Avian Mycoplasmosis Study Guide

Overview and Clinical Importance

Avian mycoplasmosis is a collective term for clinical diseases caused by pathogenic Mycoplasma species in poultry. The two most economically significant pathogens are Mycoplasma gallisepticum (MG) and Mycoplasma synoviae (MS). These organisms are WOAH-listed respiratory pathogens that cause substantial economic losses worldwide through decreased weight gain, reduced feed conversion efficiency, decreased egg production, increased embryo mortality, carcass condemnation, and treatment costs.

Mycoplasmas are unique bacteria belonging to the class Mollicutes. They lack a cell wall, which has important implications for antibiotic selection. They are the smallest self-replicating prokaryotes known and possess highly variable surface proteins that enable immune evasion.

Etiology and Classification

Four major pathogenic mycoplasmas affect poultry: M. gallisepticum (MG), M. synoviae (MS), M. meleagridis (MM), and M. iowae (MI). MG and MS are the most clinically significant and are WOAH-listed pathogens.

Comparison of Major Avian Mycoplasmas

Board Tip - Memory Aid: "MG = Major Gasper" (respiratory focus); "MS = Major Sweller" (joints swell). MG causes CRD/sinusitis; MS causes synovitis. Both cause airsacculitis!

Transmission and Epidemiology

Routes of Transmission

Vertical transmission (transovarian): Both MG and MS can be transmitted from infected breeder hens to embryos through the egg. This is a critical route for disease perpetuation and why breeder flock monitoring is essential.

Horizontal transmission: Occurs via direct contact with infected birds, respiratory aerosols (short distances), contaminated fomites (equipment, shoes, clothing), and movement of infected carrier birds between farms.

Important: Once MG or MS is introduced into a flock (vertically or horizontally), clinical signs may remain latent for days to months. Stress factors trigger clinical disease and rapid within-flock transmission.

Epidemiological Parameters

Clinical Signs and Species Differences

Mycoplasma gallisepticum (MG)

Chickens - Chronic Respiratory Disease (CRD)

• Respiratory signs: Coughing, sneezing, tracheal rales, nasal discharge, dyspnea
• Ocular signs: Mild to moderate conjunctivitis with frothy ocular exudate
• Production impact: Decreased feed consumption, weight loss, reduced egg production
• Course: Gradual onset, prolonged disease course; often subclinical in adults

Turkeys - Infectious Sinusitis

• Characteristic sign: Swollen infraorbital sinuses (may be so severe eyelids close)
• More severe respiratory signs than chickens: Marked nasal/ocular discharge, tracheal rales
• Wing feather soiling from wiping discharge from eyes
• Outbreaks typically occur at 8-15 weeks of age with up to 90% of birds showing signs

Game Birds and Wild Birds

• Pheasants, partridges, quail: Similar to turkeys with prominent sinusitis
• House finches (in North America): Severe conjunctivitis - eyes may swell shut leading to blindness, starvation, predation
• Game birds may die when purulent material prevents eating

Mycoplasma synoviae (MS)

Respiratory Form

• Often subclinical or very mild respiratory signs
• May have slight rales; usually no obvious clinical signs
• Airsacculitis develops especially when combined with viral infections or stress

Synovitis Form - Infectious Synovitis

• First signs: Pale/bluish comb, lameness, tendency to sit
• Swollen hock joints and footpads - warm, red, painful
• Sternal bursitis (breast blisters) from recumbency
• Birds rest around feeders and drinkers due to pain
• More severe joint lesions in turkeys than chickens

Pathology and Lesions

Gross Lesions - M. gallisepticum

Gross Lesions - M. synoviae

• Early synovitis: Creamy to viscous yellow-gray exudate in synovial structures, especially hock joints
• Chronic cases: Inspissated (thickened) exudate; pitted/eroded articular cartilage
• Respiratory form: Mild airsacculitis when combined with viral infections
• Embryos: Caseous exudate in thoracic air sacs of pipped embryos (decreased hatchability)
• Egg quality: Eggshell apex abnormalities (thin, translucent shell) - especially in Europe

