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.

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.