Milk Fever in Mares
Lactation tetany or hypocalcemia, which is similar to milk fever in dairy cows, is not common in horses, but does occur occasionally.
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, hypocalcaemia sometimes results in death.
What is the cause of milk fever?
This condition usually occurs in early lactation because of the huge drain milk production places on whole-body calcium reserves.
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 legume forage. 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.
Oversupplementation of calcium in late pregnancy can decrease the body's normal ability to mobilize calcium from bone when it is needed. Mares also use much calcium late in gestation to mineralize the foal's skeleton. Both of these factors can lead to hypocalcemia in early lactation.
What are the signs of milk fever?
The severity of clinical signs corresponds with the level of calcium in the blood. Increased excitability may be the only sign in mild cases.
More severely affected horses may appear anxious and show signs of muscle tremors, stiff gait, third eyelid prolapse, inability to chew and salivation, progressing to muscle weakness, recumbency, convulsions, and cardiac arrhythmias.
In lactating mares, if not treated, the disease may take a progressive and sometimes fatal course over 24-48 hours.
How can milk fever be prevented?
It is important to meet, but not exceed, the mare's calcium requirements during the last two to three months of pregnancy. Other electrolyte abnormalities, such as low magnesium, can also affect blood calcium.
In order to prevent hypocalcaemia from occuring or recurring, follow these guidelines:
Make sure there is sufficient calcium in the diet; the daily calcium requirement of the mare more than doubles with the onset of lactation;
Excessive amounts of high-calcium forages or supplements should not be fed in the last trimester of gestation so that the endocrine mechanism for mobilizing calcium will be functioning properly once the mare starts lactating;
The addition of a high-calcium forage, like lucerne, or calcium-supplemented trace-mineralized salt should begin with the onset of lactation; pastures with a mixture of grasses and clover are ideal for lactating mares; and
Once the mare has foaled, monitor her forage intake to make sure she is eating well. If her appetite wanes, seek advice from your veterinarian.
By Kentucky Equine Research - Last updated 16 November 2012
