West Nile Virus

From the office of the Medical Health Officer

West Nile Virus (WNv) 2007: Update, Reporting, Case Definitions, Diagnostic Testing and Personal Protection

• WNv is active on the Prairies – you may see patients with travel-acquired WNv infection
• Report to your area Medical Health Officer any patients with SUSPECT WNv neurological syndrome and any patients who have tested positive for WNv in another province or country.
• There is NO evidence of indigenous WNv activity in BC (as of the date of this letter).

1. EXPOSURE TO WEST NILE VIRUS
   
   Exposure to WNv is defined as exposure to mosquitoes in a WNv-affected area. Other potential modes of transmission have only rarely resulted in WNv transmission. These include transmission through blood transfusions (Canadian Blood Services screens all donations for WNv), organ transplantation, laboratory exposures to live WNv and vertical transmission in pregnancy.
   
   WNv continues its spread across North America. To date, there is no evidence of indigenous WNv activity in BC. However, BC residents with travel-related WNv infection have been identified this year, including 5 residents in Fraser infected following exposure on the Canadian prairies. More reports of possible infection and travel history are being investigated. Illness in nearby provinces and states was reported last year and again this year, with numbers increasing in the past few weeks. Human cases and infected horses were reported last year in Yakima and King Counties in Washington State. Yakima has recently confirmed locally-acquired, acute WNv infection in two horses this year. The US Centers for Disease Control has noted that many more human cases can be expected across the States in the next few weeks.
   
   Information for patients and healthcare professionals and ongoing surveillance results are found at the BCCDC website or in the West Nile section of the Fraser Health website. Patients may also be referred to HealthLink BC for more information on WNv and their health.
   
   If the WNv is detected in BC, we will notify physicians and the public. Within Fraser, and throughout BC, there is an active surveillance program with collection and testing of dead corvids (crows, ravens, jays, magpies) and of live mosquitoes (trapped at more than 60 sites throughout Fraser). Positive birds or mosquitoes are almost always detected before any locally-acquired human infections.
2. REPORTING (Unchanged since letter in 2005)
   
   WNv is a reportable disease in BC. Physicians are to report to the Medical Health Officer in their area:
   (1) any patient with suspect West Nile Neurological Syndrome; and
   (2) any patient who has tested positive for WNv in another province or country.
   
   The BC Centre for Disease Control Laboratory will report positive lab tests done in BC to Medical Health Officers. Follow-up questionnaires will be completed by public health in consultation with the patients’ physicians. If your patient recently donated or received a blood transfusion, please advise Canadian Blood Services by a call to 604-876-7219 or FAX to 604-879-6669.
3. WNv SUSPECT CASE DEFINITIONS (Unchanged since letter in 2005)
   
   In 2005, new National Surveillance Case Definitions were adopted. The term West Nile Fever was replaced by West Nile virus Non-Neurological Syndrome since a significant portion of affected individuals does not develop fever. For national case definitions see http://www.bccdc.org/content.php?item=183 select Information for Physicians
1. SUSPECT WEST NILE VIRUS NON-NEUROLOGICAL SYNDROME (WN Non-NS)
   
   Exposure AND at least two of: fever, myalgia, arthralgia, headache, fatigue, lymphadenopathy or maculopapular rash.
   
   Other clinical signs and symptoms identified in association with WN Non-NS include muscle weakness and gastrointestinal symptoms. For WN Non-NS classification, muscle weakness is characterized by mild, transient, unlikely prolonged symptoms that are not associated with motor neuropathy.
2. SUSPECT WEST NILE VIRUS NEUROLOGICAL SYNDROME (WNNS)
   
   Exposure AND onset of fever AND new onset of at least one of the following: encephalitis, viral meningitis, acute flaccid paralysis (eg., poliomyelitis-like syndrome or Guillain-Barré-like syndrome), movement disorders (eg., tremor, myoclonus), Parkinsonism or Parkinsonian-like conditions (eg., cogwheel rigidity, bradykinesia, postural instability) or other associated neurological syndromes.
   
   A significant and sometimes sole presenting feature may be marked muscle weakness. For WNNS classification, muscle weakness is characterized by severe (polio-like), non-transient and prolonged symptoms. Other clinical findings identified in association with WNNS include facial weakness, myelopathy, rhabdomyolysis, peripheral neuropathy, polyradiculoneuropathy, optic neuritis, acute demyelinating encephalomyelitis (ADEM). Ophthalmologic conditions including chorioretinitis and vitritis were also reported. Myocarditis, pancreatitis and fulminant hepatitis have not been identified in North America, but were reported in outbreaks in South Africa. Note: ‘Aseptic’ meningitis without encephalitis or flaccid paralysis occurring in August/September may be due to non-polio enteroviruses circulating (seasonally) at the same time.
4. DIAGNOSTIC TESTING (Only minor changes since letter in 2005)
1. Acute serum collected on presentation and convalescent serum collected 10 to 21 days later. Blood (7-10 mL) should be collected in a pink top or gold top serum separator tube and sent to BCCDC Laboratory Services for detection of antibody (Lab requisition #1806);
2. Acute plasma collected on presentation. Blood (7-10 mL) should be collected in a purple top EDTA tube and sent to BCCDC Laboratory Services for PCR molecular testing (Lab requisition #1811).
   
