West Nile Virus Disease

Diagnostic Criteria

Aves - Birds (Class):

• While infection may be asymptomatic, clinical signs which may be seen include general signs such as depression, anorexia, weakness, recumbency and weight loss, and/or neurological signs such as abnormal posture of the head or neck, ataxia, incoordination, inability to stand, tremors, circling, disorientation, impaired vision, posterior paresis (unilateral or bilateral) and disorientation. Clinical signs are variable and may include nervous and nonspecific signs. Birds may also be found dead without any clinical signs having been observed. (J26.37.w1, P30.1.w3) At necropsy, signs of trauma may or may not be present and gross lesions are not always present; suggestive findings are those of meningoencephalitis and necrotising myocarditis. (J26.37.w1)

Equidae - Horses (Family):

• WNV infection may cause a range of signs in equines from inapparent infection (no clinical signs) to severe neurological signs which may be fatal or require euthanasia. Ataxia (stumbling, staggering, incoordination, wobbly gait), weakness, paresis or paralysis and recumbency, muscle fasciculation and altered mental status are common; apparent blindness, proprioceptive defects, lip droop and teeth grinding may be noted in some horses; seizures occur only occasionally. Fever may or may not be present. Signs are non-specific and are not sufficient, in themselves, for diagnosis. (J4.218.w2, J84.7.w17, J84.7.w27, J89.16.w1, J484.35.w1, P51.49.w3)
  • Diagnosis in the living horse is based on assessment of clinical signs, epidemiology (geographical location, season, known occurrence in other horses or in birds, presence of potential vectors) and serological testing. (J89.22.w1, J484.35.w1)
  • In areas of North America where this disease has become common in horses, clinically affected animals may "look like" WN virus infection cases, particularly because of the muscle fasciculations and the hypersensitivity they often show. (J487.14.w1)
  • 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)
  • Note: clustering of cases in an area where WN virus is known to be active would increase the level of suspicion. (V.w117)
  • Pathologically, abnormalities are found in the lower brainstem and spinal cord, rather than the cerebral and cerebellar cortices (unlike in Eastern Equine Encephalitis and Western Equine Encephalitis). (J484.35.w1, P51.49.w3) 
  • For a list of other diseases which may cause similar clinical signs in horses see: Equine Surveillance for West Nile Virus - Other Diseases that look like WNV Infection

Homo sapiens - Human (Species Link Page):

• WN virus infection may be inapparent or result in a range of signs and symptoms from classical West Nile fever (fever and headache, with or without rash and/or enlarged lymph nodes, and sometimes other symptoms) to severe neurological manifestations of meningitis or encephalitis, sometimes with profound muscle weakness or acute flaccid paralysis. Other manifestations, such as hepatitis or respiratory signs, have been noted occasionally. 
  • The signs and symptoms of WNV infection are not specific to this disease and are not diagnostic. West Nile fever cannot be distinguished clinically from other viral fevers and WNV meningitis or encephalitis cannot be distinguished clinically from similar syndromes caused by other viruses. 

  (B244.w1, B241.49.w49, B243.31.w1, J84.7.w14, J93.38.w5, W170.28Jan03.wnv1)

