Tsetse sometimes spelled
tzetze and also known as tik-tik flies, are large biting flies that inhabit much of mid-continental
AfricaAfrica is the world's second largest and second most populous continent, after Asia. At about 30.2 million km² including adjacent islands, it covers 6% of the Earth's total surface area and 20.4% of the total land area...
between the
SaharaThe Sahara is the world's second largest desert, after Antarctica. At over , it covers most of Northern Africa, making it almost as large as Europe or the United States. The Sahara stretches from the Red Sea, including parts of the Mediterranean coasts, to the outskirts of the Atlantic Ocean...
and the Kalahari deserts. They live by feeding on the
bloodBlood is a specialized bodily fluid in animals that delivers necessary substances such as nutrients and oxygen to the cells and transports metabolic waste products away from those same cells....
of
vertebrateVertebrates are animals that are members of the subphylum Vertebrata . Vertebrates are the largest group of chordates, with currently about 58,000 species described. Vertebrates include the jawless fishes, bony fishes, sharks and rays, amphibians, reptiles, mammals, and birds...
animals and are the primary biological vectors of trypanosomes, which cause human sleeping sickness and animal trypanosomiasis, also known as
naganaNagana, also known as nagana pest or animal African trypanosomiasis, is a disease of vertebrate animals. The disease is caused by trypanosomes of several species in the genus Trypanosoma. The trypanosomes infect the blood of the vertebrate host, causing fever, weakness, and lethargy, which lead to...
. Tsetse include all the species in the
genusIn biology, a genus is a low-level taxonomic rank used in the biological classification of living and fossil organisms, which is an example of definition by genus and differentia...
Glossina, which are generally placed in their own family, Glossinidae.
Tsetse have been extensively studied because of their disease transmission. These flies are multivoltine, typically producing about four generations yearly, and up to 31 generations total over their entire lifespan.
Tsetse are crudely similar to other large flies, such as the
houseflyThe housefly , Musca domestica, is a fly of the suborder Cyclorrhapha...
, but can be distinguished by various characteristics of their
anatomyAnatomy is a branch of biology and medicine that is the consideration of the structure of living things. It is a general term that includes human anatomy, animal anatomy , and plant anatomy...
, two of which are easy to observe. Tsetse fold their wings completely when they are resting so that one wing rests directly on top of the other over their
abdomenIn vertebrates such as mammals the abdomen constitutes the part of the body between the thorax and pelvis. The region enclosed by the abdomen is termed the abdominal cavity...
. Tsetse also have a long
proboscisA proboscis is an elongated appendage from the head of an animal, either a vertebrate or an invertebrate. In simpler terms, a proboscis is the straw-like mouth found in several varieties of species.-Etymology:...
, which extends directly forward and is attached by a distinct bulb to the bottom of their head.
Fossilized tsetse have been recovered from the
Florissant Fossil BedsFlorissant Fossil Beds National Monument is a United States National Monument in Teller County, Colorado, that is noted for its fossils. It is located in a mountain valley just west of Pikes Peak and holds spectacular remnants of prehistoric life...
in
ColoradoColorado is a U.S. state that encompasses much of the Rocky Mountains as well as the northeastern portion of the Colorado Plateau and the western edge of the Great Plains...
. There are 23 species of tsetse flies. Diseases transmitted by tsetse flies kill 250,000–300,000 people per year.
Biology
The biology of tsetse is relatively well understood. Tsetse have been extensively studied because of their medical, veterinary, and economic importance, because the flies can be raised in a laboratory, and because the flies are relatively large, facilitating their analysis.
EntomologistsEntomology is the scientific study of insects, a branch of arthropodology...
have discovered a great deal about tsetse
morphologyIn biology, morphology is a branch of bioscience dealing with the study of the form and structure of organisms and their specific structural features....
,
anatomyAnatomy is a branch of biology and medicine that is the consideration of the structure of living things. It is a general term that includes human anatomy, animal anatomy , and plant anatomy...
,
developmentMorphogenesis , is the biological process that causes an organism to develop its shape...
, and
metabolismMetabolism is the set of chemical reactions that happen in the cells of living organisms to sustain life. These processes allow organisms to grow and reproduce, maintain their structures, and respond to their environments. Metabolism is usually divided into two categories...
.
Morphology
Tsetse fly can be seen as independent individuals in two forms: as third
instarAn instar is a developmental stage of arthropods, such as insects, between each molt , until sexual maturity is reached. Arthropods must shed the exoskeleton in order to grow or assume a new form. Differences between instars can often be seen in altered body proportions, colors, patterns, or...
larvae, and as adults.
Tsetse first become separate from their mothers during the third larval instar, during which they have the typical appearance of maggots. However, this life stage is short, lasting at most a few hours, and is almost never observed outside of the laboratory.
Tsetse next become puparia—small, hard shelled, oblongs with two distinctive, small, dark lobes at one end. Tsetse puparia are under 1.0 cm long. Within the puparial shell, tsetse complete the last two larval instars and the pupal stage.
Tsetse then emerge as adult flies. Tsetse adults are relatively large flies, with lengths of ½–1½ cm , and have a recognizable shape or bauplan so they can usually be distinguished without trouble from other flies. Tsetse have large heads, distinctly separated eyes, and unusual antennae. The tsetse
thoraxThe thorax is the mid section of the insect body. It holds the head, legs, wings and abdomen. It is also called mesosoma in other arthropods....
is quite large, while the abdomen is wide rather than elongated and shorter than the wings.
