West Nile Virus:

Old World Meets New World

West Nile virus was first isolated in 1937 from the blood of a febrile woman living in Omongo, in the West Nile District of the Northern Province of Uganda¹. The clinical features were described in the early 1950s², and a link between West Nile virus and severe central nervous system disease was established by 1957³.

At present West Nile virus is endemic to Africa, the Middle East and southwest Asia⁵ � in these areas many infections are asymptomatic or cause only a mild febrile illness⁴. Except for a small outbreak in the south of France in 1962⁶, West Nile virus did not represent a major health risk for the Western hemisphere.

Recent outbreaks of West Nile virus have occurred in Algeria in 1994², Romania and the lower Danube valley of southwest Europe in 1996-1997 (representing the first major epidemic in Europe⁵), the Democratic Republic of the Congo in 1998, Russia in 1999, Israel in 2000 and the United States in 1999-2001⁷.

The initial outbreak in 1999 (largely localised to New York City environs) is notable in that it represented the first documented introduction of West Nile virus into the New World. It was initially hoped that the virus would peter out over the harsh New York winter � however, virus-positive overwintering mosquitos were subsequently found in underground sewers and abandoned buildings in the city⁸.

Since the initial outbreak West Nile virus has spread slowly across the northeast states of the USA, reaching as far north as New Hampshire and as far south as North Carolina by the summer of 2000⁹. The virus is expected to continue spreading in the summer of 2001, perhaps extending into Canada, the Midwest and the Deep South⁹.

Currently there is no effective therapy against West Nile virus � symptoms can only be alleviated while the patient fights the infection on their own. Human trials for a West Nile virus vaccine are not expected to begin until at least 2002, although the National Institute of Health (NIH) is fast-tracking development of this agent¹⁰.

West Nile virus is a member of the Japanese encephalitis antigenic complex, one of eight serologically defined viral groups (of which only six have human pathogens) in the Genus Flavivirus, family Flaviviridae^4,9. This group includes viruses responsible for Japanese, St. Louis, and Murray Valley encephalitis, Kunjin and other pathogens⁹.

Flaviviruses are comprised of a 30-35nm icosahedral core consisting of nucleocapsid protein (C) surrounding a single-stranded positive-sense RNA of about 11,000 nucleotides^10,2. The capsid is enclosed in an envelope modified by insertion of a non-glycosylated membrane protein (prM) and a glycosylated envelope protein (E)¹¹. E-glycoprotein is the most important structural protein, acting as a haemagglutinin, mediating host cell binding and eliciting most of the virus-neutralising antibodies.

Antigenically, West Nile virus can be divided into two groups¹³_. Lineage 1 viruses are mainly of West African, Middle Eastern, Eastern European and Australian origin, while lineage 2 viruses are those from Africa that have not been involved in mammalian outbreaks but are thought to be maintained in enzootic cycles¹⁴.

However, some studies have indicated that there is considerable heterogeneity among strains isolated within a single region¹⁵. This antigenic variation suggests a pattern of intercontinental exchange of West Nile virus strains, with local segregation of distinct genotypic variants due to differences in ecology¹¹.

West Nile virus has been isolated from more than 40 different species of mosquitoes, the primary vector of the disease^16,17. Of specific note are those of the genus Culex, associated with transmission of related viruses such as St Louis encephalitis virus¹⁶.

In Africa and the Middle East, the primary vector is Cx. univittatus (although some Culex, Aedes and Mimomyia species are important vectors in some areas). In Europe, the main vectors are Cx. pipiens, Cx. modestus and Coquillettidia richiardii, while in Asia Cx. quinquefasciatus, Cx. tritaeniorhynchus and Cx. vishnui are predominant¹⁷.

Isolates from the recent outbreak in New York City and surrounding areas suggest that Cx. pipiens, Cx. restuans and Cx salinarius are mainly responsible for epizootic transmission^16,19. Cx. pipiens and Cx. restuans are generally ornithophilic, while Cx. salinarius feeds readily on mammals suggesting a role as a �bridge vector�¹⁹.

