According to the Centers for Disease Control and Prevention (CDC), as of Sept. 11, 2012, a total of 2,636 cases of West Nile Virus disease have been reported nationally; with every state but two represented.1 Interestingly, approximately two-thirds of the cases have been found in just six states, with Texas topping the list at 45 percent of the total cases. There have been 118 deaths to date and this year's partial statistics surpasses the entire 2011 season during which there were 712 cases and 43 deaths.2
West Nile virus (WNV) was discovered in 1937 in Uganda in eastern Africa and was not found in the United States until the summer of 1999.3 Since then, over 30, 000 individuals in the US have been positively diagnosed with illnesses attributable to the virus.
Jay M. Lieberman, MD, medical director for infectious diseases at Quest Diagnostics and Laboratory Director of Quest's Focus Diagnostics business explains that the West Nile virus is a flavivirus. Like other flaviviruses such as St Louis encephalitis, dengue fever, and yellow fever, its main route of transmission to humans is through mosquitoes (primarily the Culex species) that have acquired the virus from infected birds. Direct human-to-human transmission is not common, although individuals have become infected through blood transfusions and solid organ transplants. Transmission during pregnancy and through breast-feeding has also been reported.
The vast majority of WNV infections are asymptomatic, Lieberman points out. About 20 percent of infections result in a flu-like illness called West Nile fever lasting from 3 to 6 days. The incubation period -- from infection to the onset of symptoms, when present -- is typically 2 to 14 days. Symptoms are generally mild and may include malaise, anorexia, nausea and vomiting, eye pain, headache, muscle pain, rash, and lymphadenopathy. Only about 1 in 150 people infected with WNV develop neuro-invasive West Nile disease, which may include meningitis, encephalitis, flaccid paralysis similar to that found in polio, and other neurologic manifestations.4
For every case of encephalitis, there might well be another 100 to 150 infections that go unrecognized, according to Lieberman. Neurologic West Nile disease is about 20 times as common in infected individuals over 50 years of age as in younger people. Other potential risk factors for neurologic involvement include compromised immunity and coexisting illness like diabetes. Hospitalized individuals have a case fatality rate of 4 percent to 18 percent, with advanced age being the greatest risk factor.5
As symptoms can range from mild to severe and may include neurological involvement, differential diagnosis can be difficult unless there is a clear history of a mosquito bite, or West Nile virus is known to be circulating in the community. Also, as the majority of WNV illnesses are mild and self-limiting, definitive diagnosis is not always necessary. However, in patients with neuro-invasive disease, or in cases where symptoms are severe, diagnosis is important to exclude potentially treatable illnesses, avoid unnecessary therapies and procedures, and for public health reasons.
Laboratory tests include testing for the virus or for antibodies developed post-exposure. Viremia occurs before antibodies appear, but direct viral tests are not usually the most valuable or appropriate strategy. The most common viral test detects WNV RNA in serum and CSF. West Nile viremia peaks before the onset of symptoms and rapidly fades to undetectable levels.
Thus, PCR assays can detect WNV RNA in clinical samples as early as several days before symptom onset, prior to seroconversion. Although these assays have excellent analytical sensitivity, they lack the clinical sensitivity of antibody tests and are typically not used alone for screening or diagnosis. By the time a patient presents with symptoms, says Lieberman, viremia may be very low. Molecular assays may have some utility in patients with delayed or absent antibody response due to immunosuppression or other factors.
The American Red Cross and other major blood suppliers test donor blood to reduce the risk of transmission through transfusion. Vulnerable populations include the very young, the elderly, pregnant women and those who are immune-compromised. The most common test used in this setting is an RNA nucleic acid amplification test (NAAT), similar to the donor tests used for HIV-1 and HCV. The goal in the donor population is to close the window period between infection and the development of antibodies. According to the American Red Cross, since screening started in 2003 there have been nine cases of suspected WNV transmitted through blood donations.6
Best Time to Test
Asked about the applications of the various tests and the best time to test, Lieberman explains further. "Generally, you just collect a specimen when the patient presents." Although viremia is detectable earlier than the immune response, serologic (IgG and IgM) assays are typically more sensitive for detecting both active and convalescent WNV infection. IgM is typically detectable at the time of initial presentation; IgG can be detected as early as seven days after illness onset and within three weeks of exposure in most infected individuals.7 Both assays (IgG and IgM) must be performed on the same specimen to help establish whether or not the infection is recent. For suspected neurologic WNV disease, CSF specimens should be collected at initial presentation. If only serum samples are to be used for diagnosis, paired specimens should be collected during acute illness and again 7 to 14 days later.7
Unlike the relatively brief IgM response to many viral infections, with West Nile virus, IgM in serum generally remains detectable for several months after infection and may even persist beyond 12 months.8 Thus, serum IgM results must be interpreted in light of the patient's clinical condition (e.g., presence of encephalitis or meningitis) and travel history, as well as regional WNV activity.8
A negative IgM result on an acute-phase specimen suggests the absence of an infection, but does not entirely rule one out, Lieberman points out. However, a positive IgM result on an acute-phase specimen, accompanied by clinical symptoms consistent with WNV infection, strongly suggests recent infection. In cases of WNV central nervous system (CNS) infection, IgM is almost always detectable on the first day of clinical illness.
