West Nile Virus Infection of the Nervous System
Pathogenesis, Immunology, and Clinical Management Douglas J. Lanska
1. PATHOGENESIS
West Nile virus (WNV) is a flavivirus belonging to the Japanese encephalitis subgroup (1,2).
This subgroup also includes the serologically closely related St. Louis encephalitis virus. Fla- viviruses are small single-stranded RNA viruses, with spherical (or more precisely, icosahedral) envelopes between 40 and 50 nm in diameter.
1.1. Epidemiology
1.1.1. Descriptive Epidemiology
WNV was first isolated in 1937 from a febrile woman in the West Nile district of Uganda (3). In the 1950s, outbreaks of a nonfatal encephalitic human WNV infection occurred in the Middle East.
The first major urban outbreaks occurred in Romania in 1996, where there were 17 deaths among 800 cases (4–7), and in Russia in 1999, where there were 40 deaths among more than 800 cases (8).
By the 1990s, WNV was recognized in Africa, Europe, the Middle East, and Asia.
In 1999, an outbreak was identified in New York City, with seven deaths among 62 cases (9–14). The epidemic coincided with the WNV-related deaths of several thousand crows, as well as the deaths of exotic birds at the zoos in the Bronx and Queens (15–17). The viral genome in this epidemic was almost identical to that of a WNV strain identified in Israel in 1998, suggesting that the strain found in New York originated in the Middle East (18,19).
WNV subsequently spread along the east coast and then progressively westward across the entire continental United States (20–26). The 2002 and 2003 US WNV epidemics were the largest arboviral meningoencephalitis epidemics ever documented in the Western hemisphere and the largest WNV meningoencephalitis epidemics ever recorded. In 2002, more than 3587 laboratory- confirmed human cases of WNV infection and 211 deaths were reported, compared with less than 150 for the 3 previous years. In 2003, a total of 8567 laboratory-confirmed cases of WNV infection and 199 deaths were reported. The case-fatality rate among recognized cases was 5.9% in 2002 and 2.3% in 2003. The lower case-fatality rate in 2003 probably reflected greater detection of milder cases, rather than a changing virulence of the virus.
Peak incidence of human WNV infection occurs in late August. The median age of patients with meningoencephalitis is about 60 yr. Approximately 9% of these patients die, and almost all of the deaths occur in people over age 50 yr.
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From: Current Clinical Neurology: Inflammatory Disorders of the Nervous System:
Pathogenesis, Immunology, and Clinical Management
Edited by: A. Minagar and J. S. Alexander © Humana Press Inc., Totowa, NJ
1.1.2. Mosquito-Borne Transmission
WNV is usually transmitted by mosquitoes that have bitten infected birds. At least 36 mosquito species can transmit WNV. The different mosquito species have (a) variable lifecycles and habit requirements; (b) are collectively more common, are less localized, and have less specific breeding requirements than species that transmit LaCrosse encephalitis; (c) may bite at different times of the day; and (d) often move into homes. These factors make WNV difficult to control with mosquito management plans.
Based on mosquito life cycles and escalation of bird infections throughout the summer, human WNV encephalitis occurs in temperate regions primarily in the late summer and early fall. The number of human cases decreases with onset of cooler weather in fall as the mosquitoes die off, even though the virus can survive through winter in mosquitoes (27,28). In warmer climates, WNV can be transmitted by mosquitoes year round.
Although more than 115 species of birds have been infected, crows, blue jays, and ravens seem to be most susceptible to the virus. In addition to crows and jays, Canadian geese, mallards, ring- necked pheasants, and various birds of prey have been infected. Birds of prey are thought to have acquired the virus most often from other birds, rather than through a mosquito vector.
Mosquito and bird surveillance has become an important component of WNV monitoring in the United States. Counts of infected mosquitoes and dead birds are important indicators of risk of human infections, and dead bird counts typically increase prior to identification of human cases (15–17,20,28–32). Horse and other mammal deaths are less useful for surveillance, because they are much less common and less often precede identification of human cases.
The first indicator of WNV activity in a county is usually a WNV-infected dead bird. An index human case is uncommon and occurred in only 4% of affected counties. Of US counties reporting human cases, the first human illness is typically preceded by reports of infected animals by a median of 1 mo.
