Chronic Respiratory Disease (CRD) / Avian Mycoplasmosis (Mycoplasma gallisepticum Infection)

Chronic Respiratory Disease (CRD) is a highly contagious, economically devastating respiratory syndrome of poultry, primarily caused by the bacterium Mycoplasma gallisepticum (MG). The disease is characterized by a chronic, often subacute to chronic course, with clinical signs including coughing, sneezing, rales, nasal discharge, and conjunctivitis. In layers and breeders, MG infection leads to significant egg production drops, increased mortality, and downgrading of carcasses in broilers. The disease is distributed worldwide and is a major concern for the commercial poultry industry, particularly in multi-age and high-density production systems. MG is transmitted both horizontally (via respiratory aerosols, contaminated equipment, and personnel) and vertically (through the egg), making eradication challenging. The disease is often exacerbated by concurrent infections with respiratory viruses (e.g., Newcastle disease virus, infectious bronchitis virus) and bacteria (e.g., Escherichia coli), leading to severe airsacculitis and colibacillosis. In turkeys, MG causes sinusitis and severe respiratory distress. The economic impact includes reduced feed conversion efficiency, increased medication costs, and trade restrictions. Control relies on biosecurity, vaccination (live and inactivated), and medication, with eradication programs in some countries.

The primary causative agent is Mycoplasma gallisepticum (MG), a small (0.2-0.3 µm), pleomorphic, cell-wall-deficient bacterium belonging to the class Mollicutes. MG is characterized by a small genome (approximately 1.0 Mbp) and requires cholesterol for growth, which is reflected in its fastidious culture requirements. The organism possesses surface lipoproteins and adhesins, such as GapA and CrmA, which mediate attachment to host respiratory epithelial cilia, leading to ciliostasis and subsequent tissue damage. MG produces hydrogen peroxide and superoxide radicals, contributing to oxidative damage and inflammation. Several strains of MG exist, varying in virulence; for example, the R strain is highly virulent, while the F strain is used as a live vaccine. MG can also infect turkeys, causing infectious sinusitis, and other avian species. The organism is highly susceptible to drying and common disinfectants but can survive in organic material for days. Co-infections with other pathogens, particularly Escherichia coli, infectious bronchitis virus (IBV), and Newcastle disease virus (NDV), are common and exacerbate the severity of MG-induced lesions.

MG infection occurs worldwide, with higher prevalence in areas with intensive poultry production and poor biosecurity. The disease affects all poultry sectors: broilers, commercial layers, broiler breeders, turkeys, and backyard flocks. In broilers, MG infection often results in increased mortality (2-10%), reduced weight gain, and increased feed conversion ratio (FCR) by 0.1-0.2. In layers, the primary impact is a drop in egg production (10-30%) and an increase in cull rate. Breeders may experience reduced hatchability and vertical transmission to progeny. The disease is most commonly introduced into a flock through infected replacement birds (vertical transmission) or by contaminated equipment and personnel (horizontal transmission). Wild birds, including sparrows and starlings, can act as mechanical vectors. Housing systems with poor ventilation, high ammonia levels, and high stocking density predispose to clinical disease. Age susceptibility varies; young birds (3-8 weeks) are more susceptible to severe respiratory signs, while older birds may show milder signs but significant production losses. Seasonality is not pronounced, but cold weather and poor ventilation increase severity. Morbidity can reach 100% in susceptible flocks, while mortality is typically low (2-10%) unless complicated by secondary infections.

MG colonizes the respiratory epithelium, primarily the trachea, air sacs, and lungs. The organism attaches to ciliated epithelial cells via adhesins, leading to ciliostasis, loss of cilia, and desquamation of epithelial cells. This disrupts the mucociliary clearance mechanism, allowing secondary pathogens, especially E. coli, to invade and cause airsacculitis and septicemia. The host inflammatory response involves infiltration of lymphocytes, macrophages, and heterophils, leading to thickening of the air sac walls and the formation of caseous exudate. In layers, MG can spread systemically to the reproductive tract, causing salpingitis and eggshell abnormalities. The organism can also cross the blood-brain barrier, leading to neurological signs in some cases. Chronic infection leads to the development of lymphoid follicles in the respiratory mucosa, contributing to the chronicity of the disease. The immune response is primarily cell-mediated, with humoral antibodies playing a role in protection but also contributing to immunopathology. Vertical transmission occurs via infected eggs, leading to infected chicks that may show signs within the first few weeks of life.