Three Ochlerotatus species (formerly in Aedes genus^20,21) have also been shown to be highly competent laboratory vectors²², however current field data does not support this observation¹⁹. Deviant populations of Oc. japonicus have been described in the United States, and variations in vector competence may explain this discrepancy.

West Nile virus has also been isolated from several species of bird-feeding argasid (soft) or amblyommine (hard) ticks¹⁶ though transmission has only been demonstrated under experimental conditions¹⁷.

Wild birds are considered to be the primary hosts for West Nile virus^{17, 23} and serve as the vertebrate reservoir in the transmission cycle of the virus²⁴. The virus and/or antibodies have been isolated from a variety of wetland and terrestrial avian species¹⁷, including Passeriformes, Columbiformes, Anseriformes and domestic Galliformes²⁵.

High, long-term viraemia sufficient to infect vector mosquitoes has been described in several species of infected birds²⁶, and the virus has been shown to survive in the organs of ducks and pigeons for 20-100 days¹⁷. Hence it is likely that wild migratory birds are the main mode of introduction of West Nile virus into new areas.

The outbreak in 1999 in New York was marked with an extensive mortality rate in birds, mainly American Crows (Corvus brachyrhynchos) that died by the thousands²⁶. Between August and December, the New York State Department of Health received reports of 17,339 dead birds, including 5,697 crows (30%). Laboratory testing of 671 of these birds indicated that 295 had West Nile virus infection²³, and this represents the first outbreak with a recognised substantial avian mortality rate²⁹.

Avian mortality began in New York�s Queens County in June (where a veterinarian noted neurologic illness in some birds with unstable gait²³), moving to the Bronx by July and to birds at the Bronx and Queens Zoos by September²⁸.

Species affected included Chilean Flamingos (Phoenicopterus chilensis), Guanay Cormorants (Phalacrocorax bougainvillea), Bald Eagles (Haliaeetus leucocephalus), Black-Billed Magpies (Pica pica), Bronze-winged Ducks (Anas specularis), Impeyan Pheasants (Lophophorus impeyanus), Blyth�s Tragopans (Tragopan blythi) and Snowy Owls (Nyctea scandia)²⁶.

It is notable that the range of species affected at the Zoos aided identification of the infectious agent as the first pathogen implicated, St. Louis encephalitis, does not cause avian mortality. Furthermore, as the emu flocks were not affected the pathogen wasn�t the other candidate, eastern equine encephalitis²⁸.

West Nile virus has only rarely been isolated from mammals, including Arvicanthis niloticus, Apodemus flavicollis, Clethrionomys glarolus, sentinel mice and hamsters, Lepus europaeus, Rousettus leschenaulti, camels, cattle, horses, dogs and Galago senegalensis¹⁷.

The 1999 outbreak in New York was, however, associated with a number of cases of equine illness in the following year (60 horses from seven states meeting the criteria for a confirmed case)³⁰ and West Nile virus was also isolated from a number of small mammals (bats, rodents, rabbits, cats, racoons and a skunk)³¹.

Mammals (including humans) are probably unimportant in maintaining transmission cycles of West Nile virus as only horses, lemurs and frogs (Rana ridibunda) have sufficient viraemia to support local circulation¹⁷.

West Nile virus is amplified during periods of adult mosquito blood feeding, via transmission between mosquitoes and birds². Infectious mosquitoes carry the virus in their salivary glands, and infect susceptible bird species during blood meal feeding. Competent bird reservoirs sustain an infectious viraemia for 1-4 days after exposure, after which they develop immunity⁴.

Mammals are considered incidental or �dead-end� hosts for West Nile virus (and many other arboviruses)³¹ and are not thought to play a role in the viral transmission cycle¹¹. To date, no cases of human-to-human or animal-to-human transmission of West Nile virus have been reported.

