Infection with one strain of certain diseases yields some protection against others, but malaria is the reverse. Being bitten by mosquitoes carrying the two most common species of the malaria parasite can lead to a bout of disease more severe than either on its own. New research on rats has revealed a possible mechanism for this combination.
Most deaths from malaria are the result of the Plasmodium falciparum parasite, but the usually milder Plasmodium vivax is by far the most common form of malaria in Asia and the Americas. Understandably, two infections in quick succession can overwhelm the immune system's defenses, but doctors have been puzzled as to why such a double dose for malaria appears to produce something even worse, a sort of synergistic effect.
Professor Sarah Reece of the University of Edinburgh has provided an answer in Ecology Letters, based on the way the body reacts to P. falciparum infections.
P. falciparum infects any red blood cells it can find, destroying millions in the process. The body responds by producing fresh cells. The problem is P. vivax only infects young blood cells. Under normal circumstances, this helps damp down the severity of a P. vivax infection. With older cells uninfected the blood has more scope to keep doing its job.
However, when a partially cleared P. falciparum infection has triggered the production of so many new cells, P. vivax is like a kid in a candy shop, surrounded by opportunities to gorge itself. In this case, however, it is the host that gets sick.
Reece demonstrated the interaction using P. chabaudi and P. yoelii, two species of malaria that predominantly infect rodents, but have similar generalist and specialist strategies to falciparum and vivax.
The discovery may open possibilities for understanding other examples where parasites interact within sick individuals, and not only when the two species are closely related. The paper observes that the “nematode Nippostrongylus brasiliensis alters resources available for Plasmodium chabaudi (rodent malaria) during co-infection of mice. Interactions also occur via the host immune response if one species interferes with, or enhances, attack on a co-infecting species.”
"Immune responses are assumed to determine the outcome of interactions between parasite species but our study clearly shows that resources can be more important. Our findings also challenge ideas that one species will outcompete the other, which explains why infections involving two parasite species can pose a greater health risk to patients." Reece said in a statement.
Such co-infections are not rare. Estimates vary by region, but in Asia as many as a third of malaria cases involve co-infection.
The paper raises questions as to whether some of the focus of malaria research, currently concentrated on fighting P. falciparum, should be shifted to P. vivax.
