West Nile fever

West Nile fever is a febrile infectious disease. It is triggered by the West Nile virus (WNV), which is transmitted by mosquitoes.

In addition to the many mild or asymptomatic cases, although rare, extremely serious cases involving nerve cell damage have also been recorded.

The West Nile virus was originally only found in tropical regions of the world. As a result, West Nile fever is classed as an “exotic infectious disease”. Since the 1990s, the virus has been increasingly detected in other areas of the world. The first cases of infection with the virus within Germany were recorded in 2019.

West Nile virus infections are often asymptomatic. About one in five infected people develop West Nile fever: a febrile, flu-like condition that usually lasts for three to seven days.

In addition to fever, people can also experience chills, headaches, back ache and fatigue. Other possible symptoms include eye pain, nausea, vomiting and diarrhea.

About half of those infected also develop a patchy, nodular rash (exanthem), which spreads from the upper body to the head, arms and legs and can sometimes be itchy.

The serious form of West Nile fever, which attacks the nerve cells, affects about one in every 100 infected people.

This condition is similar to meningitis or encephalitis and results in neurological symptoms such as:

• muscle weakness
• confusion
• paralysis
• movement disorders
• optic nerve inflammation
• epileptic seizures

West Nile fever is triggered by an infection with the West Nile virus, which is transmitted by mosquitoes.

Mosquitoes primarily transmit the virus between wild birds. However, they can also transmit it from birds to people or other animals, such as horses and dogs.

People and other mammals can become infected but cannot spread the virus. This is because the West Nile virus does not multiply enough in their blood to be absorbed by mosquitoes. Furthermore, the virus is not exhaled and is therefore not transmitted as a droplet infection.

Due to the mosquito season, most West Nile virus infections in Germany occur in late summer and early autumn. In warmer regions of the world, infections are possible over longer periods of time. In southern Europe, for example, mosquitoes transmit the virus right into November.

In rare cases, the West Nile virus can also be transmitted via blood and organ donations, breast milk or during pregnancy.

People over the age of 50 in particular have an increased risk of developing a severe case of West Nile fever.

Other risk factors include:

• cardiovascular diseases
• cancer
• kidney disorders
• diabetes mellitus
• a weak immune system
• alcohol abuse

Originally only found in the tropics, the West Nile virus has now also made its way into Europe. Mosquitoes frequently transmit the virus in southern France, northern Italy and Greece, among other areas.

In 2020, 336 West Nile virus infections were documented in the European Union, 38 of which resulted in fatality.

In 2019, the first West Nile virus infections were registered in Germany. Individual cases were also reported in each of the following years. Experts expect the virus to continue spreading in Germany as the climate conditions are becoming ever more favorable for its survival.

At present, the reported figures for Europe and Germany are still low. However, experts estimate that only just under two percent of all West Nile virus infections are diagnosed and reported. This is because many infected people have no or only mild symptoms and do not seek medical treatment.

Furthermore, doctors in Germany do not generally suspect that febrile, flu-like conditions are West Nile fever and therefore rarely test for it.

Ninety-nine percent of people with West Nile fever have no or only mild symptoms and their nerves are not affected. In these cases, the condition usually goes away by itself without any complications or long-term effects.

Of the people who develop a severe case of West Nile fever with encephalitis, 5 to 10 percent die.

Of those who survive the encephalitis, one in two will experience long-term effects. Potential symptoms include fatigue, weakness, headaches, balance issues and memory problems.

At present, there is no vaccine against the West Nile virus. The most effective way to prevent infection is therefore to ensure good protection against mosquito bites – especially in areas where the virus is widespread and at times when mosquitoes are particularly active.

People can protect themselves against mosquito bites by:

• wearing long-sleeved tops and full-length trousers when outside
• spraying themselves with mosquito repellent containing the active ingredients DEET or icaridin (picaridin) before spending time outdoors
• remaining indoors in the evenings and keeping windows without insect screens closed
• fitting windows and doors with insect screens

In the case of a suspected West Nile virus infection, doctors send a blood or liquor sample to a laboratory for testing. In the field of medicine, liquor (also known as cerebrospinal fluid) is the fluid that surrounds the spinal cord in the spinal canal.

Several laboratory analysis methods can be used to detect the virus’s genetic material or associated antibodies. The virus itself can only be detected in the first few days after infection, whereas the antibodies are not yet detectable at that point – but subsequently remain so for a longer period.

At present, there is no effective medication for treating West Nile fever. As a result, treatment aims to alleviate the symptoms, for example by administering painkillers and antipyretic (fever-reducing) medication such as ibuprofen or paracetamol.

These measures suffice for most people with West Nile fever and they usually fully recover from the condition.

Patients with pronounced neurological symptoms have to be treated in intensive care. With particularly severe cases involving brain damage, mechanical ventilation may also be necessary.

About one in 100 infected people becomes seriously ill with West Nile fever and has to be treated in intensive care.

In these people, the virus attacks and damages the nerve cells. After recovering from the infection, many of them are left with neurological difficulties and deficits, such as walking or concentration problems. To relearn these abilities or to learn to cope with daily life with the limitations, patients experiencing long-term effects can undergo rehabilitation.