This essay was written by Tony Holder and was first published in the 1995 Mill Hill Essays.
What is malaria? It is a disease that is caused by a parasite and is spread by the bite of infected mosquitoes. It is also a disease of which few in Britain or the rest of Western Europe have had experience because it is largely restricted to tropical and subtropical areas of the world. Once it was much more widespread, indeed it was present in Britain in the seventeenth century, but climate change, human agricultural activities and control measures have now confined it to those areas. In the 1950s and 1960s there was even a hope that it could be eradicated but malaria is now returning with a vengeance and there is a desperate need to develop new ways to control it.
The problem of malaria is massive. The disease is estimated to be responsible for 1-2 million deaths each year, and most of these are young children in Africa. On top of this there are several hundred million other cases reported each year that drain precious resources from health care facilities and have a tremendous impact on society and economic activity. Over forty percent of the world’s population is at risk from the disease and the incidence of malaria is increasing as the disease moves back into areas from which it had once been eliminated. Why has control broken down? Insecticide spraying campaigns to control the mosquitoes which transmit the disease have been abandoned or become ineffective as the mosquitoes have become resistant to insecticides. In addition the parasite which causes the clinical symptoms is becoming increasingly resistant to the drugs used to treat it. For example a cheap and once effective drug called chloroquine is now useless in large parts of the world. Even more worrying is the fact that there are few new drugs to take the place of chloroquine. New, radical approaches are needed to control the disease, to try to get one step ahead, or at least prevent the situation deteriorating even further. These measures will need to be at many levels, from basic research, through a better application of current knowledge, to more effective implementation of existing control strategies.
There are four main types of microorganisms that cause infectious diseases, diseases that spread from one individual to another; these are viruses, bacteria, fungi and a broad group that are collectively called parasites. Malaria is the most important parasitic disease. Most of the infectious diseases with which we in the United Kingdom are familiar are caused by viruses and bacteria, but in large parts of the rest of the world parasites cause an additional massive health burden. Parasites can be divided into two large groups: those that are a single cell and those that are multicellular, with many different cells (all living things are composed of one or more cells; each human being is made of many thousands of billions of cells). The malaria parasite is a single cell, but despite this apparent simplicity it has a complex parasitic life style, spending part of its life in a mosquito and part in a larger animal. There are different sorts of malaria parasites that have adapted to life in a variety of animals, including lizards, birds, small mammals like rats and larger animals like humans. However, only one type of mosquito (the Anopheles mosquito) can become infected by the malaria parasite.
The female Anopheles mosquito must feed on blood in order to lay eggs. When it bites to take a meal of blood the infected mosquito passes on the parasite. As soon as the malaria parasite is in the bloodstream of the human body it needs to hide within a cell to avoid the body’s defence system. It is carried by the blood circulation to the liver and enters a liver cell. Once inside it begins to change its form and most importantly each parasite multiplies many times, giving rise to several thousand new ones. These new parasites then begin the next stage of the infection. They burst out of the liver cells and home in on the red blood cells which carry oxygen around to all parts of the body. Each newly produced parasite sticks to the outside of a red blood cell and then enters it, again hiding from the body’s defences. As in the liver cell, the parasite multiplies again, giving rise to ten to twenty identical daughter parasites. In turn these burst out of the red blood cell and find new, undamaged red blood cells to invade, and so the cycle continues. It is this stage of the parasite’s life cycle that is responsible for the disease of malaria. One symptom is a fever which recurs every time the cycle of red blood cell invasion takes place, but more severe infections can lead to coma and death by blocking the blood vessels that supply oxygen to the brain with infected red blood cells. For the parasites to continue to survive they must be transmitted from one human to another and so they have to get back into a mosquito. To do this, some of the parasites within the infected red blood cells change into yet another form which, if swallowed by a mosquito when it feeds, will result in the mosquito becoming infected. This infection will ensure that the parasites can get back into the saliva of the mosquito, ready to invade other individuals.
