Trypanosoma brucei gambiense on the other hand, was first identified a year earlier by Dutton. Currently, Sleeping Sickness newly infects about 20,000 to 25,000 people each year and causes about 55,000 deaths. (Kreier and Baker 1987).
Plasmodium falciparum, probably the most common species of malaria to infect man, was named by Welch in 1897. It has been described as being ‘almost unchallenged in its supremacy as the greatest killer of the human race over most parts of Africa and elsewhere in the tropics’ (Garnham 1966). Only P.falciparum directly causes fatal disease in man, making it the most dangerous species of its Genus (Kreier and Baker 1987).
Leishmania Donovani is a Sand fly borne parasite, endemic in tropics, sub-tropics, and Southern Europe. It exists in settings ranging from rain forests in the Americas, to deserts in Western Asia, and from rural to periurban areas. New regions and settings are rapidly being colonized as the disease develops as an AIDS-associated opportunistic infection. There is an increasing interface between Leishmaniasis and more developed countries; and there has been recognition of cases of Leishmaniasis in overseas travelers (Herwaldt 1999).
The disease is slow to manifest, but unless treated usually fatal. (Kreier and Baker 1987).
Trypanosoma brucei gambiense causes African human trypanosomiasis or Sleeping Sickness, and mainly infects the human population of west and central Africa, usually transmitted to man by the tsetse fly (Glossina palpalis). It produces a chronic and fatal form of disease if left untreated. After a year or so, the parasites invade the Central Nervous System, ultimately cause brain damage that leads to the coma. Death eventually ensues (Kreier and Baker 1987).
In a similarly catastrophic fashion, about 600 million people are infected by malaria causing plasmodia worlwide, mainly in Central Africa and Asia (Mahmoud 1989). It is transmitted specifically by the bite of the female Anopheles mosquito (Kreier and Baker 1987).
Diagnosis of Leishmaniasis relies on classical microbiological methods, but molecular based approaches are being tested. Pentavalent antimony compounds have been the mainstay of antileishmanial therapy for half a century (outside of India, response rates average at 90%). Treatment of cases in India is particularly challenging, due to sheer abundance and their refractoriness to antimony. Up to 50% or more previously untreated cases in the state of Bihar are unresponsive to, or relapse after conventional antimony therapy. Lipid formations of amphotericin B represent a major advance for treating Visceral Leishmaniasis in Bihar, and remain about 100% effective. A pressing need is for the technological advances in the understanding of the immune response to Leishmania and the pathogenesis of Leishmaniasis to be translated into field applicable and affordable methods for diagnosis, treatment, and prevention of this disease. Scientists are studying the immune immunoregulation of Leishmaniasis to improve understanding of the immune response to intracellular pathogens in general and Leishmania in particular, to find out whether manipulations of the immune system could be therapeutic, and to rationalize vaccine development. The use of cytokine and other types of immunotherapy combined with chemotherapy for Leishmaniasis remains experimental, and the results have been mixed. Various types of vaccines are being investigated, but no vaccine is ready for general use to prevent Leishmaniasis (Herwaldt 1999).
Trypanosoma brucei gambiense is detected via xenodiagnosis, where the lymph glands are punctured to obtain a ‘juice’. Before invasion of the CNS, the disease is fairly curable. Currently, there is neither a vaccine nor a drug available to prevent infection with sleeping sickness. There are drugs available for treatment of the disease, but at present, the drugs are scarce, difficult to administer, and sometimes dangerous. Suramin, pentamidine, and berenil have been used to treat, and sometimes cure, sleeping sickness prior to the onset of neurological symptoms. The second stage of African trypanosomiasis is particularly difficult to treat, but in the last decade or two the introduction of organic arsenical compounds based on melarsen have greatly improved the outlook (Kreier and Baker 1987).