Four characteristics definitively separate adult tsetse from other kinds of flies:
| Proboscis |
Tsetse have a distinct proboscis, a long thin structure attached to the bottom of the head and pointing forward. |
|
| Folded wings |
When at rest, tsetse fold their wings completely one on top of the other. |
|
| Hatchet cell |
The discal medial ("middle") cell of the wing has a characteristic hatchet shape resembling a meat cleaver or a hatchet. |
|
| Branched arista hairs |
The antennae have arista with hairs which are themselves branched. |
|
Anatomy
Like all other
insectInsects are a class of living creatures within the arthropods that have a chitinous exoskeleton, a three-part body , three pairs of jointed legs, compound eyes, and two antennae...
s, Tsetse flies have an adult body comprising three visibly distinct parts: the head, the thorax and the abdomen.
The head has large eyes, distinctly separated on each side, and a distinct, forward-pointing proboscis attached underneath by a large bulb. The thorax is large, made of three fused segments. Three pairs of legs are attached to the thorax, as are two wings and two halteres. The abdomen is short but wide and changes dramatically in volume during feeding.
The internal anatomy of tsetse is fairly typical of the
insectInsects are a class of living creatures within the arthropods that have a chitinous exoskeleton, a three-part body , three pairs of jointed legs, compound eyes, and two antennae...
s. The
cropA crop is a thin-walled expanded portion of the alimentary tract used for the storage of food prior to digestion that is found in many animals, including gastropods, earthworms, leeches, insects, birds, and even some dinosaurs.- Bees :Cropping is used by bees to temporarily store nectar of flowers...
is large enough to accommodate a huge increase in size during the bloodmeal since tsetse can take a bloodmeal weighing as much as themselves. The reproductive tract of adult females includes a
uterusThe uterus or womb is a major female hormone-responsive reproductive sex organ of most mammals including humans. One end, the cervix, opens into the vagina, while the other is connected to one or both fallopian tubes, depending on the species...
which can become large enough to hold the third instar
larvaA larva is a distinct juvenile form many animals undergo before metamorphosis into adults. Animals with indirect development such as insects, amphibians, or cnidarians typically have a larval phase of their life cycle...
at the end of each
pregnancyPregnancy refers to the fertilization and development of one or more offspring, known as a fetus or embryo, in a woman's uterus. In a pregnancy, there can be multiple gestations, as in the case of twins or triplets...
.
Most tsetse flies are physically very tough. Houseflies are easily killed with a fly-swatter but it takes a great deal of effort to crush a tsetse fly.
Life cycle
Tsetse have an unusual
life cycleA life cycle is a period involving all different generations of a species succeeding each other through means of reproduction, whether through asexual reproduction or sexual reproduction...
which may be due to the richness of their food source. Female tsetse only fertilize one egg at a time and retain each egg within their uterus to have the offspring develop internally during the first larval stages, a strategy called
adenotrophic viviparityAdenotrophic viviparity means "gland fed, live birth". This is the reproductive mode of many insects such as tsetse flies, gnats, moths, fleas, and mosquitos....
. During this time, the female feeds the developing offspring with a milky substance secreted by a modified gland in the uterus. In the third larval stage, the tsetse larva finally leave the uterus and begin their independent life. However, the newly independent tsetse larva simply crawls into the ground, and forms a hard outer shell called the puparial case, in which it completes its morphological transformation into an adult fly. This lifestage has a variable duration, generally twenty to thirty days, and the larva must rely on stored resources during this time. The importance of the richness of blood to this development can be seen since all tsetse development before it emerges from the puparial case as a full adult occurs without feeding, based only on nutritional resources provided by the female parent. The female must get enough energy for her needs, for the needs of her developing offspring, and to store the resources which her offspring will require until it emerges as an adult.
Technically these insects undergo the standard
developmentMorphogenesis , is the biological process that causes an organism to develop its shape...
process of insects which comprises
oocyteAn oocyte, ovocyte, or rarely ocyte, is a female gametocyte or germ cell involved in reproduction. In other words, it is an immature ovum, or egg cell. An oocyte is produced in the ovary during female gametogenesis. The female germ cells produce a primordial germ cell which undergoes a mitotic...
formation, ovulation and fertilization, development of the egg, five
larvaA larva is a distinct juvenile form many animals undergo before metamorphosis into adults. Animals with indirect development such as insects, amphibians, or cnidarians typically have a larval phase of their life cycle...
l stages, a
pupaA pupa is the life stage of some insects undergoing transformation. The pupal stage is found only in holometabolous insects, those that undergo a complete metamorphosis, going through four life stages; embryo, larva, pupa and imago...
l stage, and the emergence and maturation of the adult.
General biology
Tsetse has three distinct symbionts. The primary symbiont is
Wigglesworthia within
bacteriocyteA bacteriocyte , also called a mycetocyte, is a specialized adipocyte found in some insect groups such as aphids, tsetse flies, german cockroaches, and many others. Mycetocyte symbionts specifically include three insect groups: Cockroaches, Homoptera such as aphid, and Coleoptera such as weevils...
, the secondary symbiont is
Sodalis intercellularly or intracellularly, and the third is some kind of
WolbachiaWolbachia is a genus of bacteria which infects arthropod species, including a high proportion of insects , as well as some nematodes. It is one of the world's most common parasitic microbes and is possibly the most common reproductive parasite in the biosphere...
.
The tsetse
Glossina palpalis is also a vector and host of
Hepatozoon petti, a parasitic Sporozoa of the
nile crocodileThe Nile crocodile or Common crocodile is an African crocodile which is common in Somalia, Ethiopia, Uganda, Kenya, Egypt, Tanzania, Zambia, Zimbabwe, Gabon, South Africa, Malawi, Sudan, Botswana, and Cameroon...
