Voies respiratoires inférieures

Once air passes the windpipe it enters the lower airway: the main bronchi, the smaller bronchioles branching off them, and finally the lungs themselves, where oxygen crosses into blood and carbon dioxide crosses out. This is the business end of the respiratory system, and disease here tends to be more serious than a problem confined to the nose or throat, simply because there is less spare capacity once lung tissue itself is affected.

The pleura, a thin membrane lining the lungs and chest wall, also belongs to this region. Fluid or air trapped between its two layers can compress the lungs from outside even when the lung tissue itself is healthy, which is part of why chest injuries and certain infections cause breathing trouble out of proportion to what shows up on a quick look at the animal.

Lower airway disease shows up differently across species. Cats develop feline asthma, a chronic inflammatory narrowing of the small airways, while dogs more often face pneumonia or chronic bronchitis. Horses are prone to inflammatory airway disease linked to dusty environments, and cattle face pneumonia outbreaks tied to stress and crowding during transport. Because the lungs have limited reserve capacity, owners should treat lower airway symptoms with more urgency than similar-sounding upper airway signs.

The trachea splits into a left and right main bronchus, one entering each lung, and these branch again and again into progressively smaller bronchioles, much like a tree trunk dividing into branches and twigs. At the very end of this branching sit the alveoli, tiny thin-walled air sacs surrounded by capillaries, where oxygen and carbon dioxide actually cross between air and blood.

The lungs sit within the chest cavity, protected by the rib cage, and are lined by the pleura, a smooth membrane that also coats the inside of the chest wall. A thin layer of fluid between these two pleural surfaces lets the lungs slide smoothly against the chest as an animal breathes in and out. The diaphragm, a muscular sheet separating the chest from the abdomen, does most of the mechanical work of breathing by contracting to draw air in.

Bird lungs differ substantially from this design, working alongside a system of air sacs that extend into the body and even into some bones, allowing air to flow through the lungs in one direction rather than in and out like a mammal's. This makes avian lower airway disease harder to localize, since infection can travel through connected air sacs into multiple body regions rather than staying confined to the chest.

Pneumonia, an infection or inflammation of the lung tissue itself, affects every companion species and often follows a viral infection that allows bacteria to take hold afterward. It is a leading cause of illness in young livestock, particularly calves and lambs, where stress from weaning, transport or crowding weakens natural defenses.

Feline asthma causes recurring episodes of airway narrowing and coughing in cats, similar in some ways to asthma in people, and tends to flare with environmental triggers like dust or smoke. Dogs more commonly develop chronic bronchitis, a long-term inflammation of the airway lining that produces a persistent cough without necessarily involving active infection.

Pleural space disease occurs when air, fluid or blood accumulates between the lung and chest wall, compressing the lung from outside; this can follow trauma, infection or, in older cats, sometimes underlying tumors. Horses commonly develop inflammatory airway disease from dusty stable environments and, less often, pleuropneumonia following long-distance transport. Poultry raised in large groups are vulnerable to respiratory viruses that damage lung tissue directly, compounded by ammonia buildup in poorly ventilated housing. Lungworm affects cattle, small ruminants and some cats, causing coughing and reduced lung function as larvae migrate through airway tissue.

A cough that sounds deep or wet, rather than harsh and dry, often originates in the lungs or lower airway rather than the throat. Rapid breathing at rest, or breathing that involves visible movement of the abdominal muscles in addition to the chest, suggests the lungs are working harder than they should to move air.

Exercise intolerance that appears suddenly, where an animal that previously ran or worked without issue now stops far earlier than usual, deserves attention, particularly in horses and working dogs. Bluish or gray gums signal that oxygen is not reaching the blood adequately and reflect a lower airway or lung problem more often than an upper airway one.

Reluctance to lie down, or a preference for sitting upright with elbows out, can mean an animal is trying to maximize chest expansion because breathing feels difficult lying flat. In cats, brief episodes of open-mouth breathing or a crouched posture with the neck extended often accompany asthma flares. In cattle and calves, a hunched posture, extended neck and grunting sound with each breath point toward pneumonia. Poultry with lower airway disease often show reduced activity, ruffled feathers and open-mouth breathing that persists even at rest.

A routine visit is appropriate for a mild, intermittent cough that has not affected appetite, energy or breathing rate, particularly if it has been present and stable for some time. Note when the cough happens, since patterns tied to exercise or excitement are useful information for your veterinarian.

Book a same-day appointment if coughing increases in frequency or intensity, breathing rate climbs above normal at rest, or an animal seems tired after activity that used to be easy. Calves, lambs or foals showing any drop in energy alongside a cough should be seen quickly, since young animals can deteriorate faster than adults with the same infection.

Treat these as emergencies: breathing that is fast, shallow or clearly labored at rest, blue or gray gums, refusal to lie down, open-mouth breathing in a cat, or sudden collapse after exertion. A horse or cow that stands with its neck extended, breathes with obvious abdominal effort, or grunts with each breath needs urgent evaluation. Pleural space problems, where fluid or air compresses the lungs from outside, can worsen quickly, so any sudden onset of severe breathing difficulty following an injury should go straight to emergency care rather than waiting overnight.

How can I tell if my dog's cough is from the throat or the lungs?

A harsh, dry, honking cough usually points to the throat or windpipe, while a deep, wet-sounding cough more often comes from the lungs. Either type deserves veterinary evaluation if it persists.

Is feline asthma the same as asthma in people?

It is similar in that airways narrow and become inflamed, though the underlying triggers and treatment differ. Cats with asthma need lifelong management rather than a single cure.

Why is pneumonia more dangerous in calves than adult cattle?

Young animals have less developed immune systems and smaller lung reserve, so infection can progress faster and cause more lasting damage to lung tissue.

Can dust in a barn or stable actually cause lung disease in horses?

Yes, chronic exposure to dusty hay and bedding is a leading cause of inflammatory airway disease in horses and can be managed largely through better ventilation and forage choices.

Does a chest injury always cause immediate breathing trouble?

Not always; fluid or air can accumulate in the chest gradually after trauma, so breathing difficulty can appear hours later even if an animal looked fine right after the injury.