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