Biological functions of selenium and vitamin E

Vitamin E

Vitamin E is an antioxidant that prevents oxidative damage to sensitive membrane lipids by decreasing hydroperoxide formation. The vitamin has a central role in protection of cellular membranes from lipoperoxidation, especially membranes rich in unsaturated lipids, such as mitochondria, endoplasmic reticulum, and plasma membranes.

Enzootic nutritional muscular dystrophy (NMD)

Congenital nutritional muscular dystrophy

Congenital NMD is rare in farm animals. Isolated cases have been reported.¹⁸

Similarly, NMD can occur in calves and lambs only a few days of age but rarely. Selenium readily crosses the bovine placenta and fetal selenium is always higher than the maternal status.⁹ There is no evidence that the weak-calf syndrome is associated with selenium deficiency. Long-term parenteral supplementation with neither selenium alone nor in combination with vitamin E had any effect on the incidence of the weak-calf syndrome.

An investigation of aborted bovine fetuses with lesions of heart failure, specifically cardiac dilatation or hypertrophy along with a nodular liver and ascites compared with aborted fetuses without such lesions and non-aborted fetuses from the abattoir found myocardial necrosis and mean selenium levels of 5.5 μmol/kg in the fetuses with heart lesions, 6.5 μmol/kg in the fetuses without heart lesions and 7.5 μmol/kg selenium in the fetuses from the abattoir.¹⁹ This suggests that selenium deficiency in bovine fetuses may cause myocardial necrosis and heart failure. Normal levels of selenium in liver and kidney tissue of bovine fetuses derived from the abattoir were 7.5± 5.2 μmol/kg and 4.4± 1.1 μmol/kg, respectively.

In pigs, NMD has been produced experimentally on vitamin E- and selenium-deficient rations but is usually only a part of the more serious complex of mulberry heart disease and hepatosis dietetica.

Vitamin E-Selenium Deficiency (VESD) syndrome

Mulberry heart disease, hepatosis dietetica, exudative diathesis and nutritional myopathy, also known as the VESD syndrome (vitamin E and selenium deficiency) occur in pigs, usually as serious diseases. Nutritional muscular dystrophy may also occur in pigs. The occurrence of edema in various tissues has also been suggested as a possibility of Se or vitamin E deficiency. Impaired spermatogenesis and increased susceptibility to the effects of swine dysentery have also been suggested as responses to reduced levels of these two substances. There is a suspicion that the problems have become more common as the pig grows more quickly, the requirements have increased and the demands for anti-oxidants is increased at the same time that the provision of fat soluble vitamins is increasingly difficult. In addition, there is very small difference between the therapeutic and toxic levels of Se and Se toxicosis has occurred in an attempt to prevent Se deficiency. A more recent complication is the realization that we have been using inorganic Se to provide Se in the diet but that in the plant most of the Se is organic in the form of L-selenomethionine, a Se analogue of the amino acid methionine.²⁰ In the pig as in other species they are believed to serve as antagonists to toxic free radicles and act in concert with other substances such as Vitamin C. Little is known about their metabolism in the pig. In the pig, there is very little transfer of fat soluble products across the placenta so there is very little reserve of vitamin E in the new born pig. Immediately after birth, the young pig gets its vitamin E from the colostrum and milk of the sow. If the sow has low body stores or is fed a ration low in vitamin E then the piglet will be very low in vitamin E when it is weaned. Each can substitute for the other in a limited way in the pig. In the pig the diet has the most influence. Diets rich in polyunsaturated fatty acids, copper, Vitamin A or mycotoxins may reduce the availability of vitamin E. As dietary vitamin A levels increase, serum and liver α-tocopherol concentrations decline, suggesting a reduced absorption and retention of α-tocopherol when weaned pigs were fed high dietary vitamin A levels.²¹ Se antagonists or crops from inherently low soil Se fields may also make the situation worse. In pigs, NMD has been produced experimentally on vitamin E- and Se-deficient rations but is usually only a part of the more serious complex of mulberry heart disease and hepatosis dietetica. Microangiopathy is most common in weaned pigs²² and may be particularly related to vitamin E deficiency.²³

There is conflicting evidence on the effect of the anti-oxidative vitamins C and E on the reproductive performance of sows. In some studies,^24,25 increasing dietary vitamin E in the diet during gestation may have increased the litter size and reduced the pre-weaning piglet mortality. A similar response has been seen following intra-muscular injection of sows with vitamin E and Se^26,27 but the injection of vitamin C has produced no improvement.^28,29 A recent study³⁰ has confirmed that there was no effect on reproductive performance of sows and growth performance of piglets when supplemented by both vitamin E and C. Vitamin E and Se given to immature gilts for flushing purposes led to the formation of fewer but larger corpora lutea after ovulation, probably due to the progression of a smaller number of follicles to the ovulatory stage.³¹ Vitamin E and the Se increased the development of the uterus but did not influence the number of piglets at farrowing.

VESD occurs naturally in rapidly growing pigs, usually during the post-weaning period (3 weeks to 4 months), particularly during the early finishing period. The lowest concentration of vitamin E in piglets was day 45 after farrowing³² but it may be that the Se status of the newborn piglets may be more important for their health than their vitamin E status. The first 3–4 weeks following the move to the finishing house is the most dangerous period for a low vitamin E level³³ and it is important to remember that there is considerable individual variation. Serum vitamin E declines after weaning and even with vitamin E supplementation it takes 2–3 days for levels to rise.^34,35 There appears to be a temporary decreased absorption of the vitamin in the immediate post-weaning period and this in turn leads to the reduction of the stored vitamin E reserves. It is usually associated with diets deficient in both Se and vitamin E and those that may contain a high concentration of unsaturated fatty acids. Such diets include those containing mixtures of soybean, high-moisture corn and the cereal grains grown on soils with low levels of Se. The feeding of a basal ration of cull peas, low in Se and vitamin E, to growing pigs can cause the typical syndrome and low tissue levels of Se are present in pigs with spontaneously occurring hepatosis dietetica. It has been shown that feeding diets that contain linseed oil unfortunately reduced the vitamin E levels in the diet but increased the skatole levels.³⁶ However, there are reports of naturally occurring mulberry heart disease of pigs in Scandinavia in which the tissue levels of Se and vitamin E are within normal ranges compared with normal pigs.³⁷ In Ireland, in spite of supplementation of pig rations with vitamin E and Se at levels higher than that necessary to prevent experimental disease, spontaneous mulberry heart disease may still occur.²³ Affected pigs have lower tissue vitamin E levels than control pigs, which suggests an alteration in α-tocopherol metabolism unrelated to dietary Se and PUFA contents.