   For suspect West Nile Virus Neurological Syndrome, also obtain:
3. Cerebrospinal fluid: 1 to 2 cc should be collected in each of two tubes without any preservatives and kept at 4 degrees C or frozen for transport to the BCCDC Laboratory (Lab requisition #1811).
   
   Label the requisitions as Suspect WN Non-NS or as Suspect WNNS. Include the patient’s illness onset date and any history of recent travel. Other agents, including Herpes simplex, should be considered in the differential diagnosis for encephalitis and appropriate diagnostic tests ordered.
5. WNv INFECTION IN HUMANS
   
   Primary WNv infection in humans is associated with a transient viraemia, detectable within 1 to 2 days after being bitten by an infected mosquito and lasting for approximately 7 days (range 1-11 days). The incubation period is generally considered to be 3 to 14 days.
   
   The vast majority of human WNv infections go undiagnosed and unreported as up to 80% of those infected do not experience any symptoms.
   
   Of the estimated 20% who develop symptoms, most will experience WN Non-NS with symptoms generally lasting 3 to 6 days. However, we now know that some of this 20% with WN Non-NS may be unwell for several weeks with fatigue, fever, rash, body aches and headaches and may have muscle weakness and memory problems that may last for months. Like gastrointestinal symptoms, memory problems may be experienced with WN Non-NS infections, but are not included in the National Case Definition.
   
   It is estimated that about 1/150 of those infected develop WNNS. Although the highest risk age group is 40 to 59 years, the relative risk of serious illness increases with age, particularly over age 50, and in the immune- compromised patient or organ transplant recipient.
   
   WNv Therapy and Vaccine
   At present, no known effective antiviral therapy exists, although ribavirin, interferon-alpha and beta and IVIG containing high titers of anti-WNv antibody are being tried experimentally. Work continues on development of a vaccine for humans, but none is currently available.
   
   WNv, Children, Sunscreen, DEET and Alternative Repellents
   DEET is an effective and safe repellent. However, no repellent is recommended for infants less than 6 months of age, and toddlers between 6 months and 2 years should only have the lowest concentration of DEET (4.75% to 10%) applied once per day. Children between the ages of 2 and 12 may have concentrations of up to 10% DEET applied to them up to 3 times per day. Note that the 10% concentration of DEET recommended for children has a much shorter duration of effectiveness (1 to 3 hours) compared with 4 to 6 hours for the 30% DEET recommended for adults.
   
   If both sunscreen and DEET are needed, sunscreen should be liberally applied first and allowed to absorb into the skin for 20 minutes. DEET may then be applied sparingly to exposed skin only.
   
   Two repellents that do not contain DEET are available and relatively effective in providing protection against mosquito bites. P-menthane-3,8-diol, a synthetic derivative of Oil of Lemon Eucalyptus (available in Canada as OFF BotanicalsTM) is comparable to low concentration (<5%) DEET and is good for short period protection, but is not recommended for children under 3 years of age. Soybean oil at 2% (available in Canada as Bite BlockerTM) is another effective repellent for shorter term protection (1 to 4 hours) and has no age-associated use restrictions. Some repellents are available, but are not recommended for personal protection against WNv because the duration of protection they provide is so short. 5% to 15% Citronella oil products are reported to provide relatively short duration of protection (20 to 30 minutes) and for safety reasons are not recommended by the Canadian Pest Management Regulatory Agency (PMRA).
   
   WNv, Pregnancy and Breastfeeding, and DEET
   It appears that WNv can be passed through breast milk, but the level of risk is still unknown. On the other hand, the health benefits of breastfeeding are well known (Health Canada).
   Women who are pregnant or breastfeeding should be advised about reducing their risk of exposure to WNv if in an area with WNv activity. There is no evidence that the use of DEET-containing insect repellents by pregnant or breastfeeding women poses a health hazard to unborn babies or children who are nursing. However, pregnant or breastfeeding women may wish to consider the use of non-chemical methods to prevent mosquito bites as a first line of defence against WNv (Health Canada). A review of insect repellents is available on the Public Health Agency of Canada website. 
6. PREVENTION AND SURVEILLANCE
   Fraser Health’s WNv brochure in English, Chinese, Punjabi, Vietnamese, Korean, Japanese and French is available at your nearest health unit, or on our website. Brochures for specific types of property uses are also available.