West Nile virus: Infectious substances pathogen safety data sheet

Section I – Infectious agent

Name

West Nile virus (WNV)

Agent type

Virus

Taxonomy

Family

Flaviviridae

Genus

Orthoflavivirus

Species

Orthoflavivirus nilense

Synonym or cross-reference

WNV^{Footnote 1}, West Nile fever^{Footnote 2}, West Nile encephalitis, Kunjin virus, WN fever^{Footnote 1Footnote 3}, West Nile disease^{Footnote 4}, and West Nile neuroinvasive disease^{Footnote 3}.

Characteristics

Brief description

West Nile virus (WNV) is an icosahedral, enveloped virus of 40 to 50 nm in diameter^{Footnote 1Footnote 3Footnote 5} and has a single-stranded, positive-sense RNA genome^{Footnote 1Footnote 2Footnote 4Footnote 5}. The size of the genome is 11 kbp, with a GC content of 50.9%^{Footnote 2Footnote 6}. The genome codes for three structural proteins and seven non-structural proteins^{Footnote 6}.

Properties

WNV belongs to the Japanese encephalitis antigenic complex^{Footnote 7}. There are nine evolutionary lineages of the virus, but only lineages I and II contain strains that are responsible for human and animal infections^{Footnote 8Footnote 9}. Within each lineage, the various strains demonstrate different genetic characteristics and virulence^{Footnote 10}. The virus is maintained primarily in bird reservoirs and are transmitted to other reservoirs and hosts using mosquito vectors^{Footnote 8}.

WNV in mosquito vectors replicates in the midgut and other tissues, and then spreads through hemolymph to salivary glands, where the virus is transmitted to a host through a mosquito bite^{Footnote 8}. Proteins in mosquito saliva have anti-coagulant properties and can also interfere with the host's immune response. WNV enters the host cell through receptor-mediated endocytosis and releases it's RNA genome into the cytoplasm^{Footnote 8}. The genome is then translated into structural proteins and is further replicated in the cytoplasm. Virion progeny are assembled within the cell and are released through exocytosis. Initial infection is usually in keratinocytes and dendritic cells^{Footnote 11}. Infection then spreads throughout the body with infected cells migrating to lymph nodes from which viremia is disseminated to visceral organs and the central nervous system.

Potential mechanisms for WNV neuro-invasion have been studied, and include increased vascular permeability, virus crossing the blood-brain-barrier, transmission through infected macrophages entering the brain, infection of endothelial cells, and infection of the spinal cord^{Footnote 8}. The infection induces inflammatory lesions and neuronal damage directly and indirectly through the host inflammation response^{Footnote 10}.

Section II – Hazard identification

Pathogenicity and toxicity

Most individuals infected with WNV remain asymptomatic^{Footnote 3Footnote 4Footnote 12}. A New York serological survey revealed that, of those infected, approximately 20% developed West Nile fever^{Footnote 13}. West Nile fever is typically a mild illness, lasting 3 to 6 days^{Footnote 1Footnote 4Footnote 12}. The main symptoms are sudden onset of fever with chills, rash, malaise, headache, backache, arthralgia, myalgia, and eye pain^{Footnote 1Footnote 3Footnote 4Footnote 11Footnote 12}. Other non-specific manifestations include nausea, vomiting, anorexia, diarrhoea, rhinorrhoea, sore throat, and cough^{Footnote 1Footnote 3}. More rare gastrointestinal manifestations include gastritis, pancreatitis, and hepatitis^{Footnote 8}. In some patients there is generalized lymphadenopathy, and cutaneous manifestations such as erythematous macular, papular, or morbilliform eruptions involving the entire body^{Footnote 1Footnote 3Footnote 12}.

The neuroinvasive form of WNV infection, characterized by meningitis, encephalitis, and/or acute flaccid paralysis, develops in less than 1% of WNV-infected individuals but is the most severe form of the disease^{Footnote 3Footnote 8Footnote 13Footnote 14}. The overall case fatality rate ranges from 4% to 14% in individuals exhibiting neuroinvasive disease, with a higher incidence of severe presentation and higher fatality rates in older populations^{Footnote 13Footnote 14}. Patients with neurological disease typically have a febrile prodrome of 1 to 7 days, which may be biphasic, before they develop neurological symptoms^{Footnote 1}. Typically, neurological patients will present with a fever, stiff neck, headache, weak muscles, gastrointestinal symptoms, disorientation, tremors, convulsions, and paralysis^{Footnote 1Footnote 3Footnote 12}. Illness duration varies from weeks to months, as more serious clinical manifestations can lead to long-term functional and cognitive difficulties^{Footnote 11}. In patients with neuroinvasive WNV, 25-40% present with erythematous rashes due to infection of epithelial skin cells^{Footnote 15}.

