Infectious Synovitis (Avian Mycoplasmosis - Mycoplasma synoviae)

Infectious synovitis, caused by Mycoplasma synoviae (MS), is a highly contagious bacterial disease of poultry characterized primarily by acute or chronic inflammation of the synovial membranes of joints, tendon sheaths, and bursae, leading to lameness, swollen joints, and respiratory signs. In addition to the classic articular form, MS can cause respiratory disease, particularly in turkeys, and can lead to eggshell apex abnormalities in layers. The disease affects commercial broilers, broiler breeders, commercial layers, turkeys, and occasionally ducks and backyard poultry. MS is distributed worldwide and poses significant economic losses due to increased mortality, decreased weight gain, poor feed conversion, reduced egg production, and increased carcass condemnation at processing. The disease is often subclinical, with clinical signs appearing under stress or concurrent infections. MS is transmitted both vertically (transovarian) and horizontally via respiratory aerosols and direct contact. Control relies on biosecurity, eradication in breeding flocks, and vaccination in some regions.

The primary causative agent is Mycoplasma synoviae, a small (0.2-0.3 µm), pleomorphic, cell-wall-deficient bacterium belonging to the class Mollicutes. It requires sterols for growth and is cultured in vitro on enriched media such as Frey's medium supplemented with swine serum and nicotinamide adenine dinucleotide (NAD). MS has a genome size of approximately 800 kb and exhibits antigenic variation through phase and size variation of surface lipoproteins, particularly the hemagglutinin VlhA, which facilitates immune evasion and persistent infection. Several genotypes and strains exist, with variable pathogenicity; some strains are highly arthritogenic, while others are predominantly respiratory. The organism colonizes the respiratory epithelium and subsequently disseminates hematogenously to synovial tissues. MS lacks a cell wall, rendering it resistant to beta-lactam antibiotics but susceptible to tetracyclines, macrolides, and fluoroquinolones. It is a fragile organism, easily inactivated by heat (56°C for 30 minutes), drying, and common disinfectants.

MS infection is prevalent in commercial poultry worldwide, with higher incidence in multi-age and high-density operations. Broiler breeders and layers are commonly affected, with vertical transmission through the egg being a major route of introduction into naive flocks. Horizontal transmission occurs via respiratory aerosols, contaminated feed, water, and fomites. The disease is often subclinical, with clinical outbreaks triggered by stressors such as poor ventilation, high ammonia, overcrowding, concurrent viral or bacterial infections (e.g., infectious bronchitis virus, Escherichia coli), and vaccination reactions. Morbidity can reach 5-15% in broilers, but in severe cases, up to 75% of the flock may show lameness. Mortality is typically low (1-10%) unless secondary infections occur. In layers, egg production may drop by 10-20%, and eggshell apex abnormalities can affect up to 30% of eggs. Feed conversion ratio (FCR) may increase by 0.1-0.2 in affected broilers. Turkeys are more susceptible to respiratory disease, with higher mortality. Wild birds and rodents can act as mechanical vectors. The disease is more common in temperate climates with poor biosecurity.

MS enters the host via the respiratory tract, adhering to ciliated epithelial cells of the trachea and air sacs via surface adhesins. It colonizes the mucosa, causing ciliostasis and mild inflammation. Subsequently, the organism invades the bloodstream (mycoplasmemia) and disseminates to synovial membranes, tendon sheaths, and bursae. In the joints, MS induces a strong inflammatory response characterized by infiltration of heterophils, macrophages, and lymphocytes. The synovial membrane becomes thickened and edematous, with hyperplasia of synovial lining cells and fibrin deposition. Chronic infection leads to villous hypertrophy of the synovium, pannus formation, and erosion of articular cartilage. The inflammatory exudate accumulates in the joint space, causing swelling and lameness. In the respiratory form, MS causes catarrhal tracheitis, airsacculitis, and pneumonia, particularly in turkeys. The organism can also localize in the oviduct, leading to eggshell abnormalities. The immune response is primarily cell-mediated, with humoral antibodies produced but not fully protective due to antigenic variation. The lack of a cell wall allows MS to evade complement-mediated lysis and some antibiotics.

