Four species of the mosquito-borne plasmodium parasite cause malaria, but scientists are now discovering that a small population of the protozoan infecting the human body could save millions of lives.
About 500-million people are infected each year by the parasite and malaria remains one of the deadliest diseases facing humanity. Malaria kills about two people, mainly children, every minute. That’s more than a million victims a year around the globe, though most victims are Africans.
Plasmodium falciparum is the only species that generally causes death if not treated. The microbes are carried by the female anopheles mosquito, which injects them with her saliva into the host from whom she is taking her blood meal.
The name malaria comes from the Italian mala aria, meaning bad air, because of the disease’s association with marshy ground, the main breeding place of the mosquito. The mosquito can fly a kilometre or so, but generally remains close to its larval development site.
Malaria vaccines stimulate the production of antibodies that bind to invading infectious cells and neutralise them. But there are several malaria strains, so the body struggles to keep sufficient quantities of antibodies that can act against all the different types. Vaccination against malaria is seldom attempted for this reason.
Now researchers have injected a group of volunteers with about 30 parasites each. The parasites are really tiny and you need a good microscope to see them, so this is a ticklish business. The researchers found that human bodies inoculated with a few parasites develop more of the special T cells that fight disease.
There are several types of T cells, but they generally activate our natural killer cells such as macrophages, one of the five kinds of white blood cells that rid our bodies of cells infected by viruses or transformed by cancer. T cells work by secreting substances known as cytokines, potent chemical messengers whose actions include encouraging healthy cell growth and generally directing cellular traffic.
After an infection, T cells direct other cells to build up numbers and create a defence against similar invaders — such as the large number of parasites introduced by a mosquito.
The volunteers were dosed with the most deadly species of the malaria parasite, Plasmodium falciparum. Some were given drugs to clear the infection after a week and some after two weeks, which is long enough for the symptoms to appear.
Results of the study published in the British journal New Scientist shows that all the subjects were protected from later malaria infection. In one person the parasites grew, but he didn’t get ill. None of the volunteers produced antibodies, but all had T-cell mediated protection.
After infection, the parasite hides in the liver, where it multiplies up to 40 000 times before it bursts out after one or two weeks and invades the blood cells, the stage when patients exhibit the high fever associated with malaria.
The huge amount of research into malaria aims to prevent this multiplication by interrupting the life cycle in the mosquito with vaccines or by direct chemical means. So inoculation with just a few parasites remains an attractive possibility for curbing the deadly disease.