Clinical manifestations of neuroinvasive WNV in horses are similar to symptoms in humans; namely meningitis, encephalitis, and acute flaccid paralysis^{Footnote 8}. In birds, symptoms include lethargy, ataxia, paralysis, tremors, and weight loss. In reptiles, clinical manifestations include weakness, gastrointestinal symptoms such as anorexia, and neurological symptoms such as unbalanced swimming, tremors, ataxia, and loss of leg control.

Epidemiology

WNV was first discovered in 1937 from the blood of a febrile woman in the West Nile region of Uganda^{Footnote 3Footnote 5Footnote 9}. WNV is now known to be enzootic in most of Africa, Southern Europe, India, the Middle East, Western and Southeast Asia, Australia (known as Kunjin virus), and North America^{Footnote 1Footnote 9Footnote 16Footnote 17}. WNV was first detected in North America in 1999, following an outbreak in New York City^{Footnote 17}. The virus spread westwards across the United States, southward into Central America and the Caribbean, and northward into Canada^{Footnote 3Footnote 17Footnote 18}, resulting in the largest epidemics of neuroinvasive WNV fever ever known^{Footnote 3Footnote 17}. In Canada, WNV was first detected in 2001^{Footnote 11}. In temperate and subtropical regions, most infections in humans occur in summer or early fall; whereas, infections in tropical regions tend to coincide with the rainy season when mosquito populations are most abundant^{Footnote 19}.

Many outbreaks of WNV have been reported globally, in endemic and non-endemic areas^{Footnote 11}. Until the early 1990s, human outbreaks of mild symptoms were reported infrequently from parts of Europe. However, since then new viral strains have increased disease incidence globally, with large outbreaks of increased clinical severity occurring. Lineage I strains are historically more virulent however, lineage II strains have caused more recent epidemics^{Footnote 10}. The largest outbreak in a single US county was reported in 2021 with 1,487 cases^{Footnote 20} . Outbreaks involving animals, often horses and birds, have also been reported in Australia, North America, Europe, and Africa^{Footnote 8}.

Environmental factors affecting infection include residing in areas with high mosquito populations, near stagnant water, or near wetlands^{Footnote 21Footnote 22Footnote 23}. Warming temperatures leading to shorter winters have increased prevalence of disease as it is favourable for mosquito vectors^{Footnote 21}. Greater severity of disease is also seen in the elderly, children, and those with comorbidities such as diabetes, renal disease, cancer, immunosuppression, and hypertension^{Footnote 24Footnote 25Footnote 26}. To prevent transmission of WNV through blood transfusion and organ donations, blood products are screened for WNV in the United States^{Footnote 18}. The United States also lowers WNV vector population and prevent epidemics by spraying insecticides to kill mosquito larvae and adults^{Footnote 27}.

Host range

Natural host(s)

Humans^{Footnote 1}, mosquitoes, ticks^{Footnote 6Footnote 17}, birds (particularly passerine species)^{Footnote 2Footnote 28}, horses^{Footnote 1Footnote 2}, alligators (Alligator mississippiensis)^{Footnote 2}, tree squirrels (Sciurus spp.)^{Footnote 23}, eastern chipmunks (Tamias striatus), eastern cottontail rabbits (Sylvilagus floridanus), lake frogs (Rana ridibunda), non-human primates^{Footnote 29}; as well as a broad range of common North American wild and domestic mammals, such as dogs, deer, feral swine, coyotes, foxes, opossums, raccoons, skunks, bats and other small rodents^{Footnote 23}. Birds are considered the most important amplifying hosts for WNV^{Footnote 30}.

Humans and most other mammals are regarded as dead-end hosts, since they do not produce sufficient viraemia to infect mosquitoes and thus do not significantly contribute to the transmission cycle ^{Footnote 3Footnote 4}.

