Horses with a higher load of mucus in their airways are more likely to have severe equine asthma. | iStockEquine researchers recently determined that horses with a higher load of mucus in their airways are more likely to have severe equine asthma (SEA). More specifically, mucus quantity scores greater than 2 out of 5, as seen during endoscopy of the lower trachea and its branches, were 3.6 times more likely to be associated with the severe form of the disease. The findings explain the links between mucus and equine asthma, said Julia Drespling, DrMedVet, a PhD candidate at the Institute of Veterinary Pathology at Freie Universitt Berlin and EquiZyt UG Laboratory, in Steinhring, both in Germany.We know from human medicine that mucus plays an important role in certain diseasesnot only as a symptom but also as a central pathogenetic (disease-causing) factor, she said. We hope that our study will help veterinarians and horse owners interpret mucus findings more accurately and initiate comprehensive diagnostics.Studying Equine Tracheal MucusDrespling and her colleagues analyzed more than 1,500 samples of bronchoalveolar lavage (BAL) fluid from horses treated at 84 different clinics across Germany. They asked the treating veterinarians to complete questionnaires about their clinical findings and observations of mucus on endoscopy. The team also collected information about each horses daily life, including any respiratory signs such as coughing, directly from the owners.The researchers used various cell counts from their BAL analyses to diagnose equine asthma and its severity. Then they compared those results to the two mucus scoresone for the amount and the other for viscosity, which relates to thickness and stickinessrecorded by the treating veterinarians.Contrary to their expectations, the mucus quantity score didnt increase consistently with every step in asthma severity. Even so, they found severe asthma much more commonly in the upper half of the mucus scale. Horses with scores above 2 were more likely to have SEA, whereas mild-to-moderate asthma was more often associated with scores of 2 or less.Further statistical analyses led the team to find that horses having quantity scores in that upper bracket were nearly four times more likely to have a severe asthma diagnosis based on BAL, Drespling said.By contrast, the viscosity scores were only weakly associated with asthma severity, meaning they dont offer reliable clues about the degree of disease, said Dresplings supervisor, Lars Mundhenk, PhD, DrMedVet, Dipl. ECVP, of the Institute of Veterinary Pathology at Freie Universitt Berlin.Both quantity and viscosity scores tended to increase with age, regardless of asthma diagnosis, the researchers said. However, they did not find any correlations between scores and body condition, breed, or sex. On average the team noticed mucus was most abundant in the spring and least abundant in autumn, and winter mucus was the most viscous.Additionally, they found that a greater quantity of mucus was associated with fewer fungal spores and fragments. That was somewhat unexpected, they said, because scientists generally suspect these foreign elements contribute to asthma severity. The abundant mucus might successfully trap particles in the nose before they descend into lower regions of the respiratory tract, the researchers explained.The Importance of Accurate Diagnoses for Horses With AsthmaDrespling and Mundhenk said that while mucus scores can only be assessed through endoscopy, they were somewhat associated with visible clinical signs. Horses with higher scores were slightly more likely to cough, produce nasal discharge, breathe harder and faster, and have lower blood oxygen levels.The findings suggest that high amounts of mucus could be a red flag for severe asthma and prompt veterinarians to investigate further, said Drespling. Our study highlights the importance of combining different diagnostic elements: clinical history, physical examination, endoscopy, and cytology, she added.The study, Endoscopically assessed mucus parameters in equine asthma: Relationship to clinical history and cytological findings data, appeared in Equine Veterinary Journal in July 2025.