• The CDC 2001 Case Definition for arboviral infections states: "Arboviral infections may be asymptomatic or may result in illnesses of variable severity sometimes associated with central nervous system (CNS) involvement. When the CNS is affected, clinical syndromes ranging from febrile headache to aseptic meningitis to encephalitis may occur, and these are usually indistinguishable from similar syndromes caused by other viruses. Arboviral meningitis is characterized by fever, headache, stiff neck, and pleocytosis. Arboviral encephalitis is characterized by fever, headache, and altered mental status ranging from confusion to coma with or without additional signs of brain dysfunction (e.g., paresis or paralysis, cranial nerve palsies, sensory deficits, abnormal reflexes, generalized convulsions, and abnormal movements)." (W170.28Jan03.wnv1)
• The CDC case definition for West Nile fever describes this as: "A non-specific, self-limited, febrile illness caused by infection with WNV, a mosquito-borne flavivirus. Clinical disease generally occurs 2-6 days (range, 2-15 days) following the bite of an infected mosquito. Typical cases are characterized by the acute onset of fever, headache, arthralgias, myalgias, and fatigue. Maculopapular rash and lymphadenopathy generally are observed in less than 20% of cases. Illness typically lasts 2-7 days." [2002 definition] (D147)
• In the USA, due to year-round transmission in some geographical regions and the reported occurrence of severe disease in persons of all ages it has been recommended that a diagnosis of WNV infection should be considered in all cases of unexplained human encephalitis and meningitis but particularly in adults 50 years old or older developing unexplained meningitis or encephalitis in late summer or early autumn (fall). (W170.Aug02.WNV1)
• It is important to consider the possibility of WNV infection in patients presenting with acute flaccid paralysis, even if other signs of central nervous system involvement are absent. (J84.9.w13)

Aves - Birds (Class):

In dead birds:

• In dead bird surveillance, testing involves isolation of infectious virus, or specific RNA detection by RT-PCR. Immunohistochemistry may also be used to detect the virus in tissues. Kidney, heart and brain are the preferred tissues for testing. (D67, D147, J5.47.w2, J133.951.w12, P48.4.w13)
• In the USA for surveillance infection is considered to be laboratory-confirmed if WN virus is isolated, WN viral RNA is detected and further validated or WN viral antigen is detected and further validated, or in live birds if seroconversion to WN virus is demonstrated by PRNT in serially-collected serum specimens (with cross-neutralisation to rule out other flavivirus infections) or IgM antibody to WN virus is detected and validated by demonstration of neutralising antibody. A laboratory-probable individual is one in which IgM antibody is detected or seroconversion to WN virus is detected in serially collected serum specimens, strongly reactive by EIA or IFA, or antigen is detected but not validated by another procedure, or a single RT-PCR test is positive for WN viral RNA. (D147)

In live birds:

• Definitive diagnosis of infection requires detection of seroconversion. Tests which may be used include HI, ELISA and PRNT. It must be remembered that in birds other than hatch-year birds, and which have not been previously bled and determined to be seronegative, presence of antibodies may not indicate recent infection. 

(D67, D72, D147, J84.8.w9, J133.951.w10, P32.1.w1, P39.2.w2)

• Further details, including criteria for CDC's definitions for "Laboratory-confirmed" and "Laboratory-probable" infection in birds in the USA are provided in: Wild Bird Surveillance for West Nile Virus

• For further information on different serological tests and virus identification see:

  • Flaviviridae- West Nile Virus (Viral Type)
    • West Nile Virus - Detection and Identification Techniques (Viral Reports)

Equidae - Horses (Family):

• Definitive diagnosis of WNV infection in horses requires detection of virus, specific viral antigen or genomic sequences from blood, CSF, tissue or other body fluid, or demonstration of seroconversion. In areas where vaccination against WNV infection may be used it is important to ensure that a full vaccination history is obtained if results of serological tests are to be interpreted correctly. Depending on the serological test used and whether other flaviviruses may be present in the geographical area, cross-reaction with other flaviviruses must be considered. (J215.April 2002.w1, D67, D147, W30.Nov01.WNV4) 
• For the purposes of West Nile virus surveillance in the USA, definitions have been agreed on "confirmed" and "probable" cases for  equine infections. (D67, D72, D147).
• Details of the criteria for laboratory diagnosis and for "confirmed" and "probable" cases in the USA are provided in:  Equine Surveillance for West Nile Virus - How to make a definite diagnosis

• For further information on different serological tests and virus identification see:

  • Flaviviridae- West Nile Virus (Viral Type)
    • West Nile Virus - Detection and Identification Techniques (Viral Reports)

