Vaccination may turn malaria parasites more virulent

Washington - It is possible that more-virulent strains of malaria might evolve if a malaria vaccine goes into widespread use, researchers at Penn State University say.

They found that malaria parasites evolving in vaccinated laboratory mice become more virulent.

The mice were injected with a critical component of several candidate human malaria vaccines that now are being evaluated in clinical trials.

“Our research shows immunization with this particular type of malaria vaccine can create ecological conditions that favour the evolution of parasites that cause more severe disease in unvaccinated mice,” said Andrew Read, Alumni Professor of Biological Sciences at Penn State.

“We are a long way from being able to assess the likelihood of this process occurring in humans, but our research suggests the need for vigilance,” Read said.

The research showed that more-virulent malaria parasites evolved in response to vaccination, but the exact mechanism is still a mystery. It was not due to changes in the part of the parasite targeted by the vaccine.

Most vaccine developers use only small sections of the malaria parasite to produce an antigen molecule that then becomes a key ingredient in a highly purified malaria vaccine. Read’s lab tested the antigen AMA-1, a component of several such vaccines now in various stages of clinical trials.

“Our laboratory experiments followed clues from theoretical studies and earlier experiments that suggested that some malaria vaccines could favour the evolution of more-virulent malaria parasites,” Read said.

 If candidate vaccines do not completely eliminate all the malaria parasites, the parasites that remain have opportunities to evolve. A mosquito then could transfer the evolved parasite from the vaccinated person into a new host -- a process called leaking.

“Leaky vaccines create a situation that further fosters parasite evolution,” Read said.

The Penn State study found that parasites causing worse malaria symptoms in unvaccinated mice evolved after “leaking” consecutively through as few as 10 vaccinated mice.

“The parasites that are able to survive in the immunized hosts must be stronger after having survived exposure to the vaccine. The vaccine-induced immunity apparently removed the less virulent malaria parasites, but left the more virulent ones,” Read added.

The study will be published in the forthcoming issue of the scientific journal PLoS Biology.