.
Systematics
Tsetse include up to thirty four species and sub-species depending on the particular classification used.
All current classifications place all the tsetse species in a single genus named
Glossina. Most classifications place this genus as the sole member of the family Glossinidae. The Glossinidae are generally placed within the superfamily
HippoboscoideaHippoboscoidea is a superfamily of Calyptratae. The flies in this superfamily are blood-feeding obligate parasites of their hosts. Five families are often placed here:* Glossinidae* Hippoboscidae...
, which contains other hematophagous families.
Species
The tsetse genus is generally split into three groups of species based on a combination of distributional, behavioral, molecular and morphological characteristics. The genus includes:
- The savannah flies: (Subgenus Morsitans, occasionally named Glossina):
- Glossina austeni (Newstead, 1912)
- Glossina pallidipes (Austen, 1903)
- Glossina swynnertoni (Austen, 1923)
- The forest flies: (Subgenus Fusca, previously named Austenia):
-
- Glossina fusca fusca (Walker, 1849)
- Glossina fuscipleuris (Austen, 1911)
- Glossina frezili (Gouteux, 1987)
- Glossina haningtoni (Newstead and Evans, 1922)
- Glossina longipennis (Corti, 1895)
- Glossina medicorum (Austen, 1911)
- Glossina nashi (Potts,1955)
- Glossina nigrofusca nigrofusca (Newstead, 1911)
- Glossina severini (Newstead, 1913)
- Glossina schwetzi (Newstead and Evans, 1921)
- Glossina tabaniformis (Westwood, 1850)
- Glossina vanhoofi (Henrard, 1952)
- The riverine flies: (Sub-genus Palpalis, previously named Nemorhina):
- Glossina caliginea (Austen, 1911)
- Glossina fuscipes fuscipes (Newstead, 1911)
- Glossina fuscipes martinii (Zumpt, 1935)
- Glossina fuscipes quanzensis (Pires, 1948)
- Glossina pallicera pallicera (Bigot, 1891)
- Glossina pallicera newsteadi (Austen, 1929)
- Glossina palpalis palpalis (Robineau-Desvoidy, 1830)
- Glossina palpalis gambiensis (Vanderplank, 1911)
- Glossina tachinoides (Westwood, 1850)
Trypanosomiasis
Tsetse are biological vectors of trypanosomes meaning that tsetse, in the process of feeding, acquire and then transmit small, single-celled organisms called trypanosomes from
infectedAn infection is the colonization of a host organism by parasite species. Infecting parasites seek to use the host's resources to reproduce, often resulting in disease...
vertebrateVertebrates are animals that are members of the subphylum Vertebrata . Vertebrates are the largest group of chordates, with currently about 58,000 species described. Vertebrates include the jawless fishes, bony fishes, sharks and rays, amphibians, reptiles, mammals, and birds...
hostsIn biology, a host is an organism that harbors a parasite, or a mutual or commensal symbiont, typically providing nourishment and shelter. In botany, a host plant is one that supplies food resources and substrate for certain insects or other fauna...
to uninfected animals. Some tsetse transmitted trypanosome species cause
trypanosomiasisTrypanosomiasis or trypanosomosis is the name of several diseases in vertebrates caused by parasitic protozoan trypanosomes of the genus Trypanosoma. Approximately 500,000 men, women and children in 36 countries of sub-Saharan Africa suffer from human African trypanosomiasis which is caused by...
, an infectious disease. In humans, tsetse transmitted trypanosomiasis is called sleeping sickness. In animals, tsetse vectored trypanosomiases include
naganaNagana, also known as nagana pest or animal African trypanosomiasis, is a disease of vertebrate animals. The disease is caused by trypanosomes of several species in the genus Trypanosoma. The trypanosomes infect the blood of the vertebrate host, causing fever, weakness, and lethargy, which lead to...
,
soumaNagana, also known as nagana pest or animal African trypanosomiasis, is a disease of vertebrate animals. The disease is caused by trypanosomes of several species in the genus Trypanosoma. The trypanosomes infect the blood of the vertebrate host, causing fever, weakness, and lethargy, which lead to...
, and
surraSurra is a disease of vertebrate animals. The disease is caused by protozoan trypanosomes, specifically Trypanosoma evansi, of several species which infect the blood of the vertebrate host, causing fever, weakness, and lethargy which lead to weight loss and anemia...
according to the animal infected and the trypanosome species involved, although the usage is not strict and
nagana is occasionally used for any form of animal trypanosomiasis.
Trypanosomes are
animalAnimals are a major group of multicellular, eukaryotic organisms of the kingdom Animalia or Metazoa. Their body plan eventually becomes fixed as they develop, although some undergo a process of metamorphosis later on in their life. Most animals are motile, meaning they can move spontaneously and...
parasites, specifically
protozoaProtozoa are a diverse group of single-cells eukaryotic organisms, many of which are motile. Throughout history, protozoa have been defined as single-cell protists with animal-like behavior, e.g., movement...
of the genus
Trypanosoma. These organisms are approximately the size of red blood cells. Different species of trypanosomes infect different hosts as can be seen in the table attached to this section. Trypanosomes range widely in their effects on the vertebrate hosts. Some species, such as
Trypanosoma theileri, do not seem to cause any health problems except perhaps in animals that are already sick.
Some
strainsIn biology, a strain is a low-level taxonomic rank used in three related ways.-Microbiology and virology:A strain is a genetic variant or subtype of a micro-organism . For example, a "flu strain" is a certain biological form of the influenza or "flu" virus...
are much more
virulentVirulence is by MeSH definition the degree of pathogenicity within a group or species of parasites as indicated by case fatality rates and/or the ability of the organism to invade the tissues of the host. The pathogenicity of an organism - its ability to cause disease - is determined by its...
