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See the Information at the
Global Network for Neglected Tropical Diseases |
Selected Parasitology and Tropical Diseases
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Selected Fact Sheets on Worm Neglected Tropical Diseases
For humans and domestic animals, many aspects of the normal dynamic balance are lost and this results in serious increases in the prevalence and incidence of parasitic burdens due to many different factors that affect transmission, vector habitat, reservoir hosts, etc.
Although we don't hear much about parasites or tropical diseases in the US, they are present, and the lack of knowledge is largely a cultural phenomenon in that we don't talk about these disgusting things or that they are associated with the poor, lack of sanitation, and hygeine. Many parasitic diseases were once prevalent in the US: malaria in New England, particularly the Conneticut river valley; filariasis in the Southeast US; and many nematode parasites including hookworm. Nematode infections remain prevalent in many parts of the US (pinworm, hookworm, etc.)
Parasitic and particularly tropical diseases shorten lives, reduce the ability to work or to attend school, and impose a lifelong burden on those countries' potential for advancement. In tropical and sub-troprical environments the consequences of parasitic infection are devastating in terms of both human morbidity and mortality as well as on agriculture and animal grazing. The consequences of these diseases are devastating and inhibit progress in many underdevelopped countries. Little wonder that many of the world's poorest countries have fallen behind in the highly competitive scramble for industrialization and economic growth. Many factors contribute to poverty in these 'third world' countries, not the least of which is the presence of a host of killing, crippling and debilitating diseases.
Children are the most severely affected especially with malaria and schistosomiasis. Malnutrition and prevention of land use are also among the biggest problems contributed to by parsitic infections. Furthermore, recent studies have documented the profound affects some parasitic infections have on the intellectual development of children; the effects are serious.
More than 500 million people in the world are affected by the tropical diseases described here; that is, one in ten of the total world population. Fully half of humanity lives in the tropical regions and is therefore at risk from those diseases - whether they are old and long-entrenched or newly re-emerging as a major public health problem.
Malaria alone is a principal cause of incapacity in the world, striking hardest during the most active agricultural season and putting a severe brake on economic growth. Other diseases too debilitate the active work force, and children of critical school age. The aspirations of the affected countries to industrialize, to join the "free market economy" and to be no longer dependent on aid or interest-bearing loans will continue to be frustrated if the richer countries do not actively assist them to overcome these scourges. Besides the moral obligation, the industrialized world would do well to consider its own self-interest: many diseases can readily be transmitted outside the tropics. Insect vectors often "hitch" a ride on airliners, and not a few people living close to European and North American airports have died from malaria.
Adaptations to symbiotic or parasitic lifestyle include those associated with:
Intestinal Nematodes at WHO
Intestinal worms are one of the most common infections in humans, especially in tropical and sub tropical countries. These parasites are widespread and highly prevalent but also relatively easy to control: spectacular results obtained prove that treating schoolchildren increases their height and weight, improve iron stores and reduces iron deficiency anaemia. Unfortunately, adequate control is still lacking in many areas and the number of infections and intensity of infection are increasing.
The Burden
~3.5 billion people are affected, 450 million are ill as a result of these infections, the majority being children ~44 million pregnant women have hookworm infections which cause chronic blood loss from the intestine and predisposes to the development of iron deficiency anaemia, sometimes of great severity, constituting a major public health problem. 65 000 deaths are directly attributable to hookworm infections 60 000 to Ascaris lumbricoides (roundworm)
- Soil-transmitted nematodes (Ascaris lumbicoides, Trichuris trichiura, Hookworms) cause morbidity in humans in different ways, by:
- affecting nutritional equilibrium
- inducing intestinal bleeding
- inducing malabsorption of nutrients
- competing for absorption of micronutrients
- reducing growth
- reducing food intake
- causing surgical complications such as obstruction, rectal prolapse and abscess
- affecting cognitive development
The Burden of Malaria
- About 40% of the world's population at risk in some 90 countries or territories
- 1.5 - 2.7 million deaths per year, the great majority of them in Africa
- 300 - 500 million clinical cases per year,
- Approx. 1 million deaths among children <5 years of age can be attributed to malaria alone or in combination with other diseases.
- Malaria exacts an enormous toll in lives, in medical costs, and in days of labour lost, especially during the agricultural season,
- Growing number of cases of severe malaria, complicated by the spread of drug resistance,
- Growing number of epidemic situations linked with war, movements of people, and degrading environments.
The Burden of Schistosomiasis
- One-sixth of the world's population is at risk
- 74 endemic countries, half of which are ranked economically as among the poorest
- 200 million persons are infected, with the highest prevalence and severity of infection occurring among school-age children,
- less than 20 000 deaths per year.
- Infection is acquired by contact with freshwater, where aquatic snails have been contaminated by faeces or urine of persons with intestinal or urinary schistosomiasis. The parasites liberated from the snails penetrate the human skin and migrate to the blood vessels of the intestines or the urinary bladder. Disease is caused by the eggs released from the parasites.
- Schistosomiasis has long been recognized as a rural disease. It is linked to traditional agricultural practices and to the necessity of using natural water bodies for domestic and recreational purposes. However, because of rural - urban migrations in recent times, it is increasingly an urban disease in slums of cities in the north-east of Brazil and in West Africa.
- Water resources developments (such as irrigation systems, small dams and reservoirs) and refugee movements in Africa, the Eastern Mediterranean and Asia, are causing introduction, spread and aggravation of schistosomiasis.
