African animal trypanosomoses

R J CONNOR AND P VAN DEN BOSSCHE

Introduction

The trypanosomoses are diseases of humans and domestic animals that result from infection with parasitic protozoa of the genus Trypanosoma. Trypanosomes parasitize all classes of vertebrates: fish, amphibians, reptiles, birds and mammals. The parasites, with the exception of Trypanosoma equiperdum, the cause of dourine, are transmitted from host to host by haematophagous vectors, and usually cause little appreciable harm to either the vector or the vertebrate host. However, several species of trypanosomes which parasitize mammals are less well adapted and commonly cause disease.

The trypanosomoses form a group of diseases, each species of pathogenic trypanosome causing the disease trypanosomosis. The course of a trypanosomal infection varies considerably and depends upon both the species of trypanosome and the host involved. Trypanosomosis is generally characterized by the intermittent presence of parasites in the blood and intermittent fever. Anaemia usually develops in affected animals, and this is followed by loss of body condition, reduced productivity and, often, high mortality.

The first report that associated trypanosomes with disease was made from India by Evans⁸¹ in 1880. He found trypanosomes in the blood of camels and horses which were affected by a disease known locally as ‘surra’, and which is now known to be caused by Trypanosoma evansi. Subsequently, Bruce³⁵ made the major discovery in Zululand, South Africa, that trypanosomes were the causal organisms of ‘nagana’, or tsetse fly disease. The early review of trypanosomes and trypanosomoses by Laveran and Mesnil,¹⁵⁵ translated from the French by Nabarro in 1907, was followed by Wenyon’s³¹⁶ contribution in 1926. Subsequent major reviews of trypanosomes and African trypanosomoses were made by Mulligan and Potts,²⁰⁴ Ford,⁸⁵ Hoare,¹¹¹ Jordan,¹³⁶ and Stephen.²⁷⁴ The considerable interest in trypanosomoses arises from their importance and the intriguing biology of the causal organisms.

Two forms of human trypanosomosis exist: Chagas’ disease occurs in Central and South America and is transmitted by blood-sucking reduviid bugs, certain small wild animals and dogs harbouring the infection. The second form is human sleeping sickness. This occurs in Africa and is transmitted by blood-sucking flies of the genus Glossina, commonly known as ‘tsetse flies’ or simply as ‘tsetse’. The majority of animal diseases caused by trypanosomes occur in the tropics. In Africa, several species of tsetse-transmitted trypanosomes cause African trypanosomoses in domestic animals, which in southern Africa are collectively known as ‘nagana’, a word derived from the Zulu ‘nakane’ meaning tsetse fly disease. An important form of trypanosomosis known as dourine, caused by T. equiperdum, also occurs in southern Africa, but has no arthropod vector (see Dourine). ‘Surra’ is transmitted by biting flies other than tsetse flies and, although it occurs in many parts of the tropics, including northern Africa, it is not present in southern Africa.

Tsetse flies have recently been discovered in Saudi Arabia, ⁷⁷ but they are only known to be of importance in Africa, south of the Sahara, where the diseases they transmit are responsible for great economic loss (see Vectors: Tsetse flies).

The large populations of wild animals, which have thrived for millennia in the tsetse-infested tracts of Africa, have evolved with these flies and the trypanosomes they transmit. Hosts and parasites have become mutually adapted and coexist in a balanced relationship. Humans first brought domestic animals into the tsetse fly belts of Africa relatively recently, and although humpless cattle of the Bos taurus type, from which the present-day taurine breeds such as the West African Shorthorn, N’Dama, Muturu and Baoule are believed to have descended, were introduced into northern and western Africa from 4 500 BC onwards, the humped Zebu, Bos indicus type, arrived some 3 000 years later, and did not reach central and southern Africa until around AD 700.⁸⁰ Goats and sheep were introduced at about the same time. Because of this relatively recent introduction, the relationship between tsetse-transmitted trypanosomes and domestic animals has not fully evolved and infection with these parasites frequently produces disease. However, the West African humpless cattle have had longer to adapt than Zebu cattle and this may explain why they possess the trait of trypanotolerance. They are able to live without drug treatment in tsetse-infested areas where other cattle die. Furthermore, some indigenous breeds of goats and sheep, such as the Dwarf goats and Djallonké sheep of West Africa and the small East African breeds, are more trypanotolerant than are exotic breeds.²¹¹

The ravages of nagana have long been recognized by the inhabitants of southern Africa, and early attempts to introduce livestock into tsetse-infested areas were unsuccessful as draught animals and other stock succumbed to the disease. The devastation which resulted from the rinderpest pandemic of the 1890s (see Rinderpest) destroyed almost entire populations of wild animals and millions of cattle. Without hosts on which to feed, tsetse disappeared from large areas. With the constraint of tsetsetransmitted trypanosomosis removed, settlement in Zimbabwe was rapid. Some 25 000 cattle had survived the rinderpest catastrophy, and after restocking...