. Tsetse seem to be unaffected by the infection of trypanosomes but it is entirely possible that the parasites alter tsetse behavior or have other effects that improve the chances of transmission and survival. These trypanosomes are highly evolved and have developed a life cycle that requires periods in both the vertebrate and tsetse hosts.
Tsetse transmit trypanosomes in two ways,
mechanical and
biological transmission.
- Mechanical transmission involves the direct transmission of the same individual trypanosomes taken from an infected host into an uninfected host. The name mechanical reflects the similarity of this mode of transmission to mechanical injection with a syringe
A syringe is a simple pump consisting of a plunger that fits tightly in a tube. The plunger can be pulled and pushed along inside a cylindrical tube , allowing the syringe to take in and expel a liquid or gas through an orifice at the open end of the tube...
. Mechanical transmission requires that tsetse feed on an infected host and acquire trypanosomes in the bloodmeal, and then, within a relatively short period, for tsetse to feed on an uninfected host and regurgitate some of the infected blood from the first bloodmeal into the tissue of the uninfected animal. This type of transmission occurs most frequently when tsetse are interrupted during a bloodmeal and attempt to satiate themselves with another meal. Other flies, such as horse-fliesInsects in the order Diptera, family Tabanidae, are commonly called horse flies. Often considered pests for the bites that many inflict, they are among the world's largest true flies. They are known to be extremely noisy during flight. They are also important pollinators of flowers, especially in...
, also can cause mechanical transmission of trypanosomes.
- Biological transmission requires a period of incubation of the trypanosomes within the tsetse host. The term biological is used because trypanosomes must reproduce through several generations inside the tsetse host during the period of incubation, which requires extreme adaptation of the trypanosomes to their tsetse host. In this mode of transmission, trypanosomes reproduce through several generations, changing in morphology at certain periods. This mode of transmission also includes the sexual phase of the trypanosomes. Tsetse are believed to be more likely to become infected by trypanosomes during their first few bloodmeals. Tsetse infected by trypanosomes are thought to remain infected for the remainder of their lives. Because of the adaptations required for biological transmission, trypanosomes transmitted biologically by tsetse cannot be transmitted in this manner by other insects.
The relative importance of these two modes of transmission for the propagation of tsetse-vectored trypanosomiases is not yet well understood. However, since the sexual phase of the trypanosome lifecycle occurs within the tsetse host, biological transmission is a required step in the life cycle of the tsetse vectored trypanosomes.
The cycle of biological transmission of trypanosomiasis involves two phases, one inside the tsetse host and the other inside the vertebrate host. Trypanosomes are not passed between a pregnant tsetse and her offspring so all newly emerged tsetse adults are free of infection. An uninfected fly that feeds on an infected vertebrate animal may acquire trypanosomes in its proboscis or gut. These trypanosomes, depending on the species, may remain in place, move to a different part of the digestive tract, or migrate through the tsetse body into the salivary glands. When an infected tsetse bites a susceptible host, the fly may regurgitate part of a previous bloodmeal that contains trypanosomes, or may inject trypanosomes in its saliva. It is believed the inoculation must contain a minimum of 300 to 450 individual trypanosomes to be successful, and may contain up to 40,000 individuals.
The trypanosomes are injected into vertebrate muscle tissue but make their way, first into the
lymphatic systemThe lymphoid system is the part of the immune system comprising a network of conduits called lymphatic vessels that carry a clear fluid called lymph unidirectionally toward the heart. Lymphoid tissue is found in many organs, particularly the lymph nodes, and in the lymphoid follicles associated...
, then into the bloodstream, and eventually into the brain. The disease causes the swelling of the lymph glands, emaciation of the body, and eventually leads to death. Uninfected tsetse may bite the infected animal prior to its death and acquire the disease, thereby closing the transmission cycle.
The tsetse vectored trypanosomiases affect various vertebrate species including humans, antelopes, bovine cattle, camels, horses, sheep, goats, and pigs. These diseases are caused by several different trypanosome species that may also survive in wild animals such as crocodiles and monitor lizards. The diseases have different distributions across the African continent and are therefore transmitted by different species of tsetse. The following table summarizes this information:
| Disease | Species affected | Trypanosoma agents | Distribution | Glossina vectors |
| Sleeping sickness — chronic form |
humans |
T. brucei gambiense |
Western Africa |
G. palpalis
G. tachinoides
G. fuscipes
G. morsitans |
| Sleeping sickness — acute form |
humans |
T. brucei rhodesiense |
Eastern Africa |
G. morsitans
G. swynnertoni
G. pallidipes
G. fuscipes |
| Nagana Nagana, also known as nagana pest or animal African trypanosomiasis, is a disease of vertebrate animals. The disease is caused by trypanosomes of several species in the genus Trypanosoma. The trypanosomes infect the blood of the vertebrate host, causing fever, weakness, and lethargy, which lead to...
— acute form |
antelope
cattle
camels
horses |
T. brucei brucei |
Africa |
G. morsitans
G. swynnertoni
G. pallidipes
G. palpalis
G. tachinoides
G. fuscipes |
| Nagana Nagana, also known as nagana pest or animal African trypanosomiasis, is a disease of vertebrate animals. The disease is caused by trypanosomes of several species in the genus Trypanosoma. The trypanosomes infect the blood of the vertebrate host, causing fever, weakness, and lethargy, which lead to...