Natural occurrence of the disease complex in pigs is not uncommonly associated with diets containing 50% coconut meal, fish-liver oil emulsion, fish scraps with a high content of unsaturated fatty acids, or flaxseed, which produces yellow and brown discoloration of fat preventable by the incorporation of adequate amounts of α-tocopherol or a suitable antioxidant. The quality of the dietary fat does not necessarily influence blood vitamin E levels, but the presence of oxidized fat reduces the resistance of the red blood cells against peroxidation. The higher requirement for vitamin E by pigs fed oxidized fat may be due to the low vitamin E content in such fat. It has recently been shown³⁸ that the inclusion of 0.3 ppm Se to the diet of post-weaning piglets resulted in better performance than non-Se supplemented diets irrespective of the level of vitamin E in the ration (up to 200 ppm).

Selenium-responsive disorders

A variety of diseases have been known as selenium-responsive disorders^3,40 because they respond beneficially to the strategic administration of selenium. These include: ill-thrift in lambs and calves on pasture; lowered milk production in cows; white muscle disease in lambs, calves, and kids; lowered fertility and embryonic death in sheep and cattle; retained fetal membranes, metritis, poor uterine involution, and cystic ovaries in cows; subclinical mastitis and impaired immune function in cattle; and prematurity, perinatal death, and abortion in cattle.³ Of these, only ill-thrift, lowered fertility, lowered milk production, and white muscle disease have been reported in New Zealand.³ The literature on the roles of selenium deficiency in grazing ruminants in New Zealand and a rational approach to diagnosis and prevention has been reviewed.³

The pathogenesis of these selenium-responsive diseases is not well understood but it would appear that the selenium deficiency is only marginal. Most investigations into selenium-responsive diseases have occurred in selenium-deficient areas in which diseases such as NMD of calves and lambs occur. The evidence that selenium deficiency in breeding ewes can result in a decline in reproductive performance has not been substantiated experimentally. Reproductive performance was not affected in ewes on a selenium-depleted diet.

Selenium-responsive unthriftiness in sheep has received considerable attention in New Zealand where the response to selenium administration has been most dramatic compared with Australia where the syndrome has also been recognized but where the response is much smaller. The oral administration of selenium to lambs in these areas results in greater body weight gains from weaning to 1 year of age compared with lambs not receiving selenium supplementation. The mean fleece weight of selenium-treated lambs is also greater.

The diagnosis of selenium-responsive unthriftiness depends on analyses of the soil, pasture and animal tissues for selenium and response trials to selenium supplementation. A deficiency state might be encountered when the selenium content of the soil is below 0.45 mg/kg, the pasture content below 0.02 mg/kg DM, the liver content below 21 μg/kg (0.27 μmol/kg) (WW), and wool concentrations below 50–60 μg/kg (0.63–0.76 μmol/kg). For the blood in selenium-responsive unthriftiness of sheep the following criteria are suggested:

The GSH-PX activity is a good index of the selenium status of sheep with a selenium-responsive disease. If measured on a regular basis, it can provide an indication of the selenium status of grazing sheep in individual flocks. Single measurements of GSH-PX activity may fail to detect recent changes in grazing area, differences in pasture species and pasture composition and alterations in the physiological state of the animals.

Subclinical selenium insufficiency

Subclinical insufficiencies of selenium in grazing ruminants are widespread over large areas of southern Australia.¹ The plasma concentrations of affected sheep flocks are low, there are no obvious clinical signs of insufficiency in the ewes and there are significant responses in wool production and fiber diameter to selenium supplementation. The incidence of estrus and fertility is not affected by selenium supplementation. Live weights at birth, in mid-lactation, and at weaning were increased in lambs born to selenium-supplemented and crossbred ewes and in lambs born as singletons. Clean fleece weight at 10 months of age was increased by 9.5% and fiber diameter by 0.3 μm in lambs born to ewes that had received supplementary selenium. Differences in fleece weight and live weight were not detected at 22 months, suggesting that subclinical selenium insufficiency in early life did not permanently impair productivity if selenium status subsequently increased.

Temporal variations in glutathione peroxidase activity in sheep can be used to identify seasons of the year with the highest risk of selenium deficiency.⁴¹ In the Mediterranean area, lambs born in the spring/summer are at higher risk to selenium-deficiency related diseases.⁴¹ Lambs born in autumn/winter are from ewes gestating during the summer, when supplementation with cereal grains is provided.

Selenium is a component of type-I iodothyronine deiodinase, which catalyzes the extrathyroidal conversion of thyroxine (T4) to the more active tri-iodothyronine (T3). Sheep grazing pastures low in selenium frequently have higher circulating T4 and lower circulating T3 concentrations than sheep receiving selenium supplementations.

When ewes grazing pastures low in selenium were supplemented thiocyanate (to cause iodine insufficiency), iodide and selenium, there was no evidence of clinical deficiencies. Growth rates of lambs were not affected by thiocyanate of their dams during mid-pregnancy, but plasma T3 and T4 concentrations were depressed in ewes receiving thiocyanate. The iodide supplementation increased thyroid hormone concentrations in ewes, but depressed plasma T3 concentrations in lambs. Supplementation of sheep grazing pastures low in selenium with both selenium and thyroid hormones improved wool characteristics, live weight gain, and blood selenium, but there was no evidence of an interaction between the selenium and the hormones. Thus, it seems unlikely that the decline in the quantity of T3 produced, or of T4 utilized for T3 production, in selenium-deficient sheep is responsible for the observed differences in the productivity of selenium-deficient and supplemented sheep. The thyroids have a major role in regulating thermogenesis and lambs born to ewes supplemented with iodide tend to have higher rectal temperatures during cold stress. The thermoregulatory ability of the perinatal lamb is not adversely affected by subclinical selenium deficiency.

In a survey of the status of vitamin E and selenium of the livers of cull ewes and market lambs raised in Ontario, selenium was present at marginal levels in 3.3% of cull ewe samples and in 43% of market lamb samples.¹⁶ Vitamin E was low to deficient in 10% of cull ewe samples and in 90% of market lamb samples. In cull ewes, there was a strong relationship between selenium and vitamin E. A large percentage of samples with marginal selenium values had adequate vitamin E, which may indicate that the sheep had access to high levels of vitamin E but received inadequate levels of supplement containing selenium.