Other host(s)

Hamsters^{Footnote 30}, mice, cats, alligators, and snakes have been infected experimentally^{Footnote 31}.

Infectious dose

The WNV dose that produced infection in 50% of hosts (ID₅₀) was found to be 0.50 plaque-forming unit (PFU) (intrathoracically inoculated) for mosquitoes and 0.66 PFU (subcutaneously inoculated) for young chickens^{Footnote 32}.

Incubation period

Ranges from 2 to 6 days, but may extend to 14 days, or as long as 21 days for patients following organ transplantation^{Footnote 1Footnote 13}.

Communicability

The main route of infection is injection of infectious material via the bite of a mosquito that has been infected by feeding on WNV infected birds^{Footnote 1Footnote 2Footnote 9}. Injection through accidental needlestick exposure has been reported as well^{Footnote 33}. Contact of infectious material with mucous membranes is another method of transmission^{Footnote 28Footnote 34}. Human-to-human transmission can occur via intimate direct contact through infected breast milk, organ transplantation, blood transfusion, and vertical transmission (intrauterine transmission from mother to child during pregnancy)^{Footnote 18}.

In animals, the primary route of infection is also via mosquito bite^{Footnote 8}. Various species of birds, which maintain WNV as part of it's lifecycle, are infected via mosquito bites, ingesting infectious material, and contact transmission^{Footnote 29}. Infection through ingesting infected invertebrates, such as shrimp, has also been observed^{Footnote 35}. In alligators, contact of fecal matter with mucous membranes has been reported as a transmission route^{Footnote 36}.

Section III – Dissemination

Reservoir

Birds, particularly passerine species (jays, finches, grackles, sparrows, and crows)^{Footnote 2Footnote 18Footnote 23}. Although other animals can be infected by WNV, only birds develop a high enough viral load to transmit the infection to a feeding mosquito^{Footnote 8}.

Zoonosis

Humans can contract WNV from the exposure of conjunctival membranes^{Footnote 28} and/or percutaneous injury to the body fluids or tissues of WNV infected birds or other mammals^{Footnote 35Footnote 37}.

Vectors

The main vectors are Culex mosquitoes^{Footnote 1Footnote 3Footnote 9}. Humans and animals can contract WNV by the bite of an infected mosquito^{Footnote 8Footnote 12Footnote 16Footnote 19}.

In North America: C. pipiens, C. restuans, C. salinarius, C. quinquefasciatus, and C. tarsalis^{Footnote 3Footnote 9Footnote 23}.

In Central and South America: C. interrogator and C. nigripalpus^{Footnote 38}.

In Africa and the Middle East: Mainly C. univittatus, but other species such as C. pipiens, C. tritaeniorhynchus, C. antennatus, and C. perexiguus are also found in the region^{Footnote 5Footnote 8Footnote 9}.

In Asia: C. vishnui, C. fatigans, C. tritaeniorhynchus, C. bitaeniorhynchus, among others^{Footnote 5Footnote 39}.

In Europe: C. pipiens, C. modestus, C. molestus, Ocheloratus caspius, C. torrentium, Anopheles maculipennis, and Coquillettidia richiardii^{Footnote 8Footnote 9}.

In Australia: mainly C. annulirostris^{Footnote 40}.

Other mosquito species such as Culex nigripalpus, Aedes albopictus, Aedes vexans, and Ochlerotatus triseriatus, may also be of importance in the transmission of WNV^{Footnote 23}.

Other arthropods such as ticks, swallow bugs, and chicken mites have been infected with WNV, making them potential vectors^{Footnote 8}.

Section IV – Stability and viability

Drug susceptibility/resistance

Ribavirin, interferon, sofosbuvir, amantadine, L-dopa, and isatin can inhibit WNV in vitro^{Footnote 3Footnote 18Footnote 41Footnote 42}. Immunoglobulins have shown inhibitory capabilities in mouse models^{Footnote 43}.

Susceptibility to disinfectants

Susceptible to disinfectants such as 3 to 8% formaldehyde, 2% glutaraldehyde, 2 to 3% hydrogen peroxide, 500 to 5,000 ppm available chlorine, alcohol, 1% iodine, and phenol iodophors^{Footnote 44}.

