Mosquitoes and the spread of dengue and Chikungunya virus

The Asian tiger mosquito, Aedes (Stegomyia) albopictus, is an efficient vector of all serotypes (distinct variations within a species of bacteria or viruses or among immune cells of different individuals) of dengue, and the three genotypes of Chikungunya virus, which causes fever, fatigue, and joint swelling. Ae. albopictus is an invasive species and over the last 30 years it has expanded its territory to more than 40% of the earth’s terrestrial landmass.

 

Mosquito Adult
Mosquito*

The Chikungunya virus is traditionally native to tropical Africa, but since 2006, there have been increasing numbers of outbreaks recorded outside of this normal range. This could be due to a mutation in the virus’s envelope gene, which increases the replication and dissemination capacity of the virus in Ae. albopictus [4].

 

During the second quarter of 2014, outbreaks of the Chikungunya virus have been detected even further afield, throughout much of the Americas, with major outbreaks occurring in several Caribbean nations, and local transmission confirmed or suspected in the United States, Panama, Venezuela, Peru, and Chile. This creates an imminent threat for humans throughout the Americas, who have had no prior exposure to this infection [5].

 

In May 2014, the first cases of Chikungunya disease were reported in Panama; most of the cases were in non-residents who most likely picked up the virus in their Caribbean countries of origin. Since then, Ae. albopictus has expanded across Panama, yet there has been no information regarding the degree of expansion or about the factors contributing to the geographic expansion of Ae. albopictus.

 

Mosquitoes Leverage Road Networks, Not Untouched Landscapes, to Migrate.

With this in mind, scientists at the Smithsonian Institution decided to examine how human activity spreads Ae. albopictus. Their findings appear in an article in the journal, “PLOS Neglected Tropical Diseases.”

 

Panamanian health authorities first detected Ae. albopictus in 2002 and kept tabs on its spread from Panama City. This comprehensive data — uncommon in many tropical nations — coupled with years of mosquito surveys by the Smithsonian Tropical Research Institute, suggest that Ae. albopictus relies on road networks to disperse.

 

The researchers created competing species distribution models (SDMs) via maximum entropy machine learning algorithms using the Maxent software package to evaluate the factors associated with Ae. albopictus expansion. An SDM based only on the road network best predicted the 2010-2013 distribution of Ae. albopictus in Panama. This was in comparison to SDMs based on climate or human population density, or even to models that included roads and other factors combined.

 

“The vector is not moving organically across the landscape,” said Matthew Miller, lead author of the study.

 

Asian Tiger Mosquitoes may also Fill Ecological Niche Left by Yellowfever Mosquito Population reduction programs.

In May 2014, Panama allowed a British company called Oxitec to release transgenic yellowfever mosquitoes (Aedes aegypti) in order to combat dengue, another disease that is vectored by mosquitoes. As mosquitoes begin to become resistant to insecticides, researchers have begun looking at alternative methods of pest control in order to prevent the transmission of disease. Oxitec mosquitoes produce offspring that are unable to survive to adulthood.

 

However, according to the Smithsonian scientists, as the Ae. aegypti populations decrease, they may be replaced by Ae. albopictus, because the Asian tiger mosquito could fill the niche that the yellowfever mosquitoes occupied.

 

“The two mosquito species are so ecologically similar that, by depressing Ae. aegypti populations, the chances that Ae. albopictus is going to competitively displace it, may increase,” said Miller. “This research is relevant to the study of introduced disease vectors everywhere.”

 

Conclusion

Understanding that the Ae. albopictus mosquito is using the Panamanian road network as a means of spreading, the authors of this research recommend that health authorities fumigate vehicles at check points. There are already a number of these check points set up throughout Panama to prevent screwworms from spreading from Colombia to North America. Checkpoint fumigation could be an effective means of preventing Ae. albopictus from reaching areas where it has not yet been detected.

 

The results also tell a cautionary tale in the face of proposals to release genetically modified Ae. aegypti (GM or Genetic Modification programs). Given that Ae. aegypti has similar demographic and dispersal patterns as Ae. albopictus [13], Ae. aegypti populations may quickly rebound via recolonization after cessation of GM programs.

 

It highlights that GM strategies might have only short-term effects on vector population size and may commit Panama to a repeated and costly program for long-term arbovirus control [20]. Additionally such programs could increase the chance that Ae. albopictus displaces Ae. aegypti, making the GM program less relevant.

*Image courtesy of Free Digital Images