Tropical diseases

听

20 February 1993

Preoccupied by Western health problems of heart disease, blood pressure, ulcers, cancers and AIDS, we rarely devote much thought to tropical diseases yet they are increasingly relevant both to Western and to the global communities

ANY serious disease is devastating to the person concerned. But if we look beyond the suffering of each individual to the social and economic implications of any particular disease, we can begin to develop a sense of perspective on the problems that humanity faces. Western diseases are the scourge of a postindustrial society, but the numbers of people who suffer from them are tiny on a global scale. The tropical diseases, on the other hand, affect thousands of millions of people. They destroy communities and undermine the vital economic structure of societies by weakening or killing the people needed to farm the land and work with the natural resources of the country. One example alone illustrates the scale of the problem 鈥� it is estimated that 2100 million people (almost half of the world population) are at risk from malaria.

Plague, the Black Death, cholera 鈥� these names are echoes from a past that seems far removed from modern life. Two of the three major plague pandemics (an epidemic on an international scale) struck in the 8th and the 14th century But the most recent plague pandemic began in about 1894 in the Chinese city of Canton, spreading to kill 6 million people in India before reaching Australia in 1900. Plague still occurs in remote areas of the world such as western Arabia, Uganda and Kurdistan but world pandemics of plague are no longer a risk. We now know how the disease, caused by the bacterium Pasteurella pestis, is spread to humans via rat fleas when living and working conditions are cramped and crowded. The improvements in public health and quarantine that have resulted from this knowledge and the development of antibiotic drugs which destroy the bacteria mean that this once most dreaded disease is no longer regarded as a threat by most of the world population

Historically, plagues followed changes in the structure of society. In Europe, the Industrial Revolution led to the migration of thousands of people from small rural settlements to large towns and cities where living conditions were unhygienic, cramped and squalid. As transport improved, conditions for plagues to spread were perfect. Better public health standards, increasing knowledge of the way in which diseases are transmitted and improved medicines have now reversed this. The old plagues have been replaced in the developed world by chronic diseases of middle age, diseases often linked to modern lifestyles. But the tropical diseases of the developing world are not simply a rerun of our own history 鈥� they also present us with the opportunity to put into practice some of the lessons from the not-so-distant past.

Many Western diseases are caused either by our diet or some other aspect of our lifestyle, or by infectious organisms such as bacteria and viruses. In contrast, most tropical diseases are caused by parasites carried from one person to another by an organism known as the vector A parasite is an organism that feeds off another living organism, its host, causing it harm. As anyone who has suffered threadworms will know some parasites live in the body of their host, while others simply visit their host to feed. And while we tend to disregard parasites as a problem, 70 per cent of us will be providing the follicle mite Demodex with a cosy home. If you have had a 鈥渂lackhead鈥�, then you are probably host to these parasitic arthropods. Imagine the effect it would have if a similar proportion of the population was infected by a parasite that caused a debilitating disease, and you begin to get an idea of the impact of tropical diseases. With one or two exceptions, the vector-borne tropical diseases do not cause death on a massive scale that would generate a world-wide outcry. Instead they result in long-term debilitating illness in huge numbers of people, with far-reaching economic consequences in medical costs and lost earnings from trade and tourism.

With 267 million people infected across 103 countries, with 1-2 million people dying as a result each year, it is not surprising that the best-known of the tropical diseases is malaria. But there are many others that also have far- reaching and damaging effects. Most of them involve parasites with complicated life cycles, needing at least two different types of host 鈥� the vector and people. And most of the diseases have a state of transmission that depends on water. An understanding of the parasite鈥檚 life cycle becomes very important when attempts are made to get rid of the diseases.

The tropical diseases occur mainly in tropical countries, but can also appear elsewhere. Malaria occurs in Europe. Cases of Chagas disease have occurred in the US and if, as predicted, the world climate warms up, who knows where they might spread? But the distribution of the diseases is not static. A look at the change in the occurrence of malaria shows this. So what affects where tropical diseases are found?

