June 9, 2003
MEMO TO: Vancouver City Council
COPY TO: City Manager
MEMO FROM: Medical Health Officer
SUBJECT: West Nile Virus Response Plan for Vancouver
This memo provides City Council with further information on the public health rationale for the West Nile Virus (WNV) Response Plan proposed by staff and to convey the urgency with respect to endorsement of the plan. The Plan developed by your staff is strongly supported by the Medical Health Officer. It adheres to the Health Canada National WNV Guidelines, which provide a framework for the development of local responses that:
(a) Minimize human disease
(b) Provide a graded response based on an assessment of the specific risks within each jurisdiction; and
(c) Minimize environmental impacts.
In some ways we are fortunate to be able to draw from the experience of our Ontario and New York counterparts in pulling together a WNV response plan for Vancouver. I have been able to draw from the excellent research and groundwork of my counterpart in Toronto, Dr. Sheila Basrur in guiding the development of the City's Plan. Although there are some obvious differences in climate, geography and mosquito ecology, the principles contained in Toronto's deliberations around a response plan are sound and transferable to our situation.
Risk of WNV-related disease:
There remain many unanswered questions that make it difficult to predict the future prevalence of WNV-related disease in humans. New data on WNV regarding its rate of spread, modes of transmission, and the effectiveness of control measures emerges regularly. Work also continues on human trials of a WNV vaccine. Although the risks to the population of the more severe outcomes from infection with West Nile Virus are low in comparison to other diseases, such as influenza, there is a valid public expectation that, where reasonable, cost-effective measures can be taken to further minimize the risk, those measures should be taken.
The proposed BC WNV response for 2003 is based on the most recent information available to BCCDC and Vancouver Coastal Health. It is difficult to predict what human impact, if any, WNV will have in Vancouver in 2003. As evidenced by this year's response in Ontario to last year's perceived under-reaction, the push for broader spray programs (including adulticiding) often follows the failure to act prudently but decisively at the earliest sign of WNV presence. I am in agreement with Dr Michael Jackson and others about the futility and wrong-headedness of broadcast spray programs against adult mosquitoes and the potentially serious environmental impacts of such programs. It would clearly not be protective of the public's health in the longer term to wipe out the natural predators of mosquitoes with broad-spectrum insecticides such as malathion, while having questionable success in reducing adult mosquito populations in the short term!
Efforts to reduce these public health risks must take place before the peak incidence of WNV disease in the summer and early fall, when it is too late to make an effective impact on the transmission of WNV by mosquitoes. Hence, although there is a degree of uncertainty, a decision on which control measures are available for use in 2003 must be made now. Although there is obvious merit in exploring the use of natural predators to reduce populations of mosquito larvae (Toronto has researched these methods and largely discounted them as ineffective in an urban setting), we do not have the luxury of solely relying on these methods this year. Despite the assertions of SPEC and others, promoting increases in the brown bat population will contribute to increased risk of rabies in B.C., as they are the predominant reservoir for rabies here!
While the overall population risk of serious illness remains low, VCH and the City of Vancouver must prepare for the possibility of human illness in 2003 and beyond. Because certain species of mosquitoes are the key transmitters of WNV to humans, efforts to control viral transmission are best aimed at these mosquitoes. VCH deems it prudent therefore, and in keeping with the experience and response of other jurisdictions, for the City of Vancouver to take steps to enhance its mosquito control activities to reduce human exposure to WNV in Vancouver for the 2003 season.
Criteria to Initiate Mosquito Control Activities:
Under the BC WNV response plan, the trigger for larval control would be confirmation of the presence of WNV in the Vancouver Coastal Health Region (Level 2b of the provincial "Arbovirus Response Plan"), with preference given to non-chemical and biological control of the larvae. The criteria for initiating larval control include: Population peaks of mosquitoes of concern; density of mosquito larvae in standing water; optimum timing for larval control; weather conditions; nature of habitat to be treated (e.g. catch basin, versus bodies of water with or without natural vegetation); product application methods/frequency.
Conclusion:
In conclusion, although I understand Council's wish to embrace more natural methods of larval control, we must be prepared to take measured but decisive action to reduce the risk of WNV in Vancouver in 2003. Sole reliance on natural predators will not provide effective mosquito control in an urban setting like Vancouver and will not be acceptable to the Medical Health Officer or the Ministry of Health Planning. The Integrated Pest Management approach recommended by your staff strikes the balance between the need to protect public health and the need to minimize impacts on the environment. We look forward to continuing to work with the Park Board staff and the Engineering staff to effectively address the risks of WNV.