Trypanosoma brucei causes African sleeping sickness

Paul R EarlFacultad de Ciencias Biológicas

Universidad Autónoma de Nuevo LeónSan Nicolás, NL 66541, Mexico

In the present warming trend, the destruction of our forests by multinational as well as national loggers is causing the world to become drier. An added notorious threat is primitive or nomadic farming. These farmers, whether in Mexico, Brazil or Kenya whose activities are supported by the World Bank, clear the land for crops and for cattle. Many nations kill trees under the banner of agriculture. Then it follows that the epidemiology changes as the environment is so changed.

This terrible business of landclearing with consequently reduced transmission of the protozoan flagellate: Trypanosoma brucei in this case is partly due to human overpopulation. The vectors are tsetse flies that live in wooded areas. Landclearing has been called “agronomic prevention” that consists of methodically clearing undergrowth from villages, watersources, paths and bridges to destroy its natural habitat. Soon only protected national parks in Africa may have trypanosomes and tsetses.

The eradication of malaria and other tropical diseases around the world with emphasis on Afroasia should include sleeping sickness. A special spraying campaign is needed for the elimination of the tsetse from the national parks like Kruger. Things were better in 1965, but DDT was much abandoned by then. Such difficult refuges as this famous park and world treasure have to be cleaned out of flies on a priority basis if eradication is to succeed. Vector control is the keynote. But also, very few effective genetic investigations of tsetse flies are ongoing. Here we advocate vector control on the worst area first basis.

We have, in Africa, a most complex ecology involving increasing rural populations. Is our goal to raise the standard of living by reducing the disease burden? Is tsetse elimination important for the cattle industry as well as for human health? Is clearing woods a wise path to take to reduce tsetse flies, or is landclearing wanton destruction. The cropper wants grain to feed his family, and this much is possible by clearing land. Then the consequence is a larger unskilled rural population.

The parasite exists as 2 morphologically identical subspecies: Trypanosoma brucei rhodesiense (East African or Rhodesian African trypanosomiasis) and T. brucei gambiense (West African or Gambian African trypanosomiasis), and T. b. brucei can be added, although it is much like T. b. rhodesiense. Human blood contains a factor that lyses animal trypanosomes. Human-invasive parasites are transmitted to human hosts by bites of infected tsetse flies (Glossina species), which are found only in Africa.

T. b. brucei

Diagram of a tsetse fly

Glossina pallidipes

Tsetse flies. G. morsitans is the most important vector in East Africa and G. fuscipes (previously known as G. palpalis) in West Africa. G. morsitans is found in savanna and woodland areas, whereas G. fuscipes is found in gallery forests along rivers and lakes. Having painful bites, tsetses will follow moving objects, be attracted by the odors like acetone and phenol and will bite through thin clothing.

Mother and son arranging a tsetse trap. Note the heavy vegetation.

Dead tsetses from a trap

impregnated with DDT

Symptoms of sleeping sickness. Infection leads to malaise, lassitude and irregular fevers. Early symptoms include fever and enlarged lymph glands and spleen, that are more severe and acute in T.b. rhodesiense infections. Early signs are followed by a range of symptoms. Within 1-2 weeks after the bite, a dusky red nodule which can reach 6 cm in size develops and lasts about 2 weeks. This primary chancer may be painful and is located often on the legs.

Woman in a stage of lethargy

The parasites escape the initial host defense mechanisms by extensive antigenic variation of parasite surface glycoproteins (VSG). This evasion of the humoral immune responses contributes to parasite virulence. During the parasitemia, most pathologic changes occur in the hematologic, lymphatic, cardiac and the CNS. Immune-mediated reactions against antigens on red blood cells, cardiac and brain tissue can result in hemolysis, anemia, pancarditis and

meningoencephalitis.

Lumbar puncture to see if tryps are in

the CNS

Treatment. If the disease is diagnosed early, the chances of cure are good. Success in the second neurological phase depends on having a drug that can cross the blood-brain barrier to reach the parasite. Four drugs have been used until now.

