Trypanotolerance of the N'Dama Cattle and West African Dwarf Sheep and Goats[edit]

Certain domestic ruminant breeds in sub-saharan Africa exhibit remarkable resistance to the effects of African trypanosomiasis: they are able to tolerate the presence of parasites while controlling parasitaemia levels and, most importantly, do not exhibit the severe anemia and production loss that are typical of infection in susceptible breeds.^[1]

Trypanotolerance trait is seen in N'Dama cattle and it is the ability of the N'Dama cattle to survive in areas of high tsetse fly endemicity, where other cattle breeds would often come down with trypanosomiasis.^[2] In areas or location of low to moderate tsetse fly challenge, typanotolerant N'Dama cattle show lower numbers of parasites in their blood and they develop less severe anemia and have also shown to be more productive.^[3]

An investigation to test the resistance of different small ruminant breeds (West African Dwarf Sheep and Goat) to an artificial infection with Trypanosoma congolense revealed that native sheep and goats had a higher natural resistance to the illness than exotic breeds, with exotic/indigenous crossbreeds falling somewhere in between.^[4] Despite persistent parasitaemia, clinical signs in the trypanotolerant West African Dwarf Sheep and Goat showed a milder illness and decreased mortality.^[5] The significance of these trypanotolerant trait is highlighted while choosing breeds of sheep and goat for selection programs. Some literature claims that trypanotolerance in small ruminants (sheep and goats) should be viewed as resilience rather than resistance because it is less apparent than in cattle.^[6]

Trypanotolerance appears to include both non-immunological and immunological pathways and is most likely multifactorial.^[7] However, physiological and nutritional factors, concurrent diseases, the presence and absence of tsetse, and intercurrent diseases all have an impact on the level of resistance displayed by typanotolerant breeds of cattle, sheep and goat.^[8]

In addition to the above listed criteria, studies have shown that the effective dose of the trypanosome parasite that these animals acquire following  the bite of tsetse fly is also a consideration. It only takes one fly bite to transmit the infection.^[9] The quantity of trypanosomes injected into an animal's skin as a result of bites determines how severe the sickness will be, thus, trypanotolerance during natural exposure may be influenced by lower infective dosages.^[10] The finding that some cattle that are highly resistant in the field don't always retain that level of resistance after being artificially infected with a specific dose of the parasite may be supportive of this hypothesis.^[11]

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