Intrinsic factors include genetic susceptibility, with some broiler lines more prone to clinical disease. Age is a factor; young birds (4-16 weeks) are more susceptible to the arthritic form, while layers may develop eggshell apex abnormalities. Immunosuppression due to infectious bursal disease virus or Marek's disease increases severity. High production stress in layers and rapid growth in broilers predispose to clinical expression. Extrinsic factors include poor biosecurity, allowing introduction of MS from infected flocks or contaminated equipment. High stocking density, inadequate ventilation leading to high ammonia levels, and poor litter conditions (wet, caked) exacerbate respiratory and joint disease. Concurrent infections with respiratory viruses (e.g., infectious bronchitis, Newcastle disease) or bacteria (E. coli, Ornithobacterium rhinotracheale) potentiate MS pathogenicity. Vaccination stress, improper vaccine administration, and nutritional deficiencies (e.g., vitamin A, selenium) also increase susceptibility. Vertical transmission is more likely during acute infection in breeders.

Clinical signs vary with the form and age. In the articular form, affected birds show lameness, reluctance to move, swollen joints (hock, stifle, and wing joints), and a characteristic 'sitting on haunches' posture. Birds may have stunted growth, poor feathering, and pale combs. In severe cases, there is depression, anorexia, and dehydration. Respiratory signs include coughing, sneezing, rales, and nasal discharge, especially in turkeys. In layers, egg production drops, and eggs may have rough, misshapen shells with apex abnormalities (thickened, ridged, or cracked). Mortality is usually low but can increase with secondary infections. In broilers, the disease often becomes apparent at 4-12 weeks of age, with a gradual increase in lameness. Flocks may have uneven growth and increased culling. In breeders, there may be a slight decrease in hatchability. Neurological signs are rare but can occur if the infection spreads to the central nervous system.

Differential diagnoses include: 1) Mycoplasma gallisepticum (MG) infection: primarily respiratory, with sinusitis in turkeys; MS causes more joint lesions. Serology and PCR differentiate. 2) Staphylococcus aureus arthritis: acute lameness with swollen joints, often with skin lesions; bacterial culture yields Gram-positive cocci. 3) Escherichia coli septicemia/arthritis: occurs secondary to respiratory disease; joint lesions with fibrinous polyserositis; culture on MacConkey agar. 4) Reovirus arthritis (viral arthritis/tenosynovitis): causes lameness and swollen hocks, but primarily in broilers; histopathology shows lymphocytic infiltration and syncytia; virus isolation or PCR. 5) Infectious bursal disease (IBD): immunosuppression, but no joint swelling; bursal lesions. 6) Marek's disease: neurological signs and visceral tumors; joint swelling rare. 7) Rickets/osteomalacia: lameness due to bone deformities, but no joint swelling; calcium/phosphorus imbalance. 8) Gout: urate deposits in joints and viscera; elevated uric acid. 9) Coccidiosis: intestinal lesions, bloody diarrhea, but no joint involvement. 10) Newcastle disease: respiratory and neurological signs, but no joint swelling. Definitive diagnosis relies on isolation, PCR, and serology.

The diagnostic approach begins with flock history and clinical signs, particularly lameness and swollen joints. A thorough necropsy of affected birds is essential, examining joints, tendon sheaths, and respiratory tract. Gross lesions include synovitis, tenosynovitis, and airsacculitis. Swabs from joints, trachea, and air sacs should be collected for mycoplasma isolation in Frey's medium. Molecular diagnosis via PCR targeting the 16S rRNA gene or the vlhA gene is rapid and sensitive. Serological screening using serum plate agglutination (SPA), hemagglutination inhibition (HI), or ELISA can detect antibodies, but false positives may occur; confirm with PCR. Histopathology of synovial membranes shows characteristic inflammatory changes. Differential diagnosis requires ruling out other causes of lameness. In breeding flocks, routine monitoring for MS is recommended, with serological testing every 2-4 weeks. If MS is suspected, immediate quarantine and confirmatory testing are implemented.

