Coryza in chickens is a contagious bacterial respiratory infection, often called infectious coryza (IC). It’s caused by the bacterium Avibacterium paragallinarum and is best recognized by swollen faces, nasal discharge, and a noticeable drop in egg production. The disease occurs worldwide and can spread rapidly through a flock, making it one of the more economically damaging respiratory conditions in poultry.
What Causes Infectious Coryza
The bacterium behind coryza, Avibacterium paragallinarum, is a fragile organism that doesn’t survive long outside a chicken’s body. It dies quickly in the environment once exposed to sunlight, drying, or common disinfectants. This means the primary reservoir of infection is other chickens, particularly carrier birds that look healthy but still harbor the bacteria in their sinuses and airways.
The disease spreads through direct contact between birds, through contaminated drinking water, and through respiratory droplets when infected chickens sneeze or cough. Introducing new birds to an existing flock is one of the most common ways coryza enters a property. If a recovered bird is added to a naive flock, that bird can silently carry the bacteria and trigger an outbreak weeks or months later. Multi-age flocks, where older and younger birds share the same space, are especially vulnerable because recovered older birds can continuously expose younger ones.
Signs and Symptoms to Watch For
The hallmark signs of coryza are facial swelling, watery or thick nasal discharge, and a foul smell. The swelling typically concentrates around the eyes and wattles, sometimes so severe that one or both eyes are forced shut. You may also notice birds breathing through their mouths, shaking their heads to clear mucus, or making gurgling sounds.
Symptoms usually appear within one to three days of exposure, which is faster than most other poultry respiratory diseases. Affected birds often stop eating, become lethargic, and huddle together. Respiratory signs generally last two to three weeks in straightforward cases. When coryza occurs alongside other infections like mycoplasma or infectious bronchitis, or when housing conditions are poor (overcrowding, poor ventilation, wet litter), the illness drags on longer and a distinctly foul odor can develop in the coop.
How Coryza Affects Egg Production
For backyard and commercial flock owners alike, the hit to egg production is often the most painful consequence of coryza. Infected laying hens typically drop 10% to more than 40% in egg output. A study of 10 coryza outbreaks in Morocco documented production drops ranging from 14% to 41%, with mortality rates between 0.7% and 10% across those flocks. The mortality figures climb when secondary infections pile on top of the initial coryza infection.
Even after birds recover, egg production may not fully bounce back to pre-infection levels for the remainder of that laying cycle. The economic losses from increased culling and reduced output make coryza a serious concern for anyone raising chickens for eggs.
How Coryza Is Diagnosed
Coryza can look similar to several other respiratory conditions in chickens, including mycoplasma (chronic respiratory disease), infectious bronchitis, and fowl cholera. The rapid onset (one to three days), facial swelling, and foul-smelling discharge are strong clues pointing toward coryza specifically, but a definitive diagnosis requires laboratory testing. A veterinarian can take swabs from the sinuses or nasal passages and either culture the bacteria or use PCR testing to confirm the presence of A. paragallinarum. Getting a confirmed diagnosis matters because the treatment approach and long-term management differ depending on the disease involved.
Treatment Options
Early treatment is critical with coryza. Because the disease moves fast through a flock, the standard approach is to start medication through drinking water immediately, even before medicated feed can be arranged. Erythromycin and oxytetracycline are commonly used antibiotics. Various sulfonamide-based medications have also been effective.
One frustrating reality of coryza is that even when antibiotics improve symptoms, the disease often recurs once medication is stopped. The bacteria can persist in carrier birds, creating a cycle of flare-ups. This is why treatment alone rarely solves a coryza problem; it needs to be paired with management changes to break the cycle. Antibiotic use in poultry is also regulated differently by country. In the United States, for example, fluoroquinolone antibiotics are prohibited in food-production animals, so treatment options must align with local laws.
Prevention and Biosecurity
Because A. paragallinarum is fragile outside the host, prevention centers almost entirely on keeping infected or carrier birds away from your flock. The single most important rule: never introduce new chickens without a quarantine period of at least two to four weeks, and ideally source birds from flocks with no history of coryza. A recovered chicken is a carrier for life, so mixing recovered birds with unexposed birds is essentially guaranteeing a future outbreak.
An all-in, all-out management system, where you raise a single age group of birds and fully clean and disinfect the coop before bringing in new ones, is the most effective way to prevent coryza from becoming an ongoing problem. Standard disinfectants readily kill the bacterium on surfaces and equipment. Keep waterers clean, avoid sharing tools between flocks or with neighboring poultry keepers, and ensure good ventilation in your coop to reduce the concentration of airborne bacteria.
Vaccines (bacterins) are available in many countries and are commonly used in commercial operations, particularly on multi-age farms where the risk of exposure is constant. Vaccination doesn’t prevent infection entirely but reduces the severity of symptoms and the production losses. The bacterium has multiple strains (serovars), so the vaccine used needs to match the strains circulating in your area to be effective. Your poultry veterinarian can advise on whether vaccination makes sense for your flock’s situation and which product to use.
Long-Term Flock Management After an Outbreak
Once coryza has moved through a flock, every surviving bird should be considered a potential carrier. If you plan to add new, unexposed birds later, you face a choice: either vaccinate the incoming birds, or depopulate the existing flock entirely and start fresh after thorough cleaning and disinfection. Mixing naive birds with recovered carriers without vaccination will almost certainly trigger another round of disease.
For small backyard flocks, this can be a difficult decision. Many keepers choose to maintain a closed flock after a coryza outbreak, meaning they stop adding new birds until the current group reaches the end of its productive life. At that point, the housing is cleaned, disinfected, and left empty for a period before starting over with healthy stock. Because the bacterium survives poorly in the environment, even a short downtime between flocks is usually enough to eliminate it from surfaces and equipment.