How estrus affects equine asthma

Researchers find that hormonal fluctuations associated with estrus can influence the severity of equine asthma.

While environmental dust is the most common trigger of severe equine asthma (SEA, also known as heaves), a study from Canada shows that hormonal fluctuations can influence the severity of the disease’s signs. In short, estrus can affect equine asthma.

Researchers at the University of Montreal examined the effects of the estrous cycle on respiratory function in five mares with severe SEA, building on studies in humans showing that reproductive hormones can exacerbate asthma.

“Research on the effects of the reproductive cycle on asthma symptoms in women was published mainly in the 1980s,” says Sophie Mainguy-Seers, DVM. “There is still controversy on this topic, but most studies reported a deterioration of symptoms, with an increase in emergency admission rate just before or during menses.”

Dust is a common trigger of respiratory problems in horses, but hormonal fluctuations associated with estrus can also contribute to asthma.
Dust is a common trigger of respiratory problems in horses, but hormonal fluctuations associated with estrus can also contribute to asthma. (Adobe Stock)

To trigger exacerbation of SEA, the mares were kept in a barn on wood shavings and fed dry hay at the start of the study. Once the mares showed visible respiratory effort at rest, researchers measured their lung function using a device called an impulse oscillometer. The researchers focused on respiratory function during two specific periods of the estrous cycle: the follicular phase, the four- to eight-day period when mares are receptive to stallions, and the luteal phase, the days after ovulation when a mare is not receptive to a stallion.

The collected data showed a slight improvement in lung function during the luteal phase of the estrous cycles. “A small improvement of the airway resistance, which corresponds to the ease with which air can travel in large airways, was noted only during inspiration in these mares with severe asthma,” says Mainguy-Seers. The researchers did not correlate these changes with clinical signs but say they will make that a focus of future studies.

How exactly sex hormones influence respiratory function isn’t clear, but Mainguy-Seers says there are a few likely avenues. “There are sex hormone receptors in the lungs in other species, therefore direct effects of estrogen and progesterone could occur in the airways. However, the localization of such receptors in the lungs has never been studied in the horse. Hormonal receptors are also present in the brain and can influence control of breathing. Sex hormones have numerous properties not related to the reproductive system.”

This study could potentially lead to changes in how mares with SEA are treated. “If our findings are confirmed and extended, perhaps a treatment regimen could be adapted in mares in which the estrous cycle notably influences the respiratory signs,” says Mainguy-Seers.

For now, however, owners can keep potential effects of estrus in mind when managing mares with SEA. “If an owner notices that her mare is worse during a certain period of the cycle, she could discuss this with her veterinarian to determine if management or treatment needs to be adjusted,” says Mainguy-Seers.

Reference: “Lung function variation during estrus cycle of mares affected by severe asthma,” Animals, February 2022.

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