In Homo sapiens - Human (Species Link Page):

Diagnosis of West Nile Virus disease is generally based on compatible clinical signs associated with the confirmed presence of West Nile virus (detection of virus or of specific viral antigen or genomic sequences in serum, CSF or tissues) or serological indications of recent infection. (B243.31.w1, B244.w1, J64.19.w1, J129.42.w1, D67, D72, W170.28Jan04.WNV1) 

It is important to remember that:

• Cross-reactions with other flaviviruses may occur with standard serological tests, which complicates interpretation of the tests. (B243.31.w1)
• In immunosuppressed individuals, development of antibodies may be delayed or may not occur. (D147)
• Virus is most likely to be detected (rarely isolated, more commonly detected by RT-PCR on the first days of illness in serum or more often in acute-phase samples of CSF. (B244.w1, J129.42.w1, D147)
  • In immunosuppressed individuals, virus or viral RNA may be detected in the serum or CSF for longer periods than normal. (D147)

In general for definition of arboviral encephalitis the accepted diagnostic categories are:

• Confirmed case: compatible clinical syndrome together with either virus-specific IgM in CSF or four-fold rise in serum antibody titre detected by ELISA, IFA, CF, HI or virus neutralization or isolation of virus from tissue, blood or CSF;
• Probable case: compatible clinical syndrome together with elevated antibody titre in a single serum sample (examples of elevated titres are >320 by HI, >256 by IFA, >128 by CF, >160 by 90% PRNT) or presence of virus-specific IgM in serum;
• Suspected case: compatible clinical syndrome occurring during a period of likely arbovirus transmission.

(J93.38.w5, D68)

In Romania in 1996 the clinical case definition used in the investigation of the epidemic was "acute aseptic meningitis, encephalitis, or meningoencephalitis of suspected viral etiology, with a CSF pleocytosis. Out of those patients meeting the clinical case criteria, 84% were subsequently laboratory confirmed with WNV infection or were considered probable, based on the presence of anti-WN virus IgM antibodies in CSF or serum samples collected in the first week of illness. It was also noted that cases were confirmed in individuals hospitalised with fever and headache, presenting with non-specific febrile illness, and in individuals with clinical respiratory infection. (J93.38.w5)

For the purposes of West Nile virus surveillance in the USA, definitions have been agreed on "confirmed" and "probable" cases for human infections. (D67, D72, D147).

The most recent (2002) CDC definition for WN Fever gives the following case classification (text copied directly) (D147):

• NOTE: it can be impossible to distinguish clinically between WN fever and dengue fever, therefore a recent travel history and appropriate serological testing, to distinguish between closely-related flaviviruses, are very important. (D147 - Appendix D)

The most recent (2004) CDC definition for neuroinvasive and non-neuroinvasive arboviral disease gives the following case classification (text copied directly) (D344, W170.12May08.wnv6) [Full text included]:

Further details are provided in: Human Surveillance for West Nile Virus

• For further information on different serological tests and virus identification see:

  • Flaviviridae- West Nile Virus (Viral Type)
    • West Nile Virus - Detection and Identification Techniques (Viral Reports)

• Literature Reports: Virus Identification Techniques (Goes to Virus Species Information page)

• Literature Reports: Clinical-Pathological Test Findings (including serology)

• Literature Reports: Pathological Findings (Necropsy / Post-mortem)

In birds (Aves - Birds (Class)):

• The differential diagnosis for encephalitis and myocarditis in birds should include eastern equine encephalitis (EEE), avian influenza and Newcastle’s disease. Other cause of nervous signs include avian encephalomyelitis virus infection, botulism and listeriosis.

