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Introduction and General Information

Diagnosis, reporting and treatment of individuals with WNV infection relies to a large extent on the efforts of medical and veterinary personnel working in general practices, referral centres and hospitals.

The following summary information is designed to assist health professionals in accessing the information that they need in order to make decisions regarding an individual patient. Full detailed information is provided on the West Nile Virus Disease and Flaviviridae - West Nile Virus pages, the literature report pages linked to those pages and the appropriate "Techniques" pages which are accessible from the "Managing for West Nile Virus Infection" flowchart. 

In order for WNV infection to be diagnosed correctly in a timely manner it is necessary for health (medical and veterinary) professionals to have an awareness of the disease and for a system to be in place for sample processing. A communication system should also be present to allow information regarding the occurrence of the disease to be communicated to the appropriate authorities and, when appropriate, to increase the vigilance of health care professionals, whether in the medical or veterinary sphere.

See: Education and Communication for West Nile Virus - Education of Health (Medical and Veterinary) Personnel 

General information:

  • West Nile virus is an arbovirus transmitted mainly by mosquitoes (Culicidae - Mosquitoes (Family)). Infection therefore is likely to be seen during the time of year when mosquitoes are most active. In temperate climates this is usually over the summer and autumn.
  • For most individuals, WNV infection is asymptomatic or so mild that illness is not recognised or reported.
  • Clinical cases of WNV infection are seen sporadically.
  • Even during an epidemic, large clusters of clinical cases are relatively rare (notable clusters have occurred in some epidemics, including in North America since 1999].
  • No signs or symptoms are pathognomonic for this disease, therefore definitive diagnosis requires laboratory testing.
  • A blood or CSF sample taken early in the course of the disease may contain detectable virus.
  • More commonly, diagnosis is based on the presence of IgM antibodies and/or on a rising titre of antibodies between two paired serum samples (in the absence of recent vaccination); paired samples should be tested at the same time.
  • In order for the results of serological testing to be meaningful it is important for the correct data to accompany the sample(s). This includes information such as when the sample was taken and how long after the onset of clinical signs the sample was taken.
  • Not all serological tests can distinguish between infection with WN virus and infection with other closely related viruses.
  • Some serological tests require species-specific reagents and therefore cannot be used for testing samples from many different species.
  • Treatment is symptomatic. For individuals with nervous signs due to WNV infection treatment is the same as for other viral encephalitides.
  • Immune serum or plasma may be useful in the treatment of individuals with neurological disease.
  • Necropsy (post mortem examination) may or may not reveal gross signs. In some individuals in which nervous signs were seen signs of trauma will be found due to e.g. ataxia and recumbency.
  • Histological lesions are normally limited to the central nervous system but sometimes in birds lesions may be seen in other organs also, such as the pancreas or liver.
  • Vaccines are presently not available for humans, although three horse (Equus caballus - Domestic horse) vaccines have been licensed for use in the USA and Canada and are commercially available [May 2008], a bird vaccine has been used in Israel and vaccines have been used in birds in North America.

For further information see:

Published Guidelines linked in Wildpro

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Clinical WNV Infection in Humans

Clinical Syndromes which may be seen with West Nile Virus Infection in Humans

WNV infection may present with a variety of syndromes, particularly West Nile Fever and West Nile Virus Encephalitis. Further information regarding the presentation of the different syndromes is presented below.

  • The majority of cases of human infection with WN virus are asymptomatic. 
  • Clinical signs are often mild and undiagnosed. 
  • Severe clinical disease with central nervous signs may be more common in the elderly.
  • Severe clinical signs (WNV encephalitis) may have been more common in recent outbreaks, whereas West Nile Fever without severe CNS signs was reported more commonly in earlier outbreaks

