Trypanosome infections and tick infestations: susceptibility in N'Dama, Gobra zebu and Gobra x N'Dama crossbred cattle exposed to natural challenge and maintained under high and low surveillance of trypanosome infections.

Susceptibility to trypanosome infections and tick infestations was assessed in 51 N'Dama, 48 Gobra zebu and 37 Gobra x N'Dama crossbred (F1) cattle exposed to field-tick infestations and natural high tsetse challenge over more than one year. From these cattle, 12 animals of each breed were randomly selected and examined parasitologically for trypanosome infections and packed cell volume (PCV) twice a week (high surveillance, group HS). In the remaining 100 cattle trypanosome infection and PCV were monitored monthly (low surveillance, group LS). Mortality rates were recorded in both groups. Tick infestation was quantified fortnightly from all animals in group HS and from four to seven randomly selected animals of each breed in group LS. In both HS and LS groups, trypanocidal drug treatment was administered to trypanosome positive animals with PCV equal to or less than 20% or when they showed clinical evidence (dullness, weight loss) of trypanosomosis. In both groups, N'Dama cattle exhibited a superior capacity to control trypanosome infections and limit tick burdens. Particularly, in group HS N'Dama cattle showed lower trypanosome infection rate, higher mean PCV value, lower requirement for trypanocide treatments and lower tick load than crossbred and Gobra cattle in the corresponding group (P < 0.05 or greater). This was also true in N'Damas in comparison with crossbreds in group LS. Unfortunately, the high mortality occurring in Gobra cattle in group LS did not allow within group comparative analysis between N'Dama, Gobra and crossbred cattle overall the study period. No death occurred in N'Dama cattle maintained under high surveillance of trypanosome infection, while approximately 8% of crossbreds and 50% of Gobras died of trypanosomosis. In group LS, all Gobra and more than 70% of crossbred cattle died. In this group, mortality in N'Dama was limited to less than 16%. In both groups, differences in mortality were significantly higher (P < 0.01) in Gobras than in N'Damas. Within breed, animals of the three breeds maintained under high surveillance of trypanosomal infection showed higher mean PCV values, lower tick burdens and required proportionally less trypanocide treatments than corresponding cattle in group LS. The infection rate in N'Dama under high surveillance was lower in comparison with N'Dama cattle in group LS. There was no significant difference in mortality between groups within the N'Dama breed. Conversely, mortality rates were lower in crossbred and Gobra in group HS than in respective cattle in group LS. It was concluded that cattle of the three breeds suffered from trypanosomosis and that trypanosome infections affected tick susceptibility. However, N'Dama showed a superior ability to limit both the pathological effects of trypanosomosis and the level of tick infestations. Therefore, considering the cost of labour and consumable equipment required for a high surveillance, use of multi-disease resistant N'Dama cattle is recommended for the low-input traditional African farming systems in areas where trypanosomosis, ticks and tick-borne diseases are constraints to livestock production. Additional comparative investigations are needed to assess the impact of high surveillance scheme of trypanosome infection in different production systems in trypanotolerant and trypanosusceptible cattle exposed to different gradients of tsetse challenge.