A cough in a poultry house is almost never one disease. By the time a flock is audibly affected, the usual picture is a virus that opened the door, a bacterium that walked through it, and an environment that held the door open. Treating the last arrival while ignoring the first two is why respiratory problems come back in the following cycle, in the same house, at roughly the same age.
Key takeaways
- Ammonia damages the tracheal cilia at levels as low as 10 ppm, well below the 20 ppm ceiling set by EU broiler legislation.
- Co-infection is the norm, not the exception: field surveys report combinations such as infectious bronchitis with H9N2, or with both mycoplasmas.
- A bird has no diaphragm and its air sacs reach into the body cavity, which is why a respiratory infection so often ends as a systemic one.
- Suspicion of avian influenza or Newcastle disease is a reporting event, not a treatment decision.
Why avian anatomy changes the whole problem
Birds do not breathe like mammals, and the difference is not academic. Air moves through the lung in one direction and is stored in a network of air sacs that extends deep into the body cavity and into some bones. There is no diaphragm and no efficient cough reflex to clear the lower airway.
Two consequences follow directly. First, an infection that starts in the trachea reaches the air sacs easily, and airsacculitis is one of the leading reasons for carcass condemnation at processing, which turns a health problem into a commercial one. Second, the bird’s main defence in the upper airway is the mucociliary escalator, a carpet of cilia moving mucus and trapped particles upward. Damage that escalator and the flock loses its first line of defence before any pathogen has done anything special.
What you are usually looking at
The clinical signs overlap almost completely between agents. Sneezing, tracheal rales, nasal and ocular discharge, swollen sinuses, drops in feed and water intake and a fall in egg production or shell quality appear across most of the list. Sorting them apart is laboratory work, generally by PCR on tracheal swabs, not an exercise in observation.
| Agent | Typical signature in a flock | Status |
|---|---|---|
| Highly pathogenic avian influenza | Sudden, steep mortality; severe depression; sharp drop in intake | Category A, notify on suspicion |
| Newcastle disease | Respiratory signs with possible nervous and digestive signs, depending on strain | Category A, notify on suspicion |
| Infectious bronchitis virus | Rapid spread, shell and egg quality losses in layers, kidney forms with some variants | Managed on farm, vaccination programme |
| Infectious laryngotracheitis | Marked dyspnoea, blood-stained mucus, distress on inspection | Managed on farm, regional vaccination rules apply |
| Mycoplasma gallisepticum | Insidious onset, high morbidity and low mortality, chronic course | Breeder-level control, PCR monitoring |
| Ornithobacterium rhinotracheale, Avibacterium paragallinarum, E. coli | Usually secondary, arriving after a viral or environmental insult | Prescription treatment, veterinary diagnosis |
| Aspergillus fumigatus | Gasping without contagion between birds; often traced to litter or hatchery | Environmental origin, not transmissible bird to bird |
Co-infection is the ordinary case. A survey of Moroccan broiler farms covering 2021 to 2023 reported infectious bronchitis virus with H9N2 as the most frequent dual infection at 14 % of cases, and infectious bronchitis with both Mycoplasma gallisepticum and M. synoviae at 10 %. Any diagnostic plan that stops at the first positive result is likely to be describing part of the problem.
Ammonia does half the damage before any pathogen arrives
This is the section that changes outcomes most often, and it is not about medicine. Mycoplasma gallisepticum colonisation causes rapid ciliostasis and loss of cilia from the epithelium, and the Merck Veterinary Manual notes that clinical signs are exacerbated by concurrent viral infection and by environmental stressors, ammonia and poor ventilation among them.
The published thresholds are lower than most people assume. As little as 10 ppm of ammonia causes excessive mucus production and damage to the cilia. In the 10 to 40 ppm range, clearance of E. coli from trachea, lungs and air sacs is reduced. From around 25 ppm upward, damage to lungs and air sacs increases further.