Intrinsic factors include genetic susceptibility, with some poultry lines being more resistant to MG infection. Age is a significant factor; young birds (3-8 weeks) are more susceptible to severe respiratory disease, while older birds may show milder signs but significant production losses. Immune status is critical; immunosuppression due to infectious bursal disease (IBD) or Marek's disease can exacerbate MG infection. High production stress in layers and breeders increases susceptibility. Extrinsic factors include poor biosecurity, which allows introduction of MG through contaminated equipment, personnel, or infected birds. High stocking density and poor ventilation lead to high ammonia levels, which damage the respiratory epithelium and increase susceptibility. Wet litter and poor sanitation promote the survival of MG and secondary pathogens. Vaccination failure, either due to improper administration or immunosuppression, can predispose to field infection. Concurrent infections with respiratory viruses (IBV, NDV) or bacteria (E. coli) are major predisposing factors for severe clinical disease.

Clinical signs vary with the age of the bird, immune status, and presence of secondary infections. In broilers, signs typically appear at 3-8 weeks of age and include coughing, sneezing, rales (moist or dry), nasal discharge, and conjunctivitis with frothy eyes. Birds may show depression, huddling, and reduced feed and water intake. Growth rate is depressed, and FCR is increased. In layers, the most prominent sign is a drop in egg production (10-30%), which may be accompanied by an increase in the number of misshapen, thin-shelled, or shell-less eggs. Respiratory signs may be mild or absent in layers, but birds may show a slight increase in mortality. In breeders, similar signs are seen, with a reduction in hatchability due to vertical transmission. Turkeys infected with MG develop infectious sinusitis, characterized by swelling of the infraorbital sinuses, nasal discharge, and respiratory distress. In severe cases, especially with secondary E. coli infection, birds may develop airsacculitis, pericarditis, and perihepatitis, leading to increased mortality. Neurological signs, such as torticollis, are rare but can occur if the organism invades the central nervous system.

Differential diagnoses include other respiratory diseases of poultry. 1. Infectious Bronchitis (IB): Caused by coronavirus, IB causes similar respiratory signs and egg production drops, but also causes misshapen, rough-shelled eggs and can be differentiated by PCR and serology (ELISA, HI). 2. Newcastle Disease (ND): Caused by avian paramyxovirus type 1, ND causes respiratory, digestive, and neurological signs; it is a reportable disease and can be differentiated by virus isolation and HI test. 3. Avian Influenza (AI): Caused by influenza A viruses, AI can cause respiratory signs, depression, and sudden death; it is reportable and diagnosed by PCR and virus isolation. 4. Infectious Coryza: Caused by Avibacterium paragallinarum, it causes facial edema, nasal discharge, and conjunctivitis, but primarily affects layers; differentiated by bacterial culture and PCR. 5. Aspergillosis: Caused by Aspergillus fumigatus, it causes respiratory distress and granulomatous lesions in lungs and air sacs; diagnosed by histopathology and fungal culture. 6. Ornithobacterium rhinotracheale (ORT): Causes respiratory disease and airsacculitis, often as a secondary pathogen; diagnosed by bacterial culture and PCR. 7. Colibacillosis: Caused by E. coli, it often complicates MG infection, causing airsacculitis and septicemia; differentiated by bacterial culture. 8. Mycoplasma synoviae (MS): Causes respiratory signs and joint lesions; differentiated by PCR and serology. 9. Turkey Rhinotracheitis (TRT): Caused by avian metapneumovirus, it causes respiratory signs in turkeys and chickens; differentiated by PCR and serology. 10. Vitamin A Deficiency: Can cause respiratory epithelial metaplasia and nasal discharge; differentiated by history and response to vitamin A supplementation.

The diagnostic approach for MG infection involves a stepwise process: 1. Flock history: Assess clinical signs, production parameters (egg production, mortality, FCR), vaccination history, and biosecurity practices. 2. Clinical observation: Examine birds for respiratory signs, conjunctivitis, and sinus swelling. 3. Gross necropsy: Perform necropsy on affected birds, looking for tracheitis, airsacculitis, and caseous exudate in the respiratory tract. 4. Serology: Collect serum samples for ELISA (e.g., IDEXX MG ELISA) or rapid serum agglutination (RSA) test. Positive results indicate exposure but not necessarily active infection. 5. Molecular detection: Use PCR (conventional or real-time) on tracheal swabs or tissue samples to detect MG DNA. This is highly sensitive and specific. 6. Bacterial isolation: Culture MG from tracheal swabs or air sac lesions using specialized media (e.g., Frey's medium). This is definitive but slow (1-3 weeks). 7. Histopathology: Examine tracheal and air sac tissues for characteristic lesions, including lymphoid infiltration and epithelial hyperplasia. 8. Differential diagnosis: Rule out other respiratory pathogens using appropriate tests (e.g., PCR for IBV, NDV, AI). 9. Confirm diagnosis: Combine clinical, serological, molecular, and isolation results to confirm MG infection.

