# First Detection of Usutu Virus in Harbor Seals (Phoca vitulina)

## TL;DR

This paper reports the first detection of Usutu virus in harbor seals, highlighting its potential threat to marine mammals.

## Contribution

The study documents the first cases of Usutu virus infection in harbor seals and emphasizes its relevance in marine mammal health.

## Key findings

- Fatal neurological signs and non-suppurative encephalitis were observed in infected harbor seals.
- Two asymptomatic USUV RNA and antibody-positive seals were identified in the same rescue center.
- The detected USUV sequences belonged to lineages known to circulate in birds in Germany.

## Abstract

The zoonotic Orthoflavivirus Usutu virus (USUV) is distributed throughout Germany, putting hosts at a considerable risk of infection nationwide. Besides birds as reservoir hosts, a broad range of accidental hosts is suspected. However, there are few reports documenting the progression of USUV-associated disease. This case report describes the course of fatal USUV infections in three harbor seals (Phoca vitulina) from a rescue center on the North Sea coast in Germany. Corresponding samples were analyzed using (histo-)pathological, immunohistochemical, molecular and phylogenetic methods. The most prevalent findings in clinically affected animals were neurological signs and non-suppurative encephalitis. All animals were found dead or had been euthanized due to animal welfare reasons within 30 h after the onset of clinical signs. Blood samples taken from another 37 young harbor seals from the same rescue center in the same year revealed two further asymptomatic USUV RNA and antibody-positive animals. The sequences were found to belong to USUV lineages Europe 2 and Africa 3, which are known to circulate in birds in Germany. This case report highlights the importance of USUV as a potential diagnosis for neurological impairments in marine mammals and documents the first cases of USUV infection in harbor seals.