Symptoms for diagnosis of Malaria caused by Plasmodium falciparum include recurrent fevers, and an enlarged spleen; as well as the demonstration of parasites in thick or thin blood films. The drug quinine was known for centuries in Peru, before being imported to Europe in the first half of the 17th century. It is very useful in treating the erythrocytic parasites, and is still often used in cases of cerebral malaria where parasites have become resistant to other drugs. However, because of the association of quinine with blackwater fever, general treatment is now given by synthetic drugs such as amodiaquine, pyrimethamine, or chloroquine. Chloroquine and another drug called proguanil taken regularly in lower dosages can prevent clinical attacks of malaria. If not treated, this strain of malaria will result in rapid death.
Avoiding contact with infective mosquitoes can too prevent malaria. Since all the vector species of Anopheles feed in the evening or late at night, this can be done by screening the windows and doors of houses with fine mesh netting, and by the use of mosquito nets over beds. The mosquitoes can also be destroyed where adequate funds are available and malaria eradicated, by spraying houses with long lasting insecticides and by draining the swamps, etc, where they breed. Thanks to the efforts of local health authorities, governments, and the WHO, much progress has been made in eradicating malaria where it was previously a scourge, but much remains to be done (Kreier and Baker 1987).
In terms of human public health significance, Visceral Leishmaniasis causes life threatening systematic infection. Tropical Medicine Clinicians are often baffled by the complexities of Leishmaniasis, and by the apparently innumerable possible combinations of different Leishmanial syndromes, species, and geographical areas of acquisition of infection, each combination varying by clinical presentation, ease of diagnosis, natural history, and response to therapy (Herwaldt 1999).
Trypanosoma brucei gambiense too has a profound effect through Sleeping sickness on the health of a large portion of sub-Saharan Africa. Furthermore, the economic impact of sleeping sickness is significant owing to the dramatic reduction in the labour force and resulting decrease in economic productivity. This in turn has countless added direct and indirect impacts on the quality of life of people in affected countries. The economic impact is further increased because sleeping sickness can reduce cattle production 20-40% in areas where the disease is epidemic. It is estimated that cattle-production losses alone amount to $2.7 billion each year. (Kreier and Baker 1987).
Similarly, The strain of Malaria that Plasmodium falciparum induces has a profound effect on human public health, and will kill rapidly unlike other strains which will cause recurrent fevers which may weaken the immune system. Wide areas of the population may be affected and subsequently wiped out in this way in a short space of time, thus affecting the population and economy (Kreier and Baker 1987).
Looking at all the evidence, it is quite difficult to determine which of these three animal protists have the most profound effect on human health. While they are all deadly unless treated rapidly, the protozoa causing sleeping sickness has the more profound effect of exhibiting in the victim a chronic illness, which causes it’s effect on the population to be far more drawn out. While Malaria is a disease causing problems over wide populations, one advantage is that it can be easily stopped in its tracks if people are educated regarding the use of screens and nets. At the moment, Leishmaniasis is proving to be particularly problematic, not only due to the wide variety of species carrying the parasite, but also due to the complex and wide ranging symptoms visible in different areas of the world. Governments, local health authorities, and organizations like World Health must carry on investing in research if the problems caused by these microscopic organisms are to be resolved.
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References
W.G. Hale, J.P. Margham, & V.A. Saunders (1995), page 241, Collin’s Dictionary of Biology, HarperCollins Publishers, Glasgow.
Herwaldt (1999), 1191-99 Vol 354, Seminar Leishmaniasis, The Lancet.
Mahmoud (1989), 1015-22 Vol 246, Parasitic Protozoa and Helminths: Biological and Immunological Challenges, Science.
Kreier and Baker (1987), Parasitic Protozoa, Allen and Unwin Publishers, London, Hemel Hempstead, and North Sydney.
Hoare (1949), Handbook of Medical Protozoology, Bailliere Tindall and Cox, London.
Krapelou (1987) Parasitic Life Cycles, Springer-Verlag Publishers, New York Inc.
