WEST NILE VIRUS IN EUROPE: Epidemiology, drivers of transmission and an integrated approach to surveillance and control.
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West Nile Virus (WNV) is a mosquito borne virus that is maintained in nature in a bird-mosquito-bird transmission cycle. Mosquitos can occasionally infect humans and other vertebrate animals like horses and crocodiles. Most West Nile virus infections are characterized by subclinical signs like fever, headache, and weakness; however, infections can also progress to more severe neurological manifestations and death. It was initially discovered in the West Nile province of Uganda and the virus is widely distributed in Africa, Europe, Australia, and Asia. In recent years, sporadic cases and major outbreaks have increased in Europe, especially in countries around the Mediterranean Sea, the South-Eastern region, and Central Europe. In addition, recent pattern also shows the geographical spread of the virus towards the Northern part of Europe. The increased geographical spread and occurrence of WNV infections in Europe makes it a public health concern. Therefore, this literature review provides an overview of West Nile Virus infection in general, evaluating the spread across Europe and factors that influenced the distribution. The spread of the virus can be influenced by the migratory behavior of birds. Migratory birds can cause re-introduction of the virus in an area and the host competence among resident birds can also have an impact on the continued circulation of the virus. The Culex species of mosquito is the most ubiquitous species in Europe, and it plays a significant role in the transmission of WNV. It has two different biotypes which differ in their physiological and behavioral capacity affecting transmission dynamics in different regions. Also, climate change is an important environmental driver for WNV transmission. Higher temperatures play an important role in the growth rate of mosquito population, viral replication rate and contact rate between host and mosquitos. Precipitation creates favorable conditions for interaction between mosquito and hosts and affects the larval stage of the mosquitos. The interaction between environmental drivers, reservoir host and mosquito vector are complex. Hence, an integrated approach to surveillance, prevention and control can help mitigate the impact of the virus within Europe and this is further discussed in the review. Vector surveillance of mosquitos can help early identification of WNV circulation. Dead bird and active bird surveillance can provide information about the potential risk of introduction of WNV within an area. Also, active, and passive surveillance of horses can help monitor the virus circulation and ensure prompt identification of outbreaks within the equine and human population. Human syndromic surveillance and routine screening of blood transfusion and organ transplantation is also an effective strategy for WNV surveillance. Regarding control, vector management of both larva and adult mosquito population are efficient control measures. Vaccinations are only currently available for horses, hence more preventive methods against WNV are advised. To conclude, for the different surveillance, prevention, and control strategies to work. It needs to be an integrated approach that is tailored to each country epidemiological situation to optimize efficiency and rational use of resources. This would involve a multisectoral collaboration between veterinarians, epidemiologist, public health workers, public institutions, and other allied professionals to ensure early detection of the virus, appropriate control, and an efficient response during an outbreak.