Infection with Trypanosoma evansi (Surra)

Intermittent fever accompanies each rise in parasitaemia, followed by progressive anaemia, weight loss, jaundice, weakness and lethargy. Oedema collects in the legs, brisket and belly; urticarial skin plaques and small haemorrhages on the eyelids, nostrils and anus may appear. Camels develop a rough coat, and abortions occur in camels and buffalo. Nervous signs are common in horses and in dogs can resemble rabies. Necropsy findings are nonspecific: emaciation, anaemia, fluid in the chest and abdomen, petechiae on internal organs, and enlarged lymph nodes and spleen.

Surra is caused by Trypanosoma evansi, a flagellated blood and tissue protozoan of the subgenus Trypanozoon. It looks identical to T. brucei but is passed on mechanically and cannot develop in tsetse flies. Parasites multiply in the blood and also settle outside the vessels, including in the central nervous system. Successive waves of parasitaemia bring recurrent fever, and infection causes progressive anaemia, loss of condition, oedema and wasting. The parasite suppresses immunity, which can worsen other infections and interfere with vaccination campaigns such as those against foot and mouth disease and haemorrhagic septicaemia. In advanced cases invasion of the nervous system leads to progressive hind-limb paralysis before recumbency and death.

The host range is very wide. Camels and horses are most severely affected; cattle and buffalo usually carry chronic infection and serve as reservoirs, and dogs are highly susceptible. The parasite occurs continuously from North Africa through the Middle East to South-East Asia, and in Central and South America; camels are the main host in the Middle East and Africa. Biting flies, chiefly tabanids and Stomoxys, carry it mechanically, so outbreaks follow fly activity, for example in the rainy season. Carnivores are infected by eating fresh infected meat, and reused needles spread it during mass treatments. Stress from malnutrition, pregnancy or work increases susceptibility. WOAH lists the infection.

Horses: African horse sickness, equine viral arteritis, equine infectious anaemia, dourine and chronic parasitism. Camels: tsetse-transmitted trypanosomosis, anthrax and chronic parasitism. Cattle: babesiosis, anaplasmosis, theileriosis, haemorrhagic septicaemia and anthrax. Dogs with nervous signs: rabies. Because no sign is pathognomonic, diagnosis needs trypanosomes seen in blood or lymph node smears (with buffy-coat concentration), PCR, or serology such as CATT/T. evansi or ELISA; PCR is needed to tell it from T. brucei.

Blood, spleen, lymph nodes, central nervous system, skin and subcutaneous tissue, serous membranes, reproductive tract

No vaccine. Detect and treat infected animals, give preventive trypanocides to at-risk animals, protect stock from biting flies, avoid reusing needles, and quarantine with serial testing by CATT then ELISA at 40-day intervals before moving animals.

WOAH card: untreated horses and camels near 100% fatal, death in 2 weeks to 4 months. Desquesnes 2013: camel surra is more often chronic than in horses, often 2-3 years, but can kill acutely. Drug resistance, notably to diminazene, can defeat treatment.