Development of diagnostics for T. evansi in camels and water-buffaloes: perspectives in vaccine development.

biotec at goliat.ugr.es biotec at goliat.ugr.es
Thu Feb 27 12:57:06 EST 1997


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.



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