The body is not defenceless to invasion by microorganisms. It has a sophisticated series of defences that are designed to deal with different sorts of infectious diseases. These defences are largely of two sorts; specialised cells and specialised proteins. One type of specialised proteins are antibodies which can stick to invading microorganisms and disable them or allow other specialised cells to kill them. These specialised cells and proteins constitute the immune system which has evolved to protect the body from microorganisms. One feature of the immune system is that it is more effective at repelling and killing invading microorganisms if it has been previously exposed to that particular organism. Once one attack has been successfully repelled the defences against that organism are strengthened so that any further attack is dealt with more effectively. This is the basis of vaccination against disease. Vaccination has proved to be a very effective way to deal with some bacteria and viruses, for example whooping cough, tetanus, poliomyelitis and measles. Either an inactivated or crippled microorganism, or an important part of it, is introduced into the body. The immune system recognises the material in the vaccine to be foreign and therefore responds to try to eliminate it. Next time the immune system encounters this foreign material it recognises it much more quickly. Vaccination therefore seeks to prime the defences so that they are more prepared and can be called upon more quickly to deal with an attack from the microorganism.
How can the deadly cycle of infection and transmission of the malaria parasite be controlled? One area of current research is to try to understand how the body’s defences can control infection and then use this information to try to develop an effective vaccine. It is known that people who live in areas where malaria is a major problem do eventually develop a good defence to the parasite after repeated attacks of malaria. Researchers are, therefore, optimistic that a vaccine could be developed that would give a high level of protection before the first infection. This would be particularly important as a way of protecting very young children in malarious areas, or visitors (such as tourists and military personnel) who would be very vulnerable to an attack of malaria. To pursue this strategy we need to identify the vulnerable stages of the parasite and use these weak points to design approaches that can be used to combat the disease. There are two phases in the parasite life cycle when it is directly exposed to the immune system; when it has just been injected by the mosquito and has not yet reached its first hiding place in the liver, and when in order to multiply it bursts out of one cell to infect others. Indirectly, even when the parasite is hiding within a cell, the immune system may recognise that infected cell as unusual or foreign and target it for destruction.
Not only does the parasite hide inside cells but it changes its appearance at the different stages. So for example, the form that is injected by the mosquito and makes its way to the liver (called a sporozoite) is quite different from the form that binds to and invades red blood cells (called a merozoite). The form that infects mosquitoes is also very different. This means that the defences that might be effective against one form will not be effective against another and a number of vaccination strategies have therefore been developed. One strategy is to prepare the body to defend against the initial infection by the sporozoite delivered by the mosquito. The advantage of this strategy is that if successful it would prevent any multiplication of the parasite in the liver and stop the infection at this initial stage. But what would happen if the defences were incomplete and one or more parasites slipped through and got into liver cells? Because the appearance of the parasite changes, the defences that had been prepared for the first stage would not recognise the new form of the parasite and therefore would be ineffective in preventing the next stage of the parasite’s life cycle. In the same way, defences built against the form responsible for red blood cell infection would not recognise sporozoites. So a second strategy is to not try to block the initial infection, since this doesn’t cause any symptoms anyway, but to target the merozoite form that invades red blood cells. After all, this is the stage responsible for the disease. A third approach is to try to prime the immune system to recognise liver cells which become infected as being “foreign” so that specialised cells of the immune system that detect modified or abnormal cells could destroy them, and in yet another vaccination strategy the aim is to stop the parasite infecting the mosquito, thereby preventing it being spread to other humans. Such a transmission blocking vaccine would need to be administered to a large proportion of the people living in an area in order to be effective. Obviously it might be ideal to combine several of these strategies to prepare the body to defend against all the different stages of the parasite life cycle, if this could be achieved.
The surfaces of all cells are covered by proteins. The proteins on the surface of the malaria parasite are recognised as foreign by the immune system and therefore if the body can be exposed to malaria surface proteins by immunization before the parasite attacks, the immune system will be primed to destroy the parasite. For such a strategy large quantities of the malaria surface proteins must be produced and there are two ways in which this can be done.
