A four-year survey (2011-2014) of West Nile virus infection in humans, mosquitoes and birds, including the 2012 meningoencephalitis outbreak in Tunisia

Abstract

A West Nile virus (WNV) outbreak occurred in Tunisia between mid-July and December 2012. To assess the epidemiological features of the WNV transmission cycle, human cerebrospinal fluid samples from patients with suspected cases (n = 79), Culex pipiens mosquitoes (n = 583) and serum specimens from domestic and migratory birds (n = 70) were collected for 4 years (2011-2014) in the Tunisian Sahel region. Viral testing was performed by polymerase chain reaction (PCR). The WNV genome was detected in 7 patients (8.8%), 4 Culex pipiens pools, and a domestic mallard (Anas platyrhynchos). All PCR-positive samples were from the Monastir region. Phylogenetic analysis revealed that two different WNV strain groups circulated, and isolates from the reservoir (bird), vector (Culex pipiens), and dead-end hosts (humans) were closely related. The Monastir region is a hot-spot for WNV infection, and the reiterative presence of WNV over the years has increased the risk of viral reemergence in Tunisia, which highlights the need for more enhanced and effective WNV surveillance in humans with public awareness campaigns strengthened by monitoring mosquitoes and maintaining avian surveillance for early detection of WNV circulation.

Fig. 1. Geographic distribution of PCR-positive human, avian and pooled mosquito samples in the Monastir governorate, Sahel region, Tunisia. 1,

Bembla; 2, 8, Kniss; 3, Monastir; 4, Moôtmar; 5, Ouardanine; 6, Sayada; 7, Ksar Helal; 9, Agba; 10, Moôtmar- Sahline; 11, Oued khniss; 12, Ouardanine Oued el Guelta. Red circles: positive human samples. Black circles: negative human samples. White triangles: positive mosquito pools. Black triangles: negative mosquito pools. Blue square: positive bird sample. Black squares: negative bird samples

Fig. 2. Phylogenetic analysis of identified West Nile virus sequences from humans, pooled mosquitoes and birds in the Monastir governorate, Sahel region, Tunisia.

Phylogenetic analysis was performed using Bayesian analysis based on the TN93 evolution model, a gamma distribution and invariable sites. The tree includes 12 strains isolated in this study and 27 homologous nucleotide sequences from the West Nile virus NS5 gene obtained from the GenBank library. HS human sample, MS mosquito sample, BS bird sample