July 12, 2005
A study of blood samples from residents of communities in southwestern Manitoba showed that 3.2 per cent of people tested had antibodies to West Nile virus (WNV).
Most of those who tested positive for West Nile virus (85 per cent) were not aware they had been infected.
The West Nile virus sero-survey was conducted by Manitoba Health, the Public Health Agency of Canada, the Assiniboine and Brandon regional health authorities, the University of Manitoba and Red River College in the spring of 2004.
A random telephone survey was conducted in five areas of southwestern Manitoba including Brandon, Deloraine, Killarney, Minnedosa and Virden. Survey participants were also asked to give a blood sample.
The study found that in communities with higher numbers of
WNV-infected Culex tarsalis mosquitoes (Virden, Deloraine and Killarney),
4.8 per cent of the residents were infected, compared to 0.5 per cent of
residents in communities where the numbers of infected mosquitoes were lower
(Brandon and Minnedosa). Similar percentages of men and women were infected, and
people 60 years or older were as likely to have been infected as those between
18 and
59 years old.
The study showed that people who spent time outside during the peak Culex tarsalis mosquito times (dusk and dawn) were at greater risk of infection. It also showed the level of knowledge of how WNV is spread in Manitoba is quite high, but the knowledge of the protective measures is somewhat lower. Only one-third of people indicated that reducing time outside between dusk and dawn was a protective measure against WNV.
These results are preliminary findings. Further analysis and peer review publication is planned. The findings from this study will help guide the WNV program in Manitoba.
Manitobans can reduce the risk of mosquito bites by:
reducing the amount of time spent outdoors during peak mosquito hours between dusk and dawn;
wearing light-coloured, loose-fitting clothing; and
using appropriate mosquito repellent.
Manitobans can reduce the number of Culex tarsalis mosquitoes around their home by emptying standing water that may collect in backyards, old tires, children’s toys, pet bowls, wading pools, stagnant ponds or under flower pots.
For more information on West Nile virus, visit http://www.gov.mb.ca/health/wnv or http://www.westnile.gc.ca.
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WEST NILE VIRUS SERO-SURVEY
In 2003, Manitoba identified 142 human cases of West Nile virus (WNV). The Assiniboine Regional Health Authority identified 61 of these cases (43 per cent). The Brandon Regional Health Authority identified eight cases (six per cent).
The objectives of the West Nile virus blood survey were to compare the presence of WNV antibodies through blood tests of people in southwestern Manitoba who live in areas with different levels of mosquito activity and infection rates. Researchers also tried to determine the potential risk factors for infection in these populations, to assess levels of knowledge, beliefs and attitudes about WNV, and to determine the protective factors associated with risk reduction for WNV within these populations.
A telephone survey was conducted in five areas in southwestern Manitoba. The areas selected for the study were Brandon, Deloraine, Killarney, Minnedosa and Virden. More than 1,500 individuals completed the survey and 80 per cent of these individuals had their blood tested for WNV antibodies. The survey included questions about outdoor activities, occupation, protective measures taken and knowledge related to West Nile virus.
Blood samples were taken at the Westman Laboratory in Brandon and in laboratories in Virden and Minnedosa in the Assiniboine Regional Health Authority. Testing for West Nile virus antibodies was conducted at the Public Health Agency of Canada’s National Microbiology Laboratory in Winnipeg. The Manitoba Health West Nile virus program mosquito data for 2003 was utilized in the study.
Overall, 3.2 per cent (39 out of 1,200) of residents had evidence of previous WNV infection. Most of these individuals reported no fever symptoms (62 per cent) during the summer of 2003 and 85 per cent did not know they had a WNV infection.
In communities where there were higher numbers of WNV-infected Culex tarsalis mosquitoes during the summer of 2003, 4.8 per cent of the residents were infected, compared to 0.5 per cent in communities where the number of WNV-infected mosquitoes were low.
Similar percentages of men (2.7 per cent) and women (3.5 per cent) were infected. People 60 years or older were as likely to have been infected (3.1 per cent) as those between 18 and 59 (3.4 per cent).
Almost all study participants knew WNV was spread by mosquitoes (93 per cent). Most people knew mosquito repellent containing DEET (74 per cent) and long sleeves and pants (60 per cent) were protective measures. However, few people indicated that reducing time outside between dusk and dawn (31 per cent) would reduce the risk of WNV.
Many study participants reported using protective measures never or rarely including:
The study also showed that spending at least two hours outside from 4 to 8 a.m. or from 5 to 9 p.m. increased the risk of having antibodies to WNV. Rural residents, individuals living in areas with higher numbers of infected mosquitoes and individuals less than 60 years were more likely to spend more time outside during these times.
The findings presented here are preliminary and require further analysis and peer review. The findings are consistent with current program direction and if they hold true, will provide important information for future program planning. The findings in relation to knowledge of protective measures indicate that increased emphasis may need to be placed on strategies to increase knowledge with regard to reducing mosquito exposures during high risk times of day.
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