Disorders of magnesium and phosphorus metabolism (TV4102, Lecture 36 Notes)

List neurological and muscular dysfunctions of hypomagnesaemia

- Hyperirritability

- Muscle stiffness

- Incoordination

- Convulsions

Discuss the aetiology of hypomagnesaemia

- True veterinary emergency

- Subclinical disease to sudden death

- Associated with low [Mg] in the cerebrospinal fluid and blood

- Common in 4-6yr old beef cows

- Risk factors may include soil K : Ca + Mg ratio > 0.07-0.08, weather, plant species and lactating cows

- Amount and concentration of Mg in the extracellular fluid is dependent on GI absorption and subsequent urine excretion

List clinical signs of hypomagnesaemia

- Characterised by tetany and convulsions

- Occurs most commonly in animals grazing fresh, lush, rapidly growing pastures, often heavily fertilised with nitrogen and/or potassium

- Cow is found dead, often after inclement and/or stormy weather, with disturbed soil around its limbs, indicating paddling/seizure activity

- Cows become irritable, aggressive, and show signs of muscle twitching

Discuss subclinical hypomagnesaemia

- Associated with an increased incidence of milk fever (hypocalcaemia), lowered milk production, weight loss, and diarrhoea

- No apparent clinical signs until acute disease is precipitated by adverse factors

Discuss acute cases of grass tetany

- Initially hyperaesthesia with ear twitching, teeth grinding, salivating, frothing at the mouth, muscle tremors and aggression

- Progress to incoordination with a "staggering" gait, tetanic muscle spasms and recumbency

- HR and RR are markedly increased, with the absolute intensity of the heart sounds being greatly increased

- Recumbent cattle are often convulsing or have periodic seizures

- During convulsive episodes paddling movements of the limbs, opisthotonos, nystagmus, chomping of the jaws, frothing at the mouth, pricking of the ears, retraction of the eyelids and exophthalmos

Discuss subclinical/chronic cases of grass tetany

- Within 6 weeks of calving, associated with a long-term Mg deficiency

- Thin, rough-coated with reduced DM intake and poor milk yield

- Nervous and reluctant to be herded or milked

- Udder oedema may be present

- Pallor of mucous membranes due to anaemia

Discuss diagnosis of hypomagnesaemia

- Clinical signs and history

- Confirm with serum or urine sample

- Use Grass Tetany Index (GTI)

- Sample vitreous or aqueous humour post-mortem

Discuss treatment of hypomagnesaemia

- Slow IV Ca + Mg solution

- Subcutaneous Mg sulphate, reduce the risk of relapse

- Consider light sedation

- Cow turned into sternal recumbency and left undisturbed

- Follow up with 100 g MgO daily for the first 1-2 days, then 50 g for another 5+ days

Discuss prevention of hypomagnesaemia

- Total diet should contain 2.5 g/kg DM of magnesium

- Start Mg supplementation at least 2-3wks prior to calving

- Consider individual drenching, incorporating MgO into concentrate mix, dusting of pastures, treating hay with MgO (50 g/cow/day), soluble Mg salts in drinking water, intra-ruminal Mg bullets or lick blocks

Compare and contrast hypomagnesaemia and hypocalcaemia

- Convulsions + tetany (Mg) vs dull + depressed (Ca)

- Beef (Mg) vs dairy (Ca)

- True emergency (Mg)

- Herd outbreak (Mg) vs sporadic cases (Ca)

- No hormonal control (Mg)

- Rapid death (Mg)

- High heart rate loud (Mg) vs soft (Ca)

- No body reserves (Mg)

Discuss milk tetany

- Uncommon

- Condition in 2-4 month-old veal calves fed high-milk diets with little or no other feed

- Hypersensitivity with neurological signs

Discuss transit tetany/recumbency

- During or immediately after prolonged transportation and stress

- Well-fed cows during late pregnancy

- Present with recumbency and gastro-intestinal stasis, may proceed coma and death

- Risk factors include heavy body condition, advanced pregnancy, heavy feeding prior to transport, deprivation of feed and/or water for more than 24 hrs during transport, unrestricted access to water and exercise immediately after arrival

Discuss treatment and management of transit tetany

- May respond to intravenous infusions with calcium, magnesium, phosphorus and dextrose

- Several days prior to transport feed moderately on a restricted diet containing adequate Ca and Mg

- Longer journeys provide adequate rest periods during which cattle are being fed and watered

Discuss phosphorus deficencies

- Depraved appetite leads to bone chewing (increased botulism risk)

- Herbage P is lowest in the dry season when feed is poor

- Supplement in wet season when demand highest with high levels of energy and protein for fast growth and high production

List disorders of hypophosphataemia

- Periparturient hypophosphataemia (PPH)

- Osteomalacia and deprived appetite (adults)

- Rickets (calves)

- Poor milk yield, ill-thrift and infertility

- 'Physiological' hypophosphataemia (in cows at peak lactation)

Discuss periparturient hypophosphataemia

- Postparturient haemoglobinuria or a complication of hypocalcaemia

- Concurrent low serum [P] is common in cows with milk fever and recovers rapidly following treatment with Ca

Discuss osteomalacia

- Usually occurs in lactating beef cows on marginal, P-deficient soils

- Excess Ca, iron or aluminium exacerbates any P deficiency

- Presents with weight loss, osteoporosis and depraved appetite (pica), animals are ill-thriven, may be slab-chested, have gait abnormalities and suffer pathological fractures

- Pica (eating bones or licking rocks/dirt) is characteristic

- Supplement accordingly

Discuss rickets

- Younger animals in which the epiphyses have not yet fused

- Due to primary P deficiency, inadequate vitamin D or certain feeds that are P deficient

- Causes widening of the metaphyses, particularly of the metacarpal and metatarsal bones, causing moderate lameness

Discuss the downer cow

- Present as peri-parturient cow in sternal recumbency, unable to rise

- Primary concern is direct complications of prolonged recumbency, especially local tissue injury from compression of the limbs (compartment syndrome) and musculoskeletal damage due to struggling to rise

- More than 3-6 hours results in ischaemic necrosis and muscle/nerve damage in the underlying hindlimb, may be irreversible after 12 hours

- Perform clinical (especially udder, vaginal and rectal exam) and musculoskeletal exam

- Depressed or hyperaesthetic cows have a poor prognosis

Discuss management of downer cows

- Treat adequately for hypocalcaemia, hypomagnesaemia and hypophosphataemia

- Move onto a surface that provides good and secure footing

- Ensure dry, clean comfortable lying area (either indoors or outside)

- Turn the animal over every 3-6 hours to prevent pressure damage, may be lifted several times daily for as long as practical

- Providing ad lib good-quality food and fresh water

- Stripping of all four quarters