Physical inactivation

Inactivated by heat (50 to 60°C for at least 30 minutes), ultraviolet light, gamma irradiation, ELISA wash buffer at 37°C, and 25 μl 4x NuPAGE^® LDS Sample Buffer mixed with 10 μl NuPAGE^® Reducing Agent^{Footnote 45Footnote 46}.

Survival outside host

Low temperatures preserve infectivity, with stability being greatest below -60°C^{Footnote 45}. Can survive up to 72 hours at 4°C in a solution containing 30-150 ppt NaCl^{Footnote 36}. When added to ELISA wash buffer there is a 10-fold decrease in titre per 24 hour period at 28°C^{Footnote 46}. In blood transfusions, WNV is able to survive maximally for 5 days in platelets, 33 days in red blood cells, and 44 days in fresh frozen plasma^{Footnote 47}.

Section V – First aid/medical

Surveillance

Monitor for symptoms^{Footnote 8}. Confirmation diagnosis via virus isolation from blood or cerebrospinal fluid during the viraemic phase^{Footnote 1Footnote 3Footnote 12Footnote 14Footnote 18}. Other methods of detection include PCR^{Footnote 12Footnote 14Footnote 23}, haemagglutinin inhibition^{Footnote 1Footnote 4Footnote 28}, plaque reduction neutralization^{Footnote 1Footnote 4}, compliment fixation, indirect immunofluorescence assay^{Footnote 4}, and IgM capture ELISA^{Footnote 1Footnote 4Footnote 28}.

Note: The specific recommendations for surveillance in the laboratory should come from the medical surveillance program, which is based on a local risk assessment of the pathogens and activities being undertaken, as well as an overarching risk assessment of the biosafety program as a whole. More information on medical surveillance is available in the Canadian Biosafety Handbook (CBH).

First aid/treatment

Currently there is no treatment of proven efficacy for WNV fever^{Footnote 3Footnote 18}. Supportive therapy for encephalitis cases includes intravenous fluid, electrolyte management, assisted respiration if needed, anticonvulsants, management of cerebral oedema, and prevention of secondary bacterial infections^{Footnote 1Footnote 14}. Studies have assessed ribavirin, interferon, osmotic agents, gamma globulins, steroids for treatment of WN fever in open trials, but more definitive evidence is needed to determine their efficacy^{Footnote 1Footnote 12Footnote 14}.

Note: The specific recommendations for first aid/treatment in the laboratory should come from the post-exposure response plan, which is developed as part of the medical surveillance program. More information on the post-exposure response plan can be found in the CBH.

Immunization

None currently available. Two inactivated vaccines and one DNA vaccine are available for horses, but human vaccines are currently not available^{Footnote 4Footnote 8Footnote 12}. However, a number of candidates are in clinical trials, but none has progressed past phase II clinical trials^{Footnote 3Footnote 4Footnote 48}.

Note: More information on the medical surveillance program can be found in the CBH, and by consulting the Canadian Immunization Guide.

Prophylaxis

None currently available. The most effective preventative measure is to avoid mosquito bites^{Footnote 3Footnote 12Footnote 14Footnote 17Footnote 23}. There are no chemoprophylactic measures for individuals suspected of being in contact with WNV.

Note: More information on prophylaxis as part of the medical surveillance program can be found in the CBH.

Section VI – Laboratory hazard

Laboratory-acquired infections

Eighteen cases were reported up until 1980, with no deaths^{Footnote 49}. Post-1980, five more laboratory-acquired cases were reported; three cases of workers who acquired WNV following percutaneous inoculation while handling fluids and tissues infected with WNV^{Footnote 35Footnote 38}, and two cases of mucous membrane exposure to infectious material from an animal^{Footnote 28Footnote 35}.

Note: Please consult the Canadian Biosafety Standard (CBS) and CBH for additional details on requirements for reporting exposure incidents.

Sources/specimens

Blood^{Footnote 3Footnote 4Footnote 14Footnote 23}, cerebrospinal fluid^{Footnote 1Footnote 3Footnote 4Footnote 14Footnote 18Footnote 23}, tissues^{Footnote 4Footnote 14Footnote 18Footnote 23Footnote 28}, infected arthropods^{Footnote 1Footnote 4Footnote 14}, oral and cloacal swabs, feather pulp^{Footnote 23}, and urine^{Footnote 50}.

Primary hazards

Bites from infected mosquitoes, exposure of mucous membranes, and autoinoculation with infectious material are primary hazards associated with exposure to WNV^{Footnote 2Footnote 28Footnote 38}.

