In malaria-endemic regions, humans are often infected repeatedly with the Plasmodium parasite, and the consequences of such multiple infections are under intense study. In contrast, little is known about possible co-infection and its consequences in the Anopheles mosquitoes that transmit the disease. A new study reports that not only can individual mosquitoes accumulate infections from multiple blood feeds, but also that an existing malaria infection makes mosquitoes more susceptible to a second infection, and that infections reach higher densities when another strain is already present.
The researchers set up cages of female Anopheles mosquitoes and allowed them at defined times to feed on mice infected with two different Plasmodium strains. They found that mosquitoes can accumulate mixed strain malaria infections after feeding on multiple hosts, and found that 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 large parasite numbers do not appear to kill the insects, and as 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 (including human) hosts, with implications for the evolution of parasite virulence and the spread of drug-resistant strains.
Existing Infection Facilitates Establishment and Density of Malaria Parasites in Their Mosquito Vector. (2015) PLoS Pathog 11(7): e1005003. doi: 10.1371/journal.ppat.1005003
Very little is known about how vector-borne pathogens interact within their vector and how this impacts transmission. Here we show that mosquitoes can accumulate mixed strain malaria infections after feeding on multiple hosts. We found that parasites have a greater chance of establishing and reach higher densities if another strain is already present in a mosquito. Mixed infections contained more parasites but these larger populations did not have a detectable impact on vector survival. Together these results suggest that mosquitoes taking multiple infective bites may disproportionally contribute to malaria transmission. This will increase rates of mixed infections in vertebrate hosts, with implications for the evolution of parasite virulence and the spread of drug-resistant strains. Moreover, control measures that reduce parasite prevalence in vertebrate hosts will reduce the likelihood of mosquitoes taking multiple infective feeds, and thus disproportionally reduce transmission. More generally, our study shows that the types of strain interactions detected in vertebrate hosts cannot necessarily be extrapolated to vectors.