Bacteria
Mycobacteria do not stain with routine cytologic stains and can be difficult to visualize. However, careful examination of
the cells and background material reveals the presence of distinctive negatively stained thin rod present both intra- and
extracellularly. The organisms can be confirmed by acid fast staining. Siamese cats appear to have increased susceptibility
to mycobacteriosis. Mycobacteriosis often induces a granulomatous or pyogranulomatous inflammatory response rather than the
suppurative response seen against most bacteria. Langhans' multinucleate giant cells and large epithelioid macrophages are
often seen.
Actinomycosis/Nocardiosis infection is often seen as subcutaneous masses or intrathoracic lesions due to penetrating wounds.
The thin organisms have a characteristic branching beaded filamentous appearance that often appear in large mats that resemble
amorphous debris.
Clostridia can be identified as large gram positive rods (1 x 4um) that often contain a clear, oval spore at one pole of the
bacterial. Clostridium is associated with cellulites, penetrating wounds, and GI overgrowth.
Yersinia pestis is an uncommonly detected organism, the zoonotic potential of this organism makes identification of the organism
crucial. Yersinia pestis is a gram-negative bacillus cytologically recognizable as bipolar coccobacilli present both intra-
and extracellularly with large numbers of degenerate neutrophils. In cats, pneumonic plague occurs in about 10% of cases and
can be seen with or without the classic bubonic presentation.
Fungal
Coccidioides immitis is a soil-borne dimorphic fungus, found mainly in the arid, acid-soil regions of the United States (Arizona
and California) and in some regions of South America. In endemic areas, infected appears to be relatively common while development
of clinical signs is relatively uncommon. It is primarily a respiratory pathogen and infection is by inhalation of spores.
The incubation period for development of respiratory infection is 1-3 weeks after exposure. Disseminated disease occurs after
primary lung infection, especially in dogs. Boxers and Doberman pinschers may be predisposed to disseminated disease. Until
recently, cats were thought to be resistant to infection with Coccidioides, but both susceptibility to infection and development
of clinical signs has been reported in endemic areas. In both dogs and cats with disseminated disease, lesions in long bones
(especially the metaphyses area) and skin lesions are common. Coccidioidomycosis is induces a pyogranulomatous or granulomatous
inflammation. Coccidioides immitis spherules (sporangium) are large organisms seen extracellularly. Spherules range in size
from 10 to 100 μm in Romanowsky-stained preparations and contain a thick double-contoured wall with finely granular, blue-green
protoplasm. Occasionally internal endospores of 2 to 5 μm may be seen. Organisms are scarce in cytologic preparations and
multiple slides may need to be examined to find the organism. Due to the organism's large size, scanning is best done at
low magnification (e.g., 10X). Mycelia may rarely be seen in tissue.
Blastomyces dermatitidis is a soil-borne, dimorphic fungus. The mycelial phase occurs in nature and the yeast form in vivo,
although hyphal stages may be seen (albeit rarely). It probably has worldwide distribution, although the number of cases
reported outside of North America is relatively small. A second strain has been isolated from cases in Africa and probably
represents a distinct serotype with geographic diversity. The endemic area in the United States includes the middle western,
southeastern and Appalachian states. Blastomyces can infect numerous tissues, but the lung is the most frequently involved
organ in primary infection. Infection tends to be via inhalation of spores. Although direct puncture and skin wounds can
lead to cutaneous lesions. most cutaneous lesions are derived from pulmonary infection. In dogs, infection more often occurs
in young, large-breeds. Blastomyces is uncommon in cats, however when seen in cats the disease tends to be systemic rather
than localized and Siamese appear to be over-represented in the literature. The incubation period for Blastomycosis is relatively
long and variable (weeks to months). The extracellular yeast forms are dark blue, round, and 5 to 20 μm in diameter, with
a thick biconcave wall having a granular internal structure. Broad-based budding may be seen. The organisms are likely found
in aggregates of mucus and necrotic debris and induce granulomatous or pyogranulomatous inflammation.
