Old fronts, new frontiers

  • 29/09/1997

Old fronts, new frontiers while it is generally accepted now that the resurgence of malaria was largely due to poor public health infrastructure and development of resistance by the vector and the parasite, lack of knowledge was a major factor that had been ignored. The new global strategy to control malaria aims tocorrect this problem. The Secunderabad conference presented recent developments in various areas of research across the world.

The problems in malaria control are manifold. The traditional approach was to control breeding of mosquitoes with insecticides like ddt and benzene hexachloride ( bhc ), and to use quinine-based drugs for clinical treatment. But mosquitoes soon became resistant to ddt and bhc , and certain Plasmodium species developed an immunity to chloroquine.

In fact, P falciparum has grown resistant to most drugs. Its resistance to the sulphonamide/pyrimethamine combination, used as a second line of treatment where chloroquine is ineffective, is becoming widespread. Mefloquine is the only effective single dose drug in cases where falciparum is resistant to many drugs. Surprisingly, tests of falciparum sensitivity to mefloquine in places as widely separated as Thailand and Tanzania have revealed that chloroquine-resistant falciparum has developed resistance to mefloquine as well.

The search for alternatives to fight malaria has followed three distinct lines. First, conventional methods such as the development of new or alternative (herb-derived) drugs and use of insecticides other than ddt and bhc in aerosols and mosquito repellents. Second, innovative developments such as use of insecticide-treated bednets and dna vaccines. Third, alternative methods such as genetic engineering to make Anopheles species less fertile or resistant to Plasmodium , tinkering with the genes of the parasite to make it non-patho-genic (ineffective) and bioenvironmental vector control - where fish, worms, bacteria and fungi that feed on or infect larvae are used to check mosquito breeding.

All these methods have advantages as well as disadvantages. Given the wide variation in local conditions in which malaria exists, no one approach is likely to succeed in all places. More research is needed to make these methods effective in the long term. This also requires better knowledge of the mechanism of transmission of the disease in high intensity, medium intensity and low intensity areas.

Disease transmission But knowledge about disease transmission depends on how much is known about various species of the vector and the parasite, an area yet to be fully explored. One approach is to seek vector species which do not get infected and to investigate their resistance to the parasite. For instance, it was not known previously that all species of Anopheles are not carriers of the malarial parasites Plasmodium falciparum and Plasmodium vivax . But research has shown that different Plasmodium species infect different hosts and that some species of Anopheles do not get infected with certain species of Plasmodium .

Sibling (sister) species of An culicifacies ( a , b , c and d ) may not be equally strong vectors of P vivax and P falciparum, the two parasite species found commonly all over India. Siblings of strain a are very strong vectors of falciparum and vivax , while siblings of strain b are poor vectors of falciparum and resist infection by vivax altogether. Further research into the immunity of these species will lead to a better understanding of parasite transmission. Work on this aspect has been done in India by the Malaria Research Centre of the Indian Council of Medical Research and the Tata Institute of Fundamental Research, Mumbai, as well as in Europe and the us .

A method for cultivation of P vivax has been developed by C F Golenda at the Walter Reed Army Institute of Research in the us , a breakthrough which would enable close examination of different stages in the parasite's life-cycle and its biology at the molecular level. This would facilitate development of vaccines against malaria.

A second approach is to investigate natural immunity to malaria among humans. For instance, it is common knowledge that people who suffer from sickle-cell anaemia (a genetically transmitted recessive trait common among some Africans in which red blood cells appear like sickles, reducing their ability to transport oxygen in the body) are not susceptible to malaria. But what makes some people - who do not have such anaemia - immune to the parasite? Research in this area is also in its infancy. However, progress on other fronts has provided information which can be used to better tackle the disease.

New medicines
Where most conventional drugs have singly failed to check falciparum for long, ethnic medicines have come to the rescue in the fight against malaria. Of the various herbal drugs identified, the drug artemisinin, extracted from the Chinese plant Artemisia annua , has proved invaluable in reducing deaths due to cerebral malaria. Three derivatives of artemisinin - artesunate, arteether, and artemether - have been successfully used to treat malaria across the world. Clinical investigations in China, Myanmar, Vietnam and Thailand have confirmed the efficacy of artemisinin against vivax and falciparum .

These drugs can be administered by various means, including capsules, oral suspensions, and intramuscular or intravenous injections. Of these, the most difficult to administer is artemisinin, since it is almost insoluble in water and difficult to absorb. Artesunate, which is available in oil solutions or suspensions, acts the fastest. But it is more effective when used in combination with other drugs. All these drugs have few side-effects, and toxicity has been noted in animals, but has so far not been reported in humans.

Artemis annua has been adapted to Indian conditions by the Central Institute of Medicinal and Aromatic Plants, Lucknow. Arteether, derived from artemisinin extracted from these plants, has been tested on malaria patients by the Central Drug Research Institute, Lucknow. With a success rate comparable to that in other countries, India may soon depend on these drugs to treat severe cases of malaria. Currently, these drugs are only available to hospitals, since their indiscriminate use may allow the malaria parasite to develop resistance.

Philip Rosenthal and others of the department of medicine, University of California, usa , have tried a different approach in developing an antimalaria drug. The malaria parasite derives nourishment from rbc s of the host by degrading haemoglobin (the protein which enables transport of oxygen) and using the products (amino acids) to synthesise its own proteins. Rosenthal has found that certain chemicals (phenothiazines) inhibit the enzyme that the parasite uses to break the haemoglobin molecule, which can be used to arrest the development of the parasite in rbcs.

