Plasmodium falciparum (revisited)

This is the seventh and final post in a series of blog posts written by students from my third year Evolutionary Parasitology unit (ZOOL329/529) class of 2014. This particular post was written by Brianna Barwise about how Plasmodium falciparum - the most deadly strain of malaria - give their mosquito host a sugar craving (you can read the previous post about how the Emerald Cockroach Wasp acquires its skills for wrestling a cockroach into a submissive zombie here).

Photo by AFPMB

Ever felt like you had no control over your sugar cravings? Well, for mosquitoes infected with the apicomplexan parasite Plasmodium falciparum, that can certainly be the case. A recent study conducted by Vincent Nyasembe and colleagues have found that parasitized Anopheles gambiae mosquitoes have a significantly increased sugar uptake and attraction to nectar sources. Mosquitoes infected with the oocyst and sporozoite stages of P. falciparum showed a respective 30% and 24% increase in attraction to plant odours than those that were uninfected, with a respective 70% and 80% increase in probing activity. The study also revealed an increase in sugar uptake of those mosquitos infected with the oocyst stage of the parasite.

The relationship between these microscopic parasites and their pesky insect hosts is actually one of upmost importance to people. Plasmodium falciparum causes malaria in humans, and is transmitted by female mosquitos of the Anopheles genus. Anopheles gambiae is one of the most efficient malaria vectors and with the majority of malarial death being caused by P. falciparum, developing our understanding of this host/parasite relationship is crucial.

Behavioural manipulation of hosts by parasites to enhance their own survival and transmission rates has been well documented - from viruses that alter the egg-laying behaviour of wasps to hairworms that cause their landlubber cricket hosts to plunge themselves into water. Previous studies of Plasmodium parasites have shown they manipulate vector behaviour, with infected vectors having an increased attraction to their vertebrate hosts for a blood meal. However, this is the first study to demonstrate changed behaviour in mosquito vectors towards nectar sources.

Feeding on nectar is critical for the survival of malaria vectors. Increased attraction to nectar sources and sugar uptake could be explained by parasite manipulation to increase available energy for its own metabolism and improve survival of its vector (and thus likelihood the parasite will be transmitted). Further, an increased attraction to vertebrate hosts during non-transmissible stages of the parasite's development would be disadvantageous as it increases the risk of vector mortality. Thus, selective pressure would favour the parasite to drive a preference for nectar feeding during this time.

Alternatively, the change in behaviour could be attributed to a compensation made by the mosquito itself for the energy deficit created by the parasite. Generally speaking, parasitic infection inflicts energetic costs in the host vector, which leads to a decrease in reproductive potential and reduction in lifespan. Nyasembe suggests that it is possible the increased probing is to satisfy its own metabolic demands along with that of the parasite growing inside it.

Therefore, further study is required to establish whether the increased attraction to nectar sources and sugar uptake is a physiological adjustment by An. Gambiae in response to infection or if it is direct behavioural manipulation by the parasite. Either way, unlike much of humanity, at least they have some excuse for consuming an excess of sugar.

Reference:
Nyasembe, V., Teal, P., Sawa, P., Tumlinson, J., Borgemeister, C. & Torto, B. 2014. Plasmodium falciparum Infection Increases Anopheles gambiae Attraction to Nectar Sources and Sugar Uptake. Current Biology 24: 217-221.

This post was written by Brianna Barwise