An evaluation of the trace mineral status of beef cows in Ontario found that 96% of cull cows were deficient in blood selenium.^42,43 Based on analysis of serum samples from cattle in Iowa and Wisconsin, subclinical selenium deficiency is common in the cattle population.⁴⁴ The serum levels may be adequate for reproductive performance but marginal for optimal resistance to mastitis or for adequate transfer of selenium to the calf.

Reproductive performance

The published information on the effects of vitamin E and selenium deficiency or of dietary supplementation with one or the other or both on reproductive performance in farm animals are conflicting and controversial. Reproductive performance is complex and dependent on the interaction of many factors. Reproductive inefficiency is likewise complex and it is difficult to isolate one factor like a deficiency of vita min E or selenium as a cause of reproductive inefficiency. Conversely, it is difficult to prove that supplementation with these nutrients will ensure optimum reproductive performance.⁴⁵

Resistance to infectious disease

Many studies have examined the role of selenium and vitamin E resistance to infectious disease. Most of the evidence is based on in vitro studies of the effects of deficiencies of selenium or vitamin E or supplementation with the nutrients on leukocyte responses to mitogens, or on the antibody responses of animals to a variety of pathogens. The status of selenium and vitamin E in an animal can alter antibody response, phagocytic function, lymphocyte response and resistance to infectious disease. The administration of vitamin E and selenium during the dry period can influence mammary gland health and milk cell counts in dairy ewes.⁵² In general, a deficiency of selenium results in immunosuppression and supplementation with low doses of selenium augments immunological functions. A deficiency of selenium has been shown to inhibit:

Vitamin E and selenium have interactive effects on lymphocyte responses to experimental antigens.⁵⁴

Vitamin E supplementation of transport-stressed feedlot cattle is associated with reduced serum acute-phase protein concentrations compared with control animals.⁵⁵ Supplementation of the diet of cattle arriving in the feedlot with vitamin E had beneficial effects on humoral immune response and recovery from respiratory disease.⁵⁶

The parenteral administration of selenium and vitamin E during pregnancy in dairy cows has a positive effect on the increase of selenium and vitamin E concentrations in blood, increase of selenium and immunoglobulins concentrations in colostrum and an increase of T3 concentration in blood on the day of parturition.⁵⁷ In addition, there was a trend toward a decreased incidence of clinical mastitis.

Blood abnormalities

In young cattle from areas where NMD is endemic and particularly at the end of winter housing, the erythrocytes have an increased susceptibility to hemolysis following exposure to hypotonic saline. During clinical and subclinical white muscle disease in calves, there is a significant increase in both the osmotic and the peroxidative hemolysis of the erythrocytes. This defect is thought to be the result of alterations in the integrity of cell membranes of which tocopherols are an essential component. Abnormalities of the bone marrow associated with vitamin E deficiency in sheep have been described and abnormal hematological responses have been described in young growing pigs on an experimental selenium- and vitamin E-deficient diet. Vitamin E deficiency in sheep results in increased hemolytic susceptibility of erythrocytes, which may provide a basis for a single functional test for vitamin E deficiency in sheep.

Anemia characterized by a decreased packed cell volume, decreased hemoglobin concentration and Heinz body formation has been observed in cattle grazing on grass grown on peaty muck soils in the Florida everglades. Selenium supplementation corrected the anemia, prevented Heinz body formation, increased the body weight of cows and calves and elevated blood selenium.

Equine degenerative myeloencephalopathy

Equine degenerative myeloencephalopathy, which may have an inherited basis, has been associated with a vitamin E deficiency. The vitamin E status is low in some affected horses and supplementation with the vitamin was associated with a marked reduction in the incidence of the disease. However, serum vitamin E and blood GSH-PX activities determined in horses with histologically confirmed diagnosis of the disease compared with age-matched controls failed to reveal any differences and the findings did not support a possible role for vitamin E deficiency as a cause. Foals sired by a stallion with degenerative myeloencephalopathy and with neurological deficits consistent with the disease during their first year of life had lower plasma levels of α-tocopherol when the levels were determined serially beginning at 6 weeks to 10 months of age than age-matched controls. Absorption tests with vitamin E revealed that the lower α-tocopherol levels were not due to an absorption defect.

Equine motor neuron disease

This is a neurodegenerative disease of the somatic lower motor neurons resulting in a syndrome of diffuse neuromuscular disease in the adult horse.⁷¹ Case-control studies found the mean plasma vitamin E concentrations in affected horses were lower than that of control horses. Adult horses are affected with the risk peaking at 16 years of age. In addition to the role of vitamin E depletion, other individual and farm-level factors, contribute to the risk of developing the disease.

Generalized steatitis

Steatitis in farm animals and other species may be associated with vitamin E and/or selenium deficiency. Most cases in horses have involved nursing or recently weaned foals. Generalized steatitis in the foal has been described as either generalized cachexia due to steatitis alone, or as a primary myopathy or myositis with steatitis of secondary importance. The terms used have included steatitis, generalized steatitis, fat necrosis, yellow fat disease, polymyositis, and muscular dystrophy. The relationships between steatitis and vitamin E and selenium deficiency in the horse are not clear and there may be none. Many more clinical cases must be examined in detail before a cause–effect relationship can be considered.

Nutritional muscular dystrophy

A simplified integrated concept of the pathogenesis of the NMD would be as follows. Diets deficient in selenium and/or vitamin E permit widespread tissue lipoperoxidation leading to hyaline degeneration and calcification of muscle fibers. One of the earliest changes in experimental selenium deficiency in lambs is the abnormal retention of calcium in muscle fibers undergoing dystrophy and selenium supplementation prevents the retention of calcium. Unaccustomed exercise can accelerate the oxidative process and precipitate clinical disease. Muscle degeneration allows the release of enzymes, such as lactate dehydrogenase, aldolase and creatine phosphokinase, the last of which is of paramount importance in diagnosis. Degeneration of skeletal muscle is rapidly and successively followed by invasion of phagocytes and regeneration. In myocardial muscle, replacement fibrosis is the rule.

In calves, lambs, and foals, the major muscles involved are skeletal, myocardial and diaphragmatic. The myocardial and diaphragmatic forms of the disease occur most commonly in young calves, lambs, and foals, resulting in acute heart failure, respiratory distress, and rapid death, often in spite of treatment. The skeletal form of the disease occurs more commonly in older calves, yearling cattle, and older foals and results in weakness and recumbency, is usually less severe and responds to treatment. The biceps femoris muscle is particularly susceptible in calves and muscle biopsy is a reliable diagnostic aid.