Serology: ELISA is the most common test, with positive results indicating exposure. A four-fold rise in titer between paired sera (2-3 weeks apart) indicates active infection. The rapid serum agglutination (RSA) test is a quick screening test but can have false positives. Hemagglutination inhibition (HI) test is less commonly used but can be specific. Molecular: Real-time PCR is highly sensitive and specific for MG detection, with a detection limit of as few as 10 copies of DNA. It can be performed on tracheal swabs, air sac exudate, or egg yolk. Microbiology: MG can be isolated on Frey's medium supplemented with swine serum and NAD, incubated at 37°C in 5% CO2 for 3-10 days. Colonies are small with a 'fried egg' appearance. Blood chemistry: Non-specific, but may show elevated heterophil counts and decreased lymphocyte counts. Coccidiosis lesion scoring is not applicable unless concurrent coccidiosis is suspected. Mycotoxin feed assays may be performed if feed quality is a concern, but are not directly related to MG.

Imaging is not routinely used in poultry diagnostics, but radiography can be used in research settings to assess air sac lesions. In live birds, radiographs may show thickening of the air sac walls and increased opacity in the thoracic cavity. Ultrasonography is rarely used. Gross necropsy photography is essential for documenting lesions, including tracheal congestion, caseous exudate in the air sacs, and peritonitis. These images are valuable for diagnostic records and educational purposes.

Gross necropsy findings include: tracheitis with excess mucus and congestion, airsacculitis with thickened, cloudy air sacs and caseous exudate, and in severe cases, fibrinous perihepatitis and pericarditis (due to secondary E. coli). In layers, there may be salpingitis and egg peritonitis. Microscopic histopathology: Tracheal epithelium shows loss of cilia, epithelial hyperplasia, and infiltration of lymphocytes, plasma cells, and macrophages. Air sacs show thickening due to edema, fibrin deposition, and inflammatory cell infiltration. In chronic cases, lymphoid follicle formation is seen. In the lungs, there may be interstitial pneumonia. In the reproductive tract, there is inflammation of the oviduct with infiltration of inflammatory cells. Special stains (e.g., Giemsa) can demonstrate the presence of mycoplasma organisms on the epithelial surface.

Treatment of MG infection is challenging due to the lack of a cell wall, making beta-lactam antibiotics ineffective. Antibiotics that are effective include tetracyclines (e.g., oxytetracycline, doxycycline), macrolides (e.g., tylosin, tilmicosin), fluoroquinolones (e.g., enrofloxacin, but use is restricted in some countries), and pleuromutilins (e.g., tiamulin). Treatment is typically administered via drinking water or feed. For example, tylosin can be given at 500 mg/L drinking water for 3-5 days, or oxytetracycline at 200-400 g/ton feed for 7-10 days. Tiamulin is effective at 125-250 mg/L drinking water for 3-5 days. It is important to note that antibiotic treatment reduces clinical signs and shedding but does not eliminate the infection. Supportive therapy includes vitamins (A, D3, E, C) and electrolytes to reduce stress. In severe outbreaks, depopulation may be considered, especially in breeder flocks, to prevent vertical transmission. Vaccination with live (F strain, ts-11, 6/85) or inactivated vaccines can be used as a control measure, but vaccination does not prevent infection, only reduces clinical signs and shedding. Biosecurity measures, including all-in/all-out management, cleaning and disinfection, and control of personnel movement, are critical for preventing spread.

The prognosis for MG infection depends on the severity of the outbreak, the presence of secondary infections, and the production type. In broilers, with appropriate antibiotic treatment and management, mortality can be limited to 2-5%, but FCR may be permanently increased by 0.1-0.2. In layers, egg production may recover to near-normal levels within 4-6 weeks, but the drop can be up to 30%, and some birds may not return to full production. In breeders, hatchability may be reduced, and vertical transmission can lead to infected progeny. In turkeys, sinusitis can be chronic and debilitating. The long-term prognosis for the flock is guarded, as MG infection is lifelong, and birds remain carriers. Eradication is difficult, and the flock may be depopulated to prevent spread to other flocks. With good management and vaccination, the impact can be minimized, but the infection cannot be eliminated.

