Late Summer Weight Loss

Weight loss in late summer is a fairly common problem. It can have many different causes and pinpointing the reason is obviously central to successful resolution.

from: University of Minnesota

Cumulative effects of dehydration is a common cause. With mild dehydration the horse’s body will pull water from the tissues into the blood to restore normal concentrations of sodium and other solutes.  Since the drive to drink from the brain ultimately originates in higher osmotic pressures in the blood, the brain thinks that all is well and the end result is tissue dehydration and weight loss.

Inadequate sodium intake is the most common cause of dehydration.  If you don’t know for sure how much salt your horse is taking in, and that the amount is adequate for baseline needs + sweat losses, there’s a good chance this is the problem.  As a start, provide 2 oz of salt plus an additional oz of salt or a dose of balanced electrolyte providing at least 10 grams of sodium for each hour of formal work.

With the trend today of reduced deworming frequency, parasites can sneak up on your horse between the common routine interval dewormings in the spring and fall.  Activity of all types of parasites is increased in the warm weather and you have the added threat of stomach bots in fly season. Waiting until the first solid freeze to deworm for bots ensures there will be no further transmission but by then larvae picked up early in the season have abraded the stomach and grown quite large (each up to 3/4 inch long).

from: University of Florida

Does anyone really believe a stomach full of bots causes no problems?

Abdominal distention or coat changes are additional clues but won’t necessarily be present. Bot infestations are not detectable on fecal exams and tapeworms rarely are. Fecals are best for strongyles but manure must be freshly passed and kept cool until examined or the eggs will hatch. Mailed in fecals are a waste of time and money. When you deworm, be sure not to underdose. Underdosing is the major cause of resistance to deworming drugs.

Another issue in late summer is declining pasture quality. Even if still green, mature grasses drop in caloric value and protein. More succulent and higher protein clovers often do not tolerate high heat well. Weight loss can begin long before pastures appear “dead”. Supplement with a high quality grass hay or  grass with up to 10-15% alfalfa. A sure sign your horses needs supplemental hay is when they begin eating it.  Most horses will choose good grass over good hay any time.

Horses in their teens that begin losing weight late in summer/early fall with no apparent reason may be early cases of PPID – pituitary pars intermedia dysfunction, aka Cushing’s disease.  There is a natural rise in ACTH and other hormones from this area of the pituitary which occurs at this time.  In normal horses it is small and of no consequence but early PPID cases have an exaggerated rise.  Weight and muscle loss is one of the consequences. Blood tests are needed for diagnosis.

Weight loss can also be a nonspecific sign of many significant disorders.  If the horse does not respond to addressing the more common issues, always involve your veterinarian.

Eleanor Kellon, VMD


About Dr. Kellon

Graduate of University of Pennsylvania Veterinary School. Owner of Equine Nutritional Solutions,, industry and private nutritional consultations, online nutritional courses. Staff Veterinary Expert at Uckele Health and Nutrition.
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