Evening flights target dengue-spreading mosquitoes

Part of the Mosquito Research and Control Unit's strategy involves aerial operations over mosquito-breeding areas on Grand Cayman, Cayman Brac and Little Cayman.
Mosquito Plane

 Aircraft have begun evening patrols spraying insecticide in the West Bay area in a bid to reduce the chances of another outbreak of dengue fever in Grand Cayman. 

The Mosquito Research and Control Unit is making twice-weekly flights in an effort to kill off large numbers of the dengue-spreading Aedis-aegypti mosquito. The air operations, which began last month, coincide with the onset of rainy season when mosquitoes are a constant nuisance. 

The 5pm flights, timed to target the most active period for the dengue-spreading mosquitoes, were introduced for the first time last year as part of an effort to prevent the spread of the potentially deadly fever. The planes have been used in populated areas before, but only later in the evening to target a different type of mosquito.  

There have been no recent reported cases of dengue and Alan Wheeler, assistant director of the mosquito unit, described the flights as control operations. 

The aerial operations support ground crews that do the bulk of the work, going door-to-door to spray larvae-killing insecticide and clear stagnant water in discarded tyres and buckets, where the mosquitoes breed. 

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The evening flights, which buzz low over homes, initially prompted a handful of complaints. But Mr. Wheeler said most residents were happy to put up with the inconvenience once the reasons were explained. 

There were more than 40 incidences of dengue fever in the Cayman Islands last year, the highest on record. 

The disease was brought to the territory by residents or visitors bitten in neighbouring countries where dengue is endemic. 

The flu-like symptoms associated with the disease don’t show for up to 10 days after the victim is bitten, meaning they could be unaware they had contracted the illness until it was too late to contain the spread. 

Further research and trials are ongoing on an alternative strategy of introducing genetically modified male mosquitoes into the Cayman Islands in an effort to crash the local population. 

The male mosquitoes, developed by a team from Oxford University in the UK, are modified so they are sterile. The idea is to release enough of them so that most wild females will mate with a sterile male and this will gradually reduce the population. 

Female mosquitoes only mate once in a lifetime and previous trials have suggested that the introduction of the sterile males, which do not bite, could ultimately cause a significant reduction in numbers of mosquitoes found here. 

Mr. Wheeler said the main work right now was being done by on-the-ground officers, targeting dengue spreading mosquitoes. 

“Right now, dengue fever is our main concern.” 

“We are surrounded by dengue-endemic countries and we are getting toward the time of year where we have a lot of tourists coming back to the islands from these places, potentially carrying the virus. 

“The danger is that they are then bitten by mosquitoes here, which then transmit it to locals. Last year was the worst year we have ever had.” 

Since the outbreak of dengue fever in Cayman started last September, 43 cases were reported before the Public Health Department declared dengue season over in March. The first locally-contracted case of dengue in the Cayman Islands was recorded in 2010. In that year, four of the seven cases reported in Cayman were found to have been locally contracted. One local case was reported in 2011.  

Last year, for the first time, the Public Health Department confirmed that dengue fever could be considered endemic in Cayman. 

The approach to controlling the spread of dengue fever has been to deploy officers who go door-to-door and lay insecticide in a zone around the area of the outbreak. 

“Last year, the main foci of the outbreak was in West Bay. Aircraft were used to supplement ground control operations in this area,” Mr. Wheeler said. 

He said it was highly likely that the Cayman Islands would see additional cases of dengue fever this summer and controlling the mosquito population was a necessary measure to limit the spread of any outbreak. 

“We know we are heading into the time of year when people will be coming back with dengue fever and we want to keep the mosquito population as low as possible to limit the spread.” 

He said the bulk of the leg work was done by a team of 15 officers. “They go out every day and check yards, bring back samples and report on problem areas.” 

He said buckets, dog bowls and any yard junk, including old tyres, that held fresh water were potential breeding areas for mosquitoes.  

“The used tyre trade is often cited as being responsible for the spread of the Aedis-aegypti mosquito around the world,” Mr. Wheeler added. 

Mosquito plane Cayman

Light aircraft are spraying insecticide in urban areas across the Cayman Islands.

Low flying Mozzie plane

An insecticide spraying plane dips low over a West Bay home during a flight last week.
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  1. An aggressive approach is certainly justified to reduce risk of dengue transmission. An aerial assault, however, may offer only limited rewards against the vector of the dengue virus. History teaches us that the greatest successes against Aedes aegypti were realized by targeting houses with periodic inspections and treatments, as necessary. This would certainly cost far more in the short term, but it may be considerably more effective. Every case of dengue averted would help pay for such a campaign.