ASM: Parasites sans frontiers

Alan Cowman's group is at the forefront of research into the world's most deadly parasitic disease - malaria. His research has helped us understand how the malarial parasite evades both the human immune system and the lethal effects of anti-malarial drugs, and the mechanism by which it can invade and remodel the human red blood cell.

The fundamental biological findings published by his group over many years are now informing efforts to identify vaccine candidates targeted at the red blood cell invasion stage.

Cowman will present the Rubbo Oration at this year's ASM annual meeting and will concentrate on his group's latest work, which is attacking the problem from two different angles.

Malaria is caused by obligate intracellular parasites of the Plasmodium genus. Infection is characterised by recurrent fever, chills, headache, muscle ache, vomiting and other flu-like symptoms, all of which can be very incapacitating.

Cowman's group works on Plasmodium falciparum, which causes the most severe form of malaria. Individuals infected with this species may develop complications such as brain disease (cerebral malaria), severe anemia and kidney failure, resulting in coma and death.

Cowman works directly on the P. falciparum parasite because it causes major disease and because there is no model organism. Many important questions about malaria are parasite-specific and cannot be addressed in other systems.

"That means it is very hard to work on," he says. "The parasite is difficult to genetically manipulate and we tend to have fewer tools available than in other areas of microbiology. So, it takes us a lot longer to progress.

"There are mouse models of malaria, but unfortunately they are not so relevant for the things we are looking at."

Fortunately, global efforts in prevention and research have been greatly increased recently due mainly to increased funding in particular from the Bill and Melinda Gates Foundation.

However, no vaccine is currently available to prevent malaria and groups like Cowman's are working very hard to reverse the situation.

---PB--- Understand thine enemy

The first stage of malaria infection begins when an infected female Anopheles mosquito partakes of a human blood meal. In doing so, it injects malarial sporozoite forms from its saliva into the host bloodstream. The sporozoites rapidly migrate to the liver where they infect hepatocytes within 30 minutes of the bite.

There they hang out and multiply for six to 15 days, before sneaking out of the liver as merozoites wrapped in liver cell membranes and re-entering the bloodstream to invade red blood cells.

The merozoites mature and replicate within the erythrocytes, periodically bursting out in lots of 16-32 to invade fresh red blood cells in a process that takes less than a minute. Several such amplification cycles of infection and parasite escape occur, correlating with the classical waves of fever symptoms.

In fact, this blood stage of infection accounts for all the morbidity and mortality associated with malaria. In the absence of drug treatment, infected individuals can die quite quickly from cerebral malaria. Others may die later from overwhelming anemia or organ failure.

In surviving individuals, the parasites may further differentiate into a form that is infectious for mosquitoes, thus permitting the parasite life cycle to continue.

This invasion and occupation of the human erythrocyte by malarial merozoites is key to the disease progression and has been a major focus of Cowman's research over many years. Cowman wants to understand exactly how the parasite invades and his group has published key findings on the process.

"We know that Plasmodium uses a range of ligands to bind and activate invasion of the red blood cell it infects, and that different strains of parasite express different ligands," he says. "Basically, this diversity is one of the parasite's ways of getting around the host immune responses.

"In addition, the malaria parasite has selected polymorphisms on the red blood cell surface over thousands of years, and developed mechanisms to get around those polymorphisms to invade."

The process is obligatory and parasite-specific, making the proteins involved potential targets for novel treatments.

The data generated by this fundamental research on erythrocyte invasion by the parasite feeds directly into identifying potential vaccine candidates. Cowman, together with his colleague Dr Brendan Crabb, received funding recently from the Bill and Melinda Gates Foundation to compile the best vaccine candidates to target the red blood cell/parasite interactions.

"The biggest obstacle to such a vaccine working is variation - that is, antigenic diversity within the parasite. There is a huge amount of selective pressure on the parasite to generate this diversity to outsmart the host defences and continue its life cycle. "This parasite is a master burglar, as different forms of the malarial parasite have different keys to locks on the red blood cell surface and can use these separately and in different combinations.

