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Calcium Metabolism and associated Disorder in Cattle
 

Calcium is vital element of the body accounting for around 1.5% body weight and involved in vital function of body including skeletal muscle contraction, heart contraction, neuro transmission etc.

In general, mammals including cattle have total plasma concentration around 10 mg / dl. About 50% of calcium is bound to serum albumin and rest is in ionised form. All the mentioned vital functions are very sensitive to change in the later.

Calcium Homeostasis

Calcium Homeostasis is mentioned by 3 factors - Paratharmone, Calcitonin and the vitamin D (1.25 dihydroxy Vit D3, otherwise called as Calcitriol).

The main action of Calcitriol is to promote intestinal absorption of calcium. It stimulates intestinal epithelial cells to produce calcium binding proteins. In addition, Calcitriol activates bone osteoclasts to cause bone lysis for release of Calcium.

Paratharmone responds to low levels of plasma ionised Calcium and acts on osteoblasts and osteocytes to stimulate pumping out of calcium which restores plasma calcium levels.

Calcitonin protects against hypercalcemia. It is released from thyroid gland, promotes renal calcium excretion and inhibits bone mobilisation of calcium.

Milk fever

Hypocalcemia means reduction in blood calcium. In dairy cows, the major hypocalcemic disease is associated with parturition and early lactation - referred to as Parturient Paresis or Milk fever.

Milk fever may occur from about 24-72 hours after parturition. It is characterised by initial signs like restlessness, excitability and anorexia, but later, animal becomes down and unable to rise followed by coma and death. Death can ensue within 12 hours of onset of clinical signs.

Pathophysiology of Milk Fever

n dairy cows, parturition is accompanied by sudden increase in calcium sequestration for the production of milk. The calcium requirements rises to 2-5 times that of dry period. The daily calcium out flow through milk will not be matched with calcium availability. This places major strain on homeostatic mechanism. As plasma calcium falls, there is compensatory increase in paratharmone and calcitriol, but it takes time for both to exert their full effect i.e., bone calcium mobilisation by paratharmone takes at least a week and improved efficiency of calcium absorption by influence of calcitriol takes a day or two. So, nearly all animals at parturition develop hypocalcemia, but high yielders tend to develop milk fever. Milk fever is also common in older cows which have less efficient gut absorption and slower bone metabolism.

Dietary condition, during dry period play a major role in predisposing for milk fever. Considerable evidences documented have clearly shown that high calcium and phosphorous intake during dry period significantly increases the incidence of milk fever. High dietary phosphorous is also found to inhibit the production of calcitriol. Where as low dietary calcium and phosphorous levels during dry period are regarded as protective to greater extent. In other words, incidence of milk fever is lowest when the diets contain less calcium and phosphorous than required to meet the dry cows daily needs (approximately 25 g.).

Low level Calcium diet prior to parturition lower serum calcium level and this initial drop in serum calcium stimulates paratharmone release and bone calcium metabolism. Hence, the calcium mobilizing system is geared up for the demands of lactation immediately after parturition.

Other dietary factors may also have some influence on milk fever. It has been suggested that high propositions of alkaline components in the diet (Na, Ca, K, Mg) in relation to acidic components (Cl, K, Ca, S) was conducive to milk fever. In other words, the incidence of milk fever increases as the sum of cations minus anions increases.

Complication of Milk Fever

Cows which are prone to milk fever are also at high risk for dystocia, retention of fetal membranes, metritis and mastitis resulting in reproductive failure and economic losses. Even a successful treatment of milk fever cannot eliminate the risk of its complications. So prevention is always better, not only to prevent milk fever but, also to reduce the incidence of post partum metabolic and productive complications.

Prevention of Milk Fever

Preventive measures are designed to counteract the causative factors:

1. Under feeding the calcium to the dry cows prior to calving greatly prevents the milk fever. As mentioned earlier, an initial hypocalcemia due to such dietary change will be corrected rapidly within 1-2 days and prepares the cow being able to meet entire lactational calcium demand from bone sources of calcium without developing milk fever.

2. Use of Vitamin D3 prior to calving has shown beneficial effects in preventing milk fever beyond doubt. Vitamin D3 helps in mobilisation of bone calcium, also regulates calcium and phosphorous absorption from the gut, therapy augments the natural homeostatic system to maintain plasma calcium.

References:

Major source: Naylor & Ralson, "Large Animal Clinical Nutrition", Mosby Year Book 1991

Source : VETCARE