Page 56 - Differential Diagnosis in Small Animal Cytology, The Skin and Subcutis

Inflammatory Lesions

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6.5 Inflammation Caused by Infectious Agents
VetBooks.ir General information

• Inflammation may be caused by various infectious agents, such as bacteria, fungi, yeasts,
algae, protozoa, and/or parasites. Cytological evidence of these organisms, especially when
intracellular or associated with the inflammatory cells, is considered proof of infection.
• The absence of microorganisms on the smear does not rule out infection; therefore, additional
diagnostic investigations (e.g. culture testing, PCR) may be considered in selected cases.

Bacterial infections
The presence of intracytoplasmic bacteria (mainly within neutrophils) is the key finding to con-
firm infection and distinguishing this from sample contamination.

• Bacteria can be differentiated based on their morphology, staining characteristics and some-
times arrangement.
• Rods or bacilli (e.g. Proteus spp., Pseudomonas spp., etc.) are elongated and vary in size
from small to relatively large, thin or thick. Rod bacteria are usually Gram negative,
with the exception of Corynebacterium spp., which is Gram positive.
• Cocci are round (spherical) and can be found in chains (e.g. Streptococcus spp.) or
groups (e.g. Staphylococcus spp.). Most cocci are Gram positive.
• Filamentous bacteria, e.g. Nocardia spp., Actinomyces spp.
• Negatively staining rods: Mycobacterium spp.
• Bacteria commonly cause a neutrophilic inflammatory response, often associated with the
presence of degenerate forms. The exception to this is Mycobacterium spp., which more fre-
quently cause a pyogranulomatous or granulomatous response.

Fungal, yeasts or algae infections
• Aetiological agents: fungi, yeasts or algae.
• Partial identification can be attempted by morphological examination (e.g. Cryptococcus
spp., Malassezia canis or M. pachydermatis), but culture or PCR testing is required for pre-
cise characterization.
• They often elicit a neutrophilic and/or macrophagic inflammatory response, with a variable
eosinophilic component.
• The hyphae can be found phagocytosed by the macrophages and/or surrounded by the
inflammatory cells.

Protozoal infections
• Aetiological agents; for example:
• Leishmania spp.
• Toxoplasma gondii.
Protozoa often elicit a mixed but predominantly macrophagic inflammation. Plasma cells
may also be seen associated with Leishmania infection.
Parasitic infections
• Aetiological agents; for example:
• Mites (e.g. Sarcoptes spp., Demodex spp., Cheyletiella spp.)
• Dirofilaria repens.
Parasites usually trigger a neutrophilic to pyogranulomatous inflammation with a variable
eosinophilic component.