Ecologists have shown that organisms can adapt and change to exploit any new niche that becomes available to them. Some organisms, known as opportunists. are particularly good at this. Any piece of bare ground will be colonised quickly by opportunistic plants which grow fast and produce seeds that can lie dormant for a long time until conditions are suitable for them to grow. In many ways diseases can be considered in the same way. Just as the plagues of old rampaged through the slums of the new industrial towns, modern tropical diseases look set to take advantage of any fresh environments that people provide.

Diseases on the move

No tropical paradise

THE social state of much of the world is changing rapidly. For centuries, most of the world鈥檚 people lived in rural communities, but in the past 20 years or so people have flocked to the new cities of the developing world. The rate at which people are moving from the countryside to urban centres suggests that by the year 2000 more than half of the 6.3 billion people living on the Earth will dwell in a city. Urbanisation on this sort of scale is a phenomenon not previously seen, and the management of the health implications will have massive repercussions for the world community

A move from the country to an urban environment might at first thought imply a great improvement in health standards. In cities there is usually a higher standard of health care than in poor rural areas. In cities we do not expect people to bathe and defecate in the same water supply that they have to rely on for drinking. And the rivers, streams and ponds so vital in the life cycles of vector organisms seem to be missing in a city environment. On the face of it, urbanisation looks as if it could be the end of tropical diseases 鈥� but unfortunately, things are not that simple.

To begin with, cities mean many, many people living and working close to each other (just as in the historical plagues). As the spread of any infectious disease is related to the number of infectious people, the number of people available to be infected and the contact rate between them, it is easy to see how an urban situation is likely to increase rather than decrease the spread of diseases. Secondly, a steady stream of people moving into a city from different areas will import a whole variety of pathogens. People are always most vulnerable to new disease forms that they have not met before 鈥� so the potential for epidemics is high.

City dwellers often earn higher wages than rural workers, but a move to urban dwelling does not mean an increased standard of living for the vast majority of migrants. Around each city centre vast sprawling 鈥渟hanty towns鈥� develop where people unable to afford expensive city housing build homes from whatever they can find. There is usually no piped water, no rubbish disposal and inadequate sanitation. These suburban sprawls often extend out into the countryside, so that people have the worst of both worlds 鈥� both city and rural diseases are rife.

There are even circumstances where the health care itself spreads disease. Chagas disease can be eradicated in rural areas, because the large assassin bugs that spread it have a slow life cycle and do not move readily from place to place. In rural areas, when the houses are sprayed with pesticide the bugs are killed and the disease-causing organisms dies too.

As new bugs do not immediately invade, control is possible. But in cities, Chagas disease is spreading rapidly as a result of blood transfusions. People move in from the country unaware that they are carrying the disease and sell their infected blood to raise money. This is then given to others, increasing the numbers infected with a disease that is incurable once the protozoan parasite is in the body

The movement of people away from the country means that tropical diseases such as schistosomiasis will become rarer, but many diseases are adapting to take advantage of the new situations. Some of the types of mosquitoes that carry malaria, yellow fever, filariasis and dengue breed rapidly in tiny amounts of water (for example, in a puddle in a footprint, a water-filled bottle top) and by completing their life cycle in a matter of days the population increase can be massive. Some will even breed in polluted water such as sewage ditches and blocked drains, making shanty towns an ideal environment

Even developments that are devised and planned for the good of the community can provide new environments for the disease-causing organisms. Reservoirs, hydroelectric power schemes, irrigation ditches 鈥� all of these can provide a whole new lease of life to vectors of the tropical diseases.

The rapid and easy movement of people, not just from one country to another but from continent to continent, gives all diseases access to new environments. With changes in the world climate and the freedom of travel now available, the time may well come when some of these so-called 鈥渢ropical diseases鈥� are laying waste to a new and completely vulnerable population in the developed world.

Over the past 50 years, methods for treating and preventing bacterial and viral illnesses have improved enormously. A combination of antibiotic drugs and vaccination programmes have greatly reduced the incidence and death toll of many diseases. Smallpox, one of the ancient plagues, has now been eradicated worldwide as a result of a huge vaccination programme (see Inside Science No 53). But progress of the same sort has not as yet been made in eradicating vector-borne tropical diseases.

