Pathogen and transmission
West Nile fever is a viral disease that occurs primarily in birds. But it can also be transmitted to humans, to other mammals (such as horses, rabbits and squirrels) and to further vertebrates. The West Nile virus is usually transmitted via mosquitoes. In Europe, the Culex pipiens and the Culex modestus mosquitoes seem to play a major role in the virus’s spread. But various further types of Aedes (A.) mosquito are also potential carriers such as A. albopictus, A. japonicus, A. vexans and A. cinereus. All of these mosquitoes are found in Switzerland.
Humans cannot be infected directly with West Nile virus by already infected persons or animals. But cases have been reported in the USA of the virus’s transmission through blood transfusions and organ transplants and from mother to child during pregnancy.
Pathology
Around 80 per cent of persons who are infected with the West Nile virus present no symptoms. But 20 per cent of those infected will develop flu-like symptoms with a high fever and sometimes a rash on the trunk of the body after an incubation period of two to 14 days. The disease will generally pass of its own accord in a few days or weeks. Complications such as encephalitis or meningitis are rare, and only develop in fewer than one per cent of cases (and primarily in older patients). In very rare cases (i.e. 0.1 per cent of those infected), such complications end in death.
There is no specific treatment for West Nile fever. Only the symptoms can be treated.
Distribution and frequency of occurrence
The West Nile virus is found on all continents, and is primarily spread by migratory birds. Various outbreaks of the disease have been recorded since 1950 in Africa, Asia, the Middle East and the Mediterranean region. Such outbreaks have been irregular and of limited duration. By contrast, the importing of the virus to New York in 1999 has led to the virus’s spread throughout the USA. The pathogen has now also further spread and established itself in an area extending from Canada to Venezuela. In more recent times, cases of West Nile fever have also been reported in Europe (in Italy in particular), with several hundred recorded in 2022. The West Nile virus was first detected in native mosquitoes in Switzerland in August 2022.
Switzerland is home to various types of mosquito that are capable of West Nile virus transmission. Since West Nile fever is a zoonosis – a disease that affects both humans and animals – the FOPH works closely with the Federal Food Safety and Veterinary Office (FSVO) in monitoring the virus’s occurrence.
Prevention
No vaccine against West Nile fever is available for human use. So preventive recommendations are centred largely on the usual protection from mosquito bites.
- Actions to deter mosquitoes: avoid having standing or stagnant water on your balcony or in your garden, as these are mosquitoes’ preferred breeding conditions.
- Individual protection: The best way to avoid West Nile virus infection is by taking physical precautions, such as keeping mosquitoes out with mosquito nets and wearing long-sleeved and skin-covering clothing. Chemical preventive measures such as mosquito repellent should not be used around infants of under 12 months; and with small and older children, such products should only be used by an adult and sparingly (avoiding the hands, eyes and mouth). Such sparing application applies to adults, too. Wherever possible, physical prevention should be preferred to the chemical alternatives. Individual protection is particularly advised for older persons and those with a weakened immune system, as such persons have a higher risk (if infected) of developing a more serious case of the disease.
- Contact your GP if you fall ill after being bitten by a mosquito and feel that you need medical attention. All cases of West Nile fever in Switzerland are subject to mandatory reporting.
For further information on the global distribution of West Nile fever and ways and means to avoid infection, consult your GP or a doctor specialising in travel medicine (see also the links below).