West Nile Virus Encephalitis and/or aseptic meningitis:-

  • "Headache, high fever, neck stiffness, stupor, disorientation, coma, tremors, convulsions, muscle weakness, paralysis, and, rarely, death." (W170.Nov01.WNV2)
  • Additional signs and symptoms seen commonly include fatigue, nausea, vomiting, chills, photophobia and myalgia.(J84.7.w9, J98.352.w1)
  • Profound muscle weakness has been a notable feature of some patients.
  • Typical arboviral encephalitis::
    • Symptoms of "fever, malaise, sore throat, nausea and vomiting, lethargy, stupor, coma and convulsions", with signs including "stiff neck, signs of meningeal irritation, tremors, convulsions, cranial nerve palsies, paralysis of extremities, exaggerated deep tendon reflexes, absent superficial reflexes, and pathological reflexes." (B252.32.w32)
    • Patients may present with abrupt onset fever, convulsions and other signs of central nervous system involvement. More typically, non-specific symptoms such as fever, abdominal pain, vertigo, sore throat and respiratory symptoms progress quickly to headache, meningeal signs, photophobia and vomiting. Lethargy, somnolence and intelligence deficits may indicate involvement of deeper structures while more severely affected patients will be obviously disoriented and may be comatose. Common signs include tremors, loss of abdominal reflexes, cranial nerve palsies, hemiparesis, monoparesis, difficulties swallowing and frontal lobe signs. Convulsions and focal signs may be present early in the course of the disease or develop later. (B251.198.w198)
    • "Headache, drowsiness, fever, vomiting and stiff neck are the usual presenting symptoms. Tremors, mental confusion, convulsions and coma may develop rapidly. Paralysis of the extremities occasionally occurs." (B247)
  • Severe WNV infection in the USA 1999-2002: Among patients hospitalised with severe disease, fever, weakness, gastrointestinal symptoms and a change in mental status were seen. Severe muscle weakness and flaccid paralysis were features of several cases. A rash (maculopapular or morbilliform) affecting the neck, trunk, arm or legs was seen in only a minority of patients. Neurological presentations recorded included ataxia and extrapyramidal signs, cranial nerve abnormalities, myelitis, optic neuritis, polyradiculitis and seizures. (W170.Aug02.WNV1)

West Nile Fever:-

  • Fever and headache, usually with rash (often maculopapular and mainly on the trunk) and/or lymphadenopathy. May be accompanied by a variety of other signs and symptoms including:
    • gastrointestinal signs (nausea, vomiting, loss of appetite, diarrhoea), 
    • abdominal pain,
    • arthralgia
    • muscle pains,
    • backache,
    • chills,
    • sore throat, 
    • flushed face,
    • weakness, 
    • ocular pain,
    • congested conjunctiva,
    • splenomegaly
    • hepatomegaly
  • (J84.5.w2, J91.5.w1, J101.59.w1, J101.64.w1, J123.31.w1, J129.42.w1, B241.49.w49, B242.w1, B243.31.w1, P31.6.w1)

Hepatitis (rare but may be fatal). (B251.198.w198)

Severe pancreatitis (rare, although acute pancreatitis may be responsible for the acute abdominal pain sometimes seen with West Nile fever). (B251.198.w198)

Further information on clinical signs is provided in:

CDC case definitions for arboviral meningitis and for West Nile fever are provided in: 

Clinical Pathology in West Nile Virus Infection in Humans

  • Clinical pathological findings such as biochemistry and haematology are non-specific. There may be a mild leucopenia with relative lymphocytosis and a left shift.
  • CSF samples in patients with nervous signs may reveal pleocytosis and raised protein levels, but again these findings are non-specific; they may suggest a viral aetiology.