1.1.3. Other Modes of West Nile Virus Transmission
Documented transmission can also occur through blood transfusions (33–40), organ transplanta- tion (34,36,37,41), breastfeeding (42), and through the placenta (43), although the proportion of cases transmitted by these routes is less than 0.5%. There is no information to suggest that ticks or other vectors have any role transmitting the WNV infection identified in the United States, nor is there evi- dence to suggest that WNV can be transmitted to people by consumption of infected animals.
Twenty-three cases of confirmed transfusion-related WNV transmission were documented in the United States in 2002, compared with just two cases in 2003 (44). Since July 2003, blood-collection agencies in the United States have been using investigational WNV nucleic-acid amplification tests to screen all blood donations and have been quarantining and retrieving potentially infec- tious blood products. Preliminary data indicate that this approach is successful in preventing most cases of WNV transmission through transfusion of blood and blood products. In both of the 2003 cases of transfusion-associated WNV infection, the WNV-contaminated blood had screened negative during initial minipool testing. Later, a retrospective examination of the individual dona- tions comprising these minipools found that two donations contained low levels of WNV. It is currently not feasible to test individually all blood donations in the United States, but individual- donation testing may be considered in areas with a high incidence of WNV infection. In addition, more sensitive methods of minipool testing should be developed. In the meantime, clinicians should continue to investigate cases of WNV infection in people who have received blood trans- fusions and report cases with suspected transfusion-associated illness.
1.1.4. Risk Factors for Human West Nile Virus Infection
Risk factors for human WNV infection include standing water (e.g., flooded basements),
which can support in-home mosquito breeding (7). WNV is most likely to produce encephalitis or
death in those over age 50 yr, as well as those with weakened immune systems, but people of any age can develop severe neurological disease from WNV infection (23,46).
1.1.5. Prevention of West Nile Virus Infection
Prevention efforts are directed at limiting exposure to mosquitoes (30,47). To avoid mosquito bites, adults should apply insect repellant containing no more than 35% of the active ingredient, diethyltoluamide (DEET). Children should use products containing no more than 10% DEET.
Adults and children should consider staying indoors at peak mosquito biting times (dawn, dusk, and early evening) and should wear long-sleeved shirts and long pants when outdoors. Window and door screens should be maintained in good repair. Stagnant or standing water should be elimi- nated around homes to eradicate mosquito egg-laying sites; this includes water in flower pots, buckets, old tires, clogged rain gutters, and bird baths.
Hunters are at greater risk from mosquito bites than from cleaning or eating potentially infected birds. Hunters can minimize their risk by taking precautions to avoid mosquito bites, avoiding shooting or handling sick birds, wearing gloves while handling and cleaning game, and thoroughly cooking any game meat.
1.2. Pathology
1.2.1. West Nile Virus Meningoencephalitis
With WNV meningoencephalitis, the brain may be grossly normal or show evidence of mild cerebral edema (48,49). Histological findings include (a) variable neuronal necrosis in the grey matter with neuron loss, neuronal degeneration, neuronophagia, and microglial and polymorphonuclear leukocytic infiltration; (b) microglial nodules composed primarily of lymphocytes and histiocytes and presenting predominantly in the grey matter; (c) variable mononuclear perivascular inflammation (perivascular cuffing); (d) scattered mononuclear leptomeningeal infiltrates; and (e) focal mononu- clear inflammation of the cranial nerve roots, especially in the medulla (11,48,50–53).
The pathological changes in the central nervous system (CNS) result from viral replication in neurons and glia, as well as a cytotoxic immune response to infected cells (54,55). CD8 T- lymphocytes predominate over CD4 lymphoctyes in the microglial nodules, perivascular infil- trates, and meningeal and cranial nerve infiltrates (55). B lymphocytes are found primarily in areas of perivascular inflammation (55).
