Photo courtesy of the CDC
Repeat infection with malaria parasites might make mosquitoes more dangerous, according to a study published in PLOS Pathogens.
The research showed that individual Anopheles mosquitoes can accumulate infections with different strains of malaria parasites, an existing malaria infection makes mosquitoes more susceptible to a second infection, and infections reach higher densities when another strain is already present.
These phenomena could promote the spread of drug-resistant malaria, according to investigators.
Laura Pollitt, PhD, of the University of Edinburgh in the UK, and her colleagues conducted this research.
They wanted to determine if and how mosquitoes can be infected with different strains of Plasmodium parasites, how such heterogeneous parasites interact in the insects, and whether such interactions affect the transmission of malaria to vertebrate hosts.
The investigators set up cages of female Anopheles stephensi mosquitoes and allowed them, at defined times, to feed on mice infected with 2 different Plasmodium chabaudi strains—AJ and ER.
This study design allowed the team to examine how the presence of a co-infecting strain affects parasites that enter the vector first and second, and to test whether co-infection impacts mosquito survival.
The investigators found that mosquitoes can accumulate mixed-strain malaria infections after feeding on multiple hosts. And parasites have a greater chance of establishing a secondary infection if another Plasmodium strain is already present in a mosquito.
Moreover, the presence of the primary infection facilitated replication of the secondary infection while the first infection developed as normal. This resulted in doubly infected mosquitoes having substantially higher parasite loads.
The investigators noted that the large parasite numbers do not appear to kill the insects. And, because it is expected that mosquitoes carrying more parasites are more likely to transmit malaria to vertebrates, mosquitoes taking multiple infective bites might disproportionally contribute to malaria transmission.
This, in turn, would increase rates of mixed infections in vertebrate hosts, with implications for the evolution of parasite virulence and the spread of drug-resistant strains.
