Introduction

Plant poisonings and mycotoxicoses in stock affecting the haemopoietic system are not of great economic importance in southern Africa, apart possibly from nitrate and prussic acid toxicities which at times can inflict severe losses in cattle and sheep. Nevertheless, it is often necessary to consider some of these toxicoses as differential diagnoses for important tick-borne and infectious diseases which threaten livestock in our region.

The poisonings under discussion can be grouped roughly under plants and fungi which cause either haemolysis and/or biochemical alterations of mature red blood cells, or those which are responsible for bone-marrow and lymphoid tissue suppression. In animals suffering from intoxication by the former group of plants and fungi, tissue respiration is mostly affected, and kidney and liver function are often disturbed, while in the latter group, affected animals are frequently subjected to impaired immunological response, bleeding tendencies and sometimes also anaemia.

Enzootic icterus

Geelsiekte

Enzootic icterus, also referred to locally as geelsiekte, is a chronic copper poisoning of sheep and goats, following prolonged ingestion of relatively high levels of phytogenous copper in large parts of the semi-arid Karoo and the southern Free State Province.^{3, 28, 29, 77, 78}

The disease was first noticed in 1924, when sheep transported from the Karoo to the Veterinary Research Institute, Onderstepoort, died acutely 10–30 days after arrival, showing severe icterus and anaemia.²⁹ Since then enzootic icterus has been noticed regularly and many theories have been put forward regarding its aetiology. Brown^{6, 8, 9} suggested that enzootic icterus and geeldikkop were different manifestations of the same disease entity. Both conditions basically arose from aberrations in the selective permeability of cell membranes and the active transport mechanisms across cell membranes.⁹ Key dehydrogenases were inactivated with catastrophic effects on metabolism.^{6, 7}

As the liver copper content of sheep suffering from enzootic icterus was conspicuously elevated,^{9, 35} copper was investigated as a possible aetiological factor in the genesis of the disease. Neethling et al.,⁸⁷ however, concluded that it was unlikely, under conditions prevailing in the Karoo (including those parts where the plants contain up to 28 ppm of copper or more) that chronic copper poisoning would occur if the molybdenum status of the animal was adequate. As a result of the high copper values and the similarities of the necropsy findings of enzootic icterus to those of chronic copper poisoning, Erasmus³⁵ was tempted to equate enzootic icterus with chronic copper poisoning and to consider them as basically the same condition. Another factor, apart from copper and molybdenum, that affects the metabolism of sheep in the Karoo, is selenium. Brown and de Wet¹⁰ demonstrated the presence of potentially dangerous amounts of selenium in various plants from geeldikkop and enzootic icterus areas. Sheep emanating from such areas had higher levels of selenium in the body tissues than those of control sheep.⁹ A theory was put forward that selenium may predispose sheep to geeldikkop and enzootic icterus by disrupting enzyme systems, especially those connected with transfer mechanisms and the glycolytic cycle.^{6, 76} It was also suggested that geeldikkop and enzootic icterus were autoimmune diseases precipitated by aberrant selenoprotein antigens.^{86, 87} Recently, it was shown that geeldikkop and enzootic icterus are two distinct entities with different aetiologies. Geeldikkop is a complex disease caused by Tribulus terrestris plants,⁷² and enzootic icterus is a chronic copper poisoning.³

Bath³ discussed at length many of the epizootiological factors playing a role in enzootic icterus. He, as well as other authorities,^{28, 77, 78} reported relatively high copper levels in certain plants on farms where enzootic icterus occurs regularly. Bath³ reviewed the literature on enzootic icterus and chronic copper poisoning and came to the conclusion that the type and pH of the soil may influence copper uptake by plants. For example, low pH levels inhibit both phytogenous selenium and molybdenum, and it is well known that the former enhances copper accumulation in the liver and the latter depresses it.^{98, 99}

According to Bath,³ enzootic icterus would appear to occur more frequently on farms with doleritic ridges, and he concluded that the high copper and low pH levels in these ridges may be important contributory factors in the epizootiology of the disease. He, as well as Pope,⁹⁹ pointed out that other elements, such as sulphur, zinc, manganese, iron, calcium, phosphorus and cobalt can be involved in this complex imbalance.⁹⁹

Copper analyses, done on the livers of 36 adult Merino sheep originating from Middelburg in the Karoo and used in geeldikkop and facial eczema experiments, showed marked elevation, some of which had values of up to 544 ppm on a wet basis.⁷² Apart from slight khaki-brown discolouration of the liver and kidneys, no clinical signs or other macroscopical changes compatible with fatal enzootic icterus were seen in these animals. Microscopically, the hepatic changes of these subclinical cases were characterized by infiltration of mild to moderate numbers of macrophages laden with a homogenous khaki-brown lipofuscinous pigment in the portal triads; slight bile ductular proliferation and portal fibroplasia...