1.2.2. West Nile Virus Poliomyelitis
Early case reports and case series generally attributed the flaccid paralysis accompanying WNV infection to an inflammatory neuropathy similar to Guillain-Barre syndrome (GBS) (56,57). Most of these initial reports did not have supporting electrophysiological studies, and none reported spinal cord pathology. GBS cannot explain the clinical, laboratory, and electrophys- iologic abnormalities in most of these cases. GBS generally presents with symmetric weakness, is frequently accompanied by sensory abnormalities or paresthesias, is associated with elevated cerebrospinal fluid (CSF) protein, but without CSF pleocytosis, and has associated electrophysio- logical findings that are consistent with a predominantly demyelinating neuropathy.
The clinical, laboratory, and electrophysiological abnormalities in some patients with asym-
metric flaccid paralysis result from a polio-like syndrome with involvement of spinal cord ante-
rior horn cells and motor nerve axons. Cases of poliomyelitis in patients with WNV infection
have been identified in the United States since July 2002 (34,49,50,58–63). Two previous reports
also suggested that the acute flaccid paralysis of WNV infection resulted from myelitis, with elec-
trophysiological support in one case, but without pathologic support in either (64,65). Subsequent
reports have supported these findings (50,51,53,66) and documented an overlapping spectrum of
meningitis, encephalitis, and myeloradiculitis. Pathological findings have included loss of ante-
rior-horn neurons, accompanied by gliosis, macrophages, neuronophagia, microglial nodules,
chronic perivascular inflammation, and mild lymphocytic infiltration of anterior horn roots (49,51,53,67). In addition, a polio-like syndrome has been previously reported with Japanese encephalitis virus, a flavivirus closely related to WNV (68,69), and with tick-borne flavivirus infections in continental Europe, including Far Eastern tick-borne encephalitis and Central Euro- pean encephalitis (70). Moreover, pathological studies in animals have demonstrated lesions of the ventral spinal cord grey matter and the spinal motor neurons, with an absence of peripheral nerve lesions, in birds, horses, and nonhuman primates infected with WNV (71–73).
A few recent studies have suggested an alternative or concomitant mechanism for some cases of acute flaccid paralysis associated with WNV (i.e., acute anterior radiculitis) (61,62). Previous electrophysiological studies have, in fact, localized the abnormality to either the anterior horn or the ventral nerve roots. Magnetic resonance imaging (MRI) studies of cases of polio-virus poliomyelitis have shown increased signals in the anterior horn, whereas some cases of WNV- associated flaccid paralysis have instead demonstrated intradural nerve-root enhancement (61).
2. IMMUNOLOGY 2.1. Viral Proteins
Flavivirus structural proteins are now designated E (envelope), C (core), and M (membrane-like), replacing older terminology of V3, V2, and V1, respectively (74).
The E protein is the major constituent of the viral surface and is oriented parallel to the viral sur- face (75,76). It is a class II viral fusion protein that mediates both the binding of WNV to target-cell receptors and the entry of WNV into target cells (75,77,78). The E protein differs structurally from the spiky projections of class I viral fusion proteins seen in orthomyxoviruses, paramyxoviruses, retro- viruses, and filoviruses (76). Also, unlike class I viral fusion proteins, the E protein itself is not prote- olytically cleaved for activation but instead requires cleavage of an accessory protein (76). Fusion in WNV and other viruses (e.g., alphaviruses) that use class II fusion proteins is faster and less temperature-dependent than fusion in viruses with class I fusion proteins (76).
Each E protein is folded into three domains:
1. an antigenic domain that carries the N-glycosylation site,
2. a domain responsible for pH-dependent fusion of the E protein to the endosomal membrane during uncoating,
3. a domain important for binding to target cells and postulated to contain the receptor-binding site (77).
2.2. Target Cell Binding, Viral Entry, and Replication
Arboviruses are inoculated directly into the bloodstream or subcutaneous tissue. Initial WNV replication occurs in the skin and regional lymph nodes, followed by a primary viremia that seeds the reticuloendothelial system and a secondary viremia, following replication in the reticuloen- dothelial system (54,55). Depending on a number of host factors, including the integrity of the blood–brain barrier (BBB), the CNS may be seeded during the secondary viremia (55).
On the other hand, flaviviruses bind to a specific cell-surface protein and then enter target cells by a vesicle-mediated process (i.e., so-called receptor-mediated endocytosis) (54,77,79,80).