In foals with NMD, there is a higher proportion of type IIC fibers and a lower proportion of type I and IIA fibers than in healthy foals. The type IIC fibers are found in fetal muscle and are undifferentiated and still under development. During the recovery period, fibers of types I, IIA, and IIB increase and the proportion of type IIC fibers decreases. A normal fiber type composition is present in most surviving foals 1–2 months after the onset of the disease.

Acute NMD results in the liberation of myoglobin into the blood, which results in myoglobinuria. This is more common in horses, older calves, and yearling cattle, than in young calves whose muscles have a lower concentration of myoglobin. Hence, the tendency to myoglobinuria will vary depending on the species and age of animal involved.

Subclinical selenium insufficiency

Selenium deficiency affects thyroid hormone metabolism and may explain the cause of ill-thrift. The conversion of the iodine-containing hormone, thyroxine (T4) to the more potent triiodothyronine (T3) is impaired in animals with low selenium status and iodothyroninedeiodinase is a selenoprotein which mediates this conversion.⁷²

VESD syndrome and others

The pathogenesis of mulberry heart disease, hepatosis dietetica, exudative diathesis, and muscular dystrophy of pigs is not yet clear. Vitamin E and Se are necessary to prevent widespread degeneration and necrosis of tissues, especially liver, heart, skeletal muscle, and blood vessels. Se and vitamin E deficiency in pigs results in massive hepatic necrosis (hepatosis dietetica), degenerative myopathy of cardiac and skeletal muscles, edema, microangiopathy, and yellowish discoloration of adipose tissue. Myocardial and hepatic calcium concentrations are increased in pigs with mulberry heart disease.^73,74 In addition, there may be esophagogastric ulceration, but it is uncertain whether or not this lesion is caused by a Se and/or vitamin E deficiency. Anemia has also occurred and has been attributed to a block in bone marrow maturation, resulting in inadequate erythropoiesis, hemolysis or both. However, there is no firm evidence that anemia is a feature of Se and vitamin E deficiency in pigs. The entire spectrum of lesions has been reproduced experimentally in pigs with natural or purified diets deficient in Se and vitamin E, or in which an antagonist was added to inactivate vitamin E or Se. However, in some studies, the Se content of tissues of pigs that died from mulberry heart disease was similar to that of control pigs without the disease.

The extensive tissue destruction in pigs may account for the sudden death nature of the complex (mulberry heart disease and hepatosis dietetica) and the muscle stiffness that occurs in some feeder pigs and sows of farrowing time with muscular dystrophy. The tissue degeneration is associated with marked increases in serum enzymes related to the tissue involved. An indirect correlation between vitamin E intake and peroxide hemolysis in pigs on a deficient diet suggests that lipoperoxidation is the ultimate biochemical defect in pigs and that vitamin E and Se are protective.

Acute enzootic muscular dystrophy

Affected animals may collapse and die suddenly after exercise without any other premonitory signs. The excitement associated with the hand-feeding of dairy calves may precipitate peracute death. In calves under close observation, a sudden onset of dullness and severe respiratory distress, accompanied by a frothy or blood-stained nasal discharge, may be observed in some cases. Affected calves, lambs, and foals are usually in lateral recumbency and may be unable to assume sternal recumbency even when assisted. When picked up and assisted to stand, they feel and appear limp. However, their neurological reflexes are normal. Their eyesight and mental attitude are normal and they are usually thirsty and can swallow unless the tongue is affected. The heart rate is usually increased up to 150–200/min and often with arrhythmia, the respiratory rate is increased up to 60–72/min and loud breath sounds are audible over the entire lung fields. The temperature is usually normal or slightly elevated. Affected animals commonly die 6–12 h after the onset of signs in spite of therapy. Outbreaks of the disease occur in calves and lambs in which up to 15% of susceptible animals may develop the acute form and the case fatality approaches 100%.

Subacute enzootic muscular dystrophy

This is the most common form in rapidly growing calves, ‘white muscle disease’ and in young lambs, ‘stiff-lamb disease’. Affected animals may be found in sternal recumbency and unable to stand but some make an attempt to stand. If they are standing, the obvious signs are stiffness, trembling of the limbs, weakness and, in most cases, an inability to stand for more than a few minutes. The gait in calves is accompanied by rotating movements of the hocks and in lambs a stiff, goose-stepping gait. Muscle tremor is evident if the animal is forced to stand for more than a few minutes. On palpation the dorsolumbar, gluteal and shoulder muscle masses may be symmetrically enlarged and firmer than normal (although this may be difficult to detect). Most affected animals retain their appetite and will suck if held up to the dam or eat if hand-fed. Major involvement of the diaphragm and intercostal muscles causes dyspnea with labored and abdominal-type respiration. The temperature is usually in the normal range but there may be a transient fever (41°C, 105°F) due to the effects of myoglobinemia and pain. The heart rate may be elevated, but there are usually no rhythmic irregularities. Following treatment, affected animals usually respond in a few days and within 3–5 days they are able to stand and walk unassisted.

In some cases, the upper borders of the scapulae protrude above the vertebral column and are widely separated from the thorax. This has been called the ‘flying scapula’ and has occurred in outbreaks in heifers from 18 to 24 months of age within a few days after being turned out in the spring following loose-housing throughout the winter. The abnormality is due to bilateral rupture of the serratus ventralis muscles and has been reported in a red deer.⁷⁵ Occasionally, the toes are spread and there is relaxation of carpal and metacarpal joints or knuckling at the fetlocks and standing on tip-toe, inability to raise the head, difficulty in swallowing, inability to use the tongue and relaxation of abdominal muscles. Choking may occur when the animals attempt to drink. In ‘paralytic myoglobinuria’ of yearling cattle, there is usually a history of recent turning out on pasture following winter housing. Clinical signs occur within 1 week and consist of stiffness, recumbency, myoglobinuria, hyperpnea, and dyspnea. Severe cases may die within a few days and some are found dead without premonitory signs. In rare cases, lethargy, anorexia, diarrhea and weakness are the first clinical abnormalities recognized, followed by recumbency and myoglobinuria.

Congenital muscular dystrophy has been described in a newborn calf.¹⁸ The calf was still recumbent 13 h after birth, had increased serum creatine kinase and decreased serum vitamin E and selenium levels. Recovery occurred following supportive therapy and vitamin E and selenium.

Subcapsular liver rupture in lambs has been associated with vitamin E deficiency in lambs usually under 4 weeks of age.⁷⁶ Affected lambs collapse suddenly, become limp, and die within a few minutes or several hours after the onset of weakness.