It has been suggested that West Nile virus circulates in Europe in two transmission cycles (sylvan and urban) involving different species and populations of mosquitoes^{25, 32}. The dominant sylvatic (rural) cycle consists of wild, usually wetland birds and ornithophilic mosquitoes while the urban cycle consists of synanthropic/domestic birds and mosquitoes that feed on both birds and humans (i.e. Cx. pipiens/molestus)¹⁷.

Vertical transmission of West Nile virus has been reported in thoracically inoculated Cx. tritaeniorhynchus under laboratory conditions³³, and in Cx. univittatus under field conditions in the Rift Valley province of Kenya¹⁸. Transovarial transmission has been described in Cx. tritaeniorhynchus, Ae. Aegypti and Ae. albopictus though only under laboratory conditions¹⁷.

The most likely mechanism for vertical transmission involves virus entry into the fully formed egg during oviposition � while this is not as efficient as direct ovarian colonisation (as epitomised by members of the Bunyaviridae), it is a source of virus persistence in areas where amplifying hosts are temporarily absent or immune¹⁸.

Migratory birds have been implicated as the introductory hosts of West Nile virus into new regions. Outbreaks in temperate regions have generally occurred in late summer or early fall, coinciding with the arrival of large numbers of migratory birds²⁶. They have also occurred among humans living in/near wetlands³⁴, where high numbers of birds come in contact with high concentrations of ornithophilic mosquitoes.

West Nile virus and antibodies to the virus have been found in the blood of a number of migratory bird species including the Barred Warbler (Sylvia risoria) in Cyprus and the Turtle Dove (Streptopelia turtur)²⁶ in Slovakia. Viraemia of sufficient magnitude and duration to infect vector mosquitoes has been reported in several bird species²⁶ �this may be related to physiologic stresses of migration as these stresses predispose immunosuppression and replication of West Nile virus in rodent models^{26, 27}.

There is a high level of similarity between all of the US West Nile viruses and a strain isolated in Israel from the brain of a dead goose in 1998, differing only by 2 of 1278 nucleotides^14,31. This strain was notably associated with high bird pathogenicity under laboratory conditions¹⁴. Hence it is not unlikely that birds from the Mediterranean region served as the source of introduction via normal migration, displacement from normal range by storms, or importation (legal and/or illegal)²⁶.

There are a few caveats to this suggestion, including the relatively low numbers of migrating birds � concurrence of an infected migrant, ornithophilic mosquitoes and avian amplifying hosts seems improbable²⁶. The likely pattern of migration would also involve coastal regions of West Africa � strains found in the US are significantly different from the West African strain¹⁴ expected in birds from that area.

Bird migration is also potentially implicated in the spread of West Nile virus across North America. Four major routes followed by birds that congregate in the New York City area have been described � the south-eastern US route, the circum-Gulf route, the trans-Gulf route, and the Caribbean island-western North Atlantic route²⁶.

Hence species that may be infected with West Nile virus have the potential to reach every part of the south-eastern United States, Mexico and Central America, the Caribbean Islands, and South America during migration south to wintering sites, and nearly every part of North America during migration north to breeding sites²⁶.

Most West Nile virus infections in humans are generally asymptomatic (particularly in regions where the virus is endemic)^{36, 37}. Incubation period is 5-15 days⁷ and the resulting illness ranges in severity from mild illness to meningoencephalitis.

Disease is typically mild and characterized by fever, headache, backache, generalized myalgia, arthralgia and anorexia^{11, 17}. Other features include pharyngitis, conjunctival infection, respiratory symptoms and gastrointestinal symptoms including nausea, vomiting, diarrhoea and abdominal pain⁷. Diffuse lymphadenopathy is also common.

About 50% of cases develop a rash during or immediately following the febrile phase, and this lasts for 1 week^{7, 11}. This rash may be roseolar or maculopapular, is usually non-pruritic, involves the chest, back and upper limb and generally resolves without desquamation¹¹.