Further information is provided in: 

Pathological findings in West Nile Virus Infection in Humans

  • Gross lesions are not always evident at necropsy.
  • Histopathological findings in the CNS (brain and spinal cord) may include diffuse inflammation, small haemorrhages, perivascular cuffing, neuronal degeneration. (B240.14.w14, B244.w1)
    • Histopathology does not allow definitive diagnosis as similar findings may be seen with other arboviral encephalitides.(B245.29.w29, B253.6.w6)
    • Immunohistochemical staining or RT-PCR may allow definitive diagnosis of WNV infection.(J84.6.w3, J84.6.w4)

Further information is provided in: 

Differential Diagnoses in Humans

For West Nile Fever:

  • Typhus fever, rubella, leptospirosis, infectious mononucleosis, dengue, measles,  exanthemata due to ECHO and Coxsackie viruses, sandfly fever, chikungunya and o'nyong-nyong fever and other causes of fever  (P31.6.w1, J101.64.w1, B240.14.w14, J129.42.w1)

For West Nile encephalitis, meningitis or meningoencephalitis:

  • Other arboviral encephalitides: encephalitis due to WNV infection is clinically indistinguishable from other viral encephalitides such as Eastern Equine Encephalitis (EEE), Western Equine Encephalitis (WEE), Venezuelan Equine Encephalitis (VEE), LaCross Virus Encephalitis (LAC), Japanese encephalitis (JE). (B245.29.w29, B251.198.w198, B252.32.w32, B253.6.w6)
  • Other causes of meningitis and encephalitis: 
    • Bacterial meningitis
    • Other viral causes of meningitis
    • Protozoal meningitis
  • Herpes simples encephalitis, brain abscess, bacterial meningitis (differentiated by serial CSF examination), leptospirosis, neurosyphilis, Lyme disease, cat-scratch fever. (B251.198.w198)
  • Arboviral encephalitides may be distinguished from other causes of meningitis and encephalitis by:
    • CSF findings (generally pleocytosis (mononuclear cells generally predominate, but there may be more polymorphonuclear leucocytes initially) and raised protein levels but normal glucose). (B247)

    Further information is provided in West Nile Virus Disease - Literature Reports for Similar Diseases (Disease Reports)

Samples to be taken for Definitive Diagnosis of West Nile Virus Infection in Humans

  • Serum is generally used for detection of specific antibodies.
    • Paired acute and convalescent samples are required to demonstrate a rise in antibodies. 
    • Serum may also be tested for the presence of WN virus but this is rarely detected.
  • Cerebrospinal fluid may be tested for virus and for antibodies in individuals with signs of central nervous system involvement.
  • In fatal cases samples of brain, spinal cord, pancreas, liver, heart, lung, spleen and kidney may be taken for histopathological examination and special tests such as immunohistochemical staining or RT-PCR..

Further information is provided in: 

Details of samples required, shipping and handling conditions should be obtained prior to sending specimens to a laboratory for testing. Details of requirements for human samples sent to CDC are provided in: West Nile Virus - Detection and Identification Techniques (Viral Reports) - Specimen Sampling & Shipping

For reporting persons with suspected WNV infection and for submitting samples for diagnosis, local procedures should be followed. Links to state and local websites in the USA are provided on the CDC website (Links: State and Local Government Sites - W170.Aug02.WNV2).

Treatment of West Nile Virus Infection in Humans


  • There is no specific antiviral treatment available.
  • Treatment is supportive.

Further information is provided in:

Prognosis / Long-Term Outcome following West Nile Virus Infection in Humans

  • Convalescence may last one to several weeks and is generally longer in adults than in children.
  • Following infection with central nervous signs convalescence may be prolonged.
  • Long term sequelae may be seen following infection with central nervous signs. (W27.13Nov01.wnv1)

Further information is provided in:

Prevention of West Nile Virus Infection in Humans

  • There is no vaccine available for human use at the present time (May 2008).
  • Personal protective measures may be used including screening buildings, avoiding spending time outside at dusk, wearing long sleeves and trousers (pants) and the use of insect repellents.

