Dipping into mare’s milk

Iodine is essential for producing thyroid hormone and the normal function of the thyroid gland — a key factor in metabolism and the normal development of growing animals. A fetus completely relies on iodine from its mother during gestation. For horses living on the Prairies, there’s very little information about iodine levels in their serum (blood) or milk, or about their levels of selenium — a trace mineral that contributes to thyroid hormone production.

Members of Card’s team include theriogenology resident and graduate student Dr. Maria Lopez, epidemiologist Dr. Tasha Epp and summer research student Mikayla Swirski, along with Dr. Bernard Laarveld, an expert in effects of plant glucosinolates, and Dr. Nadia Cymbaluk — an equine nutrition expert.

In addition to the milk samples, Card and her research team analyzed mares’ blood samples to determine iodine and thyroid hormone levels. They also used hair from the horses’ manes to assess the horses’ selenium levels. The researchers were shocked by the results of their 2018 preliminary analyses:   

• 27 of the 37 horses (73 per cent) and 17 of the 45 foals (38 per cent) tested from 16 different farms had marginal or low serum iodine levels
• Eight of the 37 mares’ milk samples (22 per cent) had inadequate amounts of iodine, in spite of all farms providing their horses with access to iodized salt blocks
• Using an analysis of mane hair for selenium levels, 22 of the 25 mares (88 per cent) tested were deficient in selenium

“People spend a lot of money on feed. Horses, especially pregnant mares, aren’t getting what they need,” says Card. “Horse owners should consider getting a serum iodine, mineral panel, and mane hair selenium concentration [tests] done on their horses, because these deficiencies are so common in Western Canada.” 

Iodine’s potential link to foal disease

The researchers hope this study will bring them closer to understanding and preventing congenital hypothyroidism dysmaturity syndrome (CHDS), a devastating disease found in Western Canada that results in the death of newborn foals. CHDS was also reported in the state of Washington where mares grazing in pastures with wild mustard plants were reported to deliver CHDS foals.

Previous research, conducted by WCVM pathologist Dr. Andy Allen, indicated that foals born with CHDS suffer from primary hypothyroidism, a condition where the thyroid gland is unable to make sufficient amounts of thyroid hormone. Allen reported that a lack of mineral supplementation and nitrate exposure are risk factors for CHDS. Nitrate is also known to suppress iodine uptake.

Card and her research team recently reported that the dams of CHDS foals also have impaired thyroid function. Although a mare carrying a CHDS foal will often appear healthy, her foal is born with serious issues including hypothermia, an overshot lower jaw, contracted tendons, severe bone defects, and sometimes an enlarged thyroid (goiter).

What’s key to preventing CHDS?

Card wants to understand how to prevent the disease, but the researchers still need more information: “We need to know more about the levels of important minerals that affect thyroid function, such as iodine and selenium, along with and what role ingestion of toxic plants and weeds had in CHDS.”

“Even if an adult horse has impaired thyroid function [hypothyroidism], it is difficult to diagnose as clinical signs are not very evident,” explains Lopez. “We believe that CHDS in foals may be caused by low selenium and iodine levels in the mothers during pregnancy.”

Data from the 2018 study shows that some mares simply don’t get enough iodine in their diet, even when they had access to salt blocks during pregnancy, which results in iodine deficiency in their developing foals. Dietary selenium deficiency would also contribute to CHDS because selenium is needed for the thyroid to capture iodine to make thyroid hormone.

Lopez had shown that feeding mares certain plant compounds called glucosinolates (GSL) also interferes with their iodine uptake and utilization. GSL compounds are found in plants from the Brassicaceae family, which includes oil seeds crops such as canola, mustard and rapeseed. These oil seed plants may spread to adjacent pastures or oat fields. Common weeds, such as members of the wild mustard family that contain GSL, are commonly found in horses’ hay as well as in their pastures across Western Canada and in Washington.

GSL metabolites have been shown to prevent the thyroid gland as well as the mammary gland tissue from taking up iodine, which is why milk samples are so significant to this study. Since the mammary gland tissues have similar iodine transporters required for iodine uptake, measuring serum iodine, thyroid hormone, and milk iodine provides a more accurate assessment of the mare’s iodine status and a clearer picture of her overall thyroid health.

Researchers speculate that the combination of factors such as GSL in horses’ feed, nitrates in the feed and water, and low concentrations of iodine and selenium in western Canadian soil leading to deficiencies may cause primary hypothyroidism in horses — a condition that’s especially significant for pregnant mares since it may result in CHDS foals.

While there are no clear signs indicating primary hypothyroidism in mares, some subtle indications include a lack of energy in older broodmares, difficult births, lengthier gestation periods, and giving birth to CHDS foals. Close to her foaling date, an affected mare’s udder may also bag up (fill with milk) but then become smaller with decreased milk production.

“We still don’t know enough about milk iodine levels in mares or the relationship with serum/milk iodine and thyroid hormones in post-foaling mares, and in order to know what is abnormal in CHDS mares and foals, we must first know what is normal,” explains Lopez. “This is where we’re hoping to make a difference by figuring out these relationships.”

Once researchers have determined the current levels of iodine in western Canadian horses, they hope to use this information to better understand the relationship between iodine status in mares with healthy pregnancies and in those mares with CHDS-affected foals.

“We will be able to make evidence-based recommendations about mineral supplementation for pregnant mares,” says Card.

Although there’s still a lack of information available on CHDS, Lopez is hopeful that something as simple as blood, hair and milk samples will contribute to disease prevention. She’s asking veterinarians and horse owners to send her milk and mare/foal serum samples, as well as mane hair samples from CHDS- affected mares and foals and non-affected herdmates.

“If we can start by answering the small questions, we can start to develop a plan to prevent this syndrome and hopefully reduce the number of CHDS foals which occur each year.”

For more information about submitting study samples, contact Dr. Claire Card (claire.card@usask.ca) or Dr. Maria Lopez (maria.lopez@usask.ca). Mikayla Swirski of Langley, B.C., is a second-year veterinary student. Her story is part of a series of articles written by WCVM summer research students.

This article originally appeared in the Spring 2019 Edition of Horse Health Lines, the official publication of the Townsend Equine Health Research Fund at the WCVM.