— chronic form |
cattle
camels
horses |
T. congolense |
Africa |
G. palpalis
G. morsitans
G. austeni
G. swynnertoni
G. pallidipes
G. longipalpis
G. tachinoides
G. brevipalpis |
| Nagana Nagana, also known as nagana pest or animal African trypanosomiasis, is a disease of vertebrate animals. The disease is caused by trypanosomes of several species in the genus Trypanosoma. The trypanosomes infect the blood of the vertebrate host, causing fever, weakness, and lethargy, which lead to...
— acute form |
domestic pigs
cattle
camels
horses |
T. simiae |
Africa |
G. palpalis
G. fuscipes
G. morsitans
G. tachinoides
G. longipalpis
G. fusca
G. tabaniformis
G. brevipalpis
G. vanhoofi
G. austeni |
| Nagana Nagana, also known as nagana pest or animal African trypanosomiasis, is a disease of vertebrate animals. The disease is caused by trypanosomes of several species in the genus Trypanosoma. The trypanosomes infect the blood of the vertebrate host, causing fever, weakness, and lethargy, which lead to...
— acute form |
cattle
camels
horses |
T. vivax |
Africa |
G. morsitans
G. palpalis
G. tachinoides
G. swynnertoni
G. pallidipes
G. austeni
G. vanhoofi
G. longipalpis |
| Surra Surra is a disease of vertebrate animals. The disease is caused by protozoan trypanosomes, specifically Trypanosoma evansi, of several species which infect the blood of the vertebrate host, causing fever, weakness, and lethargy which lead to weight loss and anemia...
— chronic form |
domestic pigs
warthogThe Warthog or Common Warthog is a wild member of the pig family that lives in grassland, savanna, and woodland in Sub-Saharan Africa. In the past it was commonly treated as a subspecies of P...
(Phacochoerus aethiopicus)
forest hogs (Hylochoerus spp.) |
T. suis |
Africa |
G. palpalis
G. fuscipes
G. morsitans
G. tachinoides
G. longipalpis
G. fusca
G. tabaniformis
G. brevipalpis
G. vanhoofi
G. austeni |
Human trypanosomiasis
Human African trypanosomiasis, also called sleeping sickness, is caused by trypanosomes of the
Trypanosoma brucei species. This disease is invariably fatal unless treated but can almost always be cured with current medicines, if the disease is diagnosed early enough.
Sleeping sickness begins with a tsetse bite leading to an inoculation in the sub-cutaneous tissue. The infection moves into the
lymphatic systemThe lymphoid system is the part of the immune system comprising a network of conduits called lymphatic vessels that carry a clear fluid called lymph unidirectionally toward the heart. Lymphoid tissue is found in many organs, particularly the lymph nodes, and in the lymphoid follicles associated...
, leading to a characteristic swelling of the lymph glands called
Winterbottom's sign. The infection progresses into the blood stream and eventually crosses into the
central nervous systemThe central nervous system is the part of the nervous system that integrates the information that it receives from, and coordinates the activity of, all parts of the bodies of bilaterian animals—that is, all multicellular animals except sponges and radially symmetric animals such as jellyfish...
and invades the
brainThe brain is the center of the nervous system in all vertebrate and most invertebrate animals—only a few primitive invertebrates such as sponges, jellyfish, sea squirts and starfishes do not have one. It is located in the head, usually close to primary sensory apparatus such as vision, hearing,...
leading to extreme lethargy and eventually to
deathDeath is the permanent termination of the biological functions that sustain a living organism. Phenomena which commonly bring about death include old age, predation, malnutrition, disease, and accidents or trauma resulting in terminal injury....
.
The
Trypanosoma brucei species, which causes the disease, has often been subdivided into three sub-genera that were identified based either on the vertebrate hosts which the strain could infect or on the virulence of the disease in humans. The trypanosomes infectious to animals and not to humans were named
Trypanosoma brucei brucei. Strains that infected humans were divided into two sub-species based on their different virulences:
Trypanosoma brucei gambiense was thought to have a slower onset and
Trypanosoma brucei rhodesiense refers to strains with a more rapid, virulent onset. This characterization has always been problematic but was the best that could be done given the knowledge of the time and the tools available for identification. A recent molecular study using
restriction fragment length polymorphismIn molecular biology, restriction fragment length polymorphism, or RFLP , is a technique that exploits variations in homologous DNA sequences. It refers to a difference between samples of homologous DNA molecules that come from differing locations of restriction enzyme sites, and to a related...
analysis suggests that the three sub-genera are polyphyletic, so the elucidation of the strains of
T. brucei infective to humans requires a more complex explanation.
Other forms of human trypanosomiasis also exist but are not transmitted by tsetse. The most notable is American trypanosomiasis, known as
Chagas diseaseChagas disease is a tropical parasitic disease caused by the flagellate protozoan Trypanosoma cruzi. T. cruzi is commonly transmitted to humans and other mammals by an insect vector, the blood-sucking insects of the subfamily Triatominae most commonly species belonging to the Triatoma, Rhodnius,...
, which occurs in
South AmericaSouth America is a continent situated in the Western Hemisphere, mostly in the Southern Hemisphere, with a relatively small portion in the Northern Hemisphere. The continent is also considered a subcontinent of the Americas. It is bordered on the west by the Pacific Ocean and on the north and east...
, caused by
Trypanosoma cruzi, and transmitted by certain species of the
ReduviidaeReduviidae is a large, cosmopolitan family of predatory insects in the suborder Heteroptera...
