Otodectes cynotis are the most common small
should be considered in cytology-negative cases if the index animal cutaneous parasites. Definitive diagnosis is based on
of suspicion is high. cytologic demonstration of the organisms. Cheyletiella is
demonstrated by pressing a piece of transparent tape against
Rickettsial Diseases areas with crusts, placing the tape on a microscope slide, and
Anaplasma spp. (Fig. 91.6) and Ehrlichia spp. are occasion- examining it microscopically. Demodex spp. are most com-
ally found within the cytoplasm of cells in the peripheral monly detected in deep skin scrapings and follicular exu-
blood, lymph node aspirates, bone marrow aspirates, or dates; Cheyletiella spp., S. scabiei, and N. cati are detected in
synovial fluid (see Chapter 95). Morulae of these genera can wide, more superficial scrapings. O. cynotis or its eggs are
be found in different cell types. Wright-Giemsa stain is supe- detected in ceruminous exudates from the ear canals.
rior to Wright or Diff-Quik stain for the demonstration of
morulae. Rickettsia rickettsii in endothelial cells lining vessels Systemic Protozoal Diseases
can be documented by immunofluorescent antibody stain- The most common systemic protozoal diseases and the cyto-
ing (see Immunologic Techniques, p. 1432). logic appearance and location of these agents are described
in Chapter 98. Cytologic demonstration of these agents leads
Fungal Diseases to a presumptive or definitive diagnosis of the disease.
Arthrospores and conidia of dermatophytes can be identified Wright-Giemsa or Giemsa staining of thin blood films
cytologically. Hairs plucked from the periphery of a lesion should be used to demonstrate Leishmania spp., Trypano-
are covered with 10% to 20% potassium hydroxide on a soma cruzi, Babesia spp., Hepatozoon americanum, and
Cytauxzoon felis. Collection of blood from an ear margin
vessel may increase the chances of demonstrating the proto-
zoans found in blood, particularly Babesia spp. and C. felis.
T. gondii and Neospora caninum cause similar syndromes
in dogs, but their tachyzoites are difficult to distinguish
morphologically; immunocytochemical staining or PCR is
required to differentiate these agents. These protozoans can
also be distinguished by evaluating for seroconversion
because antibodies are specific to each agent. With the excep-
tion of T. gondii and N. caninum, systemic protozoans are
rare or regionally defined in the United States. See Chapter
98 for further discussion of these agents.

Viral Diseases
Rarely, viral inclusion bodies are detected cytologically after
staining with Wright-Giemsa. Distemper virus infection
causes inclusions in circulating lymphocytes, neutrophils,
and erythrocytes of some dogs. Rarely, feline infectious peri-
tonitis virus results in intracytoplasmic inclusions in circu-
lating neutrophils. Feline herpesvirus 1 (FHV-1) transiently
results in intranuclear inclusion bodies in epithelial cells.
TISSUE TECHNIQUES

FIG 91.6 Tissues collected from animals with suspected infectious dis-
Anaplasma phagocytophilum morula (arrow) in the eases can be evaluated by several different techniques. Tissue
cytoplasm of an experimentally infected cat. samples should be aseptically placed in appropriate transport