A note on exophthalmia in Theileria annulata infection: Clinical presentation, pathobiology, and the central role of cytokine-mediated response.

Exophthalmia is an important but poorly documented complication of tropical theileriosis caused by Theileria annulata, especially in neonatal and young calves. It involves forward displacement of the eyeball, periorbital swelling, conjunctival congestion, excessive tearing, and enlarged regional lymph nodes. Systemic signs like fever, anaemia, depression, and weakness often occur. The condition arises through complex pathogenetic mechanisms linked to the schizont stage of the parasite, where infected leukocytes transform into rapidly dividing, invasive cells. These cells infiltrate retro-orbital tissues, creating inflammatory masses that pressure the orbital cavity, resulting in globe protrusion. Simultaneously, inflammatory cytokines such as TNF-α, IL-1, IL-6, and IFN-γ are released excessively, causing endothelial damage, increased vascular permeability, and oedema. Enlarged lymph nodes block venous and lymphatic drainage, worsening fluid build-up and infraorbital pressure, which all contribute to the development of exophthalmia in tropical theileriosis.