Regular solutions
The development of new drugs proves costly in monetary as well as human terms. Moreover, the parasite ultimately develops immunity to drugs. For this reason, companies and institutions had stopped screening of new antimalaria drugs in the past. Among insecticides, synthetic pyrethroids - which are used in mosquito repellent mats and coils and to impregnate bednets - have proved to be the best alternative. So far, immunity of mosquitoes to these chemicals and their toxic effect on humans has not been reported. But, given their current wide use, these facts need to be confirmed through investigation and long term study (see box: How safe are mats and coils? ).

While insecticide-treated bednets have been widely used to protect children and vulnerable pregnant women from the malaria vector, especially in Africa, researchers have questioned their indiscriminate use (see box: Are bednets reliable? ).

Tinkering with genes
Perhaps the most effective but controversial solution lies in genetic engineering and molecular biology. Genetic engineering has been used to produce mosquitoes which do not allow the parasite to develop in the gut; to reduce the ability of the parasite to invade mosquito and human cells, and to induce immunity among humans through vaccines. Although this poses a problem because of the number of species of Anopheles and Plasmodium , considerable work has been done in this area in the us . In India, this frontier is being explored by, among others, the Centre for Applied Genetics at Bangalore University and the Indian Institute of Science, Bangalore.

Researchers led by Anthony James at the department of molecular biology and biochemistry, University of California, usa , have developed genetically engineered An gambiae with dna which has the capacity to interfere with the development of the parasite in the mosquito's gut. This breaks its life-cycle and since the parasite fails to survive it cannot be transmitted to humans. In another experiment, the original dna of the mosquito was altered to such an extent that it became resistant to infection by the parasite. This strategy has not been successful on a large scale and is not feasible since it would require introduction of millions of mosquitoes into the environment. It is also unlikely to be accepted by environmentalists, who question experimentation with the natural balance of species in an ecological niche, where results cannot be anticipated.

The most exciting contribution of molecular biology and genetic engineering is the development of dna vaccines. The first dna vaccine was formulated in October 1992 in usa . Based on recognition of a protein (circumsporozoite protein or csp ) found on the parasite's cell surface, a dna vaccine was successfully administered to human volunteers in July 1997 and is still being tested. In early August 1997, a key protein (antigen) which helps the parasite to invade liver cells and rbc s in humans was identified by Robert Mernard and others at the New York University Medical Centre, usa . Called trap (thrombospondin-related anonymous protein), the antigen is also related to the ability of Plasmodium to migrate from the mosquito's gut to its salivary glands. If scientists are able to discover a way to block production of trap in the parasite, it would also break its life-cycle.

dna vaccines have many advantages over conventional ones: they can be prepared faster and more easily, formulations are easy to modify, and confer a more balanced and longer-lasting immunity. They have a long shelf-life at room temperature - an invaluable feature in developing countries where refrigeration facilities are often not available. But there are some drawbacks: recombinant dna is introduced into the body through certain strains of viruses. These carriers sometimes do not 'deliver' the goods. dna vaccines, like other antimalaria vaccines, can only provide immunity against a stage in the life-cycle of a particular species of Plasmodium . Scientists are therefore working on a multi-antigen vaccine which can provide immunity against different stages of P falciparum, which causes fatal malaria.

Alternative solutions
The resurgence of malaria was a result of the failure of vector control methods using conventional insecticides and drugs, and ecological changes. It renewed interest in the environmental and biological methods of control, including exploitation of mosquito larvae predators like indigenous fishes, bugs and mosquitoes (such as Toxorhynchites species), parasites like worms and fungi, and bacteria which produce toxins that kill the larvae.

The advantages of biological control agents is their ability to kill target species at low doses, safety to non-target organisms, non-toxicity to mammals (including humans), little evidence of development of resistance by target species, easy application in the field and inexpensive production. However, the production costs of biocides are sometimes higher than for conventional insecticides.

There are a number of formulations made from bacteria like Bacillus thuringiensis israelensis (Bti) and Bacillus sphaericus , that have been used in field trials and found effective in controlling breeding of Anopheles. In spite of the proved safety of these formulations, environmentalists and people are sceptical about introducing them in clean water sources where Anopheles breeds. Moreover, their application is not easy, since several types of mosquitoes breed extensively in small water bodies like rainwater puddles. However, inexpensive and effective formulations, like the fizzy Culinex tablet based on Bti and B sphaericus , are being used to control breeding of Culex mosquitoes in India, Brazil, Sri Lanka and Tanzania, according to Norhert Becker of the German Mosquito Control Association, Waldsee, Germany.

Fish like Gambusia affinis , originally native to America, and Poecilia reticulata and Tilapia mossambicus have been successfully used to reduce Anopheles larvae in India, Africa and South and Central America. In a major field experiment in Pondicherry in India, a native Indian fish, Aplocheilus blochii , has been used to control larvae in wells and found to be more hardy than Gambusia . Guppy and silver and common carp have also been tried to control mosquito breeding in paddy fields. But use of fish is problematic, since the puddles in which mosquitoes breed dry up very soon.

A number of fungi have potential as biological control agents, including Coelomomyces , Lagenidium , Metarhizium and Leptolegnia . But the main constraint is that they cannot survive when free of their host. Recently, a fungus that mosquito larvae feed on has been discovered in Goa. Larvae die after ingesting its spores (see box: A fungus called mrc -367 ).

Perhaps the safest and most viable technique for reducing larvae in paddy fields is to grow the water fern Azolla pinnata . Full coverage by Azolla reduces mosquito egg hatchability by 30 per cent. Since it takes a couple of weeks to grow, the fern could be useful in controlling breeding in areas where long duration rice varieties are grown.

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