Chickens get respiratory infections through three main routes: breathing in airborne pathogens from nearby sick birds, direct contact with infected animals or contaminated surfaces, and exposure to environmental conditions that weaken their natural defenses. Their unique respiratory anatomy, which includes a network of nine air sacs connected to hollow bones, makes them especially vulnerable once a pathogen enters the airway.
Why Chickens Are Uniquely Vulnerable
Unlike mammals, chickens don’t have flexible, expandable lungs. Instead, their lungs are stiff and fixed in place, and breathing is powered by a system of balloon-like air sacs that push air through the lungs in a continuous flow. Chickens have nine of these air sacs, filling a large proportion of the chest and abdominal cavity and even connecting to air spaces inside the bones. This design is efficient for oxygen exchange, but it also means a pathogen that enters the airway has access to a much larger internal surface area than it would in a mammal. An infection that starts in the windpipe can quickly spread deep into the body.
Airborne Transmission
The most common way chickens pick up respiratory infections is simply by breathing contaminated air. Sick birds shed viruses and bacteria through nasal discharge, saliva, and feces. When those materials dry, they become dust particles that stay suspended in coop air and are inhaled by flockmates. This is the primary route for some of the most serious poultry diseases, including Newcastle disease, infectious bronchitis, avian influenza, and the bacterial infection known as infectious coryza.
Crowded housing accelerates this process. The closer birds are to each other, the higher the concentration of pathogen-laden droplets in shared air. In poorly ventilated coops, these particles accumulate rather than being flushed out, turning the entire space into an efficient transmission zone.
Direct Contact and Contaminated Surfaces
Birds also become infected through direct physical contact with sick flockmates or with surfaces contaminated by their secretions. Shared waterers, feeders, and roosting bars can harbor pathogens. Virus-containing scabs shed by birds with fowl pox, for example, can sit on surfaces and infect other chickens through skin wounds, eyes, or the respiratory tract.
Humans can act as mechanical carriers too. Walking through an infected area and then entering a healthy flock’s coop on contaminated boots or clothing can introduce pathogens. Equipment shared between flocks, unwashed hands, and even vehicle tires are all documented routes. Mosquitoes and wild birds serve as biological vectors for certain diseases, particularly fowl pox and avian influenza.
The Major Pathogens
Viral Infections
Several viruses target the chicken respiratory tract, each with distinct characteristics. Newcastle disease causes hoarse chirping in chicks, watery nasal discharge, labored breathing, and facial swelling. It comes in mild, moderate, and highly aggressive forms, with mortality ranging from 10 to 80 percent depending on the strain. Infectious bronchitis affects chickens exclusively and can also damage the reproductive tract, reducing egg production. Avian influenza ranges from mild strains causing low mortality to highly pathogenic forms that can kill close to 100 percent of a flock, producing blue combs and wattles along with severe respiratory distress.
Infectious laryngotracheitis tends to hit chickens 14 weeks and older. The first sign is usually watery eyes, followed by coughing, sneezing, and head-shaking as birds try to clear mucus plugs from their windpipes. Breathing becomes visibly difficult.
Bacterial Infections
Infectious coryza, caused by a bacterium, produces a foul-smelling, thick discharge from the nostrils and eyes along with facial swelling and rattling breath sounds. Mortality is typically under 20 percent, but the real damage comes from reduced egg production and from opening the door to secondary infections that make everything worse.
Mycoplasma is one of the most insidious bacterial threats. It attaches to the lining of the respiratory tract using specialized surface proteins that let it glide from the tips of the tiny hair-like structures in the airway down to the cell surface. Once attached, it triggers inflammation and cell death, all while managing to avoid fully breaching the airway’s protective barrier. This means it can establish a chronic, smoldering infection that’s difficult for the bird’s immune system to clear. Infected birds may appear healthy for stretches, then flare up during times of stress, shedding bacteria to flockmates the entire time.
How Environment Sets the Stage
A pathogen needs an opening, and poor environmental conditions provide one. Ammonia is the single biggest air-quality threat in chicken housing. It builds up from decomposing droppings and litter, and at levels as low as 10 to 40 parts per million, it measurably reduces a chicken’s ability to clear bacteria like E. coli from its air sacs, lungs, and windpipe. In practical terms, ammonia damages the protective lining of the airway, stripping away the first line of defense against inhaled pathogens. If you can smell ammonia when you open the coop, levels are already high enough to cause harm.
Ventilation is the primary tool for controlling ammonia, moisture, and airborne pathogen concentrations. In commercial settings, recommended minimum air exchange rates start at 0.10 cubic feet per minute per bird during the first week of life and increase steadily to around 0.90 cfm per bird by eight weeks. For backyard flocks, the principle is the same: fresh air needs to move through the coop continuously without creating cold drafts directly on the birds. A well-sealed coop with controlled inlet openings near the ceiling draws cool air upward, lets it warm as it travels across the ceiling, and allows it to descend gently toward floor level. Random air leaks around doors, windows, and fan openings create cold spots and uneven ventilation that stresses birds while failing to clear contaminated air.
The Role of Stress
Stress directly compromises a chicken’s immune defenses. When chickens are exposed to cold temperatures, overcrowding, transport, or sudden changes in routine, their bodies produce elevated levels of corticosterone, the avian equivalent of the stress hormone cortisol. The relationship between stress and disease susceptibility is complex: short bursts of mild cold can actually stimulate parts of the immune system, temporarily boosting disease resistance. But sustained or severe stress pushes the balance the other way, increasing susceptibility to infections and promoting the spread of bacteria throughout the body.
Cold stress in particular has been shown to increase bacterial translocation, meaning bacteria that might normally stay contained in one area of the body can move to places they don’t belong. For flocks already carrying a low-level Mycoplasma infection or living in a coop with marginal ventilation, a cold snap or other stressor can be the trigger that turns a subclinical infection into a full-blown outbreak.
Recognizing the Signs
Respiratory infections in chickens share a cluster of recognizable symptoms. Look for rales (a crackling or rattling sound when the bird breathes), sneezing, watery or bubbly eyes, fluid or mucus from the nostrils, open-mouth breathing, gasping, and head shaking. Some infections cause visible facial swelling or swelling around the eyes. Birds often become quiet and lethargic because the effort of breathing leaves little energy for normal activity. Egg production typically drops.
The specific combination of signs can point toward a cause. Foul-smelling nasal discharge suggests infectious coryza. Bloody mucus coughed up or found on coop walls points toward laryngotracheitis. Blue or darkened combs and wattles with high mortality suggest highly pathogenic avian influenza. But many respiratory diseases look similar in their early stages, and multiple infections often occur simultaneously, with one pathogen weakening the bird’s defenses enough for a second to take hold. Accurate diagnosis usually requires laboratory testing, not visual assessment alone.
Human Health Considerations
Most chicken respiratory diseases do not infect people, but avian influenza is a notable exception. Human infections occur primarily through direct contact with infected birds (alive or dead) or with environments contaminated by their secretions and droppings. Handling, culling, and processing infected poultry are the highest-risk activities. Symptoms in humans range from mild flu-like illness or eye inflammation to severe respiratory disease. Properly cooked poultry and eggs pose no transmission risk. The “high pathogenicity” or “low pathogenicity” labels used for avian influenza describe how the virus behaves in chickens, not how dangerous it is in people.