In foals, muscular dystrophy occurs most commonly during the first few months of life and is common in the first week.⁶ The usual clinical findings are failure to suck, recumbency, difficulty in rising and unsteadiness and trembling when forced to stand. The temperature is usually normal but commonly there is polypnea and tachycardia. The disease in foals may be characterized by an acute, fulminant syndrome, which is rapidly fatal, or a subacute syndrome characterized by profound muscular weakness. Failure of passive transfer, aspiration pneumonia, and stunting are frequent complications. In the subacute form, mortality rates may range from 30 to 45%.⁶

In adult horses with muscular dystrophy, a stiff gait, myoglobinuria, depression, inability to eat, holding the head down low, and edema of the head and neck are common. The horse may be presented initially with clinical signs of colic.

In pigs, muscular dystrophy is not commonly recognized clinically because it is part of the more serious disease complex of mulberry heart disease and hepatosis dietetica. However, in outbreaks of this complex, sucking piglets, feeder pigs and sows after farrowing, may exhibit an uncoordinated, staggering gait suggestive of muscular dystrophy.

Subclinical nutritional muscular dystrophy occurs in apparently normal animals in herds at the time clinical cases are present. The serum levels of creatine phosphokinase levels may be elevated in susceptible animals for several days before the onset of clinical signs; following treatment with vitamin E and selenium the level of serum enzymes returns to normal. Grossly abnormal electrocardiograms occur in some animals and may be detectable before clinical signs are evident.

Mulberry heart disease

Usually seen in pigs from a few weeks to 4 months of age. These pigs are nearly always the best of the group and it may be that this rate of growth increases the demand for vitamin E and Se.

In mulberry heart disease, affected animals are commonly found dead without premonitory signs. More than one pig may be found dead. When seen alive, animals show severe dyspnea, cyanosis and recumbency and forced walking can cause immediate death. In some outbreaks, about 25% of pigs will show a slight inappetence and inactivity, these are probably in the subclinical stages of the disease. The stress of movement, inclement weather, or transportation will precipitate further acute deaths. The temperature is usually normal, the heart rate rapid and irregularities may be detectable. The feces are usually normal. A good ‘classical outbreak’ has been described.⁷⁷

Hepatosis dietetica

In hepatosis dietetica, most pigs are found dead. Very few cases show other signs. In occasional cases, before death there will be dyspnea, severe depression, vomiting, staggering, diarrhea and a state of collapse. Some pigs are icteric. Outbreaks also occur similar to the pattern in mulberry heart disease. Muscular dystrophy is almost a consistent necropsy finding in both mulberry heart disease and hepatosis dietetica but is usually not recognized clinically because of the seriousness of the two latter diseases. Clinical muscular dystrophy has been described in gilts at 11 months of age. About 48 h after farrowing, there was muscular weakness, muscular tremors, and shaking. This was followed by collapse, dyspnea, and cyanosis. There were no liver or heart lesions. In experimental Se and vitamin E deficiency in young growing pigs, a subtle stiffness occurs along with a significant increase in the creatinine phosphatase (CPK) and serum glutamic-oxaloacetic transaminase (SGOT) values.

Myopathy

Selenium status

Although information on the critical levels of selenium in soil and plants is accumulating gradually, the estimations are difficult and expensive. Most field diagnoses are made on the basis of clinicopathological findings, the response to treatment and control procedures using selenium. The existence of NMD is accepted as presumptive evidence of selenium deficiency, which can now be confirmed by analyses of GHS-PX and the concentrations of selenium in soil, feed samples, and animal tissues. Tentative critical levels of the element are as follows:

Three tests can be used to assess selenium status in cattle and sheep: serum and whole blood selenium and glutathione peroxidase activity.⁷⁸ Serum selenium responds more rapidly to the administration of selenium than whole blood selenium. There is a similar delay in glutathione peroxidase activity to selenium supplementation. Blood or serum selenium status is most consistently measured at the herd-level. Interlaboratory differences in thresholds for deficiency exist and results should be considered based on laboratory-specific guidelines.

The recommended blood selenium reference ranges for New Zealand livestock have been used in several publications (see Table 30.7).^40,79,80

Table 30.7 Selenium reference range to determine selenium status of sheep and cattle in New Zealand

Reference ranges for selenium and vitamin E in serum, blood, and liver of sheep and goat in the USA are available.²

Glutathione peroxidase

There is a direct relationship between the GSH-PX activity of the blood and the selenium levels of the blood and tissues of cattle, sheep, horses, and pigs.¹ The normal selenium status of cattle is represented by whole blood selenium concentration of 100 ng/mL (1270 nmol/L) and blood GSH-PX activity of approximately 30 mU/mg hemoglobin.

There is a high positive relationship (r = 0.87–0.958) between blood GSH-PX activity and blood selenium concentrations in cattle. Blood selenium levels less than 50 ng/mL are considered as selenium-deficient, while levels between 50 and 100 ng/mL (126.6 nmol/L) are marginal and greater than 100 ng/mL are adequate.¹ Comparable whole blood levels of GSH-PX are deficient if less than 30 mU/mg hemoglobin, marginal if 30–60 mU/mg and adequate if greater than 60 mU/mg hemoglobin.¹ There is some evidence of variation in GSH-PX activities between breeds of sheep; levels may also decrease with increasing age. Low levels in some breeds of sheep may also be a reflection of adaptation to low selenium intake because of low levels of selenium in the soil and forages.

The GSH-PX activity is a sensitive indicator of the level of dietary selenium intake and the response to the oral or parenteral administration of selenium.⁵ Because selenium is incorporated into erythrocyte GSH-PX only during erythropoiesis, an increase in enzyme activity of the blood will not occur for 4–6 weeks following administration of selenium. Plasma GSH-PX will rise more quickly and will continue to increase curvilinearly with increasing dietary selenium levels because it is not dependent on incorporation of the selenium into the erythrocytes. The liver and selenium concentration and serum GSH-PX activity may respond to changes in dietary selenium more rapidly than either whole blood selenium or erythrocyte GSH-PX activity. The response in GSH-PX activity may depend upon the selenium status of the animals at the time when selenium is administered. Larger increases in the enzyme activity occur in selenium-deficient animals. The GSH-PX activity in foals reflects the amount of selenium given to the mare during pregnancy.

