physiology of hypocalcemia Flashcards
what is hypocalcemia
- AKA: milk fever, post parturient hypocalcemia, parturient paresis
- metabolic disorder due to insufficient calcium in the blood
- calcium supply is controlled by the parathyroid gland: mobilising calcium from the bone when blood calcium is low and mineralising or excreting it where supply is in excess
how is blood calcium maintained
- lowered blood calcium stimulates parathyroid hormone secretion from parathyroid gland
- PTH stimulates osteoclasts to release calcium from bone (slow)
- PTH increases calcium resorption from urine in kidney. stimulates calcitriol formation (fast)
- calcitriol promotes calcium absorption from small intestine (moderate)
takes 2-3 days to fully regulate drastic changes in calcium
discuss calcium demand in dairy cattle
- due to high milk yield, Ca demand is elevated
- older cows respond more slowly to drastic changes
- incidence is higher in Jersey and Guernsey breeds
- can also affect older beef cows (4th + calvers), especially dairy crosses, but more common in autumn calvers
- phosphorus in excess of calcium –> inhibits absorption of calcium (ideally 2:1 Ca:P)
- high dietary cations (Na, K) or low Mg can inhibit calcium mobilisation
what is the role of calcium in the body
- used for muscle contraction (actin and myosin filaments)
- decreases neurone Na permeability
- mediates ACh release
what are the general effects of hypoCa (not clinical signs)
- increased neuromuscular irritibility
- decreased smooth muscle contraction
- decreased skeletal muscle contraction
- reduced cardiac muscle contractility
explain the link between metabolic alkalosis and hypocalcemia
- metabolic alkalosis predisposes the cow to milk fever
- metabolic alkalosis inhibits the body’s response to PTH by altering the conformation of the PTH receptor in the target tissues (so they are less sensitive to PTH)
- the lowered PTH response in the kidneys reduces renal Ca reabsorption (but critically also inhibits calcitriol which is whi dietary absorption of calcium is no upregulated)
- inhibited PTH response also effects calcium loss
describe the incidence of hypocalcemia in dairy cattle
- average annual incidence in UK dairy herds is 7-8% (most at subclinical level)
- individual farms may have a much higher prevalence especially when calving at pasture
- can occur 6 weeks before to 10 weeks after calving (most 24 hours before to 48 hours after)
- it can also be triggered by other stressful situations such as handling, transporting or housing
- in very high yielding cows, during oestrus or several weeks/months into calving
discuss hypocalcemia in sheep
- usually seen in ewes in late pregnancy but can also occur in lactation (bone growth)
- can occur when ewes are brought down off hill grazing onto improved pastures prior to lambing
- can occur in stressful situations (handling, transport, housing or winter shearing)
what are some uses for calcium in the body
- muscle contraction
- immune function
- nerve impulse transmission
- bones and teeth
how do the effects of hypocalcemia reduce muscle contraction
- increased neuromuscular irritability
- decreased smooth muscle contraction
- decreased skeletal muscle contraction –> effects posture and gait
- reduced cardiac muscle contractility
discuss clinical signs of hypocalcemia
- progress over 12-24 hours
- early stages include teeth grinding, muscle tremors, stiff legs, straight hocks, paddling of the feet when standing
- often missed in the early stage
- progresses to muscle weakness, cow lies down with a characteristic kink (S bend) in the neck, later the head is held against the chest, gut stasis causes bloat and constipation
- becomes comatose and lies on her side, ruminal bloat and/or paralysis of resp muscles causes death over a few days if untreated
- uterine inertia (leading to calving problems and/or still birth) prolapse of the uterus, inhalation of rumen contents when cast causing pneumonia and pressure damage to nerves and muscles
list some secondary promblems of milk fever
- metritis
- cull within 1 month
- dystocia
- retained afterbirth
- LDA
- ketosis
- mastitis
in order of least to most likely
how might hypocalcemia increase the risk of clinical mastitis
the teat sphincter consists of smooth muscle which must contract to close the teat end
if low blood calcium reduces teat sphincter contraction the teat canal remains open allowing environmental pathogens to enter the mammary gland (esp if lying down)
give examples of hypocalcemia in other species