First phase treatments: 1/ Suramine, discovered in 1921, is used in treatment of the initial phase of T. b. rhodesiense. There are certain undesirable effects, especially indigestion. 2/ Pentamidine, discovered in 1941, is used in treatment of the initial phase of T. b. gambiense sleeping sickness. In spite of some undesirable effects, it is well tolerated by patients. Aventis dropped pentamidine. Future production is problematic.

Second phase treatments: 3/ Melarsoprol, discovered in 1949, is the only drug available on the market to treat the advanced stage of sleeping sickness, no matter which parasite is the cause. It is the last of the arsenicals. The undesired effects are drastic; they include reactive encephalopathy allergic nature that is often fatal. Survivors suffer serious neurological sequelae. Furthermore, there is considerable resistance to melarsoprol. 4/ Eflornithine, registered in 1990, is the alternative to melarsoprol. It is effective only against T.b. gambiense. Production ceased in 1999 and Aventis gave the licence to WHO, which has undertaken several initiatives to seek a new source of production.

Antigenic variation and genetics. How do tryps evade the immune response? Trypanosomes are covered with a surface coat of a monomolecular layer of a glycoprotein called variable surface glycoprotein (VSG). This protein is being changed every 5-7 days, resulting in a new variable antigen type (VAT). Immunoglobulins directed against these VSGs recognize the trypanosome binding of antibodies that will lead to cell lysis by complement and the demise of these tryps.

At least 100 different VATs that have been identified in T. equiperdum at least 1000 different genes that are present in the genome of T. brucei. Trypanosomes have a very high capacity for homologous recombination. Different members of the VSG gene family may recombine to form new variants. Cytologically unseen recombination here is by the polymerase chain reaction, and meiosis is INFERRED by flow cytometery DNA content so that due circumspection

is called for.

There are 3 size classes of chromosomes: a) minichromosomes of 50–150 kilobases (kb) containing repeat sequences and basic copies of VSG genes, b) the intermediate chromosomes of 200–900 kb containing VSG expression sites and c) 11 megabase chromosomes of 1-6 Mb thought to contain all housekeeping genes. The megabase chromosomes are said to be diploid and inherited in Mendelian fashion. Nevertheless. meiosis which must contain a reduction division is not evinced, thus discussion of sex and evolution in tryps might be premature.

Haploid + haploid chromosomes immediately reduced is zygotic meiosis. HAPLOID means one set (1n) of chromosomes as in ferns and thousands of protozoa and metazoa. Diploid (2n) reduced to the haploid state (1n) is gametic meiosis in which the 2 haploid gametes fuse (fertilize) to create or restore the diploid set of chromosomes. The role of the DNA kinetoplast and more cytology in trypanosomes is only partially

explained today.

PLEASE BE CAREFUL ABOUT WHAT IS MEANT BY MEIOSIS. WHEN THE WORD RECOMBINATION ENTERS, CROSSINGOVER HAD OCCURRED (?). WHERE IS THE EVIDENCE FOR THIS LIKE CHIASMATA? MEIOSIS CONTAINS A REDUCTION OF THE CHROMOSOME SET WHICH CAN HAPPEN BEFORE (GAMETIC) OR AFTER (ZYGOTIC) FERTILIZATION.THE SEXUAL DRAMA THAT MAY WELL TAKE PLACE IN THE TSETSE GUT IS UNKNOWN. THE SYSTEM OF DNA MINICIRCLES IN KINETOPLASTS IS UNIQUE.

Tomorrow more credible cytogenetic data will be presented on chromosomes, and perhaps transgenic tsetse flies incompetent of parasite transmission will have been created. More, susceptible and refractory families of flies are already known.These and other sophistications that—somehow—are related to making vaccines are not necessary for vector control. The grant money flows into genetics and vaccines as if these disease problems required more new advanced technology. Vector control, including release of sterile male tsetses, is proven.