Certain Diseases Affect Miniature Horses
Lais R.R.Costa, MV, MS
Clinical Instructor of Equine Medicine
Diplomate, American College of Veterinary Internal Medicine

Miniature Horses are graceful, intelligent and spirited horses that come in a very small, but well-proportionate size.  The Miniature Horse breed is predisposed to certain diseases, such as dental problems, colic due to feed impaction, and complicated foaling.  This does not suggest that any or even most Miniature Horses will have these conditions; however, owners should be able to identify these disorders early to prevent problems.

 

Several reproductive problems occur in Miniature Horse mares.  Miniature mares display behavioral anestrus (absence of signs of “heat”) during lactation more commonly than large breed mares.  Weaning the foals at about four months of age may be necessary.  The mares usually show signs of “heat” (estrus) four to six weeks after weaning the foal.  Prolonged “heat” lasting eight to 14 days (normal five to seven days) is another reproductive abnormality that occurs more commonly in Miniature mares than in large breeds.  Miniature mares show more defined seasonal estrus cycles than large breed mares.  Therefore, the seasonal anestrus lasts longer and the Miniature mares will rarely cycle during the months of late fall, winter and early spring.  Most commonly, Miniature mares start showing “heat” in April.

 

The gestational length of Miniature mares ranges from 320 to 340 days, which is shorter than that of large breeds (normally 320 to 365 days). Health maintenance care for the pregnant Miniature mare is the same as that for large breeds, including nutrition, vaccination and deworming programs.  Because of their small size, Miniature mares, especially if it is the first gestation, may have birthing difficulties requiring veterinary assistance.  Special attention should be given to passive transfer of immunity from the mare to the foal in the colostrum (antibody-rich milk).  Mares may drip some colostrum prior to parturition, and although it may appear to be a small amount of colostrum lost, that amount may be sufficient to cause the foal to be deprived of maternal protection and to become susceptible to infection (neonatal septicemia).  Miniature mares sometimes do not “bag up” before foaling, making it difficult to predict parturition.  When the lactation is delayed, it may be too late to provide the maternal protection necessary, so the foal is at risk to develop neonatal infections.  Another fairly common problem in Miniature mares is “inverted teats” (when the teats are short and the udder swells, the teats become “buried”).  The foal is unable to nurse despite vigorous attempts.  The foal may appear to be nursing, when in fact the foal cannot latch on to the teat. The foal may eventually get tired and weak because of the lack of colostrum and the lack of nutrition.  Assuring good passive transfer to the newborn foal is very important.  Your veterinarian can check the foal for adequate passive transfer with a blood test, which should be performed within 24 hours of age.  If there is inadequate passive transfer, additional treatment is necessary to provide passive immunity to the newborn.

 

The Miniature Horse foals are predisposed to some limb deformities that may result from laxity in ligaments and tendons or bone immaturity.  Bone immaturity, which affects the knees and hocks, should be identified as soon as possible by radiographs.  When the bones are not completely mineralized, they are not strong enough to bear the foal’s weight and can get damaged and become permanently deformed.  If the bones are mineralized, the best treatment for the soft tissue laxity is exercise to strengthen the ligaments and tendons.  Other flexural abnormalities involving ligaments and tendons can occur.

 

Miniature Horses of any age may “lock” their stifle joints and walk “stiffly on the hindlimbs.”  This condition may be a result of lateral luxation of the patella and is more common in Miniature Horses than in the large breeds of horses.  Miniature Horses appear to have a congenital predisposition to develop lateral luxation of the patella. 

 

Colic is a very generic term to indicate “bellyache” (abdominal discomfort).  It is usually associated with problems in the gastrointestinal tract. Numerous diseases cause colic in horses; however, Miniature Horses are predisposed to intestinal feed impactions, fecaliths (rock-like fecal balls) and enteroliths (mineralized intestinal “rocks”).  It is believed that this predisposition is associated with the small diameter of their intestines (in relation to the size of the feed particles), the tendency to have drier stool, and difficulty in maintaining good dental care.  Feeding a diet that promotes soft, moist stools (“laxative diet”) such as fresh green grass as the main source of fiber in their diet is beneficial.  Miniature Horses can be fed hay; however, the type of roughage needs to be balanced.  Other options for softening stool include bran mash with or without mineral oil.  Ask your veterinarian for the best recommendation. 

 

The issue of colic highlights the importance of dental care.  Horses’ teeth continue to grow and must wear off at a similar rate during mastication.  Tooth wear is not completely even, and the cheek teeth need to be periodically checked and filed (floating of the teeth) to correct the uneven wear and points.  Miniature Horses commonly have malocclusion (when the upper arcade does not match well with the bottom one, so they have either “parrot mouth“ or “monkey mouth”) which can be associated with abnormal wear and interfere with mastication. Moreover, the close association of tooth roots with the paranasal sinuses predispose Miniature Horses to diseases of the paranasal sinuses such as sinusitis and sinus cysts.  Miniature Horses should have their teeth examined by a veterinarian and properly “floated” twice a year.  Because of their small mouth, the regular sized floats are too large and special floats are required.

 

Miniature Horses, ponies and donkeys are predisposed to serious metabolic imbalances as a result of stress and anorexia.  Equine hyperlipemia is a condition that develops as a result of negative energy balance and leads to mobilization of fat from the fat reserves.  This causes the accumulation of excess fat in the blood, which overwhelms the liver and can result in liver failure.  The negative energy balance results from lack of nutrition (going off feed) or a higher energy requirement than the intake.  There are several conditions that can lead to negative energy balance, including stress, transportation, concurrent/primary disease that may cause the Miniature Horse to decrease or stop eating, and pregnancy and lactation.  The treatment of equine hyperlipemia often requires intensive care and parenteral nutrition (a special mixture of the nutrients that is given into their bloodstream).  The survival rate of this condition, even with treatment, is only 20 to 60%.  If untreated, the hyperlipemic horses become more and more depressed, uncoordinated and usually die within 10 days.  Early intervention is imperative.  Whenever a Miniature Horse becomes anorexic (off feed) for more than 24 hours, call your veterinarian at once.  Your veterinarian will try to determine why the horse is anorexic, and may provide intravenous fluids containing glucose in order to avoid the fatty liver failure.

 

Owners should be aware that Miniature Horses can suffer from the same conditions as large breeds of horses; however, some conditions are more of a concern in the Miniature Horse breed because of their smaller size.