(J26.37.w1, B32.31.w14, B36.20.w20, B36.21.w21, B36.22.w22, W43.Jan04.wnv6)

In horses (Equus caballus - Domestic horse):

• Diseases which may cause similar signs in horses include the other arboviral encephalitides (Western Equine Encephalitis, Eastern Equine Encephalitis, Venezuelan Equine Encephalitis), Equine herpes virus 1 (EHV-1), central nervous system protozoal infections, encephalopathy due to diseases of other organ systems (e.g. hepatic encephalopathy), various toxicities including botulism, focal spinal cord lesions, Wobbler syndrome and,  most importantly, rabies. 

(J4.218.w2, J14.44.w1, J89.16.w1, W43.Jan04.wnv4)

In humans (Homo sapiens - Human):

• For clinical signs of typical West Nile Fever, diseases such as typhus fever, infectious mononucleosis, leptospirosis, dengue,  measles, rubella, exanthemata due to ECHO and Coxsackie viruses, a wide variety of arthropod-borne viral infections such as dengue, sandfly fever, chikungunya and o'nyong-nyong fever, and unidentified summer fevers must be considered.
• In cases of West Nile Virus encephalitis, other viral encephalitides must be considered (Eastern equine encephalitis (EEE), Western equine encephalitis (WEE), St. Louis encephalitis (SLE), California encephalitis, Venezuelan equine encephalitis (VEE), Japanese B encephalitis (JE)) as well as aseptic meningitis, lymphocytic choriomeningitis, and nonparalytic poliomyelitis; severe forms from cerebrovascular accidents, brain tumours, brain abscess and intoxications and a variety of other viral and toxic causes of encephalitis.
• In individuals with acute flaccid paralysis, differentiation must be made from Guillain-Barré syndrome, poliomyelitis, stroke and myopathy.

(J84.9.w13, J101.64.w1, J129.42.w1, J214.267.w9, B240.14.w14, B252.32.w32, P39.4.w3, P31.6.w1)

Specific Medical Treatment (Antiserum, Antidote, Anti-(viral/bacterial/fungal) etc.)

ANTISERUM

• Treatment with intravenous immunoglobulins (IVIG) has been used in humans, sometimes with a marked positive effect.
• Experimentally IVIG from Israeli donors, but not from USA donors, was shown to have a protective effect against WNV infection in mice; the difference was presumed to be related to a higher level of natural immunity in the Israeli donors, since WN virus has been endemic in the Middle East for many years.
• A clinical trial of IVIG (Omr-IgG-am ™) for use in certain cases of WNV infection started September 2003 in the USA [data from January 2004]. (W483.Jan04.wnv1)
• Administration of neutralizing antibodies to hamsters (Mesocricetus auratus - Golden Hamster) was shown to give protection against subsequent subcutaneous inoculation with WN virus.
• Administration of human WN virus-immune gamma globulin to mice post-inoculation provided a significant degree of protection and in wild-type mice, but not in immunodeficient mice, some protection was shown even when therapy was initiated after virus would have reached the CNS.
• Two products, one plasma-based (from Lake Immunogenics, Inc.) and one serum-based (from Novartis Animal Health), have been granted conditional licences for use in horses in the USA. 

ANTIVIRAL DRUGS

• Antiviral drugs to date have not proved useful for the treatment of Flaviviridae (Virus Family) infections. Some protective and therapeutic effects results have been seen with interferon alpha-2b in vitro and with ribavirin in a mouse model and in at least one in vitro model. Ribavirin has been used in the treatment of human patients but no controlled studies have been completed for the use of ribavirin or interferon alpha-2b in humans. Other drugs have been tested for their efficacy in vitro.

