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TRYPANOSOMIASIS Glossinidae
(Tsetse Flies) & Reduviidae (Bugs) (Contact) Please
CLICK on
Image & underlined links for details: GLOSSINIDAE
The
Glossinidae are all potential vectors of different species of trypanosomes
that cause Sleeping Sickness in
humans and mammals, but only a few are attracted to humans. They resemble stable flies of the genus Stomoxys with their
large stout proboscis projected forward (See Heads: Fig. 2). But they are larger and make a shrill
buzzing sound when entering enclosures, such as automobiles. Adult flies hold their wings crossed over
the abdomen when at rest (Fig. 3),
there is a hatchet- or cleaver-shaped cell on the wing (Fig. 4), and the shape of their larvae and
pupae are uniquely stout and rounded (Fig.
5) A distinctive
reproductive process allows tsetse flies to hatch their eggs one at a time
within the mother's uterus where a single larva completes development (Fig. 6). The larva is fed with special "milk glands" inside
the uterus until mature after which it is placed in dry shaded soil near a
water source. Pupation occurs in the
soil and adults emerge 20-60 days later depending on season and species. Different
species of Protozoan trypanosomes, are transmitted among the Glossina species. They are parasitic in the blood stream and
tissues of vertebrates. Matheson
(1950) noted that most of the many trypanosome species are dependent on an
invertebrate host such as the tsetse flies to complete their life cycle. For humans the most important species
vectored by Glossina, are Trypanosoma gambiense, T. rhodesiense and T. brucci. In Africa wild game animals
can serve as reservoirs of the trypanosomes for some species, although their
importance in perpetuating the disease is probably not great. Medical Importance and Adult Fly
Behavior Considerable
behavioral information has been obtained from Tsetse flies because of their
great importance to human health. All
species can serve as vectors of Trypanosomiasis,
but few are natural vectors because they rarely feed on humans. Also the amount of contact the flies have
with infected reservoir hosts determines their threat. In Africa the human population suffers
greatly from the disease, with estimates given by the World Health
Organization of some 400,00 new cases every year accompanied by about 55,000
deaths. The two most important
subspecies involved are Trypanosoma brucci
gambiense and T. brucei
rhodesiense. Although
difficult to distinguish each produces different clinical symptoms and
different epidemiologies. A summary of
important characteristics by Service (2008) is given as follows: "Both
male and female tsetse-flies bite people, a large variety of domesticated and
wild mammals, and sometimes reptiles and birds. No species of tsetse feeds exclusively on one type of host but
most show definite host preferences, often associated with host
availability. For example, in East
Africa Glossina swynnertoni feeds mainly on wild pigs and G. morsitans on wild and
domesticated bovids as well as on wild pigs, whereas in West Africa G.
morsitans feeds mainly on warthogs.
In East Africa G. pallidipes
feeds principally on wild bovids, while in West Africa G. palpalis feeds predominantly on reptiles and
humans, and in West Africa. Glossina
tachinoides feeds on humans and bovids, but in southern
Nigeria it feeds predominantly on domestic pigs. Tsetse-flies blood-feed about every 2-3 days, although in cool
humid conditions it may be about every 10 days. Feeding is restricted to the daytime and vision, as well as
olfactory cues emanating from host breath and urine, are important in host
location, dark moving objects being particularly attractive. On pale-skinned people, such as
Caucasians, tsetse-flies often bite through dark clothing such as socks,
trousers and shorts in preference to settling on the skin. During feeding blood sucked up the
proboscis passes to the crop and later to the mid-gut, where digestion
proceeds." Trypanosoma spp. Development Cycle (Derived from Service 2008) The male and
female tsetse flies, both of which bite, become infected with trypanosomes
when drawing blood from an infected person or animal. Trypanosomes then pass through the fly
esophagus to the crop and peritrophic tube that lines the middle intestines. After 9-11 days the trypanosomes penetrate
the middle section of the peritrophic membrane and pass across into the space
between the membrane and migrate to the proventriculus. Then they pass down the food channel to
the proboscis and up into the salivary duct to the salivary glands, where
they develop into epimastigotes
and multiply. Varying times after a
blood meal (18-35 days) the flies become infective and the metacyclic trypanomastigotes are passed to a
vertebrate host during a blood meal.
Sometimes when Trypanosoma
species that cause Nagana only
in animals are found in tsetse flies they may be mistakenly believed to be a
threat to humans. There are also
different forms of Sleeping Sickness, the most common being Gambian and
Rhodesian. The Gambian form is more
chronic and less severe than the Rhodesian
form. Tsetse Fly Range (see Map) Adult tsetse
flies range over a variety of landscapes during wetter seasons, but remain
closer to water sources during dry seasons.
Their habits are largely synchronized with the availability of animal
herds. Service (2008) separated the
flies into three main groups depending on their range and types of
habitat: (1) Fusca group (Forest flies), (2) Morsitans group (Savanna flies) and (3) Palpalis group (Riverine & forest
flies). Fusca Group.-- There are 13 species of Glossina in this group of very large flies. All except one (G.
longipennis) inhabit dry forests of East Africa and equatorial
forests of West and West-central Africa.
