Mosquito behavior may be immune response, not parasite manipulation

Malaria-carrying mosquitoes appear to be manipulated by the parasites they carry, but this manipulation may simply be part of the mosquitoes' immune response, according to Penn State entomologists.

Sporozoites, the infections malaria parasites found in the mosquitos salivary glands.

Credit: © The Read Group, Penn State

"Normally, after a female mosquito ingests a blood meal, she matures her eggs and does not take another one until the meal is digested," said Lauren J. Cator, postdoctoral fellow in entomology and a member of the Center for Infectious Disease Dynamics, Penn State. "If infected, however, mosquitoes will wait to eat until the parasites developing within the gut mature and migrate to the salivary glands."

It was thought that fasting until malaria could be transmitted was beneficial to the malaria parasite because if the female mosquito was not feeding, she was not being swatted. The return of hunger seemed to correlate with the migration of parasites to the salivary glands. The hungrier the mosquitoes are, the more they feed and the more chances to find new hosts.

Aedes aegypti

Nyamuk Aedes aegypti berasal dari Afrika Timur, kemudian menyebar kearah timur dan barat, di daerah tropis dan subtropis pada batas lintang 40¬o lintang utara dan 40¬o lintang selatan.
Nyamuk ini tersebar luas di daerah tropis dan subtropis di asia tenggara, terutama di daerah perkotaan. Penyebaran nyamuk kelingkungan pedesaan disebabkan adanya perbaikan sarana transportasi dan pengembangan sarana suplai air sampai kepedesaan.
Ketinggian dari pemukaan laut merupakan faktor yang paling penting bagi penyebaran nyamuk Aedes aegypti. Di India, nyamuk Aedes aegypti berada pada ketinggian 0 – 100 meter dari permukaan laut. Di negara negara asia tenggara penyebarannya hanya sampai ketinggian 1500 meter.

Bacterial infection in mosquitoes renders them immune to malaria parasites

Scientists funded by the National Institute of Allergy and Infectious Diseases (NIAID), part of the National Institutes of Health, have established an inheritable bacterial infection in malaria-transmitting Anopheles mosquitoes that renders them immune to malaria parasites. Specifically, the scientists infected the mosquitoes with Wolbachia, a bacterium common among insects that previously has been shown to prevent malaria-inducing Plasmodium parasites from developing in Anopheles mosquitoes. Before now, researchers had been unable to create mosquitoes with a stable Wolbachia infection that passed consistently from mother to offspring.

Using bacteria to stop malaria

Mosquitoes are deadly efficient disease transmitters. Research conducted at Michigan State University, however, demonstrates that they also can be part of the solution for diseases such as malaria. A study in the current issue of Science shows that the transmission of malaria via mosquitoes to humans can be interrupted by using a strain of the bacteria Wolbachia in the insects. In a sense, Wolbachia would act as a vaccine of sorts for mosquitoes that would protect them from malaria parasites. Treating mosquitoes would prevent them from transmitting malaria to humans, a disease that in 2010 affected 219 million people and caused an estimated 660,000 deaths. "Wolbachia-based malaria control strategy has been discussed for the last two decades," said Zhiyong Xi, MSU assistant professor of microbiology and molecular genetics. "Our work is the first to demonstrate Wolbachia can be stably established in a key malaria vector, the mosquito species Anopheles stephensi, which opens the door to use Wolbachia for malaria control."
First, Xi's team successfully demonstrated how Wolbachia can be carried by this malaria mosquito vector and how the insects can spread the bacteria throughout the entire mosquito population. Secondly, researchers showed that the bacteria can prevent those mosquitoes from transmitting malaria parasites to humans.
"We developed the mosquito line carrying a stable Wolbachia infection," Xi said. "We then seeded them into uninfected populations and repeatedly produced a population of predominantly Wolbachia-infected mosquitoes."
The basis for Xi's latest findings is connected to the success of his work using Wolbachia to halt Dengue fever. For this research, Xi focused on the mosquito species Aedes albopictus and Aedes aegypti. This work helped launch a global effort to develop Wolbachia-based strategies to eliminate dengue and other diseases.
The key to the malaria research was identifying the correct species of Wolbachia -- wAlbB -- and then injecting it into mosquito embryos. Out of the thousands of embryos injected by research associate Guowu Bian, one developed into a female that carried Wolbachia. The mosquito line derived from this female has maintained Wolbachia wAlbB infection with a 100 percent infection frequency through 34 generations. The number could grow higher as this is simply the last generation the researchers have bred thus far, Xi said.
The team then introduced various ratios of Wolbachia-infected females into a noninfected mosquito population. In each case, the entire population carried the bacteria in eight generations or less.
Using this promising approach to tackle malaria -- the biggest vector-borne disease -- gives scientists and world health officials another important tool to fight malaria.
Once Wolbachia has been released into a mosquito population, it is quite possible that it won't need to be reapplied, making it more economical than other methods like pesticide or human vaccine. This adds special value to the feasibility of this control strategy, considering most of the malaria endemic areas are suffering from poverty, Xi said.

Mosquitoes are deadly efficient disease transmitters. Research conducted at Michigan State University, however, demonstrates that they also can be part of the solution for diseases such as malaria.

Story Source: Michigan State University.

Journal Reference:

1. G. Bian, D. Joshi, Y. Dong, P. Lu, G. Zhou, X. Pan, Y. Xu, G. Dimopoulos, Z. Xi. Wolbachia Invades Anopheles stephensi Populations and Induces Refractoriness to Plasmodium Infection. Science, 2013; 340 (6133): 748 DOI: 10.1126/science.1236192