There are several ways to attack tropical diseases. Drugs could be developed to destroy the parasitic organisms that cause most tropical diseases, and in many cases there are at least prototype chemicals which could be effective. Drug companies, however are unwilling to invest large sums of money developing and testing drugs that impoverished countries will be unable to afford. Most of the countries severely affected by tropical diseases are extremely poor 鈥� not least as a result of the diseases themselves, which weaken a large proportion of the population. Some people have suggested that the drugs for treating tropical diseases in these countries might be tested less rigorously so that they can be produced more cheaply 鈥� after all, why worry about whether a drug may cause cancer in 20 years time when it enables people to have 20 years of active, healthy life they would otherwise have been denied? Others argue that any reduction in the standards of testing drugs below those used in the developed world would be unethical. Add to this the problems of administering multiple doses of drugs to millions of people either in scattered rural communities with no health care or in teeming city slums and it seems unlikely that the route to eradicating tropical diseases will come through drugs.

Vaccines are another potential weapon against tropical diseases. They have been remarkably successful against diseases such as smallpox, polio and measles. But many of the same arguments that apply to the development of drugs also apply to the development of vaccines. There is an effective vaccine against yellow fever, but it is difficult to deliver 鈥� so yellow fever is still a problem. Vaccines work by 鈥減riming鈥� the immune system, so that when our bodies meet the living, infective disease organism they already have the tools to deal with it. Parasites that live inside the body of their host have evolved ways of evading the immune system, and so prevent their hosts from destroying them. They may live inside the host cells, or they may keep changing their own external coat so that antibodies made by the host do not recognise them any more. This makes the task of producing a vaccine difficult, and in some cases virtually impossible. The Colombian biochemist Manuel Patarroyo has recently developed a vaccine against malaria, which the WHO now has on trial in parts of Africa. It has shown some promise and if these tests in harsh conditions prove successful, it could be the first vaccine licensed against the disease. But on the whole, a combination of hard-nosed commercial reality, scientific difficulty and the circumstances of the people affected by the diseases mean that vaccines are never likely to be the whole answer to the problem

Attacking the vector

Insecticide sprays

ANOTHER line of attack is vector control. The organisms causing tropical diseases rely heavily on vectors, usually insects or other arthropods, to give them access to their hosts. If the vector organism can be destroyed, the parasite cannot continue its life cycle and cause disease. This approach has been tried against the mosquito-borne diseases, particularly malaria. Such diseases can be tackled in two ways: by attacking either the larvae stage of the mosquito life cycle or the adult stage. To destroy mosquito larvae, the rivers, ponds and streams where they breed have been sprayed with pesticides including DDT. This type of treatment is regarded as a long-term measure and can be successful, but only if repeated and maintained for a long time.

Spraying a low level of insecticide over entire cities seems to be moderately successful as a means of interrupting the development of some diseases. Approaches such as these work best if they are coordinated by the central government of a country because the attack is not then piecemeal. It is a waste of resources for one area to eradicate mosquitoes, for example, if millions more fly in immediately from the next region. For this reason, spraying to destroy adult mosquitoes is a measure usually undertaken only in an emergency

Hand-in-hand with vector control, and in many ways part of it, is environmental manipulation or modification. In the control of almost every disease, improved public and environmental health has played a major part, regardless of other techniques used. Clean drinking and washing water supplies greatly reduce the incidence of diseases such as schistosomiasis. Effective sewage disposal removes the breeding sites of disease-causing organisms, while rubbish disposal not only reduces the numbers of rats, fleas and other pests but also gets rid of cans, bottle tops and other containers which fill with water and act as breeding sites for insects such as mosquitoes. Measures like this last one might seem quite trivial yet when Singapore passed a law making it illegal for anyone to breed mosquitoes on their property, it became illegal to have any standing water on a property where mosquitoes might breed. As a result, the incidence of malaria has been reduced, by individuals each taking care of their own small environment.

On a grander scale, environmental manipulation involves schemes for draining marshlands and covering drains and water storage tanks. Biological pest control measures such as stocking lakes and ponds with fish which eat mosquito larvae and putting bacterial insecticides such as B. thuringiensis (see Inside Science No 43) in water supplies are having some measure of success. When all the difficulties are considered, it can appear that the tropical diseases hold all the aces, but there are success stories such as the programme against onchocerciasis.

Where do we go now?

