The West Nile Virus is transmitted by an unusually large number of mosquito species for Flaviviruses. Possible vectors include a multitude of different mosquito species, in particular Culex, Aedes and Ochlerotatus. ). Within the genus Culex, these are most notably Culex pipiens, C. quinquefasciatus, C. molestus, C. restuans, C. salinarius und C. tarsalis. There is repeated evidence of local mosquitoes transmitting West Nile Fever in Austria. [Kolodziejek J, Seidel B, Jungbauer C, Dimmel K, KolodziejekM, Rudolf I, Hubálek Z, Allerberger F, Nowotny N. West Nile Virus Positive Blood Donation and Subsequent Entomological Investigation, Austria, 2014. PloS One. 2015; 10(5): e0126381. Available at: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4427133/, query from 9th May, 2017].
Culex pipiens, as a parasite vector in birds and humans, is the most important vector in the transmission chain between birds and humans in Austria. The virus can survive the winter in this common mosquito and begins its reproduction cycle inside the insect the following spring. Mosquitoes of the Culex genus spend the entire winter in sheltered spaces, such as basements, barns, tunnels and caves. The West Nile Virus is transferred via a mosquito bite, following an incubation period of 10-15 days in the female mosquito. One of the most important factors for the presence of the virus are higher temperatures: while the virus titre increases only very gradually in the insect at 18 °C, the virus proliferates rapidly at 30 °C and the probability of a transmission is high.
Isolated for the first time in a female patient with fever in Uganda’s West Nile District in 1937, the West Nile Virus was only present in Africa and parts of Asia and Southern Europe before 1999. Additionally, the first cases of encephalitis caused by the West Nile Virus lineage 1 in humans, cats, dogs and horses in North America also appeared in 1999. Since then, the virus has spread across the entire North American continent. It has been isolated in the USA, Canada, Mexico, the Caribbean and in Central and South America, to date.
The dominant West Nile Virus genotype in Europe is lineage 2. There have only been a few larger local outbreaks, in addition to the sporadic infections recorded in Southern, Central and Eastern Europe, such as in Bucharest (393 infections, 17 fatalities) in 1996, in Volgograd (826 infections, 40 fatalities) in 1999 and in Northern Greece (292 infections and 35 fatalities) in 2010.
The Disease: West Nile Fever
About 80 % of infections are asymptomatic. Only 20 % of patients show influenza-like symptoms with sudden, high fever (= mild West Nile fever), muscle pain, headaches, gastrointestinal symptoms, possible cold symptoms, swollen lymph nodes and rashes. The incubation period is 2-8 days. The most prominent symptoms will subside within 7-10 days.
West Nile meningitis or encephalitis may follow in individual cases (0.7 %). Neuroinvasive disorders with disorientation, coordination disorders, swallowing problems, extreme fatigue and dizziness combined with altered behaviour and mental states are all typical signs. These are followed by hepatitis, myocarditis (inflammation of the heart muscles), nephritis (inflammation of the kidneys), pancreatitis (inflammation of the pancreas) and splenomegaly (enlarged spleen), coupled with a long period of recovery. Patients with compromised immune systems and individuals aged over 50 have a higher risk of developing the severe form of the disease. In places where West Nile fever outbreaks have occurred abroad, it has been observed that individuals over 50 years and people who spend large amounts of time outdoors suffered above average infection Levels.