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.

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.”

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 MainguySee­rs 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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