Prospects are poor

TROPICAL diseases present us with a potential global catastrophe. The poorest nations are consistently being made poorer by the ravages of these diseases, and as more and more people flock to the cities the problems of urbanisation will only add to an already heavy burden. Much work is being done. Individual countries have initiated control systems that have had some quite remarkable successes. At the international level, the World Health Organization (WHO) has representatives from a total of 130 member states and it spends 650 million dollars a year on world health problems. It acts as a coordinating body for international action on tropical diseases and is involved in projects such as the successful oncho-cerciasis scheme.

What seems to be emerging is that no one line of attack will work to eradicate tropical diseases. The effect that these diseases have on the economies and food production of many countries is so great that it affects the economy of the entire world. If this can be translated into recognition of tropical diseases as a world problem needing world support and resources to overcome it, a solution may be possible.

Increasingly it appears that the basis of any solution to the problem will be surveillance. There are proven control measures for most tropical diseases. They cannot be applied in a blanket way due to both practical considerations and expense. What is needed is the targeting of resources when and where they are needed. If trends in vector organisms are monitored, effective measures can be taken before numbers increase out of control and outbreaks of vector-borne diseases can be minimised. As well as ground-based surveillance, new space technology may help. Medsat is a proposed new satellite being developed by NASA. It is seen as being an 鈥渆nvironmental eye-in-the-sky鈥� providing information for the benefit of the international community. For example, mosquito numbers usually rocket shortly after the rainy seasons, when there are plenty of places to breed. By monitoring exactly when, where and how much rain falls in one place, Medsat will predict where the rain will fall next. This should allow more accurately targeted control measures.

Along with vaccination programmes in severely affected areas and the use of drug treatment, these measures would lead to a gradual decline in the numbers of affected individuals. This in turn would further reduce the likelihood of infection. In the long term, we may be able to look forward, if not to the complete eradication of tropical diseases, at least to a substantial reduction in their impact on the lives of millions of people 鈥� but only if an unprecedented degree of international cooperation and financing can become a reality.

What is malaria?

MALARIA is caused by Plasmodium, a group of parasitic protozoa with a complicated life cycle involving people and mosquitoes. It is transmitted to people by the bite of an infected Anopheles mosquito, the vector organism. Only the females bite humans because they need a blood meal before they lay their eggs. Malaria begins with flu-like symptoms between 8 and 30 days after the infected bite, and then progresses to a regular cycle of severe fevers, shaking chills and drenching sweats. These coincide with the stages of the multiplication of the parasites and the destruction of the red blood cells. There are several stages in the life cycle of the parasite where it can be attacked, sometimes with more success than others.

In many parts of Africa, people have become at least partly immune to malaria, acting as carriers without symptoms of the diesease. Thus although it is estimated that 267 million people are infected with the disease, there are only about 107 million actual clinical cases per year. But when new people move into an area 鈥� say, in search of work 鈥� they are rapidly infected and local epidemics occur. This has been happening a lot recently with the increased exploitation of the resources of the Amazon basin and also in Ethiopia, Madagascar, Sri Lanka and the Solomon Islands. Malaria is typical of many tropical diseases in being parasitic and being transmitted as part of a complex life cycle which involves two hosts and water.

Onchocerciasis control: a success story

WEST AFRICA is an area where millions of people have become blind over the years as a result of Onchocerciasis. People are bitten by the blackfly Simulium and infective larvae enter the blood stream. The adults develop in the body and then reproduce to release larval worms or microfilariae into the blood stream. When these microscopic larval worms make their way to the eyes they cause damage and eventually blindness. Whole families can become blind and need leading around by anyone, even a small child, who has retained their sight.

In the past 20 years, transmission of onchocerciasis has been greatly reduced by the Onchocerciasis Control Programme set up by a group of 11 nations. This programme has involved the regular and controlled release of biodegradable insecticides into rivers to destroy the blackfly larvae. By largely eliminating the vector organism the chain of infection has been broken. This has been backed up recently by the use of a new drug which can kill the larval worms in infected people, thus reducing the problem even more. In fact, in the original areas where the larvicide was used, the level of infection is down from around 1 million at the beginning of the project to almost zero. Now that the insecticide applications have stopped, blackfly populations have increased again but they are not carrying onchocerciasis as there is no infected blood to suck.

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