(J19.68.w1, J64.19.w1, J80.77.w3, J84.7.w31, J84.8.w2, J100.182.w1, J100.188.w1, J100.188.w2, J106.55.w2, J221.86.w1, J128.13.w1, J133.951.w29, J258.326.w1, J259.2.w1, J280.55.w1, B244.w1, D67, W170.Aug02.WNV1, W479.Jan04.wnv3, W479.Jan04.wnv4, W483.Jan04.wnv1)

• Literature Reports: Specific Medical Treatment

General Nursing and Surgical Techniques

• General supportive care is indicated to limit inflammation and pain.
• In patients with central nervous system signs symptomatic and supportive treatment should be given as for other viral encephalitides.
• Intravenous nutrient and fluid support may be required if the affected individual is unable to eat.
• Respiratory support may be required.
• In horses, specific consideration should be given to the prevention of self-inflicted injury and the provision of physical (sling) support for severely ataxic individuals.

(J4.218.w2, J4.222.w1, J5.36.w8, J64.19.w1, J14.45.w1, J84.7.w27, J89.16.w1, J89.22.w1, J101.64.w1, J123.31.w1, J215.24.w1, J257.168.w3, J258.326.w1, J259.2.w1, J484.35.w1, B100, B222.34.w34, B240.14.w14, B243.31.w1, B249.11.w1, P51.48.w1, P51.49.w3, W170.Nov01.WNV2, W170.Aug02.WNV1) 

• Literature Reports: Nursing and Supportive Care

• Not applicable for this disease.

• Literature Reports: Surgical Treatment - not applicable for this disease

Vaccination & Prophylactic Treatment

• Research into vaccines has progressed rapidly over the last few years.
• Four vaccines have been given USDA approval for use in horses in the USA; three are commercially available, and have also been licensed for use in Canada:
  • 1) A killed virus product (West Nile - Innovator, Fort Dodge Animal Health) for use in horses, requiring an initial course of two injections, has been developed and was given a full licence by USDA in February 2003.This vaccine also has been used in other species.
    • The reported vaccine efficacy in properly vaccinated horses is 94%.
    • The vaccine has been shown to be immunogenic in llamas and alpacas.
    • In birds, variable seroconversion has occurred following extralabel use of this vaccine.
  • 2) A recombinant vaccine (RECOMBITEK® Equine West Nile Virus vaccine, Merial Animal Health Ltd.) for use in horses, expressing WN virus prM/E genes in a canarypox vector, with a primary course of two injections, was granted a full license by USDA in December 2003.
  • 3) A Flavivirus chimera vaccine, for use in horses, (PreveNile, Intervet) containing West Nile virus pre-membrane (prM) and envelope (E) genes (from the NY99 strain) in a backbone of yellow fever (YF) 17D vaccine virus), was granted a full license by USDA in August 2006.
  • 4) A DNA vaccine for use in horses (from Fort Dodge Animal Health) was given a full licence by USDA in 2005 but is not commercially available.
• A killed virus requiring a course of two injections has been used to protect goslings in commercial flocks in Israel.
• A live attenuated vaccine has been used experimentally in mice and geese with apparent success.
• Recombinant and DNA vaccines are under development.
  • Work is progressing on the development of chimeric vaccines combining West Nile structural protein genes with a dengue type 4 virus or a yellow fever virus. In mice these chimeric vaccines have provided effective protection against challenge with WN virus. In baboons the yellow fever chimeric vaccine has been shown to result in the development of high levels of neutralising antibodies. In rhesus monkeys the WN/DEN4 chimeras have been shown to result in development of moderate-to-high levels of neutralising antibodies and protection against challenge with wild-type WN virus. 
  • Work has been carried out on the use of DNA vaccines, with initial results a) using a recombinant plasmid expressing the WN virus prM and E structural proteins showing that neutralizing antibodies can be elicited, and protection from challenge, and b) with a vaccine encoding the WV virus capsid protein (WNVCp) showing development of antibody (IgG) response and also cellular immune responses.
  • Protection has been shown for Corvus ossifragus - Fish crow vaccinated once intramuscularly with a DNA vaccine, but not for these crows vaccinated once orally or for Corvus brachyrhynchos - American Crow vaccinated once intramuscularly or orally.
  • Protection (lower viraemia) has been shown in Buteo jamaicensis - Red-tailed hawk following vaccination with a DNA-plasmid vaccine.
  • Good immune responses have been seen in condors inoculated with DNA-plasmid vaccine.
  • A recombinant envelope (E) protein vaccine has been shown to give protection against experimental infection with WN virus in mice.
• It is possible that in some cases individuals who have been vaccinated against other flavivirus infections may have a degree of immunity to disease caused by WNV infection.
• Vaccination could be useful in a situation in which large numbers of nonimmune humans were moving into an area known to have a very high rate of transmission of WN virus and for protection of people in known "at risk" groups (e.g. the elderly and people living in areas with evidence of high rates of virus transmission).
• Note: until recently there was apparently no need for a vaccine against West Nile virus. Within the historical range of WNV, most infections were subclinical or resulted in mild febrile disease; the incidence of infections involving neurological complications and fatal infections were very low in humans while outbreaks of encephalitic infection in horses were rare.

