The Anopheles gambiae mosquito is the most important vector of human malaria, which kills half a million people a year. And now, an experimental study reveals that the malaria parasite isn’t the only human pathogen that these bloodsuckers are capable of passing on: They’re also transmitting diseases that are normally spread by fleas. The findings are published in Proceedings of the National Academy of Sciences this week.
The spotted fever bacterium Rickettsia felis is transmitted by the cat flea Ctenocephalides felis. In cats, rodents, and possums, rickettsiosis is largely asymptomatic. But Rickettsia felis was recently implicated as a human pathogen: It was first reported in humans back in 2002, and by 2011, there were more than 70 human cases spanning multiple continents. In sub-Saharan Africa, human patients suffering from this poorly understood infection are usually diagnosed with “fever of unknown origin.”
Until now, the cat flea has been the only confirmed biological vector. But researchers have been noticing how human Rickettsia felis infections are prevalent in malaria-heavy regions, and they occur more often during the rainy season when mosquito activity is high. Meanwhile, infection risk is the lowest in regions where Anopheles gambiae was absent or rare, such as Tunisia and Algeria.
So, an international team led by Philippe Parola of Aix-Marseille University explored the possibility that malaria-carrying Anopheles gambiae can also transmit Rickettsia felis. The researchers allowed mosquitoes to feed on either sheep’s blood or cellular media that were both infected with Rickettsia felis. For up to two weeks after they fed, the pathogenic bacterial DNA was found in the tissue of the mosquitoes.
After the mosquitoes were infected, the team also found Rickettsia felis DNA on the sugar-soaked cotton that was used to feed the mosquitoes. That means they can probably transmit bacteria through their saliva when they bite. In addition to the salivary glands, the team also found the bacteria in the ovaries of infected mosquitoes. Although, they don’t seem to transmit the infection to their offspring.
In another experiment, mosquitoes were allowed to feed on either infected or uninfected mice. Then, after infected mosquitoes fed on healthy mice, the mice developed the infection.
To be a vector, arthropods like mosquitoes, fleas, and ticks must be able to maintain disease transmission in the open environment, and that includes having just the right behavior and biology for the microorganism to develop its life cycle within the pesky arthropod. While researchers don’t quite understand the transmission cycle of Rickettsia felis just yet, these findings do suggest that Anopheles gambiae mosquitoes are capable of transmitting the pathogen in the wild.