Page 63 - Differential Diagnosis in Small Animal Cytology, The Skin and Subcutis

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6.8 Injection Site and Foreign Body Reaction
VetBooks.ir Inflammatory process triggered by injection, most typically vaccinations or other foreign bodies.

Clinical features

• Caused by either endogenous or exogenous foreign body (FB) material.
• Endogenous FB material: keratin. This inflammatory process is often observed
associated with follicular cysts or cystic follicular tumours. It occurs following the
rupture of the cystic wall and exposure of the keratin to the surrounding tissues.
• Exogenous FB material: vaccine adjuvant, plant material, suture material, surgical
swabs or sponges, etc.
• Solitary or multiple nodules in any area of the body. When induced by vaccinations/injec-
tions, nodules are most commonly observed in the intrascapular area or hindlimb muscles.
• Spontaneous regression is possible but surgical exploration/excision is often required
and carries a good prognosis.

Cytological features
• Cellularity is variable, often moderate to high.
• Background: variably haemodiluted and proteinaceous. The foreign material is not always
found. If present, it may be found in the cytoplasm of the macrophages or extracellularly,
surrounded by the inflammatory cells.
• Inflammatory cells are often mixed. Recent or infected processes are often pyogranuloma-
tous and a mixture of macrophages, degenerate neutrophils and variable numbers of eosino-
phils are found.
• Chronic inflammations are characterized by numerous macrophages alongside small
lymphocytes and plasma cells. Both epithelioid macrophages and multinucleated giant
inflammatory cells can be present.
• In vaccine/injection-associated reactions, small lymphocytes may be numerous and eosinophils
can be found in significant numbers.
• Variable numbers of reactive fibroblasts can be present.
• Tissue necrosis may occur.

Differential diagnosis
Pyogranulomatous or granulomatous inflammation caused by infectious agents (e.g. fungi,
bacteria)

Pearls and Pitfalls
Occasionally, Langhans cells can be seen in chronic granulomatous inflammations. They repre-
sent a variant of the multinucleated giant cells. Their nuclei are typically positioned at the per-
iphery of the cell, in a horseshoe type of arrangement. They can be found in fungal infections,
mycobacteriosis or in any other case of granulomatous reaction.