(J1.40.w9, J2.36.w4, J2.38.w1, J4.225.w2, J4.225.w3, J4.225.w5, J4.225.w6, J4.226.w1, J5.47.w1, J13.65.w2, J20.314.w1, J64.19.w1, J70.23.w5, 70.25.w7, J80.75.w1, J84.7.w12, J84.8.w1, J84.8.w14, J84.9.w15, J87.39.w3, J90.1999.w2, J91.66.w1, J91.66.w2, J91.67S2.w2, J91.67S2.w3, J100.184.w1, J115.13.w2, J133.951.w5, J133.951.w26, J133.951.w27, J135.99.w1, J215.24.w1, J216.167.w1, J219.14.w2, J258.326.w1, J259.2.w1, J484.38.w1, J489.28.w1, J490.5.w1, B240.14.w14, B241.49.w49, B243.31.w1, B244.w1, P9.2004.w12, P39.3.w3, P39.3.w10, P39.4.w4, P51.49.w2, P51.52.w1, P51.51.w1, P503.1.w8, P507.2005.w8, W27.06Feb02.wnv1, W27.17Sept02.wnv1, W30.Nov01.WNV3, W30.28Jan04.WNV2, W348.20Jan03.wnv1, W479.Jan04.wnv2, D148, D149, D150)

• Literature Reports: Vaccination Regimes

• Literature Reports: West Nile Virus Disease Virus Vaccine Development

Antiviral drugs:

• No prophylactic treatment is available for this disease, although both human recombinant interferon alpha-2b and ribavirin, applied prior to infection of cells with the virus, have been shown to protect Vero cells in vitro.

(J84.8.w2, J128.13.w1)

Passive immunisation:

• Injection of WN virus-immune serum 24 hours prior to infection has been shown to protect hamsters (Mesocricetus auratus - Golden Hamster) challenged with the virus.
• Injection of either mouse WNV-immune serum or human WN virus-immune gamma globulin has been shown to provide protection in mice although in immuno-deficient mice this effect was time-limited depending on the initial dose of immune serum. (i.e. antibodies alone were not sufficient to eliminate infection)

(J80.77.w3, J84.8.w14)

Personal protective measures:

• Personal protective measures, including the use of DEET insect repellent and general avoidance of mosquitoes are effective for reducing risk of WNV infection and are particularly important for the elderly and immunocompromised, who have the greatest risk of severe infection. 

(J257.168.w3, J258.326.w1, P48.4.w9)

• Literature Reports: Prophylactic Treatment (other than vaccines)

Environmental and Population Control Measures

• The most effective long-term control strategies are likely to involve the reduction of vector breeding by habitat management.
• Larvicides control mosquitoes before they become dispersed and may keep mosquito populations at low levels, reducing the risk of transmission of arboviruses.
• During an epidemic, use of adulticides may be required to reduce the population of vector mosquitoes.

(B240.14.w14, B241.49.w49, B243.31.w1, B244.w1, J115.13.w2,