, members of the
HemipteraHemiptera is an order of insects most often known as the true bugs , comprising around 50,000–80,000 species of cicadas, aphids, planthoppers, leafhoppers, shield bugs, and others...
.
Animal trypanosomiasis
Animal
trypanosomiasisTrypanosomiasis or trypanosomosis is the name of several diseases in vertebrates caused by parasitic protozoan trypanosomes of the genus Trypanosoma. Approximately 500,000 men, women and children in 36 countries of sub-Saharan Africa suffer from human African trypanosomiasis which is caused by...
, also called
nagana when it occurs in bovine cattle or horses or
sura when it occurs in domestic
pigsPIGS is a four letter acronym that can stand for:* PIGS , Phosphatidylinositol glycan anchor biosynthesis, class S, a human gene* PIGS , the economies of Portugal, Italy , Greece and Spain...
, is caused by several trypanosome species. These
diseaseA disease is an abnormal condition affecting the body of an organism. It is often construed to be a medical condition associated with specific symptoms and signs. It may be caused by external factors, such as infectious disease, or it may be caused by internal dysfunctions, such as autoimmune...
s reduce the growth rate,
milkMilk is a white liquid produced by the mammary glands of mammals. It is the primary source of nutrition for young mammals before they are able to digest other types of food. Early-lactation milk contains colostrum, which carries the mother's antibodies to the baby and can reduce the risk of many...
productivity, and strength of
farmA farm is an area of land, or, for aquaculture, lake, river or sea, including various structures, devoted primarily to the practice of producing and managing food , fibres and, increasingly, fuel. It is the basic production facility in food production. Farms may be owned and operated by a single...
animals, generally leading to the eventual
deathDeath is the permanent termination of the biological functions that sustain a living organism. Phenomena which commonly bring about death include old age, predation, malnutrition, disease, and accidents or trauma resulting in terminal injury....
of the infected animals. Certain species of cattle are called
trypanotolerant because they can survive and grow even when infected with trypanosomes although they also have lower productivity rates when infected.
The course of the disease in animals is similar to the course of sleeping sickness in humans.
Trypanosoma congolense and
Trypanosoma vivax are the two most important species infecting bovine cattle in
sub-Saharan AfricaSub-Saharan Africa as a geographical term refers to the area of the African continent which lies south of the Sahara. A political definition of Sub-Saharan Africa, instead, covers all African countries which are fully or partially located south of the Sahara...
.
Trypanosoma simiae causes a virulent disease in swine.
Other forms of animal trypanosomiasis are also known from other areas of the globe, caused by different species of trypanosomes and transmitted without the intervention of the tsetse fly.
Tsetse vector ranges mostly in the central part of Africa.
Control
Tsetse control has been undertaken to reduce the incidence of the diseases the flies transmit. Two alternative strategies have been used in the attempts to reduce this African
trypanosomiasisTrypanosomiasis or trypanosomosis is the name of several diseases in vertebrates caused by parasitic protozoan trypanosomes of the genus Trypanosoma. Approximately 500,000 men, women and children in 36 countries of sub-Saharan Africa suffer from human African trypanosomiasis which is caused by...
. One tactic is primarily medical or veterinary, targeting the disease directly using monitoring, prophylaxis, treatment, and surveillance to reduce the number of organisms that carry the disease. The second strategy is generally entomological, and intends to disrupt the cycle of transmission by reducing the number of
fliesFließ is a municipality in the Landeck district and is located5 km south of Landeck on the upper course of the Inn River. It has 9 hamlets and was already populated at the roman age; the village itself was founded around the 6th century. After a conflagration in 1933 Fließ was restored more...
.
The idea of tsetse control implies a change in the relationship between people and these insects. Prior to the twentieth century, people in
AfricaAfrica is the world's second largest and second most populous continent, after Asia. At about 30.2 million km² including adjacent islands, it covers 6% of the Earth's total surface area and 20.4% of the total land area...
had largely adapted to the presence of tsetse. Human settlement patterns and agricultural practices had adapted to the presence of the fly. For example, in
EthiopiaEthiopia , officially known as the Federal Democratic Republic of Ethiopia, is a country located in the Horn of Africa. It is the second-most populous nation in Africa, with over 82 million inhabitants, and the tenth-largest by area, occupying 1,100,000 km2...
draft powered farming was restricted to the highland areas where the flies were absent, whereas lowland areas where tsetse are present were more sparsely populated by people living a nomadic, less agriculturally intensive lifestyle. Tsetse control is a response to changing conditions. Tsetse control has been proposed as a way of reducing the incidence of the disease in the populations living in tsetse regions, of allowing the expansion of human settlement and agriculture into new areas, and of helping people previously relocated either in forced transfers or due to
migrationHuman migration is physical movement by humans from one area to another, sometimes over long distances or in large groups. Historically this movement was nomadic, often causing significant conflict with the indigenous population and their displacement or cultural assimilation. Only a few nomadic...
.
Tsetse control efforts have been undertaken throughout the African continent, but long-term, sustainable control has rarely been achieved. Tsetse control efforts invariably are tied to the complex problems of
povertyPoverty is the lack of a certain amount of material possessions or money. Absolute poverty or destitution is inability to afford basic human needs, which commonly includes clean and fresh water, nutrition, health care, education, clothing and shelter. About 1.7 billion people are estimated to live...
,
healthHealth is the level of functional or metabolic efficiency of a living being. In humans, it is the general condition of a person's mind, body and spirit, usually meaning to be free from illness, injury or pain...
,
politicsPolitics is a process by which groups of people make collective decisions. The term is generally applied to the art or science of running governmental or state affairs, including behavior within civil governments, but also applies to institutions, fields, and special interest groups such as the...
