How West Nile Virus Infections Are Diagnosed

Specialized blood testing is used to diagnose a West Nile virus infection. This testing is aimed either at identifying the virus itself or looking for specific antibodies that have been formed against the West Nile virus.

Specific testing is done in people who are seriously ill with a suspected West Nile infection but is only rarely done in those who have the mild flu-like form of the disease.

Viral Detection

Examining blood or body fluid for the West Nile virus itself is accomplished with the polymerase chain reaction (PCR) test, which can identify actual viral RNA. However, PCR testing is not always useful for diagnosing the virus in humans.

By the time mild symptoms develop, the virus will either be gone or in very low concentrations. For this reason, PCR testing of someone with a milder infection is often negative.

However, in people who develop more severe cases of West Nile fever, the virus is much more likely to still be in the bloodstream when the time illness develops, so PCR testing tends to be more useful.

Also, PCR testing of the cerebrospinal fluid (CSF) is useful in people who have West Nile meningitis or encephalitis, because the virus is often present in the CSF in these individuals.

Antibody Testing

ELISA testing (enzyme-linked immunosorbent assay) can detect the presence of IgM antibodies that the body has made to fight off the West Nile virus.

This test is usually done twice—at the time of acute illness, and then again during the convalescent phase. Rise and fall of IgM antibody levels are usually enough to establish the diagnosis.

Diagnostic testing for West Nile virus is usually done only when it is deemed to be important to make a specific diagnosis.

Routine Lab Testing

While routine blood testing (such as blood counts and serum electrolytes) is done in almost any person who has an acute illness, these tests are not particularly revealing in a person infected with West Nile virus. 

When to Test

The large majority of people infected with the West Nile virus never have specific diagnostic testing—nor do they need it. Most people exposed to the West Nile virus either have no symptoms at all, or they develop a self-limited flu-like illness which they take care of themselves, without consulting medical professionals. 

Because there is no specific treatment for the viruses that cause such illnesses (including West Nile virus), doctors, appropriately, don’t do expensive testing to see which particular virus is causing our “cold.”

There are, however, many cases in which making a specific diagnosis is important. Fundamentally, these are the cases in which:

• The patient is very sick, and there is a risk of prolonged illness, permanent disability, or death. In such cases, doctors will do whatever testing is necessary to make a specific diagnosis. Aggressive diagnostic testing is always required when meningitis or encephalitis is present.
• Making a specific diagnosis can trigger certain public health measures, such as taking steps to reduce the mosquito or tick population, or sending out a health alert to the general population.

In making the correct diagnosis, the doctor should include (in addition to laboratory testing), taking a careful history of recent travel history, and of exposure to mosquito or tick bites. West Nile virus is not known to be spread to humans from ticks, but other similar infections certainly are.

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Potentially serious illnesses which can be confused with West Nile virus infection include:

• Other viruses can also cause meningitis or encephalitis, including herpes simplex encephalitis, varicella-zoster encephalitis, Dengue fever, Powassan virus infection, St. Louis encephalitis, Japanese encephalitis, or encephalitis due to an enterovirus.
• Several tickborne diseases can produce illnesses that may be indistinguishable from West Nile infections, including Rocky Mountain spotted fever, Lyme disease, and ehrlichiosis.
• Bacterial meningitis with pneumococcus or meningococcus can look just like any other meningitis, including meningitis caused by West Nile virus.

Many of these infections require treatment with specific antibiotics. For this reason, it is critical to make a precise diagnosis whenever somebody has a serious illness that might (or might not) turn out to be due to West Nile virus.