Autochthonous dengue outbreak in Italy 2020: clinical, virological and entomological findings

Abstract

Background: In August 2020, in the context of COVID-19 pandemics, an autochthonous dengue outbreak was identified for the first time in Italy.

Methods: Following the reporting of the index case of autochthonous dengue, epidemiological investigation, vector control and substances of human origin safety measures were immediately activated, according to the national arbovirus surveillance plan. Dengue cases were followed-up with weekly visits and laboratory tests until recovery and clearance of viral RNA from blood.

Results: The primary dengue case was identified in a young woman, who developed fever after returning from Indonesia to northern Italy, on 27 July 2020. She spent the mandatory quarantine for COVID-19 at home with relatives, six of whom developed dengue within two weeks. Epidemiological investigation identified further five autochthonous dengue cases among people who lived or stayed near the residence of the primary case. The last case of the outbreak developed fever on 29 September 2020. Dengue cases had a mild febrile illness, except one with persistent asthenia and myalgia. DENV-1 RNA was detected in blood and/or urine in all autochthonous cases, up to 35 days after fever onset. All cases developed IgM and IgG antibodies which cross-reacted with West Nile virus (WNV) and other flaviviruses. Sequencing of the full viral genome from blood samples showed over 99% nucleotide identity with DENV-1 strains isolated in China in 2014-2015; phylogenetic analysis classified the virus within Genotype I. Entomological site inspection identified a high density of Aedes albopictus mosquitoes, which conceivably sustained local DENV-1 transmission. Aedes koreicus mosquitoes were also collected in the site.

Conclusions: Areas in Europe with high density of Aedes mosquitoes should be considered at risk for dengue transmission. The presence of endemic flaviviruses, such as WNV, might pose problems in the laboratory diagnosis.

Keywords: Aedes albopictus; Aedes koreicus; DENV; follow-up; saliva; surveillance; urine.

Figure 1

Timeline of the dengue outbreak, northern Italy, 2020. Each case is indicated in rows. P represents the primary case; F1–F6 represent household contacts of P; A1 and A2 and B1–B3 represent two clusters epidemiologically-linked to P and F1–6. Each line represents a day, from July 27th to November 1st, 2020. Date of relevant events, such as the arrival in Italy of the primary case, symptom onset, laboratory diagnosis, probable exposure of a dengue case who was not resident in the affected area, duration of DENV-1 RNA detection in blood and urine, and vector control interventions and indicated in the figure.

Figure 2

Map of dengue cases, entomological surveillance and control areas. Numbers correspond to sites monitored with mosquito traps as reported in Supplementary Table 1. The red circle represents the area of 200 m of radius around primary case (site number 0), while the blue circle the area of 250 m used after the finding of autochthonous cases (site 11) out of the first surveillance area. Sites 9 (hospital) and 10 (Sovizzo municipality) are not shown on the map.

Figure 3

DENV-1 RNA load in blood and urine, reported as threshold cycles (C_T) values, and serum anti-DENV IgM and IgG antibody levels, reported as AU/mL, determined in the primary dengue case (P) and the autochthonous dengue cases (F1–6, A1, A2, B1–3) during follow-up, up to 6 weeks after fever onset. Case identification is the same of Table 1 and Figure 1.