West Nile virus: Questions

West Nile virus is transmitted by blood transfusion and organ transplantation, and rarely through breast milk and in pregnancy. Organ transmission is especially dangerous because it can come from donors without detectable viraemia into immunosuppressed recipients, which is why blood services screen donations during transmission seasons.

The other routes are not recognised means of West Nile transmission.

Because IgM does not cross the blood-brain barrier, its presence in cerebrospinal fluid signals infection within the central nervous system. In serum, West Nile IgM can persist for more than a year and cross-reacts with other flaviviruses, so paired sera and neutralisation may be needed.

The other statements misstate the specificity and timing of the assay.

Humans and horses can be infected and can fall ill, but their viraemia is too low and brief to infect feeding mosquitoes, so they cannot sustain the transmission cycle. The bird-mosquito cycle maintains the virus.

They are bitten and are fully susceptible; transfusion is a rare additional route, not the reason they are dead-end hosts.

West Nile virus is a category 3 notifiable medical condition in South Africa, reported through the routine notification system rather than the immediate emergency pathway reserved for the viral haemorrhagic fevers.

It is notifiable in humans and is not confined to International Health Regulations reporting.

Humans and horses develop too little viraemia to infect feeding mosquitoes, so they are dead-end hosts; birds are the amplifying reservoir and Culex mosquitoes the vector. A dead-end host is infected but does not pass the virus onward.

Kunjin virus is the Australian subtype of West Nile virus, classified within lineage 1b, and generally causes milder disease.

It is not a separate virus, a tick-borne agent, the African lineage 2 strain, or a vaccine strain.

West Nile virus cycles between birds, which develop a prolonged high-titre viraemia and act as amplifying hosts, and Culex mosquitoes. Humans and horses are incidental hosts.

Pigs amplify Japanese encephalitis, non-human primates maintain sylvatic yellow fever and Zika, rodents and Ixodes ticks maintain tick-borne encephalitis, and Aedes aegypti drives urban yellow fever and dengue.

West Nile virus injures the anterior horn cells (lower motor neurons) of the spinal cord, producing an asymmetric flaccid paralysis that resembles poliomyelitis and can occur without meningitis or encephalitis.

Cerebellar, sensory-nerve, posterior-column and neuromuscular-junction lesions give different clinical pictures.

West Nile virus sits in the Japanese encephalitis serocomplex, alongside Japanese encephalitis, St Louis encephalitis and Murray Valley encephalitis viruses, which shares its neuroinvasive tendency and drives cross-reactivity in serology.

Dengue, yellow fever, tick-borne encephalitis and the Spondweni group (which contains Zika) are separate serocomplexes.

Management is supportive, with intensive care and ventilatory support for the respiratory failure that follows bulbar or respiratory-muscle involvement. No antiviral has shown benefit in controlled study.

Ribavirin, favipiravir, corticosteroids and monoclonal antibodies remain investigational and are not standard care.

West Nile virus was first isolated in 1937 from a febrile woman in the West Nile district of Uganda. It emerged in New York in 1999, its first appearance in the Western Hemisphere, and spread across North America within a decade.

The other locations are associated with later spread or with related viruses rather than the original isolation.

West Nile neuroinvasive disease can injure the anterior horn cells, producing an asymmetric acute flaccid paralysis that resembles poliomyelitis.

Dengue and Rift Valley fever are chiefly febrile or haemorrhagic; chikungunya and Sindbis are arthritogenic.

Advancing age is the dominant risk factor, with neuroinvasive disease many times commoner in older adults; immunosuppression, especially in organ-transplant recipients, also raises the risk. Susceptibility to infection is similar across ages, so age drives severity rather than acquisition.

The remaining options are not established risk factors for neuroinvasion.

There is no licensed human West Nile vaccine, though effective inactivated equine vaccines are widely used. Human candidates have reached early-phase trials, but the sporadic, scattered nature of human disease has made large efficacy trials impractical.

The other statements describe vaccines that do not exist for human West Nile virus.

Wild birds develop a prolonged, high-titre viraemia and are the amplifying hosts that sustain the bird-mosquito cycle. Horses and humans are dead-end hosts that do not amplify the virus.

Pigs amplify Japanese encephalitis, not West Nile virus, and cattle are not significant amplifying hosts.

Lineage 2 is the endemic African lineage, first characterised on the continent, and drives South African equine and human disease; it has more recently seeded large European outbreaks.

Lineage 1a caused the 1999 New York outbreak and is the most widespread, lineage 1b is the milder Australian Kunjin virus, and the minor lineages are not major causes of southern African disease.

a. West Nile neuroinvasive disease (encephalitis with acute flaccid paralysis), supported by the season, the lymphocytic cerebrospinal fluid and the sick sentinel horses.

b. Advancing age is the dominant risk factor for neuroinvasive disease, with the risk in the elderly many times that of young adults.

c. IgM-capture ELISA on serum and cerebrospinal fluid (IgM in cerebrospinal fluid indicates central nervous system infection), with reverse-transcriptase PCR on cerebrospinal fluid or early serum; paired sera help address IgM persistence and flavivirus cross-reactivity, and samples are screened for other arboviruses.

d. Supportive care, with close attention to bulbar and respiratory function and intensive care with ventilation if respiratory failure develops; no antiviral is of proven benefit.

• Meningitis: an aseptic meningitis with a lymphocytic cerebrospinal-fluid pleocytosis.
• Encephalitis: altered consciousness ranging to coma, often with tremor, myoclonus and parkinsonian features.
• Acute flaccid paralysis: an asymmetric, poliomyelitis-like weakness from anterior horn cell destruction, which may occur without meningitis or encephalitis.

• Persistent IgM: serum IgM can remain detectable for more than a year, so a positive result may reflect past rather than current infection; correlate with the clinical picture and, where possible, demonstrate seroconversion in paired sera.
• Flavivirus cross-reactivity: antibody cross-reacts with other flaviviruses and with yellow fever or Japanese encephalitis vaccination, so paired serology is essential and plaque-reduction neutralisation is used to confirm the specific virus. Specimens are also screened for other co-circulating arboviruses.

A complete answer covers the transmission ecology, the human burden and the tiered laboratory approach with its pitfalls.

Epidemiology

West Nile virus is the most widely distributed flavivirus, maintained in an enzootic cycle between Culex mosquitoes and birds, the birds acting as amplifying hosts through a prolonged high-titre viraemia. Humans and horses are dead-end hosts. Transmission peaks in late summer and early autumn. The virus can also spread by blood transfusion, organ transplantation, breast milk and in pregnancy. Most infection is silent: about a quarter of infections are symptomatic and roughly 1 in 150 develops neuroinvasive disease, the risk rising steeply with age. Lineage 1a caused the 1999 North American emergence, while lineage 2 is endemic in southern Africa.

Laboratory diagnosis

Diagnosis is anchored on serology: IgM-capture ELISA on serum and cerebrospinal fluid, with cerebrospinal-fluid IgM indicating central nervous system infection because IgM does not cross the blood-brain barrier. Two pitfalls must be managed: IgM can persist for over a year, and antibody cross-reacts across flaviviruses and with vaccination, so paired sera and plaque-reduction neutralisation are used to confirm. Reverse-transcriptase PCR and culture on serum or cerebrospinal fluid are useful only in the brief viraemic window (roughly the first six days) and a negative result does not exclude infection; PCR is most valuable in immunocompromised patients with prolonged viraemia and blunted antibody. Specimens are also screened for other co-circulating arboviruses given the overlapping presentations.