6 March 2012

Irradiated parasites? Good for your health?

Malaria is one of the biggest killers of man, with 300 - 500 million cases of the disease each year, causing 2 - 3 million deaths worldwide. Sadly, the majority of these deaths are children younger than 5 in sub-Saharan Africa. The disease is caused by small, protozoan parasites from the Plasmodium family, which predominately live in their host's erythrocytes (red blood cells) as merozoites, where they replicate by dividing over and over again. Eventually, an erythrocyte becomes so full of parasites that it ruptures and they burst out into the bloodstream and invade the surrounding red blood cells. And, once inside, they begin to divide all over again. It is this bursting of red blood cells that causes the symptoms of malaria, which include fever, headaches, sweating, coughing and muscle pains.

Red blood cells that have been infected with malaria and have ruptured. Small merozoites can be seen in the surrounding plasma that will enter and infect the nearby red blood cells.

Despite scientists having a good understanding of the complex life cycle of malaria and knowing that it is spread by female Anopheles mosquitoes (male mosquitoes drink plant fluids and not blood, so cannot spread the disease), no effective vaccine has yet been developed - the best we have at the moment is drugs that can prevent malarial infection in the short term by regularly taking a drug. Chloroquine is probably the most well known example of an anti-malaria drug. However, whereas this method of prevention is useful to those living in rich, developed countries as they can afford to buy the often expensive drugs, it is not much use in the poorer countries where malaria is actually endemic. This is mainly because the people there can't afford the drugs in the first place and there is often no way of getting drugs to people in isolated villages, as many of the countries lack a proper road network. This however, may be about to change.

New research by Sanaria, supported by the Bill & Melinda Gates Foundation, has identified a possible and highly promising vaccine for malaria called PfSPZ that protected 6 out of 7 participants in a clinical trial, providing them with lasting protection for about two months. The vaccine is made by irradiating mosquitoes that have been infected with malaria and then by harvesting sporozoites (the infectious form of its life cycle) from their salivary glands. These weakened, or attenuated, parasites are then used as the basis of the vaccine. When a weakened parasite such as this, is injected into an individual it is unable to make them as sick as it could have normally, meaning that the individual's body 'learns' how to kill malaria without the usual dangers from the disease. Once their body has 'learnt' how to kill malaria parasites, it remembers for a long period of time. This means that if the vaccinated person is bitten by an infected mosquito, their body will be able to mount a much more effective immune response in a shorter period of time and they will not become as ill as they otherwise would have, meaning that they are much more likely to survive the disease.

A researcher harvesting sporozoites from an irradiated mosquito's salivary glands.

Despite showing great promise at clinical trials, this vaccine has one major and unfortunate limitation. Sporozoites are tiny and an infected mosquito usually only carries around 1, 000 of them. Therefore, unless a more efficient method of producing irradiated sporozoites is developed, it is unlikely that the drug will ever become commercially viable as a vaccine for malaria, simply due to the sheer number of mosquitoes that would need to be harvested and the amount of time and effort that this would take. So, despite its potential, PfSPZ may never make it into mass production. Still, the vaccine holds great promise and in a number of years, it may potentially save millions of lives every year.

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