Detection of WNV IgM in CSF strongly suggests acute CNS infection, as IgM does not easily cross the blood-brain barrier.9 Because the MAC-ELISA (IgM antibody capture ELISA) may be cross-reactive with St. Louis encephalitis (SLE) virus, SLE infection should be ruled out if local epidemiology suggests that it is a possibility.
WNV IgG is often detectable as early as 7 days after illness onset and persists indefinitely. Thus, a positive IgG result with a negative IGM result is consistent with past infection. A negative IgG result combined with a positive IgM result in acute-phase specimens suggests recent infection, as does seroconversion from IgG-negative to -positive status from the acute- to convalescent-phase sample.
False-positive WNV antibody results may occur in individuals infected with or recently vaccinated against flaviviruses such as SLE, yellow fever, dengue fever, and Japanese encephalitis, as well as those with previous WNV infection. The population (prevalence) should always be considered when estimating the likelihood of cross-reactivity, Lieberman emphasizes.
Laboratory testing for WNV occurs in reference labs and public health labs rather than the typical hospital laboratory. Focus Diagnostics offered the first commercially available test for WNV in the US and its FDA-cleared test kit is the most common serological test used commercially. As of this date there are no FDA-cleared tests for PCR testing.
Other tests that may be useful in diagnosis and to monitor treatment include a CBC, head CT, head MRI and a lumbar puncture (to obtain CSF).
Treatment is mostly supportive, addressing the symptoms to provide relief. For hospitalized patients the approach might be to give IV fluids, treat symptoms, provide respiratory and nutritional support, and to act so as to prevent the development of secondary infections. There are many illnesses, some unrelated, that share the symptoms associated with WNV infection. Using the wrong treatment can be worthless or even harmful. For example, using antibiotics for this viral infection can be counterproductive and contribute to the burgeoning problem of antibiotic resistance.
The best way to prevent WNV infection is to avoid mosquito bites. Public health officials recommend the use of mosquito repellant products containing DEET, wearing clothing that covers arms and legs while outdoors (especially at dusk when mosquitos tend to swarm) and draining pools of standing water where mosquitos breed.
Lieberman theorizes that this year's outbreak might be due somewhat to a mild winter followed by a hot summer allowing mosquitos to thrive. Whatever the reason, he emphasizes the need for public health vector control coupled with personal precautions, and of course continued testing of donor blood. He also worries that this year's outbreak could be a harbinger of other mosquito-borne illnesses.
Prevention is certainly still the best policy. However, if that does not work, then the laboratorian will be there to provide information so that prompt, appropriate treatment can be initiated.
Glen McDaniel is a healthcare executive, clinical lab scientist, speaker and freelance writer. His interests include mediation, leadership, change and ethics. He can be reached at firstname.lastname@example.org.
Nash D, Mostashari F, Fine A, et al. The outbreak of West Nile virus infection in the New York City area in 1999. N Engl J Med. 2001;344:1807-1814.
Mostashari F, Bunning ML, Kitsutani PT, et al. Epidemic West Nile encephalitis, New York, 1999: results of a household-based seroepidemiological survey. Lancet
Petersen LR, Marfin AA, Gubler DJ. West Nile virus. JAMA
Stramer S. Current Risks of Transfusion-Transmitted Agents- A Review Arch Pathol Lab Med. 2007; 131: 702-707
Lanciotti R. Diagnostic testing in humans -- lessons learned in the past three years. Fourth National Conference on West Nile Virus in the United States. New Orleans, Louisiana. February 9-11, 2003
Roehrig JT, Nash D, Maldin B, et al. Persistence of virus-reactive serum immunoglobulin M antibody in confirmed West Nile virus encephalitis cases. Emerg Infect Dis. 2003;9:376-379.
Petersen LR, Marfin AA. West Nile virus: a primer for the clinician. Ann Intern Med. 2002;137:173-179.