Parasitology and Tropical Diseases
Material presented here is derived from multiple sources and is for instructional purposes only

See the Information at the

Center for Disease Control (Parasitic Diseases)

Global Network for Neglected Tropical Diseases

Neglected Tropical Diseases (WHO)

WHO Tropical Disease Researh (TDR)

 


Selected Parasitology and Tropical Diseases

Intestinal Nematodes
3.5 billion affected
450 million children ill
44 million pregnant women affected

 

Malaria
40% at risk
~ 3 million deaths/yr
1 million children deaths
500 milion cases
Schistosomiasis
1/6 at risk
200 million infected
Chagas Disease
25% at in S/C. America
18 million infected
3 million chronic
Leishmaniasis
350 million at risk
12 million infected
2 miilion new cases/yr
Sleeping Sickness
55 million at risk
300,000 cases
River Blindness
120 million at risk
18 million infected
270,000 blinded/yr
Elephantiasis
120 million infected
43 million chronic or acute
Serpent of the Nile
100,000 cases

Selected Fact Sheets on Worm Neglected Tropical Diseases


Organisms living in or on another organism is normal in nature and not aberrant or unusual. All organisms are infected with some type of parasite. In fact, there are more parasites than hosts. Parasite contribution to world biomass is astonishing. Although there may only be a few different parasites per host, there could be millions of a particular species per host. Most if not all wild animals (and plants) have parasitic infections. In most cases, they do not interfere with the ability of the species to reproduce largely due to a dynamic balance in transmission, parasite burden, and host habits in terms of territorialism, dispersal, etc. Parasites and other symbiotic species play an extremely important role in ecology of a community.

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.


Parasites to be described here include protozoa and worms (helminths). These organisms have simple and complex life cycles (single vs. multiple hosts). They range in size from 5 um protozoa to intestinal roundworms (Parascaris = 50 cm and pencil thick) over a foot in length and intestinal tapeworms reaching up to and larger than 15 meters in length (D. latum is ~10 meters in length with several human Taeniids ~10 meters or more). An extreme example is Hexagonoporus which is ~30 meters in length in whales. Parasites have adapted to virtually every niche in the body and typically demonstrate niche specificity. They may live extracellularly in the gut, blood, and lymph to intracellularly in blood cells, muscle, white cells, etc.

Adaptations to symbiotic or parasitic lifestyle include those associated with:

Selected examples of adaptation:


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

 
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

Malaria at the CDC and
WHO
Malaria exerts the heaviest toll in terms of death and suffering. Some 300-500 million
persons are infected, of whom over 20 million will die during the present decade. In Africa
alone, every hour of the day 120 children aged under 5 years die from malaria. About
40% of the world's population is at risk.

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.

Schistosomiasis (or Bilharziasis) at the CDC and the WHO
Among the tropical diseases, schistosomiasis is the second most prevalent, and one of those
that is linked closely but not inevitably with agricultural and water resources development.
This blood fluke infection is closely associated with dams, irrigation canals and stagnant pools infested with water-snails which act as host to the larvae of the fluke. There are 200 million sufferers and one-sixth of the world population is at risk.

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.

Lymphatic Filariasis (Elephantiasis) at Filariasis.org and the WHO
Transmitted by mosquitos, this parasitic worm infection affects 73 endemic countries of the
tropics and sub-tropics. There are around 100 million victims, of whom 43 million suffer
from acute or chronic manifestations, including elephantiasis, hydrocele and gross
swelling of the limbs.

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 (or River Blindness) at the WHO

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


Leishmaniasis at the CDC and the WHO

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:

The Burden of leishmaniasis

 
African Trypanosomiasis at the CDC and the WHO

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


Chagas Disease at the CDC and the WHO

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


Dracunculiasis (or Guinea Worm Disease) a the CDC and the WHO

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