Usutu Virus

Usutu virus (USUV) is a mosquito-borne flavivirus belonging to the Japanese encephalitis virus (JEV) serocomplex, first identified in South Africa in 1959. Historically known as an avian pathogen, it gained prominence in Europe after causing significant mass mortality events in blackbirds (Turdus merula) starting in the early 2000s. USUV has since established endemic transmission cycles across much of Europe, with growing evidence of spillover infections in humans. While often asymptomatic, human infection can lead to severe neurological complications such as meningoencephalitis, paralleling its close relative West Nile virus (WNV). Its rapid geographic expansion, driven by climate change, globalization, and compatible bird and mosquito populations, exemplifies the dynamics of viral emergence.

Figure 1.Structure of usutu virus. (Cadar D,2022)

Introduction

For decades after its discovery, USUV was considered a low-pathogenicity virus of limited geographic range in Africa. This perception changed dramatically in 2001 when it was linked to widespread die-offs of blackbirds and other songbirds in Austria. This event marked its successful introduction and establishment in Europe. Subsequent surveillance revealed its silent spread across the continent, facilitated by resident Culex mosquito vectors and migratory bird reservoirs. Concurrently, increased testing identified a growing number of human cases, ranging from febrile illness to severe neuroinvasive disease. USUV now serves as a prime model for studying flavivirus ecology, cross-species transmission, and the potential for "pathogen pollution" – the human-mediated spread of exotic viruses into new ecosystems.

Taxonomy and Genome Structure

• Taxonomy and Structure
  USUV is a member of the genus Flavivirus (family Flaviviridae), within the Japanese Encephalitis virus serocomplex, which includes WNV, JEV, and Murray Valley encephalitis virus. The virion is spherical, approximately 50 nm in diameter, with an icosahedral capsid enclosing a positive-sense, single-stranded RNA genome. The genome is surrounded by a host-derived lipid envelope embedded with the viral envelope (E) and membrane (prM/M) glycoproteins.
• Genome and Proteins
  The ~11 kb RNA genome encodes a single long open reading frame flanked by untranslated regions (UTRs). This polyprotein is co- and post-translationally cleaved by host and viral proteases into three structural and seven non-structural (NS) proteins: C-prM-E-NS1-NS2A-NS2B-NS3-NS4A-NS4B-NS5.
  • Structural Proteins: Capsid (C), precursor Membrane (prM), and Envelope (E). The E protein is responsible for receptor binding and membrane fusion, and is the major target for neutralizing antibodies.
  • Non-structural Proteins: NS1 is involved in replication and immune evasion, NS2B-NS3 is the viral protease, NS3 also has helicase and NTPase activities, NS5 is the RNA-dependent RNA polymerase (RdRp) and methyltransferase.

Clinical Manifestations, Diagnosis, and Management

Most human USUV infections are asymptomatic, with only a small proportion of infected individuals developing clinical disease. Symptomatic cases typically present with a mild, self-limiting febrile illness characterized by fever, headache, myalgia, arthralgia, rash, and occasionally hepatitis or jaundice. Severe neuroinvasive disease occurs in a minority of cases, presenting with meningitis, encephalitis, or cranial nerve dysfunction, which may lead to long-term neurological sequelae or death.

Diagnosis of USUV infection is primarily based on laboratory testing, as clinical manifestations are nonspecific and can be confused with other flavivirus infections. Available diagnostic methods include: (1) serological testing to detect USUV-specific IgM and IgG antibodies (enzyme-linked immunosorbent assay, ELISA), (2) molecular testing (reverse transcription-polymerase chain reaction, RT-PCR) to detect viral RNA in blood, cerebrospinal fluid (CSF), or tissue samples, and (3) virus isolation from clinical samples (gold standard, but time-consuming and labor-intensive). Currently, there are no commercially available diagnostic tests for USUV in the United States, with testing limited to specialized laboratories such as the CDC Arbovirus Diagnostic Laboratory.

Anti-Usutu Virus Antibody

Usutu Virus Antigens