West Nile Virus


The information provided below is for readers based in the United States of America. Readers outside of the United States of America should seek the information from local sources.

What is West Nile Virus?

West Nile Virus (WNV) is one of several viruses that can cause disease in humans and is spread by mosquitos. In this case, the mosquitos involved belong to the genus Culex, usually, it is the species Culex pipiens. Usually, WNV does not cause disease, but when it does the condition usually consists of flu-like symptoms and a rash. In a small number of cases, the disease can become very severe and include inflammation of the connective tissue layers that surround and protect the brain (meningitis) and inflammation of the brain itself (encephalitis). Pregnancy is not known to increase the risk of becoming sick from being exposed to WNV as a result of a mosquito bite, but there certainly have been cases of WNV infection coinciding with pregnancy.

How common is WNV?

WNV is fairly rare, but the number of cases occurring each year in the United States increased substantially from 1999 to 2003. Since then, cases have decreased, but there are occasional outbreaks. During the period of 1999 to 2003, certain US states such as California, Texas, Colorado, Arizona, Illinois, New York, Nebraska, the Dakotas, and Louisiana — reported more cases than others. A notorious outbreak occurred in New York City in 1999, for example. Generally, however, outbreaks have occurred in the great plains area of the country. As of November 2017, the US Centers for Disease Control and Prevention reports 1,832 cases of WNV in the US, of which 1,210 (66%) have been what is called neuroinvasive disease, meaning including inflammation of the brain (encephalitis) or of the layers of connective tissue surrounding the brain (meningitis). The remainder (622 cases) were a more mild disease.

How is WNV diagnosed?

If doctors think that you may be infected with WNV, they will draw a sample of cerebrospinal fluid (CSF). To achieve this, a computed tomography scan (CT) of the head is taken first to make sure that the pressure of CSF around the brain and spinal cord is not too high. Then, a procedure called a lumbar puncture is performed, in which a needle is inserted through the back of the lower spine in a very safe way to draw a sample of the fluid that surrounds the spinal cord. The lumbar puncture is performed 3-8 days after the start of symptoms and the resulting CSF sample then is tested to see if it contains antibodies against the WNV. Because there are different classes of antibodies, there are different ways that the test could come out, some of which show an infection in the past, others showing the possibility of either old or new WNV infection. If testing of the CSF sample shows high levels of a type of antibody called IgM against WNV and does not show a type of antibody called IgG against the same virus, this makes it likely that the person has been infected recently with WNV. If the test comes out a different way, more testing will be needed on CSF and/or samples of blood. This testing may involve examination of body cells for changes that are caused by WNV and tests to determine whether particular genes of the virus are turned on to create a molecule called RNA.

Does WNV cause problems during pregnancy?

First of all, 80 percent of people who become infected with WNV do not have any symptoms. Most of the remaining 20 percent develop flu-like symptoms such as fever, headache, and fatigue plus a rash. In some of these cases, fatigue can persist for many months. 1 in 150 people infected with WNV develop not only the flu-like symptoms and rash but also meningitis (inflammation of the layers surrounding the brain) and/or encephalitis (inflammation of the brain).

Does WNV during pregnancy cause problems for the baby?

It is possible for WNV to enter the developing baby from the mother, but the risk of this happening appears to be extremely low. A handful of WNV cases have been reported in newborns, and pregnant women certainly should reduce their risk of getting WNV virus in the first place. This means avoiding mosquitos, applying insect repellent, and wearing protective clothing.

What to consider about taking medications when you are pregnant or breastfeeding:

  • The risks to yourself and your baby if you do not treat the WNV.
  • The risks and benefits of each medication you use when you are pregnant.
  • The risks and benefits of each medication you use when you are breastfeeding.

What should I know about using medication to treat WNV during pregnancy?

There is no specific treatment for WNV, such as an antiviral drug. Thus, treatment is supportive, including medications to combat pain and fever, such as acetaminophen in the United States and paracetamol in several other countries. These drugs are considered safe during pregnancy. Supportive treatment also may include fluids.

You may find Pregistry’s expert reports about the individual medications used to treat West Nile Virus here. Additional information can also be found in the sources listed at the end of this report.

Who should NOT stop taking medication for WNV during pregnancy?

Treatment is mostly supportive, so medications do not play a role, other than combating fever and pain.

What should I know about taking a medication for my WNV when I am breastfeeding?

Pain and fever medications, such as acetaminophen and paracetamol, are thought to be safe for those who are nursing infants. What has concerned mothers in the past is whether insect repellent could affect a nursing infant. The answer to this is that the risk of insect repellent entering breastmilk is less than the risk of the virus itself entering breastmilk, although they are both very unlikely. Thus, there is no reason to avoid medication or to stop breastfeeding. On the other hand, the rationale for using insect repellent is very strong.

What alternative therapies besides medications can I use to treat my WNV during pregnancy?

Treatment for WNV is mostly supportive, meaning that you are monitored and given fluids and other support to keep you from becoming sicker.

What can I do for myself and my baby when I have WNV during pregnancy?

Cooperate with health care providers and accept hospitalization if it is recommended for you. Also keep in mind that WNV infection is preventable, particularly through the avoidance of insect bites. The best way to do this is to wear long sleeves and long pants and to use insect repellent. Furthermore, if you go outside at times of the year when mosquitos are out, it is helpful to seek out locations that practice good mosquito control using nets and chemical measures, including pesticides.

Resources for WNV in pregnancy:

For more information about WNV during and after pregnancy, contact http://www.womenshealth.gov/ (800-994-9662 [TDD: 888-220-5446]) or visit the following link:


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General information

It is very common for women to worry about having a miscarriage or giving birth to a child with a birth defect while they are pregnant. Many decisions that women make about their health during pregnancy are made with these concerns in mind.

For many women these concerns are very real. As many as 1 in 5 pregnancies end in a miscarriage, and 1 in 33 babies are born with a birth defect. These rates are considered the background population risk, which means they do not take into consideration anything about the health of the mom, the medications she is taking, or the family history of the mom or the baby’s dad. A number of different things can increase these risks, including taking certain medications during pregnancy.

It is known that most medications, including over-the-counter medications, taken during pregnancy do get passed on to the baby. Fortunately, most medicines are not harmful to the baby and can be safely taken during pregnancy. But there are some that are known to be harmful to a baby’s normal development and growth, especially when they are taken during certain times of the pregnancy. Because of this, it is important to talk with your doctor or midwife about any medications you are taking, ideally before you even try to get pregnant.

If a doctor other than the one caring for your pregnancy recommends that you start a new medicine while you are pregnant, it is important that you let them know you are pregnant.

If you do need to take a new medication while pregnant, it is important to discuss the possible risks the medicine may pose on your pregnancy with your doctor or midwife. They can help you understand the benefits and the risks of taking the medicine.

Ultimately, the decision to start, stop, or change medications during pregnancy is up to you to make, along with input from your doctor or midwife. If you do take medications during pregnancy, be sure to keep track of all the medications you are taking.

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