Symptoms tend to last 3-6 days, followed by rapid recovery � children generally have milder symptoms than older patients¹¹. Neurological complications are rare, but may be related to underlying medical conditions (such as hypertension) that assist passage of neurotropic virus across the blood-brain barrier³⁶.

Neurological presentations include aseptic meningitis, meningoencephalitis, anterior myelitis (resembling poliomyelitis), optic neuritis and polyradiculitis⁷. Encephalitis is associated with neck stiffness, vomiting, confusion, disturbed consciousness, somnolence, tremor, abnormal reflexes, convulsions, pareses and coma¹⁷.

Other rare extra-neurological complications include hepatosplenomegaly, myocarditis and pancreatitis^{7, 17}. In the Central African Republic West Nile virus has also been linked to cases of hepatitis, including fatal disease similar to yellow fever¹¹.

In cases of fatal encephalitis, only minimal evidence of viral inflammation are seen in brain tissue, including diffuse inflammation and neuronal degeneration¹¹. In one case there was evidence of scattered microglial nodules, perivascular and perineuronal inflammation, confined to the medulla and cranial nerve roots³⁶.

Disease in horses presents as a fever and diffuse encephalomyelitis (characterised by staggering gait and weakness/paralysis of the hind legs) with a moderate to high mortality rate¹⁷. Other common symptoms reported during the New York outbreak in 1999 include ataxia (sudden/progressive), fever and behaviour changes (somnolence, listlessness, apprehension, depression or periods of hyperexcitability)³⁰.

Infected birds usually do not show any symptoms, although some strains have been associated with increased avian pathogenicity^{17, 14}. Certain species of birds are more susceptible, and present occasionally with encephalitis and death, or long-term virus persistence¹⁷. It is notable that before the outbreak in New York, a local veterinarian noted neurologic illness in some birds with unstable gait as early as July²³.

West Nile virus has a wide distribution through Africa, the Middle East, parts of Europe and the former USSR, India and Indonesia¹¹. Studies performed in the 1950s in the Nile Delta region of Egypt and Sudan indicated that human infections were extremely common, with 22% of children and 61% of young adults immune^{6, 11}.

Recent research in the Nile Delta region suggests that seroprevalence ranges from 6% in children to 40% in young adults with no major demographic differences⁷. The highly endemic nature of the virus in this area has prevented sudden epidemics to some extent, but places a high burden of infection on children who are generally asymptomatic or present with a clinically unremarkable febrile illness¹¹.

As endemic transmission declines, risk of epidemics in these areas may increase due to a larger proportion of the adult and elderly population becoming susceptible to West Nile virus. Migration of individuals from non-endemic regions to areas where West Nile is endemic³⁴, or introduction of virus to areas where the virus is not endemic and local immunity is low³⁶, can precipitate development of epidemics.

Local events can also precipitate development of an epidemic via greater prevalence of the pathogen, such as the construction of dams in Egypt and Mauritania, flooding in Somalia and heavy rain in other regions in 1999 that preceded an outbreak of West Nile among migrants in the Democratic Republic of Congo³⁴.

Due in part to difficulty in identifying all individuals with subclinical infections, the incidence of CNS complications has not been clearly defined¹¹. It is estimated that overt disease occurs in approximately 1 in every 100 infections, but it is unclear how many of these cases develop neurological symptoms³⁶.

During the 1999 New York outbreak older age was associated with a significantly higher risk of developing severe neurologic illness, and age and concurrent diabetes mellitus were significant risk factors for mortality³⁶. The case fatality rate among clinical cases of meningoencephalitis has been estimated at between 4%-13% and is generally highest in the elderly³⁶.

Note that focussing on the most severely ill patients obscures the full spectrum of West Nile disease. An unpublished serosurvey done in Queens during the 1999 New York outbreak estimated that for every hospitalised case of West Nile virus infection, there were 24 mild febrile and 110 subclinical illnesses³⁸.

