Ocular and periocular Usutu virus-associated lesions in naturally infected Eurasian blackbirds ( Turdus merula )

Usutu virus (USUV) is a zoonotic neurotropic arbovirus related to the West Nile virus that causes mortality in birds and sporadic neurologic human disease. Current research on natural USUV-associated disease lacks data on ocular involvement. This study investigated ocular and periocular tissue involvement in natural USUV infections and associated disease in Eurasian blackbirds ( Turdus merula ). Twenty-two found-dead Eurasian blackbirds were examined. USUV reverse transcriptase quantitative polymerase chain reaction (RT-qPCR) detected 12/22 infected blackbirds. Histology and immunohistochemistry for virus antigen, inflammation (anti-CD3 for T-lymphocytes), and apoptosis (anti-cleaved caspase-3 (CC3)) assessed the virus tropism and associated damage. In the eye of USUV-infected blackbirds, choroiditis was the main finding (9/12), while the pecten oculi (4/12) and optic nerves (4/12) were occasionally affected. Virus antigen was detected in the lesions. The cornea and retina lacked virus antigens and lesions. Periocular soft tissues (12/12) and eyelids (9/12) were also affected. Lesions in the choroid ( P < .001), ciliary bodies ( P < .01), and sclera ( P < .05) were significantly associated with USUV infection. In the choroid, CD3 and CC3 strongly correlated with the virus antigen scores ( P < .0001), suggesting a T-cell response and apoptosis involvement in the ocular damage. A negative correlation was identified for the virus antigen score in choroid and eyelids with USUV RT-qPCR Ct values ( P < .05). This study reports for USUV features consistent with ocular and periocular tropism and disease with a major involvement of the choroid, suggesting a primary ocular vascular spread.