ANIMAL DISEASES AND VETERINARY SERVICES
Deficiencies of the trace elements cobalt, copper, iodine, or selenium induce widespread natural diseases of animals. There is also a type of copper deficiency associated with an excess of molybdenum and this is more widespread than that arising from a simple deficiency of copper.
Cobalt deficiency or “bush sickness” occurs widely. It is characterised by loss of appetite, wasting, anaemia, failure to thrive (particularly in young animals), and eventual death, unless the deficiency is corrected. It affects mainly sheep and cattle, though it can occur in all ruminant animals and does affect goats and deer. Severe cobalt deficiency is now rare, for the deficient areas are known and are corrected by regular topdressing with cobalt sulphate. Cobalt-deficient areas have been classified as “severe”, “moderate”, or “marginal”. The classification is based on the responses of animals to cobalt and on analyses of animal livers, of pastures, and of some soils for cobalt content. The accompanying map shows the cobalt-deficient areas.
Copper deficiency is of two kinds: (i) an uncomplicated deficiency found on some peat and sandy soils; and (ii) a deficiency of copper, associated with high molybdenum content in the pasture. This occurs on most peat soils, on some pumice soils, and on some marine and river silts. Some of what is now the most highly productive dairying land in New Zealand, the reclaimed swamp lands of the Hauraki Plains and the Waikato, were almost impossible to farm economically before copper deficiency was recognised as the cause of disease. Cattle of all ages and young sheep are affected. Symptoms vary widely, depending on the severity of the deficiency and whether or not it is complicated by excess of molybdenum. Lambs and calves can suffer unthriftiness, retarded growth, ataxia, and bone fragility; and adult cattle become poorly conditioned and anaemic and their coats often lose colour. On peat lands, when there is excess molybdenum in the pasture, the characteristic symptom is a persistent, severe, debilitating scouring whenever there is a flush growth in spring or, to a less extent, in autumn. Copper sulphate mixed with the usual fertilisers is an effective, and widely used, method of control.
Iodine deficiency causing goitre (mainly in newborn animals), most often affects sheep. Goitre is likely to occur in parts of Otago, Westland and Marlborough, large parts of the Canterbury Plains, between Wanganui and Palmerston North, and in places near Waipukurau, Napier, and Gisborne.
Widespread Selenium-responsive diseases are economically important. Experiments and field trials begun in 1958 have demonstrated that selenium can prevent white-muscle disease in lambs, an associated barren-ewe problem, some forms of unthriftiness in lambs and calves, and exudative diathesis in chicks. There is evidence also that Hepatosis dietetica in pigs and paradontal disease of sheep can be controlled by the use of selenium. Live-weight responses in lambs were obtained in areas shown in the accompanying map.
Metabolic diseases cause serious losses both of dairy and of beef cattle during calving and for some time after. Hypocalcaemia, hypomagnesaemia, and acetonaemia, either singly or together, are common in mature cows and can quickly be fatal if treatment is delayed. Hypomagnesaemia, or grass tetany, attacks calves, dry and milk cows, and males. Early treatment gives good control, but relapses take place and there seems to be a district and seasonal variation in response. In beef herds Hypomagnesaemic tetany is assuming more importance and its greater incidence is often associated with improvement both of pastures and of soil fertility.
Hypocalcaemia, or lambing sickness, is common throughout New Zealand; its incidence in individual flocks may be as high as 25 per cent. Pregnancy toxaemia, also common in ewes and associated with stress (for example, multiple pregnancy, starvation over a period, a check in feeding, or unusual disturbance) can, in some seasons and under certain conditions, cause serious losses.