The sandwich ELISA is a simplified method for the estimation of GSH-PX activity and selenium concentration in bovine blood and can be used for rapid screening of the selenium status of a large number of cattle.⁸⁷ The GSH-PX activity of whole blood samples has been used to assess the selenium status of cattle in the Czech Republic.⁸⁸

The GSH-PX activity can be determined rapidly using a spot test which is semiquantitative and can place a group of samples from the same herd or flock into one of three blood selenium categories: deficient, low marginal and marginal adequate.⁸⁹ A commercial testing kit known as the Ransel Kit is now available. Because of the instability of GSH-PX plasma, GSH-PX activity in sheep, cattle, and pigs should be measured in fresh plasma or stored at –20°C (–4°F). For absolute measurements, it is suggested that pigs plasma GSH-PX activity be measured immediately after separation from the blood cells, or be assayed within 24 h under specified laboratory conditions.

Vitamin E status

Vitamin E occurs in nature as a mixture of tocopherols in varying proportions. They vary widely in their biological activity so that chemical determination of total tocopherols is of much less value than biological assay. Tocopherol levels in blood and liver provide good information on the vitamin E status of the animal. However, because of the difficulty of the laboratory assays of tocopherols, they are not commonly done and insufficient reliable data are available. Analysis of liver from clinically normal animals on pasture reveal a mean α-tocopherol level of 20 mg/kg WW for cattle and 6 mg/kg WW for sheep. The corresponding ranges were 6.0–53 mg/kg WW for cattle and 1.8–17 mg/kg WW in sheep. The critical level below which signs of deficiency may be expected are 5 mg/kg WW for cattle and 2 mg/kg WW for sheep. Tocopherol levels in the serum of less than 2 mg/L in cattle and sheep are considered to be critical levels below which deficiency diseases may occur. However, if the diet contains adequate quantities of selenium, but not an excessive quantity of PUFAs, animals may thrive on low levels of serum tocopherols. In growing pigs, the serum vitamin E levels are between 2 and 3 mg/L. In summary, there are insufficient reliable data available on the vitamin E status on animals with NMD to be of diagnostic value.

The mean plasma vitamin E levels in clinically normal horses of various ages and breeds were 2.8 μg/mL.⁹⁰ The optimal method for storing equine blood prior to α-tocopherol analysis is in an upright position in the refrigerator for up to 72 h. If a longer period is needed, the serum or plasma should be separated, blanketed with nitrogen gas and frozen in the smallest possible vial; the α-tocopherol in these samples will be stable at –16°C (3°F) for at least 3 months.

A summary of the GSH-PX activity, tocopherol and selenium levels in blood and body tissues of animals deficient in selenium appears in Table 30.6. Normal values are also tabulated for comparison.⁹¹ Both the abnormal and normal values should be considered as guidelines for diagnosis because of the wide variations in levels between groups of animals. The level of dietary selenium may fluctuate considerably, which may account for variations in GSH-PX. Selenium reference ranges to determine selenium status of sheep and cattle in New Zealand are shown in Table 30.7.

Table 30.6 Glutathione peroxidase (GSH-PX) activity and selenium levels in blood and body tissues of animals deficient in selenium

In the early stages of the subclinical form of NMD in lambs, there may be a decrease in serum selenium and glutathione peroxidase activity and an increase in the activity of aspartate aminotransferase (AST), creatine kinase (CK), and lactate dehydrogenase (LDH) compared with healthy lambs.⁹² The LDH-isoenzyme activity is useful for detection of subclinical forms of NMD because of significant increases in the activity of the LDH₅-muscle fraction.

Farmed red deer

Reference range data for liver and blood selenium in red deer are limited.^93,94 White muscle disease has occurred in young deer with blood and liver selenium concentrations of 84–140 nmol/L and 240–500 nmol/kg fresh tissue, respectively. No growth rate response to selenium supplementation occurred in 1-year-old deer when blood selenium concentrations were less than 130 nmol/L, the range in which a growth rate response would be expected in sheep.

Pigs

An increase in the activity of several plasma enzymes occurs in Se and vitamin E deficiencies of pigs. The measurement of AST, CPK, lactic acid dehydrogenase, and isocitrate dehydrogenase can be used to detect the onset of degeneration of skeletal and myocardial muscles and liver. However, these are not commonly used for diagnostic purposes because of the acuteness of the illness. The determination of the levels of Se in feed supplies, tissues, and blood of affected pigs is much more useful as an aid to diagnosis and for guidelines for supplementation of the diet.

In Se-vitamin E deficiency in pigs, serum Se values of less than 2.5 ng/mL (3.2 nmol/L), hepatic Se of less than 0.10 mg/kg (1.3 μmol/kg), plasma α-tocopherol values of <0.40 μg/mL and hepatic α-tocopherol concentrations of <0.75 μg/g of tissue are common. In a recent study, the vitamin E level was <2 ppm in 25% of pigs with gross and microscopic lesions of MHD.³⁹ In a recent study results suggested that supplementation with a surfeit level of vitamin E reduced the response to endotoxin, i.e. a reduced response to the peak levels of IL-6.⁹⁵

The diagnostic criteria for the VESD complex in pigs in New Zealand indicate that liver vitamin E concentrations >10 μmol/kg are adequate, with <2.5 μmol/kg associated with deficiency. Corresponding estimates for serum vitamin E are >2.5 μmol/L and <0.8 μmol/L, respectively. Liver Se concentrations of >2200 nmol/kg are adequate, with 1100–2200 nmol/kg being in the marginal range and <1100 nmol/kg being deficient. Deficiency levels for blood are in the range of 400–1500 nmol/L. These values must be interpreted along with the concentration of PUFAs in the diet.

There is a close relationship between blood vitamin E and resistance of erythrocytes against lipid peroxidation. The supplementation of the diet of pigs with vitamin E will increase both the serum levels of vitamin E and the resistance of the erythrocytes to lipid peroxidation.⁹⁶

Samples for confirmation of diagnosis

Nutritional muscular dystrophy

For treatment of NMD in calves, lambs, and foals a mixture containing 3 mg selenium (as sodium or potassium selenite) and 150 IU/mL of DL-α-tocopherol acetate, given IM at 2 mL/45 kg BW is recommended. One treatment is usually sufficient. Animals with severe myocardial involvement will usually not respond to treatment and the case mortality rate is about 90%. However, all in-contact animals in the herd (calves, lambs, and foals) should be treated prophylactically with the same dose of selenium and vitamin E. They should be handled carefully during treatment to avoid precipitating acute muscular dystrophy. Animals with subacute skeletal muscular dystrophy will usually begin to improve by 3 days following treatment and may be able to stand and walk unassisted within 1 week.

Animals sometimes do not respond to either vitamin E or Se or treatment with both.

In outbreaks of mulberry heart disease, hepatosis dietetica and related Se and vitamin E deficiency diseases in pigs, all clinically affected pigs and all pigs at risk should be treated individually with a combination of Se and vitamin E parenterally at first to prevent any further sudden death. It can then be followed by oral administration.

