Q&A on avian respiratory disease: Anatomy, infectious disease, and diagnosis

Birds breathe differently than mammals, and understanding that distinction is foundational to treating them well. In this Q&A, Lauren Thielen, DVM, DABVP (Avian Practice), walks through the anatomy that makes the avian respiratory system so unique, the infectious diseases most likely to affect it, and why diagnosis can be difficult. She also makes the case for computed tomography (CT) imaging as a tool that can reveal what radiographs often miss.

Editor’s note: This dvm360 Q&A has been edited and consolidated from a verbal interview to better fit a written format while retaining the substance of the original conversation.

dvm360: What are some of the most common avian respiratory diseases?

Thielen: Aspergillosis is the most common fungal disease of birds and definitely the most common fungal respiratory disease. But really, being able to recognize the difference between respiratory disease and non-respiratory disease [is important]. Birds have very unique anatomy, so things that cause a bird to present as if it can't breathe may actually have nothing to do with the respiratory tract at all. Environmental and nutritional factors [are] often why these birds become susceptible in the first place.

dvm360: What makes avian respiratory anatomy so distinct from what we see in mammals?

Thielen: Birds don't have a diaphragm, so they breathe differently from the start. Everything is connected through a network of air sacs, which make birds remarkably more efficient at oxygen exchange than any mammal could be. You can think of the air sacs like bellows—they don't exchange oxygen themselves, but they hold air so that on the second breath, the bird can move that air through the lungs and complete gas exchange. That system is what makes birds so extraordinarily efficient.

But it has its drawbacks. Those air sacs are like little balloons. There isn't much of a primary immune defense there. There aren't a lot of cells to trap and clear debris, and the blood supply is limited, so white blood cells can't easily reach an infection. That same efficiency that makes them such incredible breathers also makes them more susceptible to infection, because they lack that primary clearance mechanism.

The upper respiratory system has its own quirks, too. In birds, the sinuses can expand beyond the skull and into the surrounding soft tissue — unlike ours, which are contained within bone. But here's the design flaw: birds' sinuses only drain at the very top of the head. So mucus or pus has to fill the sinus like a fishbowl, all the way to the top, before it finally spills over and you see nasal discharge. By the time an owner notices something is wrong, the infection is often quite advanced.

dvm360: How do you approach diagnosing respiratory issues in birds? What tools and techniques do you rely on?

Thielen: My primary approach is imaging. You can use radiographs to look at the lungs, but it's really hard to visualize the air sacs on x-ray, and the sinuses are essentially impossible to see. Even with the lungs, disease often has to be quite advanced before you can clearly see what's going on.

So, what I really try to champion is the use of CT. CT is a different imaging modality, and it's a game-changer for avian patients. You can see those thin air sac walls, whether they're thickened or have plaques on them. You can look directly into the trachea, visualize mucus inside the sinuses, and pick up very subtle changes in the lungs that you would otherwise miss entirely. It really changes what's possible diagnostically.