The Zika (ZEE’-ka) virus was first discovered in monkey in Uganda in 1947 – its name comes from the Zika forest where it was first discovered. It is native mainly to tropical Africa, with outbreaks in Southeast Asia and the Pacific Islands, but also in many Latin American countries and Caribbean islands.
It is typically transmitted through bites from the same kind of mosquitoes – Aedes aegypti – that can spread other tropical diseases, like dengue fever, chikungunya and yellow fever. It is not known to spread from person to person, but it can be transmitted sexually. The World Health Organisation recently warned the mode of transmission is ‘more common than previously assumed’.
Scientists have discovered a benign bacterium called Wolbachia pipientis can completely block transmission of Zika in the species of mosquito responsible for passing the virus onto humans. Trials exposing the Aedes aegypti mosquito to the bacteria are already underway in Colombia, Brazil, Australia, Vietnam and Indonesia to help control the spread of dengue virus.
Researchers led by Jorge Osorio, a University of Wisconsin-Madison professor of pathobiological sciences and Professor Scott O’Neill of the the Eliminate Dengue Program (EDP), believe the bacteria could present a ‘novel biological control mechanism.’
‘In two of our initial study sites in Australia, approximately 90 per cent of the mosquitoes continue to be infected with Wolbachia after initial release more than six years ago’ said Professor O’Neill.
In the study, the team infected mice with Zika virus originally isolated from a human patient and allowed mosquitoes from Medellin, Columbia, to feed on the mice either two or three days after they were infected.
The mosquitoes were either harbouring the same strain of the Wolbachiabacteria (called wMel) used in field studies or were Wolbachia-free and the mice had levels of virus in their blood similar to humans infected with Zika virus.
An additional group of mosquitoes, both wild-type and Wolbachia-infected, was allowed to feed instead from a membrane containing sheep’s blood spiked with a high concentration of Zika virus. Four, seven, 10 and 17 days after the mosquitoes fed on Zika-virus-infected blood, the researchers tested them for Zika virus infection.
They assessed whether the virus had disseminated – or spread to other tissues in the mosquito, and examined whether the virus made its way to the mosquito saliva, where it must be present to be transmitted.
They found that mosquitoes carrying the bacteria were less likely to become infected with Zika virus after feeding on viral blood – and those that were infected were not capable of transmitting the virus in their saliva.
Wolbachia pipientis can be found in up to 60 percent of insects around the world, including butterflies and bees.
While not typically found in the Aedes aegypti mosquito – the species that also transmits dengue, chikungunya and yellow fever viruses – Professor O’Neill discovered in the early 1990s that it could be introduced to the mosquito in the lab and would prevent the transmission of the dengue virus.
The Zika virus belongs to the same family as dengue virus, leading scientists to believe the same could be true for this virus.
EDP hopes to see greater than 80 percent of Aedes aegypti mosquitoes in study areas harbouring Wolbachia. The study is one of the first to study Zika virus transmission dynamics using a living host.