Provide selenium and vitamin E

Over the years, both the vitamin E levels and Se levels in the diets have increased but particularly the former. This is in response to the more rapid growth rates of pigs but also the realization that pigs are coping with many more oxidative disease states. Outdoor pigs usually have sufficient of both unless the soil is Se deficient. A recent Chinese paper⁹⁹ has suggested that dietary zinc at 85 mg/kg, Se at 0.40 mg/kg, and vitamin E at 45 IU/kg is appropriate for crossbred sows.

While Se alone is protective against a greater spectrum of diseases than is vitamin E, there are situations in which vitamin E is more protective. Both Se and vitamin E should be provided when the diets are deficient in both nutrients, but this may not apply in every situation. Most of the emphasis has been on Se supplementation at the expense of vitamin E, which is more expensive and less stable. Most injectable vitamin E and Se preparations are adequate in Se but insufficient in vitamin E.

There have been several attempts to supplement weaner pigs with a vitamin E preparation. Besides individual injections, it is possible to supplement weaner pigs with water supplementation.¹⁰⁰ Pigs will usually drink even if they are not eating. A recent study¹⁰¹ showed that the supplementation of drinking water with high doses of vitamin E (150 mg of Dl-α-tocopherol acetate) was effective in maintaining serum vitamin E levels over the weaning period. This was even true when over the weaning period the intake of food was very low (it can be as low as 0.2–0.3 kg) and there is a temporary malabsorption in the intestine. It takes about 100 IU/L of water to provide a good vitamin E blood serum value.

Maternal transfer to newborn

Diseases caused by selenium deficiency are preventable by the administration of selenium to the dam during pregnancy or directly to the young growing animal. Selenium is transported across the placenta and provides protection for the neonate. Oral supplementation with selenium in beef cattle will provide enough to maintain blood levels in the dam and for adequate transfer to the fetus, which can sequester selenium when the levels are low in the dam. The colostrum of selenium-supplemented cattle also contains an adequate amount of selenium to prevent severe selenium-deficiency diseases.⁶⁸ However, by 7 days after parturition, the levels in milk may be inadequate to maintain adequate serum levels in calves. The strategic administration of selenium and vitamin E before the expected occurrence of the disease is also a reliable method of preventing the disease.

Selenium is potentially toxic

Se is toxic and any treatment and control program using it must be carefully monitored.¹⁰² Se injected into or fed to animals concentrates in liver, skeletal muscle, kidney, and other tissues and withdrawal periods before slaughter must be allowed. There is some concern that Se may be a carcinogen for man. The only tissues that appear likely to consistently accumulate more than 3–4 mg/kg of Se are the kidney and liver and these are very unlikely to constitute more than a very small part of the human diet. There have been no reports of untoward effects of Se on human health when it has been used at nutritional levels in food-producing animals. The incorporation of Se into commercially prepared feeds for some classes of cattle and pigs has been approved in some countries. A recent case in Norway showed the hazards of Se contamination in the case of an iron supplement.¹⁰³ Se toxicosis has been fairly regularly reported.^104-106

Pigs that are deficient may be more susceptible to other diseases. Pigs with NMD often have the appearance of pneumonic pigs because the diaphragm is weak and the pigs are dyspneic.

Deficient and small pigs may be more susceptible to the effects of iron and when this is given by injection there may be large numbers of dead piglets as a result of iron toxicity. In these cases, the heart lesions resemble those of MHD.

Selenium in milk supplies

The use of selenium in the diet of lactating dairy cows has caused concern about possible adulteration of milk supplies. However, the addition of selenium to the diets of lactating dairy cows at levels that are protective against the deficiency diseases does not result in levels in the milk that are hazardous for human consumption. The feeding of excessive quantities of selenium to dairy cattle would cause toxicity before levels became toxic for man.

Dietary requirement of selenium

The dietary requirement of selenium for both ruminants and non-ruminants is 0.1 mg/kg DM of the element in the diet. There may be nutritionally important differences in the selenium status between the same feeds grown in different regions and between different feeds within a region. Even within a region featuring high selenium concentrations, some feeds may contain levels of selenium below the 0.1 mg/kg minimum requirement for livestock. Thus a selenium analysis of feeds appears necessary in order to supplement livestock appropriately. Some geographical areas are known to be deficient in selenium and the feeds grown in these areas must be supplemented with selenium and vitamin E on a continuous basis. Some reports indicate that surveys have found that dairy producers are providing insufficient supplementary selenium in the ration to meet the recommended selenium intake for lactating dairy cows.¹⁴ Long-term administration of organic selenium in the form of selenium yeast provides higher blood and tissue concentrations than repeated parenteral administration of recommended therapeutic doses of inorganic selenium.⁷¹

High sulfate diets

Avoidance of high sulfate diets is desirable, but provision of adequate Se overcomes the sulfate effect.

Glutathione peroxidase activity

Whole blood GSH-PX activity is a way of monitoring Se status but is not as reliable in pigs as in sheep and cattle.

Different methods of supplementation

The prevention of the major diseases caused by selenium and vitamin E deficiencies can be achieved by different methods, including:

The method used will depend on the circumstances of the farm, ease of administration, cost, the labor available, severity of the deficiency that exists and whether or not the animals are being dosed regularly for other diseases such as parasitism. The subcutaneous injection of barium selenate, the administration of an intra-ruminal pellet and the addition of selenium to the water supply were compared in cattle; each method was effective for periods ranging from 4 to 12 months.

Muscular dystrophy

Under most conditions, NMD of calves and lambs can be prevented by providing selenium and vitamin E in the diets of the cow or ewe during pregnancy at 0.1 mg/kg DM of actual selenium and α-tocopherol at 1 g/d per cow and 75 mg/d per ewe. If possible, the supplementation should be continued during lactation to provide a continuous source of selenium to the calves and lambs. Under some conditions the level of 0.1 mg/kg DM may be inadequate. In some circumstances, the optimal selenium concentration in the feed is considerably higher than 0.1 mg/kg DM and levels up to 1.0 mg/kg DM in the feed result in increases in GSH-PX activity which may be beneficial; however, the cost-effectiveness has not been determined. Pregnant ewes being fed on alfalfa hay may require selenium at a level of up to 0.2 mg/kg DM to prevent white muscle disease in their lambs. Young growing cattle, particularly beef cattle likely to receive hay and straw deficient in selenium and those which are fed high-moisture grain, should receive a supplement of selenium at the rate of 0.1 mg/kg DM and α-tocopherol at 150 mg/d per head. If selenium-supplemented concentrates are used as part of a feeding program for dairy cows, it is not necessary to provide additional selenium by parenteral injection.

