Overview
Vector-borne diseases are caused by transmission of parasites, viruses and bacteria in humans by vectors. Most common vectors are mosquitoes and ticks, and can transfer the pathogen from one host to another. Public health units are required to conduct vector-borne surveillance to monitor trends for emerging diseases including factors that influence their occurrence. Also, surveillance data is translated into evidence-based programs and services to control insect vector populations. The primary focus of Ontario’s vector-borne management plan is on tick and mosquito population surveillance. This report summarizes the surveillance activities of the Windsor-Essex County Health Unit (WECHU) as it relates to West Nile Virus (WNV) and Lyme disease.
Lyme disease is an infectious disease spread by ticks. A common symptom is an expanding area of redness on the skin (not itchy or painful) that appears at the site of the tick bite about a week after it occurred. Other early symptoms may include fever, headache and tiredness. More severe infections may lead to joint pain, memory problems, and tiredness for at least six months.
The WECHU's role is to measure and evaluate the risk of this tick-borne disease in our area. To measure the local distribution and incidence of ticks and Lyme disease cases in Windsor and Essex County (WEC), the following three surveillance techniques are used:
Active surveillance
Active surveillance involves the dragging of a white cloth through grassy areas whereby ticks attach themselves to the cloth and can be easily spotted. Ticks collected on the cloth are sent to an accredited lab for testing. Tick dragging is performed twice yearly to identify areas in WEC that have populations of blacklegged ticks, which is the main vector for Lyme disease.
- In 2018, tick dragging was conducted at two sites (Chrysler Greenway & Ojibway Prarie Nature Reserve) in the spring and in the fall.
- 12 blacklegged ticks were found through tick dragging with one tick carrying the infectious agent of Lyme disease.
Passive surveillance
Residents submit ticks to the WECHU for identification and subsequent testing if the tick is identified as a blacklegged tick.
- 339 ticks were submitted to the WECHU in 2018. Tick species was identified for 338 ticks (99.7%) (Figure 1).
- 31 blacklegged ticks were identified to the WECHU (9.2% of identified ticks).
- Two blacklegged ticks tested positive for Borrelia burgdorferi, the bacteria that causes Lyme disease.
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Human case surveillance
Lyme disease is a reportable disease in Ontario. The WECHU investigates reported cases of Lyme disease in the region, although the number of cases fluctuates from year to year.
- In 2018, there were nine cases of Lyme disease in WEC, representing a rate of 2.2 cases per 100,000 population (Figure 2).
- Over two-thirds (67.8%) of Rapid Risk Factor Surveillance Survey (RRFSS) participants in WEC (May - August 2017) were aware that Lyme disease was transmitted from tick bites (95% CI: 48.9 to 82.2%; 283 out of 363 respondents (Table 1). RRFSS is an ongoing survey collecting information about health-related behaviours among Ontario adults 18 years of age and older.
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| Aware of: | Percent | 95% CI | Responses |
|---|---|---|---|
| Lyme disease | 86.7% | 81.4% to 90.6% | 342 out of 401 |
| Transmitted from tick bites | 67.8% | 48.9 to 82.2% | 283 out of 363 |
| Protection from tick bites when spending time outdoors (most of the time and every time) | 57.0% | 43.6 to 72.8% | 100 out of 173 |
Source: Rapid Risk Factor Surveillance Survey (RRFSS), Cycle 26 (May - August 2017)
West Nile virus (WNV) is typically spread by infected mosquitoes which receive the virus when they feed on infected birds, who often carry the disease. Although the majority of infections do not result in any noticeable symptoms, some human cases may develop a fever, headache, vomiting, or a rash. Rare instances of infection may lead to encephalitis or meningitis.
[1] Rarely the virus is spread through blood transfusions, organ transplants, or from mother to baby during pregnancy, delivery, or breastfeeding.[2] It otherwise does not spread directly between people.[3] Risks for severe disease include age over 60 and other health problems
The WECHU works with local municipalities to reduce the impact of WNV. The program includes:
Adult mosquito surveillance
This is an important component of the vector-borne disease program and involves the deployment of black-light CDC traps at various locations throughout WEC, to capture mosquitoes for testing to determine the presence of WNV in our area. These traps have a light and dry ice that attracts the mosquitoes. The mosquitoes fly into the trap, get caught in the net, and are then collected for testing. The mosquitoes trapped are counted and tested to identify the species and determine if any of the mosquitoes are carrying the virus.
- In total, 81,954 mosquitoes were caught in CDC traps deployed across WEC during the 2018 season.
- In 2018, 2.7% of mosquito pools tested were positive for WNV (Figure 3)
- Coquillettidia perturbans was the primary species captured in CDC traps, comprising 10.3% of mosquitos identified.
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Human case surveillance
WNV is also a reportable disease in Ontario. The WECHU follow-ups on reported cases of WNV in the region.
- In 2018, there were 13 human cases of West Nile Virus (WNV) illness in WEC, representing a rate of 3.1 cases per 100,000 population (Figure 4)
- Although the elderly and children age groups have the greatest risk of becoming ill with WNV, 55.0% of RRFSS participants in WEC (May - August 2017) reported there is no real difference in WNV based on age (95% CI: 43.0 to 66.4%; 193 out of 376 respondents).
- This finding indicates that residents may need to become more aware of the age-related vulnerabilities of WNV illness.
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Mosquito larviciding program
Involves applying larvicides to catch basins, standing water sites, and lagoons that hold water and breed mosquitoes. The WECHU and municipalities contracted a service provider to run the larviciding program in our area. Larval surveillance begins in May of each year.
- 133,283 treats of larvicide were applied at roadside catch basins across WEC in 2018.
- 71 treatments of larvicide were applied at 33 standing water sites in 2018