Periodically since its discovery in 1979, an ailment called congenital hypothyroidism dysmaturity syndrome (CHDS) has caused newborn foal deaths in Western Canada and the northwestern United States. Since then, around 75% of foals born in affected herds have died, and the condition’s cause remains unclear.

Now researchers in Canada are making strides in understanding what could trigger CHDS. Mariana Diel de Amorim, DVM, DVSc, Dipl. ACT, and colleagues recently became the first to identify primary hypothyroidism in a herd of mares with a history of producing CHDS foals. She presented their findings at the 2015 American Association of Equine Practitioners convention, held Dec. 5-9 in Las Vegas.

Diel de Amorim is an associate professor of theriogenology at the University of Prince Edward Island’s Atlantic Veterinary College, in Charlottetown. She worked on the study with senior investigator Claire Card, DVM, PhD, Dipl. ACT, at the University of Saskatchewan’s Western College of Veterinary Medicine, in Saskatoon.

Foals with CHDS are born to mares that appear healthy, but after a prolonged gestation (often more than a year) and sometimes dystocia (a difficult birth). Affected foals can have umbilical hernias, contracted front legs and other flexural deformities, cubital bone dysgenesis (inappropriate bone formation), mandibular prognathism (their lower jaw sticks out beyond their upper), and poor muscling. Diel de Amorim said researchers believe contributing factors include a mare’s excessive consumption of nitrates, endophytes, and goitrogens (goiter-causing substances), and deficiency in trace minerals, such as selenium.

Wondering if an overarching diagnosable condition in dams could be causing CHDS, Diel de Amorim took a closer look at primary hypothyroidism—a rare condition in horses in which the thyroid gland fails to respond to thyrotropin-releasing hormone (TRH). “Reported clinical signs of hypothyroidism include lethargy, exercise intolerance, weight gain, and a poor hair coat,” she said. “There has been speculation regarding mare’s fertility but no large study to substantiate that.”

Veterinarians diagnose hypothyroidism using a TRH stimulation test. In their study, Diel de Amorim and colleagues aimed to investigate thyroid levels in a herd of 16 mares with a history of having CHDS foals and identify the risk factors.

Key study findings included:

  • The majority, if not all, of the mares and foals were selenium-deficient (this was not a surprise, as most horses in Western Canada are fed hay only and are not supplemented with any extra feed or minerals, Diel de Amorim said);
  • The mean gestation length for mares with CHDS foals was 332 days, while mares with healthy foals was 325 days;
  • Six of the 16 foals had clinical signs of CHDS, three of which died or were euthanized;
  • Mares’ baseline thyroid levels were excessively low or undetectable in the majority of the animals; and
  • Four of seven mares that underwent a second TRH stimulation test and blood testing were considered positive for primary hypothyroidism, a phenomenon that has never been reported before.

Diel de Amorim noted that some hypothyroid mares did deliver apparently healthy foals. “However, the majority of those foals did not respond to the TRH stimulation test either,” she added.

She said mares’ age, the presence of toxins in the system, trace mineral deficiencies, and other factors likely influence their thyroid function. Therefore, further investigation is necessary.

“We don’t really know exactly how and what levels of hypothyroidism may affect mare’s fertility at this point,” she explained. “And it does seem that CHDS has several risk factors. I believe that breeders should investigate if the mare is really hypothyroid by performing a TRH stimulation test, if possible, and if this (comes back positive) they need to investigate the cause for the hypothyroid.

“Good management in terms of appropriate feeding practices and additional mineral supplementation in pregnant mares and/or animals that are in areas known to be deficient in trace minerals is crucial,” she added. “If mares are out in pasture, investigating if the pasture contains common goitrogenic plants like mustard plants should be important, as well.”