Coryza in chickens is a contagious upper respiratory infection caused by the bacterium Avibacterium paragallinarum. It produces foul-smelling nasal discharge, facial swelling, and watery eyes, and it spreads rapidly through a flock. The disease typically appears within one to three days of exposure and lasts two to three weeks, though complications can drag it out longer. While many birds recover, mortality can reach up to 48% in older flocks, especially when other infections pile on.
What Causes Infectious Coryza
The culprit is a small, rod-shaped bacterium called Avibacterium paragallinarum (formerly known as Haemophilus paragallinarum). It’s a gram-negative organism, meaning it has a thin cell wall that makes it relatively fragile outside a host. The bacterium grows slowly in laboratory conditions, taking 36 to 48 hours to form visible colonies, but it spreads efficiently between living birds.
Three recognized serovars (A, B, and C) exist, and immunity to one doesn’t necessarily protect against the others. This matters because a flock that recovered from one strain can still be hit by a different one.
How It Spreads
Infectious coryza travels through three main routes. The most common is respiratory transmission: infected birds shed the bacterium in nasal discharge, and healthy birds inhale contaminated droplets or dust particles. Direct contact, such as birds pecking at each other or sharing tight quarters, is the second route. The third is indirect transmission through contaminated drinking water or feed.
Dust is a particularly effective vehicle. Dried nasal secretions break into tiny particles that stay airborne in a coop, disrupting the protective lining of healthy birds’ airways and making infection easier. Crowded, poorly ventilated housing accelerates spread dramatically.
Symptoms to Watch For
The hallmark signs are a thick, often foul-smelling nasal discharge, puffy or swollen faces, and watery eyes. Birds typically stop eating (anorexia) and may develop diarrhea. The nasal discharge starts thin and watery, then thickens into a mucus-like or even cheese-like consistency as the infection becomes more chronic. That shift in consistency reflects how long the infection has been active rather than a change in severity.
In the Americas, some outbreaks have produced more dramatic swelling of the entire head, resembling a condition called swollen head syndrome. Egg production drops noticeably in laying hens, and birds generally look lethargic and miserable. Young birds and older flocks tend to be hit hardest.
How Coryza Differs From Other Respiratory Diseases
Several chicken diseases cause sneezing, runny noses, and swollen sinuses, so telling them apart matters. Infectious coryza produces more marked facial swelling than most other respiratory conditions. Mycoplasma infections (MG) can also swell the sinuses, but the swelling is usually less dramatic in chickens. Coryza also tends to hit faster, with a one-to-three-day incubation period, while mycoplasma infections develop more gradually.
Viral diseases like Newcastle disease, infectious bronchitis, and avian influenza can cause similar eye and nasal discharge, but they often come with additional signs like severe breathing difficulty, neurological problems, or very high mortality. Infectious laryngotracheitis (ILT) is another look-alike, but its signature is bloody mucus in the windpipe and harsh breathing sounds, which coryza doesn’t cause.
Many cases of coryza are complicated by an underlying mycoplasma infection running alongside it. When both are present, symptoms are worse and recovery takes longer. A foul odor from the nasal discharge, combined with rapid-onset facial swelling, points strongly toward coryza rather than these other conditions, but a veterinarian can confirm through lab testing.
The Carrier Bird Problem
This is the single most important thing to understand about infectious coryza: recovered birds remain carriers of the bacterium for extended periods. A chicken that looks perfectly healthy after recovering can still shed the organism and infect newcomers. Once a flock has had coryza, every bird in it should be considered a carrier indefinitely.
This creates a serious management challenge. Introducing new, uninfected pullets into a flock with carrier birds almost guarantees the new birds will get sick. It also means you can unknowingly bring the disease home if you purchase birds from a flock that had a past outbreak.
Treatment Options
Antibiotics can reduce the severity of symptoms and shorten the course of illness, but they do not eliminate the carrier state. Treated birds feel better and return to production faster, yet they still harbor the bacterium. Water-soluble antibiotics are the most practical delivery method for flock treatment since sick birds often stop eating but continue drinking. Your veterinarian can recommend the appropriate antibiotic based on what’s available and legal in your area, as regulations on poultry antibiotics vary by country and region.
Supportive care also helps. Improving ventilation to reduce dust, ensuring clean water sources, and separating visibly sick birds from the rest of the flock can slow transmission and give recovering birds a better chance.
Prevention and Biosecurity
Prevention relies on keeping the bacterium out of your flock in the first place. The most effective strategy is an all-in, all-out replacement policy: you raise one group of birds together, and when it’s time to replace them, you remove every bird before bringing in new ones. Between flocks, thoroughly clean and disinfect the housing. This breaks the chain of carrier-to-newcomer transmission.
If your flock does become infected and you want to eliminate the disease permanently, complete depopulation followed by cleaning and disinfecting is the only reliable method. That’s a difficult decision, especially for backyard flock owners attached to their birds, but partial measures leave carriers behind.
Vaccines against infectious coryza do exist and are used in commercial operations, particularly for laying hens. Because there are three serovars, vaccine programs need to cover the strains circulating in your area to be effective. Vaccination reduces the severity of outbreaks but doesn’t completely prevent infection or the carrier state.
Practical biosecurity steps that reduce risk include quarantining any new birds for at least two weeks before introducing them to your flock, avoiding sharing equipment with other poultry keepers, keeping wild birds away from feeders and water sources, and never purchasing birds from flocks with unknown health histories. Since contaminated water is a known transmission route, regularly cleaning and refreshing waterers is a simple step that makes a real difference.