Trypanosomiasis is a collective term for a group of diseases brought about by one or more of the pathogenic trypanosome species. Typically, trypanosomiasis is a wasting disease in which there is a slow progressive loss of condition accompanied by increasing anaemia (the main sign of the disease) and weakness to the point of extreme emaciation, collapse and death often due to heart failure.

There is a range of variation from the very acute disease in pigs caused by Trypanosoma simiae to the usually mild condition caused by T. brucei and T. evansi in cattle. In addition, severity of symptoms is related to the gradient of susceptibility to trypanosome infections. Generally, wild mammals and some African cattle (e.g. N'Dama, Baoule) and small ruminant (Djallonke sheep, West African Dwarf goats) breeds posses a certain degree of tolerance to the infection.

Clinical observation in the early stage relies mainly on the presence of cervical adenopathies which are the only suggestive signs, while fever, headaches, joint and muscle pains are common but unspecific complaints. As the disease evolves and the parasite invades the central nervous system, neurological and psychiatric symptoms and signs appear. Mental alteration, abnormal movements, sensory problems and wake-sleep circadian rhythm disturbances are observed.

The following description applies to chronic trypanosomiasis caused by T. congolense or T. vivax in susceptible animals.

The disease becomes apparent about seven to ten days after an infective tsetse bite: body temperature rises and the heart and the respiratory rates increase. From now onwards there is a fluctuating but continuous slow deterioration in health with a steady loss of condition. The animal becomes overtly sick with a variable appetite. Its coat becomes dull and "staring" There may be some diarrhoea in the early stages. The animal becomes anaemic (visible mucous membranes are pale). There is very often an increased secretion of tears (lachrymation). The emaciation is associated with weakness and in the final stages results in inability to stand, severe anaemia and death.

The hyperacute haemorrhagic form of trypanosomiasis caused by T. vivax, normally a chronic disease is characterized by bleeding from natural orifices of the animal body and, at post mortem, haemorrhages are very widespread and extensive. Large haemorrhages are seen in the heart, the pleural cavity, the peritoneum, the diaphragm. The disease progresses rapidly to death that there is no loss of condition.

In small ruminants (sheep and goats) symptoms are similar to those observed in cattle. Sheep and goats infected with T. brucei may show central nervous symptoms (staggering, paralysis, etc.). In sheep infected with T. brucei it may be observed infiltration with liquid of subcutaneous tissues which leads to swelling of the eyelids, the lips and the skin beneath the lower jaw.

In horses extensive subcutaneous oedema is often seen in infections caused by various species of trypanosomes (T. brucei, T. congolense, T. evansi and T. equiperdum). Progressive symptoms of an affection of the central nervous system are not uncommon in horses infected with T. brucei and T. evansi. The animals may show staggering, paralysis, stupor and symptoms similar of HAT.

In the domestic pig, T. simiae produces a hyperacute, fulminating disease. After a short incubation period death occurs very rapidly and at post-mortem examination the picture is one of a complete capillary breakdown with haemorrhages and congestion in various organs throughout the carcass.

The post mortem findings in trypanosomiasis can never by themselves lead to a certain diagnosis of the cause of death. As for the clinical signs, there is not one single specific lesion.

In acute trypanosomiasis, in addition to the lesions described above, it can be observed enlarged spleen. The lymph nodes are also enlarged and oedematous (containing more fluid than usual). The liver is enlarged and congested. The heart may show a few haemorrhages on the muscle surface. There is also likely to be more fluid than normal in the chest, lungs, abdomen and pericardium (heart sac). The kidneys are pale and swollen. Subcutaneous oedemas may be present particularly in horses and sheep.

In chronic trypanosomiasis the carcass is emaciated and often dehydrated. The skin may show pressure sores and ulcers when the animal has been unable to stand up for some time. The fat reserves under the skin have been used up and the skin is closely adherent to the underlying muscles and bones. The muscles have wasted to a remarkable degree, are pale because of anaemia and the blood is watery and pale with an increased clotting time. The heart is often enlarged and flabby because of muscle deterioration (myocarditis). Unlike the picture seen in acute disease, the lymph nodes are mostly normal or even hard, dry and reduced in size. The spleen is also normal in size or contracted with a drier pulp than normally seen. Subcutaneous oedemas may be present particularly in horses and sheep.