In the previous step we described the practical importance of counting mosquitoes. Here we examine why we plan surveillance activities and surveillance recommendations for Zika.
Planning for surveillance
Selection of the most appropriate surveillance strategies is based on the outcome or objective, and must also consider time, resources, and infestation levels. Mosquito surveillance programmes have traditionally been based on household larval surveys,1 but advances have been made to the methods of capturing adults and studies have indicated that adult traps, including the BG-Sentinel and MosquiTRAP, are more sensitive, so they should be considered as complements or alternatives to larval surveys.2
It is fundamentally important that, prior to data collection and virus screening, a surveillance strategy is developed to address the specific aims of the proposed work. Ideally, a framework should be set up with representative coverage over a wide area, which involves a limited network of carefully selected ‘sentinel’ sites spaced at fixed distances. These should take into account the flight range of Ae. aegypti, with repeated sampling over time.3 However, such an approach is very labour-intensive and, although it is essential for more in-depth studies that specifically look into the epidemiology of disease transmission, is really beyond the scope of most programmes operating in the field. Instead, surveillance epidemiologists can focus on monitoring in specific areas that they consider to be of high risk. At the planning stage it should also be considered whether other operationally-significant data should be collected, such as the incidence of other viruses that may be newly emerging or re-emerging in the area, or the insecticide resistance status of collected mosquitoes. Emergency control measures are primarily based on application of insecticides, so periodical monitoring of the susceptibility of Aedes to the insecticides most widely used can help to ensure that the emergency measures will be effective.
Surveillance of Zika
The Centers for Disease Control and Prevention has proposed the development of a local approach for enhanced surveillance for Zika,4 the recommendations of which are outlined in Table 1 and 2 below. It is important to remember that good surveillance programmes designed to detect infestation of Zika can reduce widespread transmission, and are much less costly than eradication or control programmes established after an infestation has occurred.
Considerations for your area
|Are there Aedes aegypti or Aedes albopictus mosquitoes in your area?||Areas with Aedes aegypti are at greater risk for local transmission|
|Have you had local mosquito-borne transmission of dengue or chikungunya virus?||These areas are likely at increased risk for local mosquito-borne transmission of Zika virus|
|Have you identified travel-associated cases?||Local transmission may first be identified in an area with a recent travel-associated case|
|Does your public health laboratory perform Zika virus testing?||Polymerase chain reaction (PCR) testing should be performed in suspect cases who have had symptom onset within the past 7 days.|
Strategies for enhanced surveillance to identify possible local transmission during mosquito season
|Surveillance around travel-associated cases||Household members of travel-associated cases should be interviewed about symptoms suggestive of Zika virus infection during the case investigation.|
|Investigation of unusual clusters of rash illness||Outreach to healthcare providers helps increase awareness and recognition of unusual clusters of rash illness which may indicate local transmission of Zika virus in areas with Aedes aegypti or Aedes albopictus mosquito activity.|
|Expanded testing for Zika virus among people with no known exposure||Suspected cases of Zika virus disease may include patients with acute fever and rash, with or without joint pain or conjunctivitis; when feasible, patients should be tested for Zika virus within 7 days of symptom onset.|
Table 1 & 2. Summary of enhanced surveillance proposal.4
© London School of Hygiene & Tropical Medicine