Question
Eight days after foaling, my mare suffered from recumbency, listlessness, intermittent seizures, and the inability to urinate and defecate. She was diagnosed with “milk fever” and administered intravenous calcium. She quickly perked up and seems normal now. Did my mare lack calcium in her diet? I’d like to continue to use her as a broodmare but not if she is susceptible to this again. Your thoughts?
Answer
Hypocalcemia, which is similar to “parturient paresis syndrome” (milk fever) in dairy cows, is not common in horses, but it does occur. It results from of a dangerous drop in circulating calcium, a mineral important for normal transmission of nerve impulses and for muscle contraction.
Left untreated, hypocalcemia sometimes results in death from heart failure. This condition usually occurs in early lactation because of the huge drain milk production places on whole-body calcium reserves. Hypocalcemia hit your mare at the peak of an exponential increase in milk consumption, when the foal was probably drinking 25% of its body weight by day 7, up from only 15% on day 1.
Most of the time hypocalcemia affects mares that are excellent milk producers and that are receiving only grass hay or pasture with little or no clover. Mares that are not eating well and not taking in enough nutrients can also be affected.
The reason for low blood calcium can be from a lack of calcium in the diet, but can also be the result of how the endocrine system responds to the sudden change in metabolic demands for calcium imposed by lactation. Calcium regulation is a complex process that involves at least seven organs or tissues, three hormones, and several enzymes and minerals. There are times when the body needs to be able to mobilize calcium from the bone to meet the high demands; parathyroid hormone (PTH) and vitamin D work together to stimulate this process. In cases of hypocalcemia, the hormonal response to PTH may be too sluggish to meet immediate calcium needs, and the result is low blood calcium.
The veterinarian did an excellent job of treating the mare. The quick recovery can be attributed to the intravenous calcium raising blood levels, thereby improving penetration into the deficient tissues.
This may have been an isolated episode for this mare, particularly if her problem was due to appetite depression. In an effort to prevent this from recurring, (1) make sure there is sufficient calcium in the diet; the daily calcium requirement of the mare more than doubles with the onset of lactation; (2) the addition of a high-calcium forage, like alfalfa, should begin with the onset of lactation; pastures with a mixture of grasses and clover are ideal for lactating mares; and (3) once the mare has foaled, monitor her forage intake to make sure she is eating well; if her appetite wanes, be aware that this could happen again.
