First evidence of simultaneous occurrence of West Nile virus and Usutu virus neuroinvasive disease in humans in Croatia during the 2013 outbreak

Infection. 2014 Aug;42(4):689-95. doi: 10.1007/s15010-014-0625-1. Epub 2014 May 5.


Purpose: We report on first evidence of simultaneous occurrence of West Nile virus (WNV) and Usutu virus (USUV) neuroinvasive infection in humans in Croatia during the transmission season 2013.

Methods: From June to December 2013, a total of 95 patients with clinically suspected WNV infection (WNV fever and neuroinvasive disease) were tested for WNV IgM/IgG antibodies using enzyme-linked immunosorbent assay. Twenty-six reactive samples were further tested by virus neutralization test for confirmation.

Results: WNV neuroinvasive infection was confirmed in 20 patients, while in three patients USUV neutralizing antibodies were detected. Cases occurred during the 11-week interval (from 24 July to 07 October 2013). Both WNV and USUV cases were distributed in three north-western Croatian counties. In addition to human cases, recent asymptomatic WNV infection (detection of IgM antibodies) was recorded in 9/3,460 (0.3 %) tested sentinel horses. Infected animals were recorded in two eastern and one north-western county.

Conclusions: Our results indicate co-circulation of WNV and USUV in Croatia. WNV infection could be misdiagnosed with other emerging infectious diseases presenting with neurological symptoms such as USUV infection.

MeSH terms

  • Adult
  • Aged
  • Animals
  • Antibodies, Neutralizing / blood
  • Antibodies, Viral / blood
  • Croatia / epidemiology
  • Disease Outbreaks*
  • Encephalitis Viruses, Japanese / isolation & purification
  • Encephalitis, Arbovirus / epidemiology*
  • Enzyme-Linked Immunosorbent Assay
  • Female
  • Humans
  • Immunoglobulin G / blood
  • Immunoglobulin M / blood
  • Male
  • Middle Aged
  • Neutralization Tests
  • West Nile Fever / epidemiology*
  • West Nile virus / isolation & purification


  • Antibodies, Neutralizing
  • Antibodies, Viral
  • Immunoglobulin G
  • Immunoglobulin M