Further information is available in:

Associated techniques linked from Wildpro

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Clinical WNV Infection in Equines

Clinical Syndromes which may be seen with West Nile Virus Infection in Equines
  • Most WN virus infections of horses (Equus caballus - Domestic horse) are subclinical/asymptomatic.
  • Neurological signs which are variable and not pathognomonic are seen in horses with WNV encephalomyelitis.
    • A wobbly gait due to ataxia or paresis is common, with the hind limbs affected more commonly than the forelimbs; this may be symmetrical or asymmetrical.
    • Muscle fasciculations or trembling (face, muzzle, shoulder or trunk) are common.
    • Other signs include a wide-based stance, hypermetric gait, staggering, stumbling, circling, leaning to one side, toe dragging, paralyzed or drooping lip and tooth grinding.
    • Lethargy or fearfulness may be seen, although many horses retain an interest in food and water.
  • Lacrimation has been reported although not a feature in the USA outbreaks, also blindness.
  • Fever may be found but is not a consistent feature of WNV encephalomyelitis in the horse.
  • The muscle fasciculations and hypersensitivity often seen may be highly suggestive of West Nile virus infection. (J487.14.w1)
(B526.21.w21, J4.218.w2, J4.225.w2, J84.7.w12, J84.7.w17, J84.7.w27, J85.108.w1, J87.32.w1, J89.16.w1, J484.35.w1, J487.14.w1, P32.1.w13, P51.49.w3)
  • It has been suggested that WNV infection should be suspected "for any horse with rapid onset of neurological disease in an area with confirmed West Nile virus activity during periods of high mosquito activity." (J4.218.w2)
    • This is most pertinent in horses which are unvaccinated or not fully vaccinated against WNV.
  • The CDC case definition for WNV infection in equines (D147) includes the following "compatible clinical signs", taken from the description given by Ostlund et al., (J84.7.w12):
    • "Clinical signs are associated with central and/or peripheral nervous system dysfunction. Most horses exhibit secondary CNS-derived neurological manifestations such as ataxia (including stumbling, staggering, wobbly gait, or incoordination) or at least two of the following: circling, hind limb weakness, inability to stand, multiple limb paralysis, muscle fasciculation, proprioceptive deficits, altered mental status, blindness, lip droop/paralysis, teeth grinding." (D147 - based on the description in J84.7.w12) Additionally, it is noted that "Fever is not a consistent finding." (D147)

Further information is provided in:

Clinical Pathology in West Nile Virus Infection in Equines

  • There are no consistent haematological or biochemical findings, although a mild absolute lymphopaenia may be seen and sometimes a slightly increased serum bilirubin level may be found, as may hyponatraemia and elevated muscle enzymes (secondary to trauma and prolonged recumbency). (B526.21.w21, J4.222.w1, J87.32.w1, J89.16.w1, P51.48.w1)

Further information is provided in:

Note: haematological and biochemical findings can help rule out other potential causes of CNS abnormalities, e.g. liver failure. (B526.21.w21)

Pathological findings in West Nile Virus Infection in Equines

  • N.B. Appropriate precautions should be taken when performing a post mortem examination (necropsy) to avoid possible contact of WN virus (and other zoonotic pathogens) with the mucous membranes or open wounds of the person performing the necropsy. Suggested recommendations include:
    • Wear water-resistant clothing: as a minimum a long-sleeve, water-resistant gown with a solid front.
    • Wear outer Kevlar or metal gloves with two pairs of waterproof gloves (disposable gloves covered by more sturdy household gloves) inside these.
    • Minimise the use of sharp instruments including needles.
    • Use a manual saw not a mechanical saw for collecting samples of spinal cord.
    • Avoid the possibility of aerosol contact with mucous membranes by wearing a face shield and a disposable HEPA filter "half-mask" respirator.
    • Use mosquito repellents on clothes and exposed skin.
    • (J89.16.w1, W30.Nov01.WNV4)
  • Gross necropsy findings:
    • There may be no gross lesions. 
    • Traumatic lesions may be found, associated with ataxia and struggling while recumbent.
    • Central nervous system often shows no gross changes, but submeningeal oedema, petechial or suffusive haemorrhages or congestion of the brain or spinal cord have been reported.
    • Congestion and oedema of the lungs was a common finding in an outbreak in Morocco.
    • (J4.218.w2, J85.108.w1, J86.57.w1, J87.32.w1, J89.16.w1, P32.1.w13)
  • Histological:
    • Changes indicating nonsuppurative encephalitis, such as neuronophagia, multifocal gliosis and sometimes perivascular cuffing with lymphoplasmacytic and histiocytic cells. The lesions may be most severe in the lower brain stem and spinal cord.

