Dourine

A G LUCKINS, P R BARROWMAN, W H STOLTSZ AND J J VAN DER LUGT

Introduction

Dourine is a chronic contagious disease of horses, mules and donkeys. It is characterized by swelling of the external genitalia and subcutaneous tissues of the ventral abdomen, cachexia and, in some cases, involvement of the nervous system. The disease is caused by the venereally transmitted protozoan parasite Trypanosoma equiperdum.

Trypanosoma equiperdum is essentially a tissue parasite which has developed the ability to invade and survive in the genital tract, thus enabling direct transmission from one host to another. The disease consequently differs from other African trypanosomoses in that it does not require an arthropod vector for transmission.

Dourine was described as early as AD 400 by Chiron, a Byzantine veterinarian,⁶⁰ and is probably one of the oldest recognized diseases of horses. The disease was known to the Arabs and horsemen of North Africa for centuries before it was reported in Europe in the late eighteenth century.³⁰ The dissemination of Arab horses for breeding purposes may have introduced dourine to Europe, from where it spread to many parts of the world. Rouget, in 1894, was the first to demonstrate a trypanosome in the blood of an infected horse,³⁵ while the first experimental transmission of the parasite occurred five years later.⁵⁶ The organism was named Trypanosoma equiperdum by Doflein in 1901.¹⁸

Control measures based on clinical identification,³⁵ and later by serological testing,⁶⁶ have eliminated the disease from many countries. Intensive campaigns led to its eradication in Canada⁶⁶ and the final pockets of infection in the USA were eliminated in 1949.³⁶ By 1960 most European countries were free of the disease.¹⁹ Outside southern Africa dourine still occurs in certain parts of North Africa, Russia, the Middle East, and Burma.¹⁹ Recent outbreaks have occurred in Italy.⁹

Dourine was known to farmers in Griqualand West in South Africa for several years before it was officially recorded for the first time in 1914.⁶⁴ During the same year the disease was reported from Namibia by Maag as cited by Schulz.⁶⁰

A shipment of horses from Germany to Namibia may have brought the disease to the subcontinent.³³ Schulz believed that dourine was brought into South Africa by donkey jacks imported from Canada or by transport horses returning from Namibia during the Herero wars.⁵⁸ Dourine has a scattered distribution in Namibia, Botswana, Swaziland, Lesotho and South Africa.^{8, 19, 20, 68}

Although the disease is not of great economic importance in South Africa, its continued presence necessitates strict surveillance of horses intended for breeding and export purposes.

Aetiology

Trypanosoma equiperdum is a salivarian trypanosome of the subgenus Trypanozoon, which includes Trypanosoma evansi and the Trypanosoma brucei complex.¹⁷ The mammalian blood forms of the three species have similar morphology and share common somatic antigens, which prevent their distinction by most serological tests.^{52, 70} Trypanosoma equiperdum is closely related to T. evansi, the causative organism of surra, which is endemic in North Africa,²⁸ and it is possible that they are derived from a common ancestor descended from T. brucei.¹² Evidence that they might belong to the same species comes from isoenzyme characterization that has shown isolates of T. equiperdum and T. evansi from different geographical regions form a homogeneous group,³⁹ and analysis of kinetoplast DNA, nuclear DNA and molecular karyotyping has confirmed the closeness of their phylogenetic relationships. ^{39, 40, 41, 71} A major difference between the two species is in the presence of maxicircles in the DNA of T. equiperdum and their absence in T. evansi,⁵¹ so it is unlikely that T. equiperdum could have originated from T. evansi, as suggested by Hoare,²⁸ as this would have required the reacquisition of the maxicircles.

Trypanosoma equiperdum is monomorphic and typically occurs as slender forms 15,5 to 36 μm long, with a distinct free flagellum and a subterminal kinetoplast.Occasional stumpy forms are seen.²⁹

The organism is only found naturally in the definitive host and multiplies asexually by binary fission.Trypanosoma equiperdum strains have been maintained for experimental purposes in the laboratory in animals such as mice, rats and guinea pigs for many generations.^{49, 66} These strains have enhanced virulence and produce acute disease when used to infect horses.⁴⁹ They have a tendency to produce dyskinetoplastic strains, particularly in rodents.³⁰

Although T. equiperdum is essentially a tissue parasite, parasitaemia does occur 21 to 23 days after natural infection and may persist for three to four months. Thereafter it is difficult to demonstrate the parasite in the blood of infected horses.^{6, 7}

For many years, T. equiperdum could only be maintained by serial passage in laboratory animals or developing chick embryos.⁴⁴ Recently an in vitro culture system has been developed in which a laboratory strain ofT. equiperdum can be continuously maintained in a semi-defined medium, whilst still retaining its morphological and biochemical characteristics. ^{5, 47} These in vitro systems have been utilized in studies on the efficacy of trypanocidal drugs^{11, 72} and could be used also in the study of the physiology, biochemistry and antigenic variation of the parasite.

Epidemiology

Dourine occurs in horses, mules and donkeys...