"A successful vaccine probably needs to block all those entries. We think we have now identified all these pathways and are close to identifying the best protein and domain combinations to use in a potential vaccine."

---PB--- A global challenge

The second vaccine focus of Cowman's group is a collaborative effort with laboratories in the US to develop an attenuated vaccine against malaria that targets the liver stage of the disease. Cowman's laboratory developed the ability to knock out P. falciparum genes in 1997, making the possibility of genetically attenuated vaccines a reality.

Subsequently, one of Cowman's collaborators in Seattle identified the key genes that need to be knocked out to block development of the parasite in the liver stage. Without these genes expressed, the Plasmodium develop to a certain point and then stop, meaning that in vivo the malaria parasite still gets transmitted but does not cause disease.

The attenuated vaccine project is at a more advanced stage than the invasion-targeted effort and it is hoped that clinical trials will begin soon at the Walter Reed Army Medical Centre in Washington. The trials will be the first to test a genetically attenuated live parasite in a vaccine.

"We have used genetic manipulation technology over many years to get to this point, and are confident that the attenuated organism will work as it should mechanistically," Cowman says.

"The vaccine has been validated already, both in mouse models and using gamma-irradiated merozoites. The largest obstacle to this type of vaccines now is delivery of a live parasite in developing countries. Storage, handling and distribution of the vaccine - these are all serious logistical problems that need to be overcome if this approach is going to be effective."

Current measures to prevent malaria infection and the spread of disease are still centred on vector control measures and anti-malarial drugs. The distribution of drugs and even insecticide-impregnated bednets in affected areas remains a problem, although much less so now with global funding efforts such as the Gates Foundation and the Malaria Vaccine Initiative.

Although these measures are reducing the local incidence in some endemic areas, development of a vaccine is still a priority. The on-going research commitment of scientists like Cowman therefore remains at the frontline of the continuing battle to prevent malaria infection and improve the lives of so many.

---PB--- Biography

Alan Cowman did his PhD in molecular parasitology through the University of Melbourne. He then secured a CJ Martin Fellowship from the NHMRC to pursue postdoctoral training at the University of California, Berkeley. He subsequently returned to WEHI as a senior research officer and became a Wellcome Australian Senior Research Fellow in 1988.

For his work on Plasmodium falciparum, Cowman has received many awards including the 1990 Burnet Prize, the 1992 Glaxo Award, the 1993 Gottschalk Medal for Medical Science and Biology of the Australian Academy of Sciences, the 1994 Boehringer-Mannheim Medal, the 2001 Royal Society of Victoria Research Medal, the Centenary Medal and election to the Australian Academy of Science.

The malaria menace

Malaria has plagued humans since we descended from the trees and remains one of the most severe public health problems worldwide. It kills 1-3 million people worldwide every year, with ~500 million infected and over 3 billion people living in risk areas.

Malaria, literally meaning bad air, occurs mostly in tropical and subtropical areas of the world, encompassing areas of Africa, the Americas and Asia. Sub-Saharan Africa carries the heaviest disease burden, with an estimated 90% of all human deaths due to malaria.

The most vulnerable to malarial infection are those individuals with no or little protective immunity against the disease, especially young children, pregnant women and underexposed adults. In fact, most deaths from malaria globally occur in African children under 5 years old.

Closer to home, malaria is also a serious health problem in Papua New Guinea and nearby regions, and Australian scientists have long been involved in research into malaria and other tropical diseases.

In developing countries, malaria is the third biggest killer behind HIV and TB. Malaria has a profound effect on individuals and societies it affects in terms of productive days lost, due largely to the age of its victims and subsequent projection of their future productivity to the society.

Malaria therefore places a huge economic and social burden on countries where this disease is endemic. It is said to not only be associated with poverty, but to cause poverty and hinder economic development.

Malaria transmission can be reduced in affected areas by preventing mosquito bites using personal control measures such as bednets and repellent or environmental controls including insecticide spraying.

Preventative anti-malarial drugs such as chloroquine are used, although these agents must be taken continuously to reduce the risk of infection and drug resistance is an increasing problem. In addition, such pharmaceutical measures are often too expensive or too impractical for populations in endemic areas to use effectively.