Development of diagnostics for T. evansi in camels and
water-buffaloes: perspectives in vaccine development.
Trypanosoma evansi infections cause severe disease mainly in camels
("surra") and water buffaloes. Periodically, severe outbreaks of
trypanosomiasis also occur in horses and cattle although the
organism can infect other domestic animals such as dogs, cats and
pigs. The T. evansi organism is transferred from animal to animal
when biting flies are interrupted during feeding on an infected
animal. As a result, they quickly seek another animal in order to
satisfy their food requirements and at the same time transfer
trypanosomes to that animal. T. evansi is regarded to have evolved
from the morphologically identical Trypanosoma brucei group which
include T.b. brucei infective to animals and T.b. rhodesiense, T.b.
gambiense which are also responsible for "sleeping sickness" or
"human African trypanosomiasis". While the others undergo a
cyclical development within their vector, the tsetse fly, T. evansi
has lost this ability and relies on a purely mechanical transfer of
the trypomastigote blood stage by biting flies and as a result has
spread outside the tsetse fly-belt of tropical and sub-tropical
Africa throughout North Africa, middle East, southern Russia and
China, India, SE Asia, Indonesia and Philippines. It is also found
in S. America where the disease is sometimes transmitted by
infected vampire bats.
In contrast to T. brucei, T. evansi isolates exhibit a major degree
of similarity in their antigenic components and for this reason the
development of a vaccine was contemplated, based on an
infection/treatment regimen. However, it was discovered that
trypanocidal treatment released a product which stimulated the
production of tumour necrosis factor which may cause death if
released in excess amounts and work on a live vaccine was halted
and the emphasis was moved to the development of accurate
diagnostic tests of T. evansi infection.
In a European Union funded project (TS2 071) a diagnostic test
based on the detection of trypanosome antibodies was developed - a
card agglutination trypanosome test (CATT). This CATT is now in
widespread use in the diagnosis of T. evansi infections and has
been adapted to the diagnosis of human sleeping sickness. However,
a more specific and sensitive test was still desirable and the
polymerase chain reaction (PCR) using specific portions of the
trypanosome DNA molecule was used. This test depends, firstly on
identifying portions (base pairs) of DNA, from the nucleus (DNA) or
from the kinetoplast (kDNA) which are specific for T. evansi.
Utilising the PCR reaction a small amount of DNA present in a
sample of blood or tissue can be replicated by the PCR until enough
is present to be recognised on an electrophoretic gel. A PCR DNA
test has been developed and tested under field conditions in
Thailand and Mali and is sensitive to 5 trypanosomes per 50 litres
blood, and can be performed with non-sterile blood samples which
have withstood temperatures of nearly 40oC for days. The kDNA PCR
has shown that all T. evansi isolates fall within four subtypes and
indicate a identity with T. equiperdum, the sexually transmitted
trypanosome causing “dourine” in equines.
The two unrelated findings that (a) T. evansi is capable of
producing procyclin, which implies that mitochondrial activation is
not necessary for procyclin production and (b) the dromedary camel
produces two immunoglobulins devoid of light chains which still
react with trypanosome antigens, may prove to have further
scientific importance.
For further information on this joint project of the University of
Brussels, Mahidol University of Thailand and Laboratoire Central
Veterinaire du Mali contact Prof R. Hamers, Institute of Molecular
Biology, Vrije Univ0ersiteit Brussel, Belgium.