, and
violenceViolence is the use of physical force to apply a state to others contrary to their wishes. violence, while often a stand-alone issue, is often the culmination of other kinds of conflict, e.g...
that have proved so disastrous for the African people.
The reduction of fly numbers has generally been attempted with two different aims, either total eradication from the area, or control to just reduce the numbers. Eradication has often been imagined, has repeatedly been attempted, and is still proposed—but many reasons suggest that control is a safer, cheaper, more realistic, and sustainable approach. Eradication refers to the successful killing of every tsetse, either in a region or, under more grandiose proposals, from the entire African continent. Local eradication efforts have repeatedly been undertaken and have achieved temporary success, only to fail in the long term because tsetse re-invaded (as in
ZanzibarZanzibar ,Persian: زنگبار, from suffix bār: "coast" and Zangi: "bruin" ; is a semi-autonomous part of Tanzania, in East Africa. It comprises the Zanzibar Archipelago in the Indian Ocean, off the coast of the mainland, and consists of numerous small islands and two large ones: Unguja , and Pemba...
).
All of the
economicEconomics is the social science that analyzes the production, distribution, and consumption of goods and services. The term economics comes from the Ancient Greek from + , hence "rules of the house"...
,
ecologicalEcology is the scientific study of the relations that living organisms have with respect to each other and their natural environment. Variables of interest to ecologists include the composition, distribution, amount , number, and changing states of organisms within and among ecosystems...
,
politicalPolitics is a process by which groups of people make collective decisions. The term is generally applied to the art or science of running governmental or state affairs, including behavior within civil governments, but also applies to institutions, fields, and special interest groups such as the...
, and environmental justifications for eradication have been called into question. The economic justification for eradication offsets the immense costs of the eradication campaign against the medical and veterinary benefits that accrue in perpetuity.
However, eradication campaigns may have unintended social consequences, as a successful campaign may open up lands for agriculture previously populated by nomadic hunters, which displaces the original population.
Control techniques
Many techniques have reduced tsetse populations, with earlier, crude methods recently replaced by methods that are cheaper, more directed, and ecologically better.
Slaughter of wild animals
One early technique involved slaughtering all the wild animals tsetse fed on. For example, the island of
PrincipePríncipe is the northern and smaller of the two major islands of the country of São Tomé and Príncipe lying off the west coast of Africa. It has an area of 136 km² and a population of approximately 5,000. The island is a heavily eroded volcano over three million years old, surrounded by other...
off the west coast of Africa, was entirely cleared of
feralA feral organism is one that has changed from being domesticated to being wild or untamed. In the case of plants it is a movement from cultivated to uncultivated or controlled to volunteer. The introduction of feral animals or plants to their non-native regions, like any introduced species, may...
pigs in the 1930s, which led to the extirpation of the fly. While the fly eventually re-invaded in the 1950s, the new population of tsetse was free from the disease.
Land clearing
Another early technique involved complete removal of brush and woody vegetation from an area. Tsetse tend to rest on the trunks of trees so removing woody vegetation made the area inhospitable to the flies. However, the technique was not widely used and has been abandoned. Preventing regrowth of woody vegetation requires continuous clearing efforts, which is only practical where large human populations are present. The clearing of woody vegetation has come to be seen as an environmental problem more than a benefit.
Pesticide campaigns
Pesticides have been used to control tsetse starting initially during the early part of the twentieth century in localized efforts using the inorganic metal based pesticides, expanding after the Second World War into massive aerial and ground based campaigns with organochlorine pesticides such as
DDTDDT is one of the most well-known synthetic insecticides. It is a chemical with a long, unique, and controversial history....
applied as
aerosolTechnically, an aerosol is a suspension of fine solid particles or liquid droplets in a gas. Examples are clouds, and air pollution such as smog and smoke. In general conversation, aerosol usually refers to an aerosol spray can or the output of such a can...
sprays at
Ultra-Low VolumeThe term Ultra-Low Volume is used in the context of pesticide application.Ultra-low volume application of pesticides has been defined as spraying at a of les than 5 L/ha for field crops or les than 50 L/ha for tree/bush crops...
rates. Later, more targeted techniques used
pour-on formulationsThe biological activity of a pesticide, be it chemical or biological in nature, is determined by its active ingredient . Pesticide products very rarely consist of pure technical material. The AI is usually formulated with other materials and this is the product as sold, but it may be further...
in which advanced organic pesticides were applied directly to the backs of cattle.
Trapping
Tsetse populations can be monitored and effectively controlled using simple, inexpensive traps. These often use electric blue cloth, since this color attracts the flies. Early traps mimicked the form of cattle but this seems unnecessary and recent traps are simple sheets or have a biconical form. The traps can kill by channeling the flies into a collection chamber or by exposing the flies to insecticide sprayed on the cloth. Tsetse are also attracted to large dark colors like the hides of cow and buffaloes. Some scientists put forward the idea that
zebraZebras are several species of African equids united by their distinctive black and white stripes. Their stripes come in different patterns unique to each individual. They are generally social animals that live in small harems to large herds...
have evolved stripes, not as a camouflage in long grass, but because the black and white bands tend to confuse tsetse and prevent attack.
The use of chemicals as attractants to lure tsetse to the traps has been studied extensively in the late 20th century, but this has mostly been of interest to scientists rather than as an economically reasonable solution. Attractants studied have been those tsetse might use to find food, like carbon dioxide, octenol, and acetone—which are given off in animals' breath and distributed downwind in an
odor plume. Synthetic versions of these chemicals can create artificial odor plumes. A cheaper approach is to place cattle urine in a half gourd near the trap. For large trapping efforts, additional traps are generally cheaper than expensive artificial attractants.