For further details see:

Differential Diagnoses in Equines

  • The differential diagnoses for WNV encephalomyelitis in equines include all the diseases that may result in neurological signs, particularly ataxia. This includes the other arboviral encephalitides, herpesvirus myeloencephalopathy, rabies, hepatoencephalopathy, equine protozoal myeloencephalitis (EPM), bacterial meningoencephalomyelitis, verminous meningoencephalomyelitis, leukoencephalomalacia, trauma, space occupying masses such as neoplasia and abscesses, aberrant parasite migration, and other conditions. (B526.21.w21, J4.218.w2, J89.16.w1, B249.11.w1, V.w117, W43.Jan04.wnv4)

For a complete list see: 

Samples to be taken for Definitive Diagnosis of West Nile Virus Infection in Equines

  • In the live horse: blood for serum, cerebrospinal fluid.
    • Paired serum samples should be taken, the first as soon as possible after presentation of a clinical case, the second at least seven days later. (D67)
    • Serum is more important than cerebrospinal fluid. (W30.Nov01.WNV4)
  • At necropsy: samples of brain and spinal cord (fresh and formalin fixed), also cerebrospinal fluid.(D67, W30.Nov01.WNV4)
    • Fresh brain should also be sent for rabies testing; rabies should be ruled out before testing for WNV infection is undertaken. (W30.Nov01.WNV4)

Further information is available in: 

Treatment of West Nile Virus Infection in Equines

  • Two antibody products, from Novartis Animal Health and Lake Immunogenics, Inc., have been granted conditional licences by USDA for use in treatment of WN virus infection in horses in the USA. [January 2004](W479.Jan04.wnv3, W479.Jan04.wnv4)
  • There are no specific antiviral drug treatments.
  • Treatment is supportive as for the other equine encephalitides.
    • Particular care may be required to reduce the risk of self-inflicted injury in severely ataxic and recumbent individuals.
    • Nutritional and fluid support may be required.
    • Respiratory support may be required.
    • Anti-inflammatory and analgesic drugs should be given as appropriate.
(B100, B249.11.w1, J4.218.w2, J4.222.w1, J4.225.w2, J14.44.w1, J64.19.w1, J89.16.w1, J89.22.w1, J215.24.w1, P51.48.w1)

For further details see:

Prognosis / Long-Term Outcome following West Nile Virus Infection in Equines

  • Case fatality rate (death or severe signs necessitating euthanasia) among equines with neurological signs due to WNV infection may be approximately 20-45%. (J64.19.w1, J84.7.w12, J84.7.w17, J84.7.w27, J87.32.w1, J89.16.w1)
  • Prognosis is not necessarily related to the severity of signs at the time of presentation.
  • Clinical signs rapidly progressing to recumbency has been associated with poor prognosis. (J4.225.w2, J215.24.w1)
  • Complete recovery usually within a few weeks to months; many horses which recover appear not to show any long term signs. (J215.24.w1, J84.7.w27)
    • One study of 125 horses surviving the initial illness indicated a longer average duration of clinical abnormalities, with abnormalities of gait and/or behaviour remaining in 40% of horses six months after the initial diagnosis of WNV infection. (P51.49.w1)
    • In another study, about 20% of equids had residual signs; the most persistent signs in equines not fully recovered at 30 days or more after infection were loss of body condition and decreased stamina; ataxia and stumbling were less common residual problems. (J4.225.w2)

Further information is available in:

