Mycoplasma gallisepticum (MG) is a bacterial pathogen affecting poultry worldwide, concerning both commercial producers and backyard flock owners. The organism causes a chronic, slow-onset respiratory disease that persists within a flock, leading to decreased productivity and welfare issues. This bacterium is often latent, only becoming apparent when birds are stressed or infected with other common respiratory pathogens. Understanding MG’s nature, spread, diagnosis, and management strategies is essential for protecting avian health.
The Organism and Clinical Signs
The causative agent, Mycoplasma gallisepticum, lacks a rigid cell wall, placing it in the class Mollicutes. This absence means that antibiotics targeting the cell wall, such as penicillin, are ineffective against the organism. MG establishes itself by attaching to the ciliated epithelial cells lining the respiratory tract, causing inflammation and damage. The resulting disease is known as Chronic Respiratory Disease (CRD) in chickens and Infectious Sinusitis in turkeys.
Clinical signs often indicate respiratory distress, including coughing, sneezing, and a rattling sound known as rales. Nasal discharge, ranging from clear to mucoid, is common, along with mild conjunctivitis that sometimes presents as a foamy discharge from the eyes. In turkeys, a characteristic sign is the pronounced swelling of the infraorbital sinuses, giving the bird a swollen face.
Symptom severity is variable and often depends on the presence of stressors or concurrent infections, such as E. coli or respiratory viruses. In laying hens, an economically significant sign is a drop in egg production (10 to 20%) and reduced egg quality and hatchability. Infected birds typically remain lifelong carriers, capable of shedding the organism and acting as a reservoir, even if they appear healthy.
Routes of Transmission
MG spreads through two distinct mechanisms: vertical and horizontal transmission. Vertical transmission is the most common way the infection is initially introduced into a clean flock or hatchery. This occurs when an infected hen passes the bacterium directly to her offspring through the egg, known as transovarian transmission.
Horizontal transmission facilitates the spread once the disease is established. Direct contact between infected and susceptible birds, often through respiratory secretions, is a primary route. Aerosolized droplets from sneezing and coughing allow for short-distance spread within a poultry house.
Indirect transmission is also a factor, moving the organism via contaminated objects (fomites), including equipment, feed, water, and the clothing or footwear of personnel. Wild birds, particularly songbirds like the House Finch, can carry MG and act as a source of infection for domestic poultry. Although the organism only survives for a few days outside a host, poor biosecurity practices facilitate its rapid spread.
Diagnosis and Confirmation
Diagnosing MG cannot rely solely on clinical signs because many other respiratory pathogens cause similar symptoms. Definitive confirmation requires specialized laboratory testing, typically coordinated by a veterinarian. The common diagnostic strategy combines serology and molecular testing.
Serology tests blood samples for antibodies produced in response to the infection. The Plate Agglutination Test is a rapid screening method that detects these antibodies, indicating past exposure. While crucial for monitoring flocks, serological tests may not differentiate between active and past infection, and false positives can occur.
Molecular testing, specifically Polymerase Chain Reaction (PCR), offers a direct and sensitive method for confirming the presence of the bacterium’s genetic material. PCR is performed on swabs collected from the trachea, choana, or infraorbital sinuses of live birds, or on tissue samples from necropsied birds. This method is useful for detecting active infections early and differentiating MG from other Mycoplasma species. PCR provides rapid results and is considered the most reliable tool for confirming an active infection.
Treatment and Flock Management
The primary goal of treating MG is to reduce clinical signs and minimize economic losses, as complete elimination of the bacterium from an infected bird is not achievable. Treatment involves specific antibiotics, administered under veterinary guidance, typically through feed or drinking water. Macrolides (such as tylosin) and tetracyclines are commonly used because they target the bacterium’s internal processes rather than the cell wall MG lacks.
Antibiotics alleviate symptoms and decrease transmission through eggs, but they do not clear the infection; birds remain lifelong carriers. Supportive care focuses on reducing environmental stressors like poor ventilation, high ammonia levels, and dust. Providing warmth and accessible feed and water helps birds recover from the acute phase.
For small flocks, a “test and cull” strategy is sometimes employed, removing infected birds to protect the rest of the flock. Commercial operations, particularly large layer farms, often manage the infection continuously due to the impracticality of culling large numbers. Prevention remains the most effective defense, centered on strict biosecurity protocols.
Robust biosecurity measures are essential for prevention:
- New birds must be quarantined for 30 to 60 days and tested for MG before introduction to a resident flock.
- Strict sanitation, including regular cleaning of equipment and the use of foot baths, prevents indirect spread.
- Purchase birds only from sources that participate in monitoring programs, such as the National Poultry Improvement Plan (NPIP).
- Vaccines are available and primarily used in commercial settings to reduce clinical signs and production losses, but they are not a substitute for biosecurity