Spatial distribution of Culex mosquitoes across England and Wales, July 2023

With medically important arboviruses such as West Nile virus (WNV) circulating in Europe and Usutu virus (USUV) currently present in the UK, it is imperative to identify areas in the UK at risk of establishment and spread of these viruses. Here, we describe a comprehensive nationwide field surveillance study conducted during July 2023 to map the distribution of the WNV and USUV competent vectors: Culex pipiens biotype pipiens, Culex pipiens biotype molestus and Culex torrentium, across England and Wales.

Mosquitoes were sampled for 3 trap nights (3TN) at 200 sites in rural, urban and suburban settings, selected using a lattice plus close pairs surveillance design. Field-caught samples were analysed using morphological and molecular approaches.

A total of 2157 adult mosquitoes of the Culex genus were collected. Culex pipiens biotype pipiens dominated the resident Culex populations, comprising 1478 (95.8%) out of 1543 mosquitoes with confirmed molecular species identity. Culex torrentium were present in much lower numbers, with only 38 (2.5%) identified mostly and in the central and southwestern regions. Only five of the biotype molestus (0.3%) were identified in this study; however these were found in localities outside of London and in a rural location, with the biotype previously having been associated with human-made habitats. This study also found that hybrids of the pipiens and molestus biotypes are more widespread than previously recorded. In total, 22 hybrids (1.4%) were identified from counties as far west as Cornwall and as far north as Suffolk.

Coupled with reviews of previous UK Culex sp. surveys, this study provides essential data for the vectorial component of risk modelling of WNV and USUV in the UK, furthering the country's preparedness for incursions of vector-borne diseases in the future.