Special hazards

Faecal secretions of infected birds may present a hazard to humans^{Footnote 12Footnote 28}.

Section VII – Exposure controls/personal protection

Risk group classification

West Nile Virus is a Risk Group 3 Human Pathogen and Risk Group 3 Animal Pathogen^{Footnote 51Footnote 52}.

Containment requirements

Containment Level 3 facilities, equipment, and operational practices outlined in the CBS for work involving infectious or potentially infectious materials, animals, or cultures.

Protective clothing

The applicable Containment Level 3 requirements for personal protective equipment and clothing outlined in the CBS are to be followed. At minimum, use of full body coverage dedicated protective clothing, dedicated protective footwear and/or additional protective footwear, gloves when handling infectious materials or animals, face protection when there is a known or potential risk of exposure to splashes or flying objects, respirators when there is a risk of exposure to infectious aerosols, and an additional layer of protective clothing prior to work with infectious materials or animals.

Note: A local risk assessment will identify the appropriate hand, foot, head, body, eye/face, and respiratory protection, and the personal protective equipment requirements for the containment zone must be documented.

Other precautions

All activities involving open vessels of pathogens are to be performed in a certified biological safety cabinet (BSC) or other appropriate primary containment device. The use of needles, syringes, and other sharp objects are to be strictly limited. Additional precautions must considered with work involving animals or large scale activities.

Section VIII – Handling and storage

Spills

Allow aerosols to settle. Wearing protective clothing, gently cover the spill with absorbent paper towel and apply suitable disinfectant, starting at the perimeter and working towards the centre. Allow sufficient contact time with disinfectant before clean up (CBH).

Disposal

Regulated materials, as well as all items and waste to be decontaminated at the containment barrier prior to removal from the containment zone, animal room, animal cubicle, or post mortem room. This can be achieved by using decontamination technologies and processes that have been demonstrated to be effective against the infectious material, such as chemical disinfectants, autoclaving, irradiation, incineration, an effluent treatment system, or gaseous decontamination (CBH).

Storage

The applicable Containment Level 3 requirements for storage outlined in the CBS are to be followed. Primary containers of regulated materials removed from the containment zone to be stored in a labelled, leak-proof, impact-resistant secondary container, and kept either in locked storage equipment or within an area with limited access.

An inventory of RG3 long-term storage, to be maintained and to include:

• specific identification of the regulated materials
• a mechanism that allows for the detection of a missing or stolen sample in a timely manner

Section IX – Regulatory and other information

Canadian regulatory information

Controlled activities with WNV require a Pathogen and Toxin licence issued by the Public Health Agency of Canada (PHAC). WNV is a terrestrial animal pathogen in Canada; therefore, importation of WNV requires an import permit under the authority of the Health of Animals Regulations (HAR). The PHAC issues a Pathogen and Toxin licence which includes a Human Pathogen and Toxin licence and an HAR importation permit.

The following is a non-exhaustive list of applicable designations, regulations, or legislations:

• Human Pathogens and Toxins Act and Human Pathogens and Toxins Regulations
• Health of Animals Act and Health of Animals Regulations
• Transportation of Dangerous Goods Act and Regulations
• National Notifiable Diseases (Human)
• Immediately Notifiable Diseases (Animal)

Last file update

November 2023

Prepared by

Centre for Biosecurity, Public Health Agency of Canada.

Disclaimer

The scientific information, opinions, and recommendations contained in this Pathogen Safety Data Sheet have been developed based on or compiled from trusted sources available at the time of publication. Newly discovered hazards are frequent and this information may not be completely up to date. The Government of Canada accepts no responsibility for the accuracy, sufficiency, or reliability or for any loss or injury resulting from the use of the information.

Persons in Canada are responsible for complying with the relevant laws, including regulations, guidelines and standards applicable to the import, transport, and use of pathogens in Canada set by relevant regulatory authorities, including the Public Health Agency of Canada, Health Canada, Canadian Food Inspection Agency, Environment and Climate Change Canada, and Transport Canada. The risk classification and related regulatory requirements referenced in this Pathogen Safety Data Sheet, such as those found in the Canadian Biosafety Standard, may be incomplete and are specific to the Canadian context. Other jurisdictions will have their own requirements.

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