Because of the wide host range of natural WNV transmission, the cell-surface protein target is likely to be highly conserved across different species (80). This target-cell-receptor molecule is not fully characterized, but recent work suggests that it is a 105-kDa plasma membrane-associated glycoprotein (77). Although not absolutely required, cholesterol in the target cell membrane significantly facilitates viral binding (76).
Irreversible conformational changes in the E protein and viral uncoating occurs within the
endocytic vacuoles at an acidic pH, after which the viral genome is released into the cytoplasm for
replication (54,77,79). Virus assembly takes place in the endoplasmic reticulum (75). Initially, the
E protein forms a stable heterodimeric complex with the precursor of the M protein. The precur-
sor M protein is cleaved by a protease in the Golgi system to generate fusion-competent mature
infectious virions (75).
2.3. Host Factors
Host factors can influence the ability of viruses to enter the CNS (81). WNV and other arboviruses spread hematogenously and must cross the BBB. Penetration of WNV into the CNS depends heavily on the degree and persistence of viremia (54) but is facilitated by altered BBB permeability, as from hypercarbia, hyperosmotic agents, and mechanical breach (81). Animal studies have suggested the coincident Gram-negative bacterial infections can induce WNV to invade the CNS, leading to markedly increased rates of encephalitis and death (81); this effect is apparently caused by endotoxin-stimulated release of various cytokines and secondary changes in the integrity of the BBB (81).
Virus-specific antibodies and cytotoxic T cells are important for the clearance of flaviviruses (55,80). Available evidence suggests that humoral immunity, in particular, protects against WNV infection and severe WNV-related disease (82,83). Furthermore, preliminary studies have sug- gested that patients with WNV encephalitis may benefit from intravenous immunoglobulin (Ig) from donors who have had a high frequency of WNV exposure (84–86). WNV-specific anti- body in the CSF may decrease viral replication by interfering with viral attachment to receptors on the cell surface or by preventing endosomal fusion (55). Little is known about a specific cell-mediated immune response to WNV (55).
2.4. Persistent Infection
Several studies in animals have suggested that WNV may produce persistent infection in the CNS (87–89). The pathophysiological mechanisms of this phenomenon are still poorly understood.
2.5. Vaccines
Previous infection with WNV is believed to confer lifelong immunity. Immunization of exper- imental animals with heterologous flaviviruses (i.e., Japanese encephalitis virus, St. Louis encephalitis virus, yellow fever virus) reduces the severity of subsequent WNV infection (90);
however, protective neutralizing antibodies to WNV have not been identified in human subjects following vaccination with Japanese encephalitis or dengue vaccines (91).
WNV vaccines are in development but are not expected to be available for several years. Kun- jin virus, an Australian flavivirus, is an attractive WNV vaccine candidate because it is closely antigenically related to WNV but is less virulent (92). Mice immunized with a plasmid DNA vac- cine coding for the full-length infectious Kunjin virus RNA did not develop clinical disease but did develop neutralizing antibodies and were protected against wild-type Kunjin virus and other- wise lethal doses of virulent WNV (92).
Another approach is the use of molecularly engineered live-attenuated chimeric virus vaccines, with the pre-M (membrane precursor) and E (envelope) protein genes of WNV on a backbone of dengue virus (93). A further modification of this chimeric virus vaccine was accomplished by a 30-nucleotide deletion mutation in the 3´ noncoding region of the dengue virus backbone. Such chimeric viruses induced moderate-to-high titers of neutralizing antibodies and prevented viremia in monkeys challenged with WNV (93).
3. CLINICAL MANAGEMENT 3.1. Clinical Manifestations
Most people who have been bitten by WNV-infected mosquitoes have no symptoms or only mild ones. Approximately 20% of infected individuals develop a mild illness resembling the flu.
Mild illness is referred to as West Nile fever. Symptoms of mild illness can include sudden onset of fever with malaise, anorexia, nausea, vomiting, diarrhea, headache, photophobia, neck pain and stiffness, myalgias, rash, and lymphadenopathy (4,94–97).
