Usutu virus risk to the UK: Determining local vector competence and modelling climate suitability

Usutu virus infects and causes large scale mortality in certain species of wild bird (such as blackbird and crows); but in recent years it has been recognised as zoonotic, infecting people, and causing fever and, in some cases, neuro-invasive disease such as encephalitis and meningoencephalitis.

Usutu virus is spread by mosquitoes and is emerging throughout Europe. During the warm summer of 2020, zoonotic mosquito-borne Usutu virus was detected in two species of wild birds in Greater London. Furthermore, Usutu virus was detected over a long time period and in the non-migrating house sparrow, strongly suggesting that the virus has established in the UK and local British mosquitoes are transmitting it.

The primary mosquito vector of Usutu virus in Europe, the Northern House mosquito, is present in the UK where it feeds predominantly on birds. Recent studies have shown that a laboratory colony of this species is competent to transmit the virus at warm temperatures. Therefore, our native mosquitoes may therefore be capable of transmitting it. In addition, other species of mosquito present in the UK feed on both birds and humans and may therefore be capable of spreading the virus from the bird reservoir to humans. The ability of these species to transmit Usutu virus in the wild is not known.

The overall aim of this proposal is to assess the risk to the UK of local and invasive mosquitoes transmitting Usutu virus to birds and humans, at realistic UK temperatures. Using these results, we will then mathematically model the suitability of the UK climate, in conjunction with data on the geographical range of local mosquitoes and reservoir bird species. This will yield risk maps, showing where and when Usutu virus is capable of being transmitted in the UK.

At the end of this project, we will have collected extensive data on Usutu virus and the ability of our mosquitoes to transmit it. This will put the UK in a stronger position to effectively manage an outbreak, should one occur in the next few years. Some of the data will also inform on other mosquito-borne viral threats. As demonstrated by bluetongue in northern Europe, West Nile across the world, and Zika in South America, arboviral threats can appear suddenly and spread quickly. Basic research in advance is needed if such outbreaks are to be effectively contained with minimal impact on human and animal populations.

Usutu virus is an immediate and direct threat to the UK's wild birds, urban ecology and public health. It is present in London, but we do not whether it will become more significant in future as British summer temperatures increase.

The mosquito Culex pipiens (s.l.) is believed to be the major European vector among birds. Colonised forms of UK Cx. pipiens kept at 25oC developed transmissible infections. It is not known if wild-caught Cx. pipiens can develop transmissible infections, or whether this is possible at temperatures more typical of the UK summer. In WP1, we will test wild-caught Cx. pipiens, at 19 and 23 degree Celsius, for their ability to develop transmissible infections with USUV.

Zoonotic transmission requires a mosquito to feed on a bird to become infected, and then to a person; such a species is called a 'bridge vector'. The UK has a number of potential bridge vectors (e.g. Cx. modestus, Ochlerotatus detritus, Aedes vexans) which we have shown are capable of transmitting several flaviviruses. In WP2, we will investigate the ability of these bridge vector species to develop transmissible infections of USUV, at a range of temperatures, and thereby identify lower temperature thresholds and develop temperature-based risk maps.

During WPs1&2, we will also collect adult females from the areas. These will be RT-PCR screened for USUV to confirm field virus acquisition. As added-value, we will also screen these mosquitoes for other arboviruses to identify any unknown viral circulation. In WP3, we will collect and screen wild adult mosquitoes for USUV and other arboviruses.

In WP4, we will combine these newly gained data from WPs1-3 to assess the risk of USUV circulation in the UK. We will use the temperature-transmission data from WPs1&2 to inform temperature-driven models of USUV transmission in the UK, and use these models to determine the potential distribution and optimal seasons for transmission now, and in the future climate change scenarios