hypomagnesemic tetany

Hypomagnesemic Tetanyin Ruminants

B.GOWRISHANKAR

BVN 05013

Hypomagnesemic tetany Synonyms

Grass tetany Hypomagnesemia Grass staggers Lactation tetany Wheat pasture poisoning Winter tetany Green-oat poisoning Barley poisoning

Definition:

A magnesium deficiency of the blood and cerebrospinal fluid (CSF).

Highly fatal, affecting only ruminant species.

Occurrence

Sporadic with low overall incidence (0.5-1% adult ruminant females/year)

Outbreaks in some herds (up to 20% affected) Usually early lactation animals grazing lush

pastures in spring or fall May occur in milk fed calves or lambs 2-4

months of age

(milk tetany)

Magnesium Homeostasis

Mg stores in the bovine:

– 70%: bones & teeth

– 30%: soft tissues

(intra & extra cellular fluids) Mg body stores are not very labile Milk production removes 0.15 g

Mg/liter milk 20 g/day Mg requirement must be

supplied almost entirely by diet

Etiology

• Magnesium deficiency develops when Mg output exceeds input:

1) High Mg output (early lactation)

2) Low Mg levels in diet (<0.2% of DM)

3) Inadequate Mg absorption from diet (↓availability)

1. Animal Risk Factors for High Mg Output

• Usually first 0-8 weeks of lactation

• High producers

• Older animals

• Ewes suckling multiple lambs

• Body condition:

• Very thin or over conditioned cattle

2. Factors affecting Concentration of Mg in Diet

• Cool season grasses (e.g. wheat) contain less Mg than warm season grasses.

• Cool weather (spring/fall) reduces plant tissue uptake of Mg

• High moisture content of rapidly growing pasture means low Mg concentration

Cont..

Examples of crops with low Mg concentration

– Early growth cereal crops (barley, wheat, green-oat poisoning)

– Corn stalks/stubble (winter tetany)

3. Factors affecting Mg absorption across Rumen Wall

• High dietary Potassium interferes with sodium-linked active transport of Mg across rumen wall

• Lush, high moisture pasture => rapid rate of passage => less opportunity to absorb Mg

• Rumen pH: > 6.5 decreases Mg solubility (high forage diets) < 6.5 increases solubility (higher energy/grain diets)

Cont…Age: Older animals can only absorb Mg in

rumen and large intestine. Calves < 2 mos. old can also absorb in small

intestine.Very little Mg in milkMilk fed calf < 2 mos. old can still absorb

more available MgMilk fed calf 2-4 mos. old less able to absorb

=> becomes Mg defic.

Environmental/Management Risk Factors

Often animals already on low-Mg dietClinical hypomagnesemic tetany

precipitatedby environmental or management stress:

– lactation

– sudden weather changes

– transport

– feed deprivation, sudden feed changes

When Mg drain from lactation exceeds dietary

intake/absorption:

– Mg is depleted from CSF and extracellular fluids

– Loss of neuromuscular function results

Effects of Low CSF & plasma Mgconcentrations :

Lower central & peripheral nervous membrane potentials closer to threshold: => hyper excitable /convulsions

Helps modulate Ca effect on acetylcholine release at myoneuronal junction: => Excess release of acetylcholine at myoneuronal junction in response to depolarization

Cont..

• Mg-dependent ATPase essential for relaxation phase of muscle contractions: => Sustained myofibril contractions /tetany

• Reduces PTH secretion, affecting Ca homeostasis: =>Hypomagnesemia usually accompanied by hypocalcemia

Early Clinical Signs Rapid progression:– Reduced feed intake, nervousness, ↓ milk

production– Twitching of muscles in face, shoulder, flank– Restless, irritable, +/- aggressive– Isolation from herd– Spastic, stiff, high-stepping gait– Frequent urination, bellowing– Hypersensitive to stimuli: sound (car horn)

can initiate tetany

Advanced Clinical Signs• Tetanic muscle spasms, staggering/falling

• Lateral recumbency:

• Clonic convulsions: head back, frothy salivation, jaws champing, legs paddling

• Tachycardia (<150/min) with very loud heart sounds

• Tachypnea, Pyrexia (excessive muscle activity)

Cont…• Marked nystagmus, fluttering eyelids, +/-

prolapse third eyelid

• May get up after several minutes; repeat convulsive episodes several times

• Death from respiratory failure during a seizure (30-60 min. from onset)

Tetany in sheep

Frothy salivation

Aggressiveness

Clinical Pathology

• Hypomagnesemia

– Plasma Mg < 1.2 mg/dl– CSF Mg < 1.4 mg/dl– Urine Mg (< 2.5 mg/dl)

Clinical Pathology (con’t)

• Hypocalcemia: very common

• Hypophosphatemia: common

• Hyperkalemia

• ↑ AST, CPK (muscle damage)

Diagnosis

• Often diagnosed after 1 or 2 animals “found dead”• History: – sudden death of early lactation animal on lush growing pasture in spring/fall – Soil disturbed due to convulsive episodes – 2-4 mos. old calf/lamb fed whole milk• Clinical signs• Response to Mg & Ca treatment• Low urine or blood Mg levels in 5-10 herdmates

Necropsy• Diagnostic: – Submit serum, CSF, urine & enucleated eye (vitreous humor) for Mg levels – Bone Ca:Mg ration > 100:1 in calves suffering Mg deficiency• Additional: – Agonal pulmonary emphysema, diffuse echymotic hemorrhages – Aspiration of rumen contents – Trauma/bruising of head & extremities – +/- Calcium mineral deposition in heart, spleen, arteries of lungs

Differential Diagnoses• Rabies: incoordination, behavior change, but

no tetany

• Nervous ketosis: No convulsions/tetany. Urine ketones

• Milk fever: No increase pulse amplitude. Flaccid paralysis

• Lead poisoning: Hx lead ingestion, blind

• Polioencephalomalacia: response to tx

Cont…

• Enterotoxemia (Clostridial toxins): may find toxin in GIT

• Salt toxicity/water deprivation: history

• Tetanus

• Nervous coccidiosis

• Listeriosis

Treatment

• Immediate tx with Mg & Ca

• SAFETY FIRST: May require IM tranquilizer (acepromazine)

• IV treatment

• Cattle: slow IV 500ml CBG with 5% Mg-hypophosphate

• Sheep: slow IV 50-100ml

Cont…SQ treatment:• Cattle: 200-300ml 20% CBG & Mg-sulfate solution

• Sheep: 50-100ml

• Slow response. Don’t disturb for 3-5 hours.

• Other: Mg sulfate enema (20g in warm water)

• Avoid relapse: oral Mg salt slurry (60g/day x 5-7days)

• Don’t give orally until regains swallow reflex

SQ treatment

Options for Prevention• If occurs in ewe/cow, should supplement flock/herd

immediately• Options for supplementation:1. Individual oral drenching

2. Remove from pasture &/or add supplemental Mg to grain

3. Supplement pasture with dry hay

4. Mg in free choice mineral

5. Mix Mg with molasses & spray on hay

6. Intraruminal Mg boluses (90 day release)

Options for Prevention (con’t)

• Options to manipulate pasture Mg content:

1. Top dress with Mg-rich fertilizer

2. Spray/dust pasture q 2 weeks with 20-30kg calcined magnesite/acre

3. Don’t put animals on fast growing pasture until reaches mature height of 10”

4. Select grasses with known higher Mg uptake

Cont…

5. Avoid/reduce use of Potassium-rich fertilizers

6. Feed calves good hay & starter ration

• Reduce stressors:– Adequate dietary energy– Shelter– Avoid stress, starvation, etc.

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