The regulatory ceiling in the European Union sits above the biological threshold. Council Directive 2007/43/EC requires ventilation designed and operated so that ammonia does not exceed 20 ppm and carbon dioxide does not exceed 3 000 ppm, measured at the level of the chickens’ heads, for holdings stocked above 33 kg per square metre. Compliance at 19 ppm is lawful and still measurably harmful to the airway, so treating the legal limit as a target rather than a maximum is a genuine mistake. Litter moisture, ventilation rate and stocking density are the three levers, and they are covered in more depth in our guidance on what house air quality actually costs in performance.
What to do the day something looks wrong
The sequence matters more than the speed of any single step.
- Read the data before entering the house. Daily water consumption usually falls before anything is visible, and feed intake follows. The meter is the cheapest early warning available.
- Decide the category. Sudden high mortality, or respiratory signs combined with nervous signs, puts you in notification territory. Avian influenza and Newcastle disease are Category A diseases under Regulation (EU) 2016/429: suspicion alone triggers investigation and reporting, with control measures set out in Commission Delegated Regulation (EU) 2020/687.
- Sample properly. Tracheal swabs from several birds showing signs, plus post-mortem material, give the laboratory something to work with. One bird tells you very little about a flock.
- Fix the air while you wait. Ventilation and litter correction are not a treatment, but they stop the environment from adding to the load during the days the diagnosis takes.
Treatment, and the limits the law places on it
Antivirals have no role in poultry flock management. Viral respiratory disease is prevented by vaccination programmes and biosecurity, and managed by supportive conditions once present. Antimicrobials treat the bacterial component, secondary colibacillosis or a mycoplasma infection, and only on prescription after diagnosis.
Since 28 January 2022, when Regulation (EU) 2019/6 on veterinary medicinal products and Regulation (EU) 2019/4 on medicated feed became applicable, routine preventive antimicrobial treatment of a whole flock is no longer lawful in the European Union. Metaphylaxis remains possible under defined conditions and veterinary responsibility. The practical effect is that the diagnosis has to come first, which was always the better practice and is now also the rule.
Frequently asked questions
Can respiratory disease be diagnosed from the signs alone?
No. The signs overlap almost completely between viral, bacterial and mycoplasma agents, and co-infection is common. Laboratory confirmation, generally PCR on tracheal swabs together with post-mortem findings, is what distinguishes them.
How quickly should a suspicion be reported?
Immediately, when the picture is compatible with avian influenza or Newcastle disease. Reporting is triggered by suspicion, not by confirmation, and the obligation sits with the keeper as well as the veterinarian.
Is coughing in a flock always infectious?
Not always. Dust, high ammonia and aspergillosis all produce respiratory signs without bird to bird transmission. That is one of the reasons a house environment assessment belongs in the diagnostic work, not after it.
Does vaccination make biosecurity less important?
No. Vaccination reduces clinical impact and shedding for the agents it covers; it does not prevent introduction, and it does not cover the environmental causes at all. The two work together or not at all.
What should be recorded routinely?
Daily water and feed intake per house, daily mortality, production and shell quality, house temperature, ammonia readings, and every treatment with its prescription reference. That set answers most of what a veterinarian or an inspector will ask first.
Respiratory disease is one chapter of a wider health plan
The categories, the reporting obligations and the prevention logic described here apply well beyond the airway, and they work best written down rather than carried in the head of whoever has been on site longest.
Sources: Council Directive 2007/43/EC on the protection of chickens kept for meat production (ammonia 20 ppm, carbon dioxide 3 000 ppm at head level); Regulation (EU) 2016/429 and Commission Delegated Regulation (EU) 2020/687 on Category A disease control; Regulation (EU) 2019/6 and Regulation (EU) 2019/4, applicable from 28 January 2022; Merck Veterinary Manual, Mycoplasma gallisepticum infection in poultry; University of Kentucky extension publication ASC-200 on the avian respiratory system and ammonia thresholds; survey of infectious bronchitis virus and H9N2 co-infection in Moroccan broiler farms, 2021-2023. Consulted August 2026.
Published previously, fully revised on 9 August 2026. This article is general information for poultry professionals and does not constitute veterinary advice. Diagnosis, treatment and vaccination protocols must be established by the veterinarian responsible for your flock, and notification obligations are set by your competent authority. Regulatory references apply to the European Union; verify the rules in force in your own market.