West Nile fever

The West Nile virus was first detected in North America in 1999. Since then, West Nile fever has spread around the world

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The West Nile Virus under the electron microscope

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West Nile Fever: The Story of the Discovery

The West Nile virus was first detected in 1937 in a woman with an unclear fever in the West Nile province of Uganda, after which the disease was named. When cases first emerged in North America in 1999, the disease received international attention.

West Nile Virus in the USA: Dead birds in Central Park

A doctor who had worked extensively in the tropics saw the link between bird deaths in New York's Central Park and illnesses in the surrounding metropolitan area.

She informed the military doctors of her suspicion that there might be an outbreak of West Nile fever. That could be confirmed. A mosquito infected with the virus was identified as the vector, which had traveled by plane from Tel Aviv to New York.

In a short time the disease had spread across the entire United States and is considered a prime example of "emerging infections" - diseases that are inexorably spreading and cannot be contained. Since the disease can be transmitted from animals to humans, it is counted among the so-called zoonoses. An infection of horses or birds is notifiable as an animal disease, whereas there is no obligation to notify humans.

The West Nile virus is usually transmitted to humans through a mosquito bite

© istockphoto / Andsem

Causes: how is West Nile fever transmitted?

West Nile virus is the cause of West Nile fever. It belongs to the genus of the Flavi viruses, which also include yellow fever and Japanese encephalitis.

The West Nile virus is transmitted by mosquitoes. So far, the virus has been detected in over 43 different mosquito species, especially those of the "Culex" genus. In most cases, infected birds are the hosts and serve as a reservoir for the virus. They transmit the pathogen to mosquitoes, which act as spreaders (so-called vectors) and infect the final host, i.e. humans or horses, through the bite. Direct blood contact with infected animals can also lead to the transmission of the virus.

The direct transmission route via blood is also an option from person to person. The disease can be transmitted after a blood transfusion with blood from an acutely ill patient. Therefore, the blood of blood donors after their return from risk areas is tested for the presence of a West Nile virus in order to prevent this risk. West Nile virus infections can also occur after organ transplants and infected mothers can pass the virus on to the unborn child or later through breast milk while breastfeeding.

Distribution: Where and how often does West Nile fever occur?

No other mosquito-borne virus is geographically more widespread than the West Nile virus, which has been detected on all five continents. The number of unreported cases of infected people is very high, as the symptoms of the disease are non-specific and are often confused with the flu or other diseases.

In Africa, the cases are increasing particularly in Uganda and Mozambique. West Nile fever is also widespread in Egypt, India, Southeast Asia, Israel and the Middle East.

Since the virus was first detected in America in 1999, it has spread rapidly across the United States and Canada. In 2002 there were over 4,000 registered cases and 263 deaths in the United States. In the last five years there were between 5674 and 2149 cases per year. Between 1999 and 2016, more than 46,000 people fell ill in the United States of whom over 2,000 died. All attempts to contain the virus through the consistent use of pesticides to control mosquitoes have failed.

The virus is also spreading increasingly in Europe. Current maps can be accessed online via the ecdc (European Center for Disease Prevention and Control). Since the beginning of 2019, over 400 WNV cases have been reported within the EU and countries bordering the EU. In September 2019, for the first time in Germany, a person fell ill with meningoencephalitis (inflammation of the brain and meninges), which was caused by the West Nile virus. The carrier was a mosquito. Several infected birds and horses are also known, especially in East Germany. The warm summers of 2018 and 2019 are blamed as the reason for the spread of the West Nile virus north of the Alps.

For 2020, the first WNV infection in Germany was reported on September 3, followed by reports of further infections in the Leipzig, Meissen and Berlin area. As in 2019, it is assumed that it is an infection acquired in this country by mosquitoes.

Symptoms: How does West Nile fever get sick?

In almost 80 percent of cases, the infection goes unnoticed and there are no symptoms. For the remaining 20 percent, the time between virus transmission and the appearance of the first symptoms (the so-called incubation period) is two to 14 days.

The patients suffer from flu-like symptoms with fever, headache and body aches, conjunctivitis, lymph node swelling, diarrhea, vomiting and loss of appetite. After the first attack of fever, the disease may initially improve. Then the fever rises again (biphasic course). Nearly half of the patients develop a rash towards the end of the fever phase that lasts for about a week. Overall, the disease usually lasts about a week and heals without therapy.

In some cases (around one in 150 patients) serious inflammation of the brain (encephalitis) and the meninges (meningitis) as well as acute paralysis symptoms (paralysis) can occur, which only partially or not at all recede. These serious complications can lead to death.

Symptoms of meningitis and meningitis begin, similar to other viral neurological diseases, with fever, headache, stiff neck and photophobia. Confusion and a coma can also occur.

Old people, children and people with weakened immune systems such as cancer or HIV patients are particularly affected. The older the patient, the greater the risk of the disease spreading to the brain and nervous system. Severe courses are observed especially from the age of 50. The mortality rate after encephalitis is five to ten percent. The virus can also affect other organs such as the pancreas, heart or eyes.

After a West Nile virus infection, long-term effects can occur months after the symptoms have subsided. These manifest themselves in fatigue, muscle pain or difficulty concentrating.

Diagnosis: How can West Nile fever be diagnosed?

In the first few days, the disease can only be diagnosed by directly detecting the virus in the blood. This can be done by culturing or by virus genome detection (PCR) in the blood. The immune defense needs time to react to the virus; it does not begin to form special antibodies (antibodies) until about a week after the infection. Detection of these antibodies in blood or cerebrospinal fluid (liquor) is therefore only possible after a few days.

False test results can occur because West Nile virus is similar to other viruses of the same genus (for example dengue or yellow fever virus).

Therapy: How can you treat West Nile fever?

The disease usually resolves on its own in about seven days. There is no specific therapy for treating West Nile fever. All attempts at therapy with common antiviral drugs have so far been unsuccessful. Therapy is therefore limited to relieving symptoms such as fever or body aches. Isolation is not necessary.

If the disease is severe, it makes sense to be monitored in the hospital in order to be able to react quickly if complications arise.

American researchers succeeded in isolating an antibody against the virus from mice. The aim of current studies is to modify this antibody so that it can also be used effectively and safely in humans.

Prevention: How can you protect yourself against West Nile fever?

West Nile fever vaccines are under development, but not yet available. The only prophylaxis is therefore the consistent protection against mosquito bites through appropriate clothing and insect repellent. In areas with large mosquito populations, this protection must be guaranteed all day long, as the transmitting Culex mosquito is also active during the day.

West Nile fever is currently of little importance in European travel medicine. There are few reports of cases acquired through long-distance travel. However, due to the unspecific symptoms, which can easily be mistaken for a flu-like infection, there is likely to be a high number of unreported cases.

If patients suffer from neurological symptoms after long-distance travel, West Nile fever should be considered as the cause.

Dr. med. Markus N. Frühwein

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Consulting expert:

Our author Dr. med. Markus Frühwein, has his own practice in Munich and is director of the Bavarian Society for Immune, Tropical Medicine and Vaccination e.V.

Important NOTE:
This article contains general information only and should not be used for self-diagnosis or self-treatment. He can not substitute a visit at the doctor. Unfortunately, our experts cannot answer individual questions.


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