Severe illness can occur in individuals of all ages, but only 1 out of every 150 infected individuals
develops severe disease. Severe neurologic illness may include encephalitis, meningoencephalitis, or
poliomyelitis. Symptoms of severe disease can include high fever; headache; nuchal rigidity; con- fusion and disorientation; severe muscle weakness; or paralysis, cranial nerve palsies, tremors or other abnormal movements, sensory deficits, and seizures (4,14,97–100). Some patients may develop cerebral edema, aphasia, ataxia, tremor, myoclonus, parkinsonism, cranial neuropathies, dysarthria, breathing difficulties, myocarditis, pancreatitis, or hepatitis (14,52,99,101–103).
Cases of poliomyelitis in patients with WNV infection have been identified since July 2002 (34,58,59,66). Patients were generally admitted to hospital with a 1- to 4-d history of symptoms, including fever, chills, vomiting, headache, fatigue, lethargy, confusion, myalgias, and facial and acute asymmetric painless limb weakness. Physical examination demonstrated asymmetric hyporeflexic or areflexic weakness of various extremities, generally with intact sensation. Sev- eral patients developed bladder dysfunction and acute respiratory distress requiring ventilatory support.
3.2. Differential Diagnosis
The differential diagnosis includes stroke, GBS, polyradiculitis, meningoencephaloradiculitis, encephalitis (including other arboviral encephalitises), myelitis, poliomyelitis, meningitis, and postvi- ral demyelination. WNV is now the most common cause of arbovirus encephalitis and meningoen- cephaloradiculitis in the United States but is not a common cause of GBS, polyradiculitis, myelitis, meningitis, or postviral demyelination. Even in areas of recognized WNV infection in animals, most human cases of aseptic meningitis are caused by enteroviruses and not WNV (104).
3.3. Diagnostic Workup 3.3.1. Clinical Suspicion
WNV or another arbovirus (e.g., St. Louis encephalitis virus) infection should be strongly consid- ered in patients with unexplained encephalitis, meningitis, or poliomyelitis in late summer or early fall, particularly in adults 50 yr or older (97). The local presence of other human cases or documented WNV infection in animals (e.g., mosquitoes, birds, horses) should further raise clinical suspicion.
3.3.2. Diagnostic Studies
Serological testing for WNV can be problematic for several reasons, including crossreactiv- ity between WNV and other flaviviruses, and persistence of IgM antibodies. False-positive results can occur with WNV testing because of exposure to St. Louis encephalitis virus or dengue virus or because of previous vaccination for yellow fever or Japanese encephalitis (20,90,97). Also, WNV IgM antibodies can persist for more than 1 yr, potentially producing confusion regarding whether the antibodies are a marker of current or previous infection (20,97). In the latter case, an increase in WNV-specific neutralizing antibody titer in acute and convalescent serum can confirm acute infection (97). Furthermore, some immunocompromised patients may never make antibodies.
Serum and CSF should be obtained for assay of IgM and IgG antibodies to WNV using enzyme- linked immunosorbent assays (ELISA) (20,30,97,105). Ideally, serologic testing should be per- formed for both WNV and St. Louis encephalitis virus. Positive ELISA results should be confirmed with the more specific plaque reduction neutralization test, which may take 10 d. If CSF is not obtained, paired acute- and convalescent-phase serum samples should be obtained, with the initial specimen obtained during the acute illness and the subsequent specimen obtained 7 to 14 d later.
Other laboratory findings include variable peripheral blood leukocytosis, lymphocytopenia, anemia, and hyponatremia, particularly in patients with encephalitis (86). Stool cultures and poly- merase chain reaction (PCR) studies for enterovirus are negative.
CSF analysis typically shows a lymphocytic pleocytosis, an elevated protein level, and a normal
glucose level (97). Because IgM antibody does not cross the BBB, WNV-specific IgM antibody in
CSF strongly suggests CNS infection (20,97). CSF should also be cultured and analyzed by PCR
techniques to detect WNV nucleic acid; however, because isolation and PCR testing are relatively insensitive, negative results do not exclude WNV infection (20,97).
