CRD — mycoplasmosis in poultry: symptoms, transmission and prevention
Chronic Respiratory Disease (CRD) is one of the most costly bacterial infections in poultry farming. Learn how to recognise Mycoplasma gallisepticum, what makes it worse, and how to protect your flock.
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What is CRD and what causes it?
Chronic Respiratory Disease (CRD) is a respiratory tract infection in poultry caused by the bacterium Mycoplasma gallisepticum (MG). Mycoplasmas lack a cell wall, which makes them naturally resistant to beta-lactam antibiotics such as penicillins. CRD affects mainly chickens, turkeys and quail — in both breeding and commercial flocks as well as backyard holdings.
What symptoms indicate mycoplasmosis?
Early signs include characteristic rales and gurgling sounds during breathing, sneezing, and watery or mucous discharge from the nostrils and eyes. In advanced cases the infraorbital sinuses swell visibly. The flock shows a marked drop in weight gain and egg production, increased feed consumption and deteriorating performance indices. Mortality is usually low, but production losses can be substantial. In turkeys the disease tends to follow a more severe course.
How does MG spread?
Mycoplasma gallisepticum spreads by two main routes. Vertically — the pathogen passes from an infected hen to her offspring through the egg, meaning infected chicks may already be a source of infection for an entire cohort at the hatchery. Horizontally — through direct bird-to-bird contact, aerosol, contaminated equipment, and the clothing and footwear of farm workers. The organism does not survive long in the environment, but subclinical carriage is common. Read more about transmission routes and biosecurity: Poultry farm biosecurity.
What makes CRD worse?
MG very often acts synergistically with infectious bronchitis virus (IB) and E. coli, creating the so-called CRD complex — losses are then many times higher. Factors that aggravate the disease: poor ventilation and ammonia build-up, dusty litter, heat stress, excessive stocking density and vitamin A deficiency. Microclimate and ventilation control are critical. Read more: Broiler house ventilation.
How is CRD diagnosed?
Diagnosis is made by a veterinarian based on clinical signs, post-mortem examination of dead or culled birds, and laboratory tests (serological and molecular). Serological tests (including the haemagglutination inhibition test) assess a flock’s MG status. In Poland, breeding flocks and day-old chicks from multiplier farms are subject to mandatory MG testing. Only a veterinarian can make a diagnosis.
CRD symptoms and factors that aggravate MG infection
Early recognition of symptoms and elimination of risk factors reduce production losses.
Rales and gurgling
The hallmark sign — audible rales and gurgling in the airways, more pronounced at night and during temperature fluctuations. One of the first signals of MG presence in a flock.
Sneezing and nasal discharge
Birds sneeze more frequently than usual, with watery or mucous discharge from the nostrils. When secondary bacterial infections are present the secretion becomes purulent.
Infraorbital sinus swelling
In advanced cases there is visible unilateral or bilateral swelling of the head around the cheeks and sinuses. The sign is particularly pronounced in turkeys.
Drop in egg production and weight gain
Infected laying hens reduce egg production by 10–30%. Broilers show poorer FCR and lower average body weight, which directly lowers the production result.
Ammonia and poor ventilation
Ammonia concentrations above 25 ppm damage the respiratory epithelium and facilitate colonisation by MG. Regular measurements and effective ventilation reduce this risk.
Stress and stocking density
High stocking density and heat stress weaken bird immunity and accelerate horizontal spread. Adhering to stocking norms is a straightforward preventive measure.
How to protect your flock from CRD and what to do when infection occurs
Prevention based on MG-free flocks and good biosecurity is far cheaper than treatment.
MG-free flocks
The most effective measure — purchase chicks or hatching eggs exclusively from flocks certified as free of Mycoplasma gallisepticum. The supplier's certificate must be backed by laboratory tests.
Biosecurity and the all-in/all-out principle
Populating and emptying the house in a single cycle (all-in/all-out) with thorough disinfection and a technological break eliminates residual infection. Restrict the movement of outside birds and equipment.
Microclimate control
Keep ammonia below 20 ppm, relative humidity at 50–70% and ensure effective air exchange. Keep litter dry — wet litter generates both ammonia and stress in birds.
Vaccination
Both live and inactivated vaccines against MG are available. The vaccination programme, timing and product selection are determined by a veterinarian taking into account the flock’s health status and commercial requirements.
Antibiotic treatment (veterinarian only)
When CRD is confirmed, a veterinarian may prescribe antibiotics active against mycoplasmas (e.g. tetracyclines, macrolides, fluoroquinolones). Treatment alleviates clinical signs but does not eliminate carriage. A withdrawal period applies — treatment records are legally required. Details: Treatment records and drug withdrawal.
Treatment records with DlaFerm.pl
DlaFerm.pl lets you log every drug administration — dose, route and withdrawal period — directly in the app. The documentation is ready to present during a veterinary inspection or sanitary audit. Records are linked to a specific batch of birds and poultry house.
Frequently asked questions about CRD and mycoplasmosis in poultry
What is CRD in poultry?add
CRD (Chronic Respiratory Disease) is a respiratory tract infection in poultry caused by the bacterium Mycoplasma gallisepticum. It is characterised by rales, sneezing, nasal and ocular discharge, and a drop in production performance. The disease is rarely fatal on its own but causes serious economic losses.
How does MG pass from a hen to a chick?add
Mycoplasma gallisepticum can spread vertically — through the hatching egg. An infected breeder hen transmits the bacterium into the egg, and the infected chick hatches already a carrier or develops symptoms shortly after hatching. This is why sourcing chicks from certified MG-free flocks is the cornerstone of prevention.
Can CRD be cured?add
Antibiotics active against mycoplasmas (e.g. tetracyclines, macrolides) alleviate clinical signs and reduce bacterial multiplication but do not eliminate carriage from the flock. After treatment, birds may still shed MG. The decision to treat, the choice of antibiotic and the dosage are made exclusively by a veterinarian.
What effect does ammonia have on CRD?add
Ammonia concentrations above 25 ppm damage the cilia and mucous membrane epithelium of the respiratory tract, greatly facilitating colonisation by Mycoplasma gallisepticum. Even a mild MG infection combined with high ammonia levels can produce a clinical picture resembling severe CRD. Regular ventilation checks and litter management are essential.
How does CRD differ from avian influenza (HPAI)?add
CRD is a bacterial disease caused by mycoplasmas — it runs a chronic course with low mortality. Avian influenza (HPAI) is a viral disease that can cause nearly 100% mortality within days and is notifiable to the Veterinary Inspection service. With HPAI, do not attempt to treat the flock yourself — notify your district veterinary officer immediately. More: Poultry diseases — symptom table.
How does DlaFerm.pl help with CRD?add
DlaFerm.pl allows you to record health events and symptoms in the poultry house, log every drug administration with date, dose and withdrawal period, and monitor the microclimate via IoT sensor integration (temperature, humidity, ammonia). The complete treatment record is available in one place and ready for inspection.
Sources & resources
- linkNational Veterinary Research Institute — PIWet-PIB (Puławy): poultry diseases
- linkEFSA — Mycoplasma gallisepticum: scientific opinion
- linkMerck Veterinary Manual — Mycoplasmosis in Poultry
- linkWOAH (OIE) — Avian mycoplasmosis (M. gallisepticum, M. synoviae)
- linkEU Regulation 2016/429 — Animal Health Law
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