Lambs are born with a low serum level of vitamin E but the concentration increases rapidly after the ingestion of colostrum.¹ Supplementation of pregnant ewes with α-tocopherol, either as a single IM dose (500 mg 2 weeks before lambing) or orally (150 mg daily during 3–4 weeks before lambing) results in a marked increase in the levels of the vitamin in the serum and colostrum. The vitamin E concentration in colostrum was 5–11 times higher than in milk 1 week after lambing.

Vitamin E supplementation of the feed of weaner sheep by oral drench or feed additive is effective in increasing plasma α-tocopherol concentrations. This is the most practical method for housed sheep and prevents subclinical myopathy.¹¹⁴ The IM oily injection was slow to increase plasma levels of tocopherols and did not prevent myopathy in grazing experiments. Vitamin E supplements have no beneficial effects on wool quality or quantity in grazing sheep and unless certain flocks are susceptible to vitamin E deficiency myopathy it is not recommended.

Selenium responsive reproductive performance and growth

Selenium toxicity and residues

Selenium intoxication can occur following the administration of toxic amounts of a selenium salt. The use of selenium selenite instead of sodium selenate and giving a dose of five times the intended dose resulted in a high mortality within several hours after administration.¹ Animals deficient in selenium are more susceptible to selenium toxicosis than those that are selenium-adequate. The pharmacokinetics of selenium toxicity in sheep given selenium selenite parenterally has been examined. When oral preparations of selenium and monensin are given concurrently as part of a routine dietary management practice, there is greater risk of selenium intoxication than if the selenium is given alone. Administration of monensin sodium at a constant, safe dosage enhanced the toxicity of selenium as demonstrated by increased severity of the signs of intoxication, fatalities, tissue selenium concentrations and intensified gross, histopathological, and biochemical changes. There is some concern about selenium supplementation of beef cattle being a potential source of contamination for nearby aquatic systems, but there is no evidence that this has occurred.

Selenium responsiveness

The response to selenium supplementation is proportional to the degree of deficiency and supplementation of animals that have adequate selenium intakes is unlikely to significantly improve growth rate. In New Zealand, for selenium-deficient lambs, the potential for a growth response to selenium supplementation is strongly related to blood selenium concentration.⁷⁹ Economically significant live weight gains of >10 g/d can occur when initial blood selenium concentrations are <130 nmol/L. This is the basis for the development of reference curves using blood selenium concentration to diagnose selenium deficiency and predict growth responses to lambs.⁷⁹

Although many methods of supplementation of selenium are efficacious, they can differ widely in their cost and convenience of administration. The objective of any micronutrient supplementation program should be to optimize the return on investment.⁴⁰ The least cost option which provides adequate supplementation for the required period should be recommended initially.

Veterinarians are the professionals in the best position to offer advice on cost-effectiveness supplementation.⁴⁰ To retain this position, they must provide sound recommendations based on micronutrient analysis of animal tissue and defensible reference ranges which are supported by production response data. Monitoring micronutrient status in animal tissue should be encouraged so as to ensure that regulatory requirements are met and that deficiency and excessive use are avoided. Circumvention of veterinary involvement in the diagnosis and treatment of micronutrient supplementation can lead to greater use of supplements when not indicated, higher costs to farmers and low cost-benefit ratios for the industry.

Depot and bolus preparations have revolutionized the treatment of deficiencies of cattle and sheep that are grazed extensively where there is little opportunity for frequent administration (Table 30.8). The relatively short duration of a single drench or injection of selenium salts such as sodium selenite should be noted. The use of fertilizer applications selenium prills is gaining widespread acceptance on farms with high stocking rates.

Table 30.8 Dose rates and duration for selected selenium supplements for adult cattle⁴⁰

Absorption and metabolism of calcium and phosphorus

In ruminants, dietary calcium is absorbed by the small intestine according to body needs. Whereas young animals with high growth requirements absorb and retain calcium in direct relation to intake over a wide range of intakes, adult male animals, irrespective of intake, absorb only enough calcium to replace that lost by excretion into urine and intestine, retaining none of it. Calcium absorption is increased in adult animals during periods of high demand, such as pregnancy and lactation, or after a period of calcium deficiency, but a substantial loss of body stores of calcium appears to be necessary before this increase occurs. The dietary factors influencing the efficiency of absorption of calcium include the nature of the diet, the absolute and relative amounts of calcium and phosphorus present in the diet and the presence of interfering substances. Calcium of milk is virtually all available for absorption, but calcium of forage-containing diets has an availability of only about 50%. The addition of grain to an all-forage diet markedly improves the availability of the calcium.

Phosphorus is absorbed by young animals from both milk and forage-containing diets with a high availability (80–100%), but the availability is much lower (50–60%) in adult animals. Horses fed diets containing adequate amounts of calcium and phosphorus absorb 50–65% of the calcium, and slightly less than 50% of the phosphorus present in a variety of feedstuffs. In grains, 50–65% of the phosphorus is in the phytate form which is utilizable by ruminants, but not as efficiently by non-ruminants like the horse and pig. An average availability of 70% has been assumed for phosphorus in early weaning diets for young pigs, and a value of 50% in practical cereal-based feeds as supplied to growing pigs, sows and boars.

The metabolism of calcium and phosphorus is influenced by the parathyroid hormone calcitonin and vitamin D. Parathyroid hormone is secreted in response to hypocalcemia and stimulates the conversion of 25-dihydroxycholecalciferol to 1,25-dihydroxycholecalciferol (1,25-DHCC). Parathyroid hormone and 1,25-DHCC together stimulate bone resorption and 1,25-DHCC alone stimulates intestinal absorption of calcium. Calcium enters the blood from bone and intestine, and when the serum calcium level increases above normal, parathyroid hormone is inhibited and calcitonin secretion stimulated. The increased calcitonin concentration blocks bone resorption and the decreased parathyroid hormone concentration depresses calcium absorption.

Primary calcium deficiency

No specific syndromes are recorded.

Secondary calcium deficiency

Rickets, osteomalacia, osteodystrophia fibrosa of the horse and pig and degenerative arthropathy of cattle are the common syndromes in which secondary calcium deficiency is one of the specific causative factors. In sheep, rickets is seldom recognized, but there are marked dental abnormalities. Rickets has been produced experimentally in lambs by feeding a diet low in calcium.

Samples for confirmation of diagnosis