GENERAL INTRODUCTION: RICKETTSIAL, CHLAMYDIAL AND HAEMOTROPIC MYCOPLASMAL DISEASES

Introduction

Heartwater (cowdriosis) is a tick-borne disease of cattle, sheep, goats and some wild ruminants that is caused by the rickettsia Ehrlichia ruminantium. Typically, the disease is characterized by high fever, nervous signs, hydropericardium, hydrothorax and oedema of the lungs and brain, and death. It is one of the major causes of stock losses in sub-Saharan Africa.

The first reference to what may have been heartwater was made by the Voortrekker pioneer Louis Trichardt in 1838.²²⁴ While trekking through what is today the Limpopo Province of South Africa many of his sheep succumbed to a disease known locally as ‘nintas’ three weeks after they had suffered massive tick infestation. According to evidence given by a farmer, John Webb, to the Cattle and Sheep Disease Commission of 1876 in Grahamstown, heartwater was observed in 1858 in South Africa in the northern part of the Eastern Cape Province. Because of its confusion with other local diseases of unknown aetiology that were prevalent at that time some of the earlier information regarding the occurrence of heartwater is unreliable.¹⁴⁴

The first important experimental findings came in 1898 when both Dixon⁸⁷ and Edington¹¹³ showed that heartwater disease could be induced by blood passage from infected to susceptible animals. No organisms could be demonstrated in the blood or other tissues of diseased animals but it was concluded that heartwater was caused by a living microorganism,¹⁴⁹ at that time believed to be a virus.²⁹⁶ In 1900 Lounsbury published his confirmation of the long-standing suspicion that the bont tick (Amblyomma hebraeum) was the vector of heartwater in South Africa,¹⁷⁷ but another quarter of a century elapsed before Cowdry demonstrated that the infectious agent in the tissues of infected animals and ticks was a rickettsia which he named Rickettsia ruminantium.^{76, 77} The name was later changed to Cowdria ruminantium²¹⁰ and more recently to Ehrlichia ruminantium.¹¹¹

Heartwater occurs wherever ticks capable of transmitting the organism are present (see Vectors: Ticks). The endemic area encompasses most of sub-Saharan Africa, including the islands of Madagascar, Sao Tomé, Réunion, Mauritius, and Zanzibar.^{125, 136, 137, 265} The disease is absent from the Kalahari Desert and dry coastal areas of Namibia and South Africa. Heartwater also occurs on the islands of Guadeloupe, Marie-Galante, and Antigua in the Caribbean^{63, 327} to which infected Amblyomma variegatum ticks were introduced, probably with cattle from Senegal, during the eighteenth century.^{29, 190}

The occurrence of heartwater is frequently taken for granted in the endemic areas and definitive diagnoses are usually only conducted for particularly valuable animals. This leads to the prevalence rates of the disease being under-reported.⁶² In the endemic area in South Africa mortalities from heartwater are three times greater than those from babesiosis and anaplasmosis combined.^{226, 322} From a livestock census performed by the Directorate of Animal Health, South Africa, in 1996, it was estimated that 17.5 million head of livestock were at risk in the endemic area of the country. Goats are especially threatened, and in some parts of the rural farming sector it is believed that up to 30 per cent of goats become infected with heartwater annually. It is not known how many of these die from the infection, but a substantial proportion must do so.

The economic impact of heartwater is difficult to quantify, both because of the under-reporting noted above and because the actual occurrence of the disease may be partially suppressed by a range of factors. These include the use of acaricides (in 1996 acaricides costing US$ 13 million were purchased in South Africa), antibiotic prophylaxis,²⁴⁶ immunization by infection and treatment,³³⁹ the resistance of certain animal breeds to the disease,⁶⁶ and endemic stability.³¹² An estimate of the impact of heartwater was made by the Deputy Director of Veterinary Services of the Eastern Cape Province of South Africa during 1998.²³ His assessment was that 10 per cent of all stock losses, costing up to US$ 30 million annually, were due to heartwater, and this was despite an annual expenditure of between US$ 1 and 5 million for prophylaxis and vaccination. There are estimates from Zimbabwe of annual losses over a 10-year period amounting to US$ 5.6 million per annum, which includes the cost of acaricides, milk losses and treatment.²¹³

Heartwater is a major obstacle to the introduction of high-producing animals into sub-Saharan Africa to upgrade local stock^{160, 320} and is of particular importance when susceptible animals are moved from heartwater-free to heartwater-infected areas.^{226, 293} Whatever the actual costs may be it is certain that...