Mucus in the respiratory tract plays an important role in respiratory health by trapping dust, pollen, pollutants and bacteria. Dr David Marlin discusses mucus (in detail!), what is healthy mucus, why it becomes unhealthy and what can be done to help.
Dr David Marlin
Scientific & Equine Consultant
Healthy mucus is difficult to see...
One of the key features of a healthy respiratory system is a thin layer of almost invisible mucus which lines the airways. Even when a horse is being ‘scoped, if the airways are healthy it can be very difficult to see any mucus. However if we do a tracheal wash (where saline is passed down the endoscope and into the windpipe and then aspirated (drawn out) or a deep lung wash (bronchoalveolar lavage - BAL) we can see the mucus in the sample retrieved from the lungs. In fact, if we aspirate a clear sample with no visible mucus that’s a sign that we haven’t managed to get a good sample. In contrast, when horses are sick we can often see large streams or clumps of thick, yellow, green or brown opaque mucus in the airways when we ‘scope horses or we may even see mucus at the nostrils. This is referred to as mucopurulent discharge and a sure sign that there is inflammation and or infection present. So what is mucus, what does it do and what changes it from healthy mucus to mucopurulent? Let’s start by look at healthy normal mucus.
Mucus is slimy, doesn’t mix well with water and is a secretion that is produced by specialized cells known as Goblet cells, often in groups (glands), for the primary function of lubrication and protection. Although it doesn’t mix well with water, ironically mucus is a water-based substance and is mainly formed from mucins, which are large proteins with sugar molecules stuck on them. Its this combination that makes mucus “sticky” and “slimy”. Mucus is produced in the nose, the lungs, the eyes, the gastro-intestinal tract and the reproductive tract. Mucus also serves to help prevent surfaces drying out and becoming sticky. This is especially important in the eyes and in the lungs. In the lungs the very small airways have a tendency to want to collapse inwards on themselves, especially during breathing out as the lung volume becomes smaller and the airways are compressed. Mucus and lung surfactant are vital to prevent the airways closing under high surface tension as once they collapse and shut, it is difficult to reopen them.