Serology: ELISA is commonly used for flock screening; positive results indicate exposure but not necessarily active infection. HI titers may be elevated, but cross-reactions with MG can occur. SPA is rapid but less specific. Molecular: PCR (conventional or real-time) on swabs or tissues is highly sensitive and specific; can differentiate MS from MG. Microbiology: Isolation of MS requires specialized media (Frey's) and incubation at 37°C in 5-10% CO2 for up to 14 days; colonies are small with a 'fried egg' appearance. Blood chemistry: In affected birds, acute-phase proteins (e.g., serum amyloid A) may be elevated. Complete blood count may show leukocytosis with heterophilia. In layers, eggshell quality parameters are altered. Histopathology: Synovial membranes show hyperplasia, heterophil infiltration, and fibrin deposition. In chronic cases, lymphoid follicles and fibrosis are present.

Radiography of affected joints may show soft tissue swelling, joint space widening, and periarticular osteophyte formation in chronic cases. However, radiography is rarely used in poultry practice due to cost and logistics. Ultrasonography can assess joint effusion and synovial thickening, but is also uncommon. Gross necropsy photography is essential for documentation and educational purposes. Imaging is not a primary diagnostic tool for MS; laboratory confirmation is required.

Gross necropsy lesions: In the articular form, affected joints (hock, stifle, wing) are swollen with excessive yellow-brown or caseous exudate. The synovial membranes are thickened, edematous, and hyperemic. Tendon sheaths may be distended with fibrinous fluid. In the respiratory form, there is catarrhal tracheitis, airsacculitis with caseous exudate, and sometimes pneumonia. In layers, the oviduct may show inflammation. Microscopic histopathology: Synovial membranes exhibit hyperplasia of lining cells, infiltration of heterophils, macrophages, and lymphocytes, and fibrin deposition. Chronic lesions show villous hypertrophy, fibrosis, and lymphoid follicle formation. In the trachea, there is loss of cilia, epithelial hyperplasia, and mononuclear infiltration. Air sacs show thickening and inflammatory exudate. No inclusion bodies are seen. Special stains (e.g., Giemsa) may demonstrate the organism in smears.

Treatment of MS is challenging due to the lack of a cell wall and the organism's intracellular persistence. Antimicrobial therapy can reduce clinical signs and transmission but may not eliminate the infection. Commonly used antibiotics include: Tylosin (Tylan) at 500 mg/L drinking water for 3-5 days, or 100-200 g/ton feed for 10-14 days; Oxytetracycline at 200-400 mg/L drinking water for 3-5 days, or 500-1000 g/ton feed; Doxycycline at 100-200 mg/L drinking water; Enrofloxacin (where legal) at 50-100 mg/L drinking water for 3-5 days; Tilmicosin at 75 mg/L drinking water for 3 days; Florfenicol at 400 mg/L drinking water for 3-5 days. Supportive therapy with vitamins (A, D3, E, C) and electrolytes is beneficial. In layers, improving eggshell quality with calcium and vitamin D3 supplementation may help. Biosecurity measures should be intensified, and affected flocks should be isolated. In severe cases, depopulation may be considered, especially in breeding flocks to prevent vertical transmission. Vaccination with live or inactivated MS vaccines is available in some countries and can reduce clinical disease and egg transmission.

The prognosis for individual birds is poor; they often remain chronic carriers and may be culled. For the flock, the prognosis is guarded. With appropriate antimicrobial therapy and management, clinical signs may resolve within 2-4 weeks, but the infection persists. In broilers, recovery may occur, but weight gain and FCR are permanently affected. In layers, egg production may return to near normal after 4-6 weeks, but eggshell quality may remain compromised. Mortality is usually low, but secondary infections can increase it. In breeding flocks, the disease can have long-term economic impact due to reduced hatchability and the need for eradication. Without treatment, the disease can become chronic, leading to increased culling and condemnation. The flock may remain seropositive for life.