Proteins are made from chemical compounds called amino acids joined together to form long strings. There are twenty different amino acids and the sequence in which they are joined is unique to each individual protein. In the laboratory, amino acids can be chemically joined into such strings which have the amino acids in the same sequence as parts of any selected protein. Unfortunately there are technical limitations to the length of the string that can be made in this way and so to make larger parts of proteins alternative methods are used which involve techniques of genetic engineering. If the malaria gene which carries the information for the synthesis of the protein we wish to use in the vaccine can be identified, it can be transferred to another type of cell, for example a bacterium, which will then produce large amounts of the protein.
How are the important proteins for inclusion in a vaccine to be identified? The parasite has about twenty thousand genes, each coding for a distinct protein, so finding the small number that are responsible for the proteins of interest can be a daunting task. But the search can be narrowed down. First of all we can concentrate on a particular stage in the life cycle of the parasite. The appearance of the parasite is different at each stage because a different set of cell-surface proteins is made, and therefore only the set of genes corresponding to these proteins need to be considered. Identification of the proteins that are recognised by the immune system of people who have had malaria and have therefore developed immunity, can narrow down the search even further.
Although vaccines for viral and bacterial diseases have existed for many years, this is not the case for the malaria parasite or any other parasitic disease of humans. This is in part because it was very difficult to isolate the organisms, but in the mid 1970s there were three developments that had an important impact on vaccine research. The first was being able to reproduce one stage of the parasite life cycle in a test tube. For the first time it was possible to maintain infected red blood cells in culture providing the starting material for many research investigations. The two other developments were the technologies of genetic engineering and monoclonal antibody production. Genetic engineering techniques allow any gene to be isolated and analyzed and transferred between cells of different species; monoclonal antibodies allow the proteins coded by them to be identified, characterized and purified. As a result many proteins have been identified, including those on the surface of the different forms of malaria and the corresponding genes isolated and transferred into bacteria to enable the proteins to be made in large amount.
Despite all of these advances, progress towards developing a malaria vaccine has not been straightforward. This is illustrated by research towards developing a vaccine against sporozoites, the form injected by the mosquito. A single major protein was identified on the surface of the sporozoite by using a monoclonal antibody, and the sequence of the amino acids in the protein was determined. Parts of the protein were synthesised chemically and the complete protein itself was also produced in bacteria. Both types of product were used in separate studies to immunise volunteers who then agreed to allow infected mosquitoes to feed on them. Subsequently blood samples from the volunteers were examined for evidence of parasite infection. Disappointingly no evidence of protection against infection was found; in this case the vaccination had failed.
There has been a lot of interest and publicity surrounding another malaria vaccine that has been developed in Colombia. This contains synthetic fragments of three proteins identified in the infected red blood cell, stitched together into one long fragment. This vaccine has been given to many thousands of people living in malarious areas in South America and the rate of infection of these people was measured relative to that of other groups living in the same area who did not receive the vaccine. In this case the volunteers were not tested by allowing infected mosquitoes to feed on them but by random bites from infected mosquitoes in their environment.
Combining the results of all the studies, there appeared to be protection against the malaria parasite because fewer clinical cases were found in the group which received the vaccine. These results were quite encouraging, so the Colombian vaccine has also been tested in Africa. In this case the testing was done in areas where the incidence of malaria is much higher than in South America, and in children who were much more vulnerable because their defences had not had chance to be primed by a previous infection. The results of two African studies that have been done so far have not been very promising. The degree of protection observed was quite low although it is difficult to estimate accurately because only small numbers of children could be included in the studies.
In view of these disappointing results, is a malaria vaccine a real possibility? It is still too early to say. What is clear is that for success, researchers will have to have much better knowledge of which parasite protein the immune system needs to be primed to recognise. There are a number of candidates. For example, there is currently much interest in another surface protein, this time from the surface of the merozoite stage of the life cycle. In this case laboratory studies have shown that immunization with fragments of this protein produces antibodies that block the invasion of the red blood cells by the parasites. The challenge now is to produce this protein in a vaccine that brings a vigorous response from the immune system. With this and several other current studies there must still be grounds for optimism that an effective malaria vaccine will one day be a reality and contribute to the armoury of ways of eliminating this most dangerous parasite.