A special trapping method is applied in Ethiopia, where the BioFarm Consortium (ICIPE, BioVision Foundation, BEA, Helvetas, DLCO-EA,
Praxis EthiopiaPraxis Ethiopia is an international organisation whose goal is to apply professional expertise to the problems of extreme poverty in Ethiopia. The word "Praxis" comes from the Greek language, and meaning the application of knowledge to solve problems...
) applies the traps in a sustainable agriculture and rural development context (SARD). The traps are just the entry point, followed by improved farming, human health and marketing inputs. This method is in the final stage of testing (as per 2006).
Releases of irradiated males
The
sterile insect techniqueThe sterile insect technique is a method of biological control, whereby overwhelming numbers of sterile insects are released. The released insects are normally male as it is the female that causes the damage, usually by laying eggs in the crop, or, in the case of mosquitoes, taking a bloodmeal from...
has been used to reduce tsetse populations. This technique involves the rearing of large numbers of tsetse, separation of the males,
irradiationIrradiation is the process by which an object is exposed to radiation. The exposure can originate from various sources, including natural sources. Most frequently the term refers to ionizing radiation, and to a level of radiation that will serve a specific purpose, rather than radiation exposure to...
of these flies with large doses of gamma rays to make them sterile and then release into to the wild. Since females only mate a few times in their life, generally only once, any mating with a sterile male prevents that female from giving birth to any offspring.
The sterile insect technique has recently been used on
ZanzibarZanzibar ,Persian: زنگبار, from suffix bār: "coast" and Zangi: "bruin" ; is a semi-autonomous part of Tanzania, in East Africa. It comprises the Zanzibar Archipelago in the Indian Ocean, off the coast of the mainland, and consists of numerous small islands and two large ones: Unguja , and Pemba...
, an island off the coast of
East AfricaEast Africa or Eastern Africa is the easterly region of the African continent, variably defined by geography or geopolitics. In the UN scheme of geographic regions, 19 territories constitute Eastern Africa:...
. Like other eradication efforts, early indications are that the fly numbers have been devastated, with the fly possibly extirpated (locally eradicated) from the island. A number of traps are in place to monitor the island and repress any resurgence.
Additionally, using the parasite refractory strains is another method to control the tsetse, that means providing the blood meal containing the trypanocide before releasing the sterilised males. One can also consider using the cytoplasmic incompatibility strategy to control the population of tsetse. With the development of genetic engineering, the releasing of engineered parasite refractory counterparts is another strategy to control the population of tsetse.
Etymology
The word 'tsetse' comes from
TswanaTswana or Setswana is a language spoken in Southern Africa by about 4.5 million people. It is a Bantu language belonging to the Niger–Congo language family within the Sotho languages branch of Zone S , and is closely related to the Northern- and Southern Sotho languages, as well as the Kgalagadi...
, a language of southern Africa, and, in that language, the word means fly. Recently 'tsetse' without the 'fly' has become more common in
EnglishEnglish is a West Germanic language that arose in the Anglo-Saxon kingdoms of England and spread into what was to become south-east Scotland under the influence of the Anglian medieval kingdom of Northumbria...
, particularly in the scientific and development communities.
The pronunuciation of the word differs in different regions. Many
African languagesThere are over 2100 and by some counts over 3000 languages spoken natively in Africa in several major language families:*Afro-Asiatic spread throughout the Middle East, North Africa, the Horn of Africa, and parts of the Sahel...
have an
ejectiveIn phonetics, ejective consonants are voiceless consonants that are pronounced with simultaneous closure of the glottis. In the phonology of a particular language, ejectives may contrast with aspirated or tenuis consonants...
ts sound and so a common pronunciation of the word involves two identical syllables both having this
ts sound and a shorter sound of the vowel, as
ts-eh-ts-eh. The British pronunciation of the word uses two different sounds for the two different syllables, generally
tee-tsee. In Zimbabwe, it is generally pronounced
tseh-tsee.
See also
External links
Fiction
- "Winged Death" by H. P. Lovecraft and Hazel Heald (1933). Gothic horror on the malevolent use of trypanosome
- "The In-Laws" film starring Peter Falk and Alan Arkin. Very early in the film, Vince (Falk) tells an outlandish story about a trip he took to the jungle (the "bush," as he explains), during which he witnessed tsetse flies "the size of eagles" swoop down, snatch, and carry away small children. This early scene helps establish Vince as a character of questionable reliability to Sheldon (Arkin).
Textbooks
- Maudlin, I., Holmes, P.H. & Miles M.A. (2004) "The Trypanosomiases" CAB International.
- Buxton, P. (1955) The Natural History of Tsetse Flies: An Account of the Biology of the Genus Glossina (Diptera). London, UK: H.K. Lewis & Co.
- Glasgow, J. (1963) The Distribution and Abundance of Tsetse International Series of Monographs on Pure and Applied Biology, No. 20. Oxford, UK: Pergamon Press.
- Mulligan, H. & Potts, W. (1970) The African Trypanosomiases London, UK: George Allen and Unwin, Ltd.
- Ford, J. (1971) The Role of the Trypanosomiases in African Ecology. Oxford, UK: Clarendon Press.
- Leak, S. (1998) Tsetse Biology and Ecology: Their role in the Epidemiology and Control of Trypanosomiasis. New York, NY, USA: CABI Publishing. book site
- McKelvey Jr., J. (1973) Man Against Tsetse: Struggle for Africa. Ithaca, NY, USA: Cornell University Press.