African Trypanosomiasis, Sleeping Sickness of Animals
Nagana is a very important disease of domestic livestock. According to the Food and Agriculture Organization of the United Nations (FAO), it is probably the only disease which has profoundly affected the settlement and economic development of a major part of a continent. Today, it is still endemic in more than 35 African countries and causes huge economic losses (Trypanosoma).
In cattle the pathogenesis is dominated by three features: anemia, tissue lesions and immunosuppression. The cause of anemia is complex and involves a variety of mechanisms. Although hemolysins are released by trypanosomes, intravascular haemolysis is not a prominent feature, and anemia is rather attributed to erythrophagocytosis by cells of the mononuclear phagocytic system in the spleen, bone marrow, lungs and lymph nodes. These cells are stimulated by the formation of complexes between immunoglobulin specific for trypanosomes and antigen or complements attached to red cells. Other possible contributing factors include increased haemodilution and fragility of the red cells, and a depression of erythropoiesis. Although T. congolense and T. vivax are mainly intravascular parasites they cause significant tissue lesions, notably myocarditis and myositis. The aetiology of these lesions is unknown but is probably related to the damage induced by parasite products, immune complexes and vasoactive amines to capillary endothelial cells. Finally, chronically infected animals show immunosuppression which, in association with other factors of stress such as malnutrition, pregnancy or lactation, leads to a higher susceptibility to other diseases. African trypanosomosis may follow an acute course, mainly in exotic breeds of cattle which tend to be more susceptible than local breeds. Animals suffer from intermittent fever, quickly lose weight, and may die within 3 to 4 weeks. However, the disease more frequently follows a relatively chronic course characterized by intermittent fever, anemia, lymphadenopathy and progressive emaciation. Animals which have been infected for many months or even years become cachectic, their precrural and prescapular lymph nodes being visible from a distance. African trypanosomosis is usually a herd problem. It reduces the general herd productivity and affects fertility.
T. brucei has more affinity for tissues than for blood, and may cause severe lesions in the tissues it invades. The myocardium is more commonly affected, with degenerative changes and focal necrosis of myocytes, and fibrosis. Lesions due to long-standing infection have also been observed in the pituitary, adrenals, kidneys and gonades. T. brucei is generally considered as being of little clinical importance in cattle, but may be responsible for acute and chronic infections in goats and sheep. Mixed trypanosome infections are very common in endemic areas.
Horses are very susceptible to trypanosomosis. T. brucei is certainly the most pathogenic, while T. congolense and T. vivax produce diseases similar to those seen in cattle. The earliest signs of infection are a stumbling gait, a harsh hair coat and relapsing fever. As the disease progresses, subcutaneous oedema of the limbs, thorax, abdomen and genitalia appears. Anemia is a constant feature and lymphadenitis is usually present. Keratitis and corneal opacity may develop in horses affected by T. brucei.
Pigs are refractory to infection with T. vivax, and are only mildly affected by T. congolense, T. brucei and T. suis. In contrast, they are highly susceptible to infection with T. simiae. The latter is highly virulent and may cause death in a few days.
African dog breeds are very resistant to most species of trypanosomes. Only T. brucei appears to be highly pathogenic and often produces acute disease. Clinical signs include anemia, weakness, loss of weight and development of subcutaneous oedema. Parasitic invasion of the eyes causes inflammatory reactions with reactions of pain and lacrimation. Invasion of the central nervous system with ataxia and paralysis has been reported.