Combating Aedes

One of the more interesting ways to combat mosquito-borne diseases is by infecting the mosquitoes with a parasite. The Aedes aegypti mosquito hosts the viruses responsible for dengue, zika, yellow fever and chikungunya. Female mosquitoes bite infected humans and pick up those viruses, which flourish within the mosquitoes’ cells. These diseases are passed on when the infected mosquito bites somebody else. 

Dengue causes 50-100 million infections every year while zika epidemics in the Latin America have had terrible effects on new-born babies. Yellow fever affects Brazil and Africa and chikungunya has caused epidemics across continents.

Researchers have discovered that a common bacteria, wolbachia, can inhibit virus infections in mosquitoes. The wolbachia bacteria is a parasite that comes in multiple strains. It is naturally present in 60 per cent of insect species, as well as in some nematodes (worms). But aedes is not a normal host for wolbachia. 

The bacteria works in two ways to inhibit virus infections. It seems to boost the mosquito immune system, which makes it harder for viruses to infect the insect. It also competes with viruses for key molecules like cholesterol. Viruses need cholesterol to survive and wolbachia is more efficient at consuming cholesterol. Hence it starves any virus that does infect the mosquito. 

Some strains of wolbachia present in nematodes are dangerous. They can cause inflammation that results in filaria and other diseases. But the strains used by researchers in the anti-mosquito experiments don’t harm human beings, or large animals. This makes the introduction of the bacteria to the mosquito (the bacteria is present everywhere, anyhow) safe and environmentally sustainable. 

Researchers at Universities in Melbourne and Glasgow and the Institute for Medical Research in Malaysia have worked with the World Mosquito Program (WMP) to experiment with the introduction of wolbachia to aedes populations. 

In 2011, researchers led by Scott O’Neill, a professor at the Monash University in Melbourne, Australia, started injecting wolbachia into aedes eggs, after harvesting the bacteria from fruit flies where it is naturally present. After hatching, the wolbachia-infected aedes were released into pilot areas in Queensland to mate with wild aedes populations. Over time, the disease incidences of these pilot zones were compared to nearby control areas where wolbachia-mosquitoes were not present. 

The mating of a wolbachia–infected male mosquito with an uninfected female mosquito results in sterile eggs. But wolbachia-infected female mosquitoes can breed with uninfected males, and produce offspring born carrying wolbachia. These wolbachia carrying mosquitoes spread through the population in successive generations. Over time, this brings down infection rates for endemic viral diseases. 

This method could be more effective in the long run than insecticide sprays and less likely to cause health and environmental hazards. It is also cost effective since the bacteria propagates by itself, generation by generation. 

There is statistical evidence that these experiments are working. The researchers claim Northern Queensland is almost disease-free due to this treatment. The WMP has set up projects in 12 countries and carrying mosquitoes covered populations of over 4 million people by June 2019. In Yogyakarta, Indonesia, municipal records indicated 76 per cent reduction in dengue infections in the 30 months since the release of the first batch of treated mosquitoes in a pilot zone, compared to the rates in adjoining control areas. Similarly, pilot projects in Brazil have led to a 75 per cent drop in chikungunya cases compared to untreated control sites. 

Early studies involved one strain of wolbachia, known as wMel. But wMel can’t handle really high-temperatures. Another team discovered that a different strain of wolbachia has better results in high temperature regions. This second team is led by Steven Sinkins, a vector biologist at the University of Glasgow and it introduced the high-temperature strain, wAlbB, in six pilot neighbourhoods in Kuala Lumpur. A study published in Current Biology reports that the second strain seems to thrive even in peak daily temperatures exceeding 360C. 

The WMP claims three independent risk assessments have been conducted on the wolbachia method. It says there is negligible risk associated with the release of wolbachia carrying mosquitoes; the bacteria is safe for people, animals and the environment. Hence, the concept has regulatory approval from government bodies in all countries where WMP has worked.

Other biological methods of combating aedes carry more risks. Genetically modifying mosquitoes to prevent breeding takes a very long time to affect large populations. Wiping out a local mosquito population could also have negative environmental consequences — the insects are an important part of the food chain. The WMP may now be looking to scale up the wolbachia method.