After an outbreak, implement a comprehensive monitoring program. Conduct serological testing (ELISA) every 2-4 weeks to track antibody levels. Perform PCR on tracheal or joint swabs to confirm clearance of active infection. Review and enhance biosecurity protocols, including cleaning and disinfection of houses, equipment, and footwear. Depopulate and rest after thorough cleaning and downtime of at least 2 weeks. In breeding flocks, consider antibiotic treatment of eggs (egg dipping or injection) to reduce vertical transmission. Monitor sentinel birds for signs of infection. Maintain strict all-in/all-out management. Regularly audit ventilation, litter quality, and stocking density to reduce stress. In layers, monitor egg production and shell quality. Provide nutritional support with vitamins and minerals. Document all findings and adjust vaccination programs if applicable.

Pearls: 1) Always examine the hock joints in any lame bird; swollen joints with caseous exudate are highly suggestive of MS. 2) In layers, eggshell apex abnormalities are a key indicator of MS. 3) MS can be subclinical; use PCR on tracheal swabs for early detection. 4) Serology (SPA) can give false positives; confirm with HI or PCR. 5) MS is often secondary to respiratory viruses; control underlying infections. Pitfalls: 1) Confusing MS with MG based on clinical signs; always use species-specific PCR. 2) Treating with antibiotics that are ineffective (e.g., penicillin) due to lack of cell wall. 3) Neglecting biosecurity, leading to rapid spread. 4) Assuming vaccination eliminates infection; vaccinated flocks can still shed MS. 5) Failing to consider vertical transmission in breeders; test and treat accordingly. 6) Overlooking concurrent infections that exacerbate MS.

Antimicrobial protocols (based on Plumb's and AAAP guidelines): Tylosin: 500 mg/L drinking water for 3-5 days; or 100-200 g/ton feed for 10-14 days. Oxytetracycline: 200-400 mg/L drinking water for 3-5 days; or 500-1000 g/ton feed for 10-14 days. Doxycycline: 100-200 mg/L drinking water for 3-5 days. Enrofloxacin (where legal): 50-100 mg/L drinking water for 3-5 days. Tilmicosin: 75 mg/L drinking water for 3 days. Florfenicol: 400 mg/L drinking water for 3-5 days. Withdrawal times: Tylosin 1-3 days; Oxytetracycline 3-5 days; Enrofloxacin 7-10 days; Florfenicol 6 days. Supportive therapy: Vitamin A 10,000 IU/L, Vitamin D3 2,000 IU/L, Vitamin E 100 IU/L, Vitamin C 1 g/L, electrolytes as per label. Vaccines: Live MS vaccines (e.g., MS-H) administered by eye drop or spray at 4-8 weeks of age; inactivated vaccines for breeders. Anticoccidials are not directly relevant but may be used if coccidiosis is concurrent.

Landmark studies: Kleven et al. (1972) demonstrated the pathogenicity of MS in chickens and turkeys. Landman et al. (2000) described eggshell apex abnormalities associated with MS. Recent research has focused on molecular epidemiology and vaccine development. A meta-analysis by Feberwee et al. (2009) evaluated the efficacy of live MS vaccines in reducing clinical disease and egg transmission. Consensus guidelines from AAAP and WVPA recommend a combination of biosecurity, monitoring, and vaccination for MS control. WOAH/OIE lists MS as a notifiable disease in some countries, requiring eradication in breeding flocks. Field trials have shown that tylosin and oxytetracycline reduce clinical signs but do not eliminate infection. The use of enrofloxacin is effective but restricted in many countries due to antimicrobial resistance concerns. Overall, the evidence supports a comprehensive approach including early detection, antimicrobial therapy, and vaccination to mitigate economic losses.