Enhanced surveillance of birds, mosquitoes, horses and humans is a high priority for areas affected or at high risk of being affected by West Nile virus³⁹. These include states from Maine, New Hampshire and Vermont to Texas along the Atlantic and Gulf coasts, states adjacent to states with current virus activity, Canada and other countries in the Caribbean and Central/South America³⁹.

Unlike similar viruses (such as St. Louis encephalitis), captive sentinel chickens are not a very sensitive predictor of West Nile virus activity. Very few sentinel chickens in New York became seropositive during the summer of 2000, and none of them did so before the first human cases²².

However, �Dead Crow Density� appears to be a relatively consistent marker of human risk⁴⁰. Data from the 2000 outbreak in New York showed that the number of dead crows reported per square mile was as high as 5.9 in Staten Island (where most human cases occurred). In nearby areas, some of which had human cases, density was from 0.1 to 1.5, and it was below 0.1 in all other counties where no human cases occurred⁹.

It is important to note that West Nile virus infection in birds is generally much more geographically widespread than the distribution of human cases and positive mosquito pools³⁶. American Crows also often have extensive home ranges, so the site of death may be distant from the site of infection⁴⁰.

The long-term value of this system is also unclear as natural selection for disease-resistant birds may occur, numbers of susceptible birds may become very low, or the virus may evolve leading to low or no avian mortality. Nevertheless, in areas where the Dead Crow Density is high, warning the public or implementing virus control measures can be considered before development of human cases⁹.

Methods of controlling West Nile virus have largely been focussed on mosquito control. One method is source reduction, aiming to alter or eliminate mosquito larval habitats to prevent breeding of mosquitoes that could potentially harbour the virus⁴⁰. When source reduction and water management are not feasible or have failed, chemicals may be employed to control adult and/or immature mosquito populations.

This has been performed recently in New York City where Staten Island, Queens and Central park were sprayed with the pesticide Anvil (sumithrin and piperonyl butoxide) earlier this month⁴¹. Sumithrin is a synthetic pyrethroid acting as an adulticide, while piperonyl butoxide acts to potentiate the response of sumithrin.

Another method of controlling West Nile virus that has been suggested is the use of biological organisms to control mosquitoes. Possibilities include a larvivorous fish (Gambusia), a predaceous mosquito (Toxorhynchites), predacious copepods, a parasitic nematode (Romanomermis) and the fungus Lagenidium giganteum⁴⁰.

The importance of community/public education cannot be over-emphasised. This may be useful at a number of levels including dead bird surveillance and practical mosquito control � recent data suggests that West Nile virus has become enzootic in the northeastern United States and further outbreaks of disease are not unlikely³⁶.

The emergence and establishment of an Old World virus into the northeastern United States in 1999 shows that with the growing volume of international travel/commerce, exotic pathogens can move between continents with increasing ease³⁶. It shows that �new� viral outbreaks frequently result from changes in the host range, geographic distribution, or ecology of previously known pathogens³¹.

While the exact mechanism of introduction is unknown (and unlikely to be clearly elucidated), it is becoming increasingly clear that the virus has established a firm foothold and will likely continue to spread across North America and may enter Central and South America due to the distribution of migratory birds.

This outbreak emphasises the important relationship that veterinarians, physicians, and the public health system ought to have in the surveillance of disease � the initial identification of the pathogen was a masterpiece of medical deduction involving staff from locations as diverse as the University of California at Irvine, and the Bronx Zoo.

It also highlights the importance of sound clinical medicine � investigation of the pathogen was instigated by the perceptive observations of the chief of infectious disease at the Flushing Hospital Medical Center in Queens, who contacted the New York City Department of Health⁷. Similarly, autopsies were critical in confirming the cause and nature of the West Nile virus cases.

Although West Nile virus generally only causes a few dozen cases a year, federal and state governments are still spending considerable amounts of money on prevention and control. Nevertheless, even though clinical disease in humans is vanishingly rare investment in the public health infrastructure should pay off as other exotic pathogens inevitably make their way to the New World.