Prevention of West Nile Virus Infection in Equines

  • The risk of horses being bitten by mosquitoes may be reduced by eliminating mosquitoes in barns/stables, using screens to keep mosquitoes out of barns/stables, using fans in stables/barns to discourage mosquitoes, avoiding turning horses out during the times when mosquitoes are most active, using mosquito repellents, and reducing mosquito habitat around areas where horses are kept. 
  • Three vaccines have been licensed for use in horses in the USA and Canada and are commercially available [May 2008]. 
  • It has been recommended by APHIS (USDA) that owners both vaccinate their horses against WN virus and also take measures to avoid exposure of their animals to mosquitoes. (W30.28Jan04.WNV2)
  • It has been confirmed by APHIS that the killed vaccine is safe to use (following publication of misleading articles suggesting otherwise), and a study has confirmed safety in pregnant mares. (J4.225.w4, W30.28Jan04.WNV2)

Further information is available in:

Associated techniques linked from Wildpro

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Clinical WNV Infection in Birds

Clinical Syndromes which may be seen with West Nile Virus Infection in Birds
  • Birds may be found dead.
  • May show non-specific signs: such as:
    • depression;
    • anorexia;
    • weakness; 
    • weight loss; 
    • recumbency.
  • May show neurological signs such as:
    • abnormal posture of the head or neck;
    • ataxia;
    • tremors;
    • circling;
    • posterior paresis (unilateral or bilateral);
    • disorientation;
    • impaired vision.
  • (P30.1.w3, J26.37.w1)

Further information is provided in:

Details of bird species which have been identified as definitive hosts for WN virus are found in:

Clinical Pathology in West Nile Virus Infection in Birds

  • No data available for birds. Findings in mammals are nonspecific.

Further information is provided in:

Pathological findings in West Nile Virus Infection in Birds

Click for Video: Bird Necropsy Protocol for West Nile Virus Surveillance Video Available: Bird Necropsy Protocol for West Nile Virus Surveillance:
Internet (Web) Version (Smaller files - quicker to load)
CD-ROM Version (Larger files - higher quality images)

  • "Meningoencephalitis and necrotizing myocarditis, in particular, should alert pathologists to the possible presence of WNV in birds." (J26.37.w1)

Further information is provided in:

Differential Diagnoses in Birds

Further information is provided in:

Samples to be taken for Definitive Diagnosis of West Nile Virus Infection in Birds

Further information is provided in:

Treatment of West Nile Virus Infection in Birds

  • There are no specific drug treatments.
  • Treatment is supportive, as for other arboviral encephalitides in birds. (J5.36.w8)

Further information is provided in:

Prognosis / Long-Term Outcome following West Nile Virus Infection in Birds

  • To date no long term effects have been reported.
  • Recovery may occur in clinically affected birds; this is most likely in individuals with only mild clinical signs. (P30.1.w3)

Prevention of West Nile Virus Infection in Birds

  • Mosquito netting, larvicides, biological control and ground spraying of adulticides have been used to protect birds in zoos. (P30.1.w1)
  • A killed vaccine has been developed and used for the protection of commercial goose flocks in Israel. (J133.951.w26, W27.06Feb02.wnv1)
  • A recombinant DNA vaccine is being tested for use in birds. (J91.67S2.w3)

Further information is available in:

Associated techniques linked from Wildpro Click for Video: Bird Necropsy Protocol for West Nile Virus Surveillance Video Available: Bird Necropsy Protocol for West Nile Virus Surveillance:
Internet (Web) Version (Smaller files - quicker to load)
CD-ROM Version (Larger files - higher quality images)

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Clinical WNV Infection in other Mammals

  • Based on information available, natural infection of most mammals other than humans and equines with WN virus is unlikely to cause clinical illness.
  • However, the possibility of clinical illness caused by WNV infection cannot be ruled out; clinical disease has been reported in a few cases in various mammals including dogs, cats, sheep, Oreamnos americanus - Mountain goat, Lama pacos - Alpaca and others.

Details of mammal species which have been identified as definitive hosts for WN virus are found in:

Associated techniques linked from Wildpro

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Authors & Referees

Authors Debra Bourne (V.w5)
Referee Suzanne I. Boardman (V.w6); Becki Lawson (V.w26); Dr Robert G. McLean (V.w42); Dr Josie Traub-Dargatz (V.w117); Dr Jules Minke (V.w119)

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