Susceptibility of N'Dama cattle to experimental challenge and cross-species superchallenges with bloodstream forms of Trypanosoma congolense and T-Vivax

Susceptibility to Trypanosoma congolense, T. vivax challenge and cross spec ies-superchallenges, and related effects on health and productivity were as sessed in N'Dama cattle. Twenty-five N'Dama bulls aged 3-4 years and previo usly primed with trypanosome infections through natural tsetse exposure ove r more than one year were used. The experimental herd was divided in five g roups each composed of five randomly selected animals. Group 1 was challeng ed with T. congolense, Group 2 with T. vivax, Group 3 was inoculated with T . congolense followed by a cross-superchallenge with T. vivax. Group 3 was inoculated with T. vivax followed by T. congolense cross-superchallenge. An imals in Group 5 were used as controls. Both T. vivax and T congolense cros s-superchallenges were carried out on Day 14 subsequent to respective initi al T. congolense and T. vivax inoculations. All challenges were performed b y intradermal needle inoculation of stocks of trypanosome bloodstream forms . In challenged animals (Group 1 to 4), parasitaemia profiles and packed re d cell volumes (PCV) were measured for four months. Weight changes were rec orded monthly and daily weight gain (DWG) computed. All cattle challenged w ith T. congolense became parasitaemic. Conversely, one animal in Group 2 an d two in Group 3 never displayed patent T. vivax parasitaemia. Both in sing le (Group 1), initial (Group 3) and cross-superchallenged (Group 4) cattle higher percentage of positive blood samples and higher parasitaemia level w ere obtained following I congolense than T. vivax inocula (Group 2, 3 and 4 ) (P < 0.04 or greater). Overall the pre-challenge period, PCV values and D WGs were nearly identical in the five groups. Conversely, over the post-cha llenge period, cattle singly, initially and cross-superinoculated with T. c ongolense (Group 1, 3 and 4) displayed lower PCV values and DWGs in compari son with both control animals (Group 5) and with singly T. vivax challenged cattle (Group 2) (P < 0.05 or greater). No difference in mean PCV levels a nd DWGs was found between animals in Group 2 and cattle in Group 5. It was concluded that trypanotolerant N'Dama cattle suffered more from T. congolen se and mixed congolertsel T. vivax infections, while pure T vivax: infectio n did not produce appreciable negative effects on their health and producti vity. Therefore, considering that tsetse and trypanosomosis control campaig ns are costly and are justified only when derived economic benefits exceed those of control, and also that an ample mosaic of fanning systems exists i n West Africa, species-specific trypanosome prevalence and relative impact should be assessed in various cattle populations and breeds differing in tr ypanosome susceptibility before advising any intervention. Moreover, virule nce and related effects of T. congolense and T. vivax endemic stocks on hea lth and productivity in local cattle populations should also be estimated i n order to counsel appropriate economic protection measures against trypano smosis, i.e. vector control and/or strategic use of trypanocidal drugs. (C) 1999 Elsevier Science B.V. All rights reserved.