London School of Hygiene & Tropical Medicine

Zika has caught the world by surprise. The declaration by the World Health Organization that the recent cluster of microcephaly cases and other neurological disorders constitutes a Public Health Emergency of International Concern, shows the seriousness of the situation.

The virus is transmitted by Aedes aegypti mosquitoes. Vector control should be, and is, the focus for stopping the spread of Zika. But will current control methods work?

Aedes aegypti can be found in 150 countries, making them vulnerable to future outbreaks of Zika, as well as chikungunya and dengue. The biggest current outbreak is in Brazil where Zika virus is thought to have infected between 440,000 and 1.3 million people, and there are concerns that the virus can’t be prevented from spreading to all regions of the Americas.

Currently, disease control depends on the management of mosquitoes. This relies on either insecticides or destroying breeding sites, but due to widespread insecticide resistance and the impracticality of eliminating standing pools of water on a city-wide scale, there is little hope for the containment of this disease using these methods. We therefore need to look at other options for controlling the spread of Zika.

In recent years, two new approaches have shown considerable promise of tackling dengue, which is also transmitted by Aedes aegypti: the genetic sterilisation of Aedes aegypti mosquitoes, and the infection of mosquitoes with bacteria called Wolbachia which reduces their ability to transmit viruses to people.

The sterilisation strategy involves only releasing male mosquitoes – which do not bite – and these compete with wild males to mate with wild females. Offspring do not survive to the adult stage and the wild mosquito population numbers drop. Using Wolbachia-infected mosquitoes, the ‘eliminate dengue’ project has been able to demonstrate substantial reductions in the ability of Aedes aegypti to pass dengue between people. The strategy would involve releasing large numbers of these Wolbachia-infected mosquitoes to breed with wild mosquitoes and potentially result in a mosquito population that poses less risk to humans.

If Zika spreads into parts of the world with less developed healthcare systems the financial and health cost could be enormous, so finding effective control methods are vital. A recent small trial in Brazil showed the genetic sterilisation technique could reduce local mosquito populations by 95%, while introducing the Wolbachia bacteria into mosquitoes has been shown to stop the chikungunya virus and yellow fever virus replicating.

More research is needed to see whether these two methods could work on a large scale. However, there is no known cure for Zika virus and with a vaccine anticipated to take several years to develop, we need to explore all the options to stop its spread. Controlling Aedes aegypti will go a long way to controlling the outbreak.

Publication

• Laith Yakob, Thomas Walker Zika virus outbreak in the Americas: the need for novel mosquito control methods Lancet Global Health doi:10.1016/S2214-109X(16)00048-6

Image: Aedes aegypti mosquito. Credit: VectorBase