The Problem
- A major cause of clinical morbidity for 120 million people; the second leading cause of permanent and long-term disability (World Health Report, 1995)
- An impediment to socio-economic development in 73 endemic countries of the tropics and sub-tropics
- Increasing prevalence worldwide, often because of rapid unplanned urbanization
Onchocerciasis ("river blindness") remains the second leading infectious cause of blindness and the source of enormously debilitating skin disease in 34 countries of Africa, the Arabian peninsula and the Americas. It is a major cause of social and economic deterioration throughout these endemic regions. Fear of "river blindness" drove villagers away from rich arable lands beside West Africa's rivers. Until 1987, control efforts focused on reducing the blackfly vector. With the advent of the drug ivermectin, the strategy changed to more cost-effective single annual dose treatment of the 17.6 million people still infected.
BURDEN OF ONCHOCERCIASIS
- Infection
- Population at risk: 120 million people - 96% in Africa, 4 % in the Americas and Yemen
- Numbers infected: 18 million people - 99% in Africa
- Distribution: 34 endemic countries - 28 in Sub-Saharan Africa plus Yemen, 6 in the Americas
Disease
- Blindness: 270,000 persons
- Skin disease - severe itching, dermatitis: 6 million people - dermal microfilariae: 18 million people
Socioeconomic impact
- The psychosocial impact of this debilitating, disfiguring, blinding disease includes ostracism, personal anguish and low self-esteem.
- Specific costs of the decreased productivity from blindness and other chronic disabilities are currently being quantified.
Twelve millions persons are infected and some 350 million people are at risk from this parasitic disease transmitted by biting sandflies. The leishmaniases are parasitic diseases with a wide range of clinical symptoms: cutaneous, mucocutaneous and visceral. Of its three forms, visceral leishmaniasis is the most severe.
The different forms of leishmaniasis constitute severe public health problems:
- visceral leishmaniasis (VL) is fatal when untreated;
- mucocutaneous leishmaniasis (MCL) is a mutilating disease;
- diffuse cutaneous leishmaniasis (DCL) is a disabling disease;
- cutaneous leishmaniasis (CL) is a disabling disease when lesions are multiple.
The Burden of leishmaniasis
- Leishmaniasis is a worldwide disease, affecting 88 countries: 72 are developing countries and 13 of them are among the least developed countries.
- 350 million people at risk.
- Overall prevalence: 12 million people
- Annual incidence:
- 1 to 1.5 million cases of cutaneous leishmaniasis
- 500 000 cases of visceral leishmaniasis
- 90% of all cases of visceral leishmaniasis occur in Bangladesh, India, Nepal and Sudan.
- 90% of all cases of cutaneous leishmaniasis occur in Afghanistan, Brazil, Iran, Peru, Saudi Arabia and Syria.
- Leishmaniasis is a disease linked to poverty and closely related to economic development and environmental changes such as deforestation, building of dams, new irrigation schemes, urbanization and migration of non-immune people to endemic areas
- Reduction of malaria vector control activities, malnutrition and more recently AIDs an other immunosuppressive conditions are other important risk factors. These factors require integrated control activities.
Pain, suffering and death from sleeping sickness, also know as human African trypanosomiasis (HAT), is a daily threat to more than 55 million women, men and children in 36 countries of sub-Saharan Africa. The tsetse fly transmits a blood parasite to humans, who subsequently suffer from fever, body wasting and somnolence; there is a high fatality rate. An estimated 300 000 people are affected in tropical Africa.
The Burden
- 55 million people mostly from rural areas, in 36 countries are at risk; only 3 million people under surveillance
- fewer than 25 000 cases diagnosed and treated annually; over 300 000 people infected
- if left untreated the disease is fatal
- important current outbreaks of the disease, particularly in the central African region, where some villages reach the level of 50 to 70% of individuals infected; situation calls for urgent intervention in all countries of central Africa.
This parasitic disease, transmitted by a blood-sucking bug, puts at risk 100 million people in Central and South America. About 18 million individuals are affected and over 45 000 people a year die of it. Chagas disease or American trypanosomiasis, is transmitted by triatomine vectors and by blood transfusion. In a global perspective, the disease is ranked in third place after Malaria and Schistosomiasis Control. However, elimination of transmission should be achieved in four countries by 1998.
The Burden of Chagas Disease
- 25% of the total population in Central and South America are at risk,
- one million cases per year,
- more than 45 000 deaths per year,
- some 16-18 million people are currently infected,
- 2-3 million of these may already have developed chronic complications,
- over 3 million are still in the incubation period;
- The incurable lesions of Chagas disease develop in one-third of those infected, some 10-20 years after the initial acute phase, and include:
- chronic cardiopathy (in 27% of those infected)
- chronic digestive lesions (in 6%), and
- neurological disorders (in 3%).
- Patients with severe chronic disease become progressively sick and ultimately die, usually as a result of heart failure;
- 2 740 000 Disability-adjusted life years (DALYs) are lost.
Total eradication of this parasitic disease, which is transmitted through contaminated drinking water, is within reach by the end of this century. In 1989 there were one million cases, but by 1994 just 164 941 cases were recorded in 16 countries of Africa south of the Sahara, plus India and Yemen.
THE BURDEN OF DRACUNCULIASIS
- In 1995, there were approximately 100,000 cases,
- there are 16 endemic countries in Africa south of the Sahara, plus India and Yemen,
- dracunculiasis impedes socio-economic development, especially in the most poor and remote areas.