Service (2008) reports that these flies do not often attack humans and
no species is a vector of the trypanosomes. Morsitans Group.-- There are 5 species in this group of
medium-sized flies, which usually inhabit savannahs that occur from costal
areas or the edges of forests to the drier areas. Glossina morsitans
is common in the savannahs of West, Central and East Africa, while G. pallidipesis
restricted to East and southern Africa.
G. swynnertoni occurs in
savannahs of a small larea of East Africa.
G. morsitans and G. pallidipes range from
wooded areas near the edges of forests and dry thickets, while G. swynnertoni is found primarily in dry
thickets. All of these flies are
vectors of trypanosomes, the most important being G. morsitans through most
of its range. Palpalis Group.-- There are 9 species
and subspecies of small to medium-sized flies included here. They occur all over Africa in vegetation
of wetter areas, such as forests, and along rivers, lakes and swamps. Glossina
palpalis is common in all these habitats while Glossina fuscipes is found
primarily in Central Africa and parts of East Africa. Gossina
tachinoides is a riverine species in West and Central Africa
and portions of Ethiopia and Sudan. All
species are vectors of trypanosomes. Control of Tsetse Flies Avoidance of
places where tsetse flies occur, as near rivers and certain vegetation, can
reduce infection, but it is difficult for people who use such areas for
obtaining water, fishing and gathering plant products. Elimination of reservoir animal hosts has
been deployed with success, but reliance on them for food minimizes
effectiveness. Insecticides continue
to be used to kill adult flies, but the inevitable problem of insecticide
resistance reduces effectiveness in time.
Trapping methods are being deployed because they are less expensive
(Green 1994), but the resistance problem eventually reduces effectiveness
over time also. Service (2008) noted
that genetic control with sterile tsetse male releases has been successful at
times, as flies were actually eradicated from the island of Zanzibar with
this approach (Gooding & Krafsur 2005).
He suggested that the sterile males be used against residual
populations of tsetse flies that were controlled with insecticides. Also of great
importance for control is a knowledge of where the adult flies pass most of
their time. Both fed and unfed flies
pass the night and most of the day resting in dark and humid places. Service (2008) noted that about 23 hours a
day are spent resting on vegetation.
Favored resting sites of most species are on twigs, branches and
trunks of trees and bushes, but they shy away from areas where the
temperature exceeds 35 deg. Centigrade.
During the night the flies rest on the upper surface of leaves. The height at which adults rest on trees
determines where insecticides should to be directed, and this is usually
below four meters and even at ground level in some areas. - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - Vectors of Trypanosomiasis in the Americas are all in the family
Reduviidae, and they cause a disease called Chagas,
which is treated separately (See: Chagas Disease ). = = = = = = = = = = = =
= = = = = = = = Key References: <medvet.ref.htm> <Hexapoda> Buxton, P. A. 1955. The Natural History of Tsetse Flies: an
Account of the Biology of the Genus Glossina
(Diptera). London School of Hygiene & Trop. Med. Memoir 10, London Colvin, J. & G.
Gibson. 1992. Host-searching behavior and management of
tsetse. Ann. Rev. Ent. 37: 21-40. Fevre, E. M., K. Picozzi, J. Jannin, S. C. Welburn
& I. Maudlin. 2006. Human African trypanosomiasis epidemiology
and control. Adv. Parasitol. 61: 168-221. Ford, J. 1971.
The Role of Trypanosomiasis in African Ecology: a Study of Tsetse Fly Problem. Clarendon Press, Oxford. Gooding, R. H. & E. S. Krafsur. 2005.
Tsetse genetics: contributions to the biology, systematics and control
of tsetse flies. Ann. Rev. Ent. 50: 101-23. Green, C. H. 1994.
Bait methods for tsetse fly control.
Adv. in Parasitology 34:
229-91. Jannin, J. & P.
Cattand. 2004. Treatment and control of human African
trypanosomiasis. Current Opinions in
Infectious Diseases 17: 565-71. Jordan, A. M. 1989. Man and changing patterns of the African
trypanosomiasis. IN: Demography &
Vector-Borne Diseases. CRC Press, Boca Raton, Fla. p. 47-58. Jordan, A. M. 1993.
Tsetse-flies (Glossinidae).
IN: Medical Insects and
Arachnids. Chapman & Hall, London.
p. 333-88. Matheson, R. 1950. Medical Entomology. Comstock Publ. Co, Inc. 610 p. Maudlin, I., P. H.
Holmes & M. A. Miles. 2004. The Trypanosomiases. CABI, Wallingford, England. Service, M. 2008.
Medical Entomology For Students.
Cambridge Univ. Press. 289 p Torr, S. J., J. W.
Hargrove & G. A. Vale. 2005. Towards a rational policy for dealing with
tsetse. Trends in Prasitology 21: 537-41. Legner, E.
F.
1995. Biological
control of Diptera of medical and veterinary importance. J. Vector Ecology 20(1): 59_120. Legner, E. F. 2000.
Biological control of aquatic Diptera. p. 847_870.
Contributions to a Manual of Palaearctic Diptera, Vol. 1, Science Herald, Budapest. 978 p World Health
Organization. 2003. Report of the Scientific Working Group
Meeting on African Trypanosomiasis (Sleeping Sickness).
TDR/SWG/01, Geneva. |
