July 4. Good Science News #20 Malaria vaccine: Fighting fire with fire.
Updated: Jul 10, 2021
The malaria parasite, also known as a plasmodium, has a complicated life-history. Starting at the business end of a mosquito, the cycle begins when an infected mosquito bites a person, injecting them with a form of the parasite known as a sporozoite. That then circulates in a the infected person’s blood, finally taking up residence in the liver. Within the liver cell, the plasmodium, changes form again, and ultimately ruptures from the liver cells as a merozoite.
Merozoites affect red blood cells, and it is this stage of the plasmodium that causes the manifestations of malaria. In the blood cells, the merozoites once again become schizonts, a form of plasmodium that then can produce gametocytes, the male and female form of the plasmodium.
The gametocytes circulate in the blood until they are taken up by a female mosquito. Ultimately, the gametocytes form sporozoites again, which take up residence if the mosquitos salivary glands and wait to infect the unsuspecting human.
There are researchers that are trying to develop a vaccine for malaria, and in an interesting man-bites-dog type of twist, they are using the plasmodium’s life cycle against itself. For the problem with a complicated life history, such as this one, is that if one aspect of it is disrupted, the entire cycle fails.
So just how is this done?
It begins with a deliberate inoculation of volunteers with the sporozoite. Along side the living sporozoite, the volunteers is also injected with chemicals that kill the parasite once it enters a liver cell. Despite the fact that living sporozoites are circulating in these people, they are killed before they can reach the red blood cell and cause the disease.
A distinct advantage of using the whole sporozoite within a vaccine, is that it offers the immune system a variety of targets to build antibodies against rather than just one or two. And while this has been done for some viral diseases, including yellow fever and measles, its use relative to developing a vaccine for malaria has met with only limited success.
Despite the success, there are problems that remain to be solved. One is where to get a large and continuous supply sporozoites. Until recently the sporozoites were harvested from the salivary glands of infected mosquitos, a laborious task. Work is currently underway, however, to create sporozoites outside of the mosquito, a necessity if the vaccine is going to be able to go into production. While this is a challenging prospect, efforts toward this end have been promising.
Mosquito control using pesticides and the use of mosquito netting in infected regions have been and are effective tools to combat malaria. However, insecticide- and drug-resistant mosquitos and the parasites that they carry are evolving naturally, creating a need for constant vigilance in the battle against this vexing disease.
To keep up the fight, we need new tools, and the sporozoite-based vaccine may be a highly effective one once its development has been completed.
For the remaining countries in the world that have yet to eradicate malaria, this vaccine may be just the tool that they need.