West Nile virus will likely soon arrive in BC

Following 3 years of westward spread across North America, there is a high probability that West Nile virus (WNV) will be detected in British Columbia in 2003. Physicians should become familiar with the clinical presentation of WNV infection, diagnostics, transmission, and case reporting requirements.

WNV is a mosquito-borne, RNA flavivirus related to yellow fever, dengue, and Japanese B encephalitis viruses. In nature WNV cycles between mosquitoes and birds. Historically, WNV was limited to Africa, Asia, the Middle East, and southern Europe. The virus emerged in New York in 1999 and, by late 2002, had been detected in 44 of 48 continental states in the US, and from Nova Scotia to Saskatchewan in Canada.

Eighty percent of individuals who are infected with WNV have subclinical infections. Of the 20% who are symptomatic, most will have WNV fever, with sudden onset of moderate to high fever and one or more of the following: fatigue, headache, eye pain, vomiting, myalgia, arthralgia, and rash. These symptoms last up to 6 days.

One of every 150 infections leads to severe neurological disease (WNV neurological syndrome) presenting as fever along with encephalitis, aseptic meningitis, or meningoencephalitis. Patients may present with headache, severe muscle weakness, and altered mental status. About 10% of patients with WNV neurological syndromes have acute flaccid paralysis, caused either by anterior myelitis (polio-like syndrome), or Guillain-Barre-like syndrome. Other neurological manifestations have been described. Sequelae including impaired memory, difficulty mobilizing, and muscle weakness may follow the acute illness.

There is no known effective antiviral therapy, although ribavirin and interferon-beta are being tried experimentally. There is no human vaccine.

During 2002, more than 300 symptomatic cases were reported in Canada and more than 3800 were reported in the US. The average age of patients with severe infections was about 50 years. Fatality among reported, symptomatic cases ranged from 4% to 6%.

Although most WNV infections follow a mosquito bite, transmission through receipt of contaminated blood and donated organs has been documented. A nucleic acid test to screen donated blood for WNV should be in use by Canadian Blood Service by 1 July 2003. Transmission via placenta and breast milk have also been described.

Diagnostic tests should be ordered on patients with clinical presentations consistent with WNV infection, including WNV fever or WNV neurological syndrome, during the period when WNV could be transmitted by mosquitoes in BC (spring until the first hard frost in the autumn). Appropriate specimens include:

• Acute serum collected on presentation and convalescent serum collected 14 days to 21 days later. Serum (7 mL to 10 mL) should be collected in a red-top tube and sent to BCCDC Laboratory Services.

• Cerebrospinal fluid (CSF): 1 cc to 2 cc should be collected in each of two tubes without any preservatives. It should be kept at 4°C or frozen for transport to the BCCDC laboratory.

Include the patient’s onset date on the laboratory requisition. Other agents, including Herpes simplex, should be considered in the differential diagnosis for encephalitis and appropriate diagnostic tests ordered.

WNV fever and WNV neurological syndrome are reportable in BC. Physicians must report the following to the medical health officer in their area:

• Patients who have lab-confirmed WNV fever or WNV neurological syndrome.

• Patients who have a clinical presentation consistent with WNV neurological syndrome during a period of time (spring until hard frost in autumn) when mosquitoes may transmit WNV.

Physicians should inquire about recent blood donation or transfusion and should contact Canadian Blood Services directly (phone 1 888 332-5663 local 2207; fax (604) 879-6352) if a case of suspected WNV neurological syndrome donated or received blood in the previous 4 weeks.

BCCDC and regional medical health officers will notify physicians and the public when WNV has been detected in BC. A program to monitor dead crows and adult mosquitoes for WNV will operate during the spring through autumn. Physicians should remind patients of measures to prevent mosquito bites, including wearing long, light-colored clothing and using insect repellents with DEET. Municipalities may implement mosquito control programs. Mosquito breeding can also be reduced by eliminating stagnant water around people’s homes.

—Murray Fyfe, MD, FRCPC
Physician Epidemiologist
—Muhammad Morshed PhD,
Clinical Microbiologist
BCCDC