After an outbreak, it is essential to monitor the flock for recurrence and to prevent spread to other flocks. Serial serological testing (ELISA) should be performed every 2-4 weeks to monitor antibody titers. PCR testing on tracheal swabs can be used to detect shedding. Cleaning and disinfection of the facility should be thorough, with a downtime of at least 2 weeks between flocks. Litter should be removed and disposed of properly. Biosecurity audits should be conducted to identify and correct any lapses. In breeding flocks, monitoring of progeny for MG infection is crucial. Vaccination programs should be reviewed and adjusted if necessary. Regular necropsy of sick birds should be performed to detect any secondary infections. The flock should be monitored for egg production and quality, and any deviations should be investigated.

Pearls: 1. MG infection should be suspected in any flock with chronic respiratory signs and a drop in egg production, especially if there is a history of poor biosecurity. 2. The presence of caseous airsacculitis in broilers is highly suggestive of MG complicated by E. coli. 3. In layers, a sudden drop in egg production with mild respiratory signs is a classic presentation of MG. 4. PCR is the most reliable diagnostic tool; culture is definitive but slow. 5. Antibiotic treatment can reduce clinical signs but will not eliminate the carrier state. Pitfalls: 1. Failing to consider MG in the differential diagnosis of respiratory disease, leading to misdiagnosis and inappropriate treatment. 2. Relying solely on serology, which can be negative in early infection or false positive due to vaccination. 3. Using antibiotics that are ineffective against mycoplasma (e.g., penicillin). 4. Neglecting biosecurity measures, leading to rapid spread to other flocks. 5. Overlooking secondary infections, which are often the cause of severe mortality. 6. Using live vaccines in flocks that are already infected, which can exacerbate clinical signs.

Antibiotics: 1. Tylosin: 500 mg/L drinking water for 3-5 days; or 100-200 g/ton feed for 7-10 days. Withdrawal time: 1 day for eggs, 3 days for meat. 2. Oxytetracycline: 200-400 g/ton feed for 7-10 days; or 1-2 g/L drinking water for 3-5 days. Withdrawal: 3 days for eggs, 5 days for meat. 3. Doxycycline: 100-200 mg/L drinking water for 3-5 days. Withdrawal: 3 days for eggs, 7 days for meat. 4. Tilmicosin: 75-150 mg/L drinking water for 3 days. Withdrawal: 3 days for eggs, 14 days for meat. 5. Enrofloxacin: 50-100 mg/L drinking water for 3-5 days (use restricted in some countries). Withdrawal: 3 days for eggs, 7 days for meat. 6. Tiamulin: 125-250 mg/L drinking water for 3-5 days; or 200-400 g/ton feed for 7 days. Withdrawal: 2 days for eggs, 3 days for meat. Supportive therapy: Vitamins A, D3, E, C, and electrolytes in drinking water for 3-5 days. Vaccines: Live vaccines (F strain, ts-11, 6/85) are administered by eye drop or spray to pullets at 8-16 weeks of age. Inactivated vaccines are given subcutaneously or intramuscularly to breeders and layers. Dosage and administration per manufacturer's instructions.

Landmark studies have demonstrated the efficacy of tylosin and tiamulin in reducing clinical signs and shedding of MG. A study by Kleven et al. (1988) showed that tylosin treatment reduced airsacculitis lesions in experimentally infected chickens. Vaccination with live F strain has been shown to reduce egg production losses and provide cross-protection against field strains (Evans et al., 1992). The ts-11 vaccine has been shown to be safe and effective in layers (Whithear et al., 1990). A meta-analysis by Ghanem et al. (2018) concluded that vaccination with live vaccines significantly reduces MG shedding and improves egg production. The AAAP guidelines recommend a comprehensive control program including biosecurity, vaccination, and medication. The World Organisation for Animal Health (WOAH) lists MG as a notifiable disease in some countries, and eradication programs have been successful in Scandinavia. Recent research focuses on the development of recombinant vaccines and improved diagnostic tools, such as loop-mediated isothermal amplification (LAMP) for rapid field detection.