Trypanosomoses
Author: Vincent Delespaux
Adapted from:
R.J. CONNOR and P. VAN DEN BOSSCHE, 2004, African animal trypanosomoses, in Infectious diseases of livestock, edited by
J.A.W. Coetzer & R.C. Tustin. Oxford University Press, Cape Town, 12: 251 - 295.
Licensed under a Creative Commons Attribution license.
Introduction
days), the tsetse fly becomes infective and may remain
Human and animal trypanosomosis, transmitted by
infective for the rest of its life. Transmission occurs in
tsetse flies constitutes one of the major health and
the early stage of the blood feeding, when the fly
agricultural
sub-Saharan
injects some saliva before sucking the blood of its host.
Africa. The trypanosomoses are diseases of humans
Mechanical transmission by means of biting flies,
and domestic animals that result from infection with
especially tabanids and stomoxys, but possibly other
parasitic
biting insects (including tsetse flies) are limited to T.
livelihood
protozoa
of
constraints
the
of
genus
Trypanosoma.
Trypanosomes belong to the Order Kinetoplastidae in
vivax. Mechanical transmission can
the family Trypanosomatidae. The further classification
interrupted feeding is re-started on a new host. This is
is based on the mode of transmission via faeces or
efficient among a group of animals but has little chance
saliva. Members of the former group are included in the
to occur over a distance.
section Stercoraria, and the latter in the section
mentioned
Salivaria.
transmission of Trypanosoma equiperdum in horses.
modes
of
occur when
The exception to the
transmission
is
venereal
Salient features of trypanosomoses
Trypanosomes parasitize all classes of vertebrates:
fish,
amphibians,
reptiles,
birds
and
mammals.
However, its main economic effects are found in cattle,
and to a lesser extent in sheep, goats, pigs, horses and
dogs. Two forms of human trypanosomosis exist:
Chagas' disease occurs in Central and South America
and is transmitted by bloodsucking reduviid bugs, with
certain small wild animals and dogs harbouring the
infection. The second form is human sleeping sickness.
This occurs in Africa and is transmitted by tsetse flies.
Trypanosomosis is generally characterized by the
Cyclical transmission of trypanosomes is accomplished
intermittent presence of parasites in the blood and
through the bite of an infected tsetse fly. Tsetse flies
intermittent fever. Anaemia usually develops in affected
get the infection when feeding on an infected animal.
animals, and this is followed by loss of body condition,
After completion of the parasitic cycle in the fly (15–21
reduced productivity and, often, high mortality.
Animal trypanosomosis is one of the major constraints
to animal health and production in sub-Saharan Africa
and has a major impact on people’s livelihoods. The
annual, estimated direct and indirect losses due to the
disease run into billions of dollars. It is reponsible for
the death of 3 million head of cattle every year with 50
million animals at risk in sub-Saharan Africa.
General distribution of tsetse flies and cattle in Africa
Surra is a disease that affects a wide range of host
animals, and it occurs in North Africa, the Near and Far
East, Central and South America, Philippines and
Subacute trypanosomosis in an ox: the animal has lost
weight and condition and it is dejected, with drooping ears
and flaccid tail.(Reprinted by kind permission of FGU
Consulting and Engineering GmbH, Königstein, Germany,
and the Regional Coordinator, RTTCP, Harare, Zimbabwe)
Mauritius. It is caused by Trypanosoma evansi, a
dyskinetoplastic
form
of
which
—
known
as
Trypanosoma equinum — also causes disease in
equids in Central and South America where it is known
as ‘mal de Caderas’ or ‘Murrina’.
Where does trypanosomoses occur?
African animal trypanosomosis occurs where the tsetse
What triggers an outbreak of trypanosomoses?
fly vector exists in Africa. The tsetse distribution area is
In southern Africa two epidemiological situations can
restricted to lowland rainforest and wooded savanna
be distinguished. The disease can have an endemic
regions of Africa south of the Sahara. Roughly
nature with high morbidity and low mortality in the
speaking tsetse flies can be found between the 15th
livestock
degree of latitude in the Northern hemisphere and the
trypanosomosis is found mainly in areas where cattle
30th parallel south (eastern part) or the 20th parallel
constitute the main host of tsetse and reservoirs of
south (western part). The distribution is however not
trypanosomes. Epidemic trypanosomosis, with high
uniform but often patchy. In vast areas the tsetse flies
morbidity and high mortality is found in areas where
and thus the disease, are absent, especially in East
wildlife persist as main reservoir and where livestock
Africa.
come into contact with tsetse flies transmitting
population.
Endemic
trypanosomes from the sylvatic reservoir.
livestock
Prevention and control
Based on the differences in impact of the disease on
livestock health in these two epidemiological settings,
the appropriateness of the available trypanosomosis
control tools differs. In trypanosomosis endemic areas,
trypanocidal drug use could be the most suitable
approach (in combination or not with trypanotolerant
cattle).
The main drugs used by livestock keepers are
isometamidium chloride (ISM) which has both curative
and prophylactic effects and diminazene aceturate
(DA) which has only curative properties. These drugs
have been in use for more than half a century.
Approximatly 35 million doses of trypanocides are
administered every year in sub-Saharan Africa, with
ISM, ethidium bromide and DA representing 40%, 26%
and 33% respectively. Despite the high usage of these
veterinary trypanocides, the pharmaceutical industries
have
been
unwilling
to
invest
in
research
for
developing new products, leaving farmers to rely on the
existing drugs. Due to the privatization of veterinary
services in most parts of Africa, farmers have easy
access to these trypanocides and this has resulted in
rampant misuse and under-dosage of the medications,
actions which have been blamed for the emergence of
trypanocidal drug resistance. To date, there are 17
countries in which trypanocidal drug resistance has
been reported. However, most of these reports seem to
be confined to areas where the disease is endemic.
In epidemic situations, vector control seems the most
appropriate long-term solution.