Computed tomography of the brain is usually normal but may show evidence of hydrocephalus and subependymal edema (52). T1- and T2-weighted MRI of the brain is also usually normal but may show hydrocephalus, basal ganglia, and deep white-matter edema. T2-weighted images may show petechial hemorrhages, symmetric areas of hyperintensity in the thalami, corticospinal tracts, hippocampi, cerebellum, and substantia nigra, as well as enhancement of the lep- tomeninges, the periventricular areas, or both (52,86,97,106). Diffusion-weighted MRI imaging may be more sensitive, with early changes evident on apparent diffusion coefficient maps prior to evidence of enhancement on postgadolinium T1-weighted images (106).
In patients with WNV poliomyelitis, electromyogram (EMG) and nerve conduction studies indicate a severe, asymmetric process affecting anterior horn cells, their axons, or both. Sensory amplitudes, motor distal latencies, and conduction velocities are normal. Motor amplitudes are typically 25 to 50% of normal. Recruitment is severely reduced with normal appearing motor units on EMG. Spontaneous activity is profuse 2 wk after onset of illness.
3.3.3. Centers for Disease Control and Prevention Case Classification for WNV Encephalitis and Meningitis
The US Centers for Disease Control and Prevention has published a case definition for arboviral encephalitis or meningitis that is applicable to WNV encephalitis and meningitis (Table 1) (107).
There is presently no similar case definition for WNV poliomyelitis, although the criteria are easily modified to include poliomyelitis in the same line with encephalitis and meningitis.
3.4. Prognosis and Complications
Symptoms of mild illness generally last from several days to 1 wk. Symptoms of severe dis- ease can last for weeks and, in some cases, neurologic effects can be permanent (108). Only about one-third of people who develop WNV encephalitis or meningitis are fully recovered after 12 mo.
Between 3 and 15% of patients who develop severe illness ultimately die from it, with most deaths reported in those over age 50 yr.
Table 1
Case Definition for Arboviral Encephalitis or Meningitis Probable WNV encephalitis or meningitis
Encephalitis or meningitis
Occurring when WNV transmission is likely (late summer or early fall) Supportive serology
• Single or stable (twofold change) elevated titer of WNV-specific serum antibodies; or
• Serum IgM antibodies detected by antibody-capture ELISA but with no available confirmatory results from WNV-specific serum IgG antibodies
Confirmed WNV encephalitis or meningitis Encephalitis or meningitis
Laboratory confirmation
• Fourfold or greater increase in WNV-specific serum antibody titer; or
• Isolation of WNV or demonstration of WNV-specific antigen or genomic sequences in tissue, blood, CSF, or other body fluid; or
• WNV-specific IgM antibodies demonstrated in CSF by ELISA; or
• WNV-specific serum IgM antibodies (demonstrated by ELISA) confirmed by WNV-specific IgG antibodies (demonstrated by neutralization or hemagglutination inhibition).
WNV, West Nile virus, Ig, immunoglobulin; ELISA, enzyme-linked immunosorbent assay.
3.5. Treatment
Although there is no specific treatment for WNV infection, patients with symptoms such as high fever, confusion, headaches, or muscle weakness should seek medical attention immediately.
Intensive supportive care is sometimes needed and may include hospitalization, intravenous fluids, breathing support with a ventilator, and good nursing care (105). Some preliminary studies have suggested that patients with WNV encephalitis may benefit from intravenous immunoglobulin from donors with a high frequency of WNV exposure (84,85). High-dose ribavirin and interferon α2b have some in vitro activity against WNV, but there are no controlled trials of these drugs in people.
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CME QUESTIONS
1. West Nile virus is transmitted by which of the following vectors?
A. Fleas B. Flies C. Mosquitoes D. Ticks
2. Which is not an extablished mechanism for West Nile virus transmission?
A. Blood transfusion B. Freast-feeding
C. Eating infected animals
D. Organ transplantation3. Which component of the West Nile virus is responsible for target cell binding?
A. C protein B. E protein C. M protein
D. Class I fusion protein
4. Which of the following is least likely to be the presentation of West Nile virus infection?
A. Aseptic meningitis B. Encephalitis C. Febrile illness D. Poliomyelitis
