# Rocio virus sustained circulation in Brazil: first infection case in a horse highlights the need for enhanced arbovirus surveillance

## TL;DR

A new case of Rocio virus in a horse in Brazil shows the virus is still active and evolving, highlighting the need for better surveillance.

## Contribution

The first ROCV case in nearly 40 years and full-genome sequencing of a new isolate reveal ongoing evolution and potential sentinel role of horses.

## Key findings

- ROCV was isolated and sequenced from a horse's CNS tissue, showing divergence from the reference sequence.
- The envelope protein had 31 non-synonymous substitutions, indicating significant evolution.
- Horses may serve as sentinels for ROCV, suggesting a need for enhanced genomic surveillance.

## Abstract

Rocio virus (ROCV), a neurotropic arbovirus of the Orthoflavivirus genus, caused Brazil’s largest encephalitis outbreak in the 1970s but has since been rarely detected. We report the first ROCV encephalitis clinical case after nearly 40 years and isolated and sequenced the virus from CNS tissue of a horse. Metatranscriptomics enabled full-genome sequencing, revealing divergence from the reference sequence at an evolutionary rate of 5.8–7.0 × 10⁻⁴, consistent with Orthoflavivirus. Thirty-one non-synonymous substitutions were identified, with the envelope protein most affected. Findings indicate ROCV cryptic circulation and ongoing evolution. Horses may act as sentinels, underscoring the need for strengthened ROCV genomic surveillance.

The online version contains supplementary material available at 10.1007/s00705-026-06574-9.