Histopathology

• Mucous membranes: Thickened, hyperplastic epithelium with inflammatory cell infiltration
• Lamina propria: Lymphoid hyperplasia with germinal center formation
• Chronic MG air sacs: Beaded appearance from discrete lymphoid follicles
• MS joints: Heterophil and fibrin infiltration; synovial membrane hyperplasia with villous formation

Diagnosis

Sample Collection

Best sample from live birds: Tracheal swab (most reliable for most mycoplasmas). Choanal cleft swabs also useful.

Necropsy samples: Swabs from trachea, lungs, air sacs, sinuses, oviduct, and joints (for MS).

For embryo investigation: Yolk sac swabs during last third of incubation; oropharynx and air sac swabs from affected embryos.

Diagnostic Tests

Differential Diagnoses

For MG respiratory signs: Newcastle disease, infectious bronchitis, avian influenza, infectious coryza (Avibacterium paragallinarum), avian metapneumovirus, chlamydiosis, Ornithobacterium rhinotracheale.

For MS synovitis: Viral arthritis/tenosynovitis (reovirus), Staphylococcus aureus arthritis, E. coli arthritis, Enterococcus joint infections, Riemerella anatipestifer (waterfowl).

Treatment

Important principle: Antibiotics reduce clinical signs and decrease Mycoplasma load but DO NOT ELIMINATE THE ORGANISM. Treated birds remain carriers. Treatment is NOT curative.

Antimicrobial Options

Vaccination

Vaccination is used when biosecurity measures fail to prevent infection, particularly in commercial layer and breeder flocks. Important: Vaccines reduce clinical signs and production losses but do NOT prevent infection.

M. gallisepticum Vaccines

M. synoviae Vaccines

• MS-H strain: Live, temperature-sensitive vaccine available in many countries (not commercially available in US). Eye drop administration.
• MS1: Live vaccine; fine aerosol administration.
• Bacterins: Inactivated vaccines becoming increasingly limited in availability.

Prevention and Control

Biosecurity Measures

• Source Mycoplasma-free breeding stock: Purchase chicks from MG/MS-free flocks (NPIP certification in US)
• All-in/all-out production: Prevents mixing of positive and negative flocks
• Separate incubation: Eggs from Mycoplasma-negative flocks must be incubated separately from positive flocks
• Fomite control: Change clothing/footwear between flocks; disinfect equipment
• Wild bird exclusion: Prevent contact with wild birds (especially house finches for MG)
• Quarantine: Quarantine and test new birds for 30 days before introduction

Eradication vs. Management

Eradication approach: Best option - depopulate infected flocks, thoroughly clean and disinfect facilities, leave vacant at least 2 weeks, restock with Mycoplasma-free birds.

Management approach: When eradication is not feasible - ongoing antibiotic treatment, vaccination, strict biosecurity to prevent spread. Flocks remain positive for life.

Memory Aid - "MYCOPLASMA Prevention": M = MG/MS-free stock; Y = Yearly monitoring; C = Clean and disinfect; O = One-age flocks (all-in/all-out); P = Protect from wild birds; L = Limit visitors; A = Avoid mixing flocks; S = Separate incubation; M = Monitor serology; A = Act quickly on positive results

Economic Impact and Public Health

Economic Losses

• Decreased weight gain and feed conversion efficiency
• Reduced egg production (MG-infected layers produce fewer eggs per production cycle)
• Increased embryo mortality and decreased hatchability
• Carcass condemnation at processing due to airsacculitis
• Costs of prophylaxis, treatment, and vaccination programs

Zoonotic Potential

There is NO evidence that M. gallisepticum or M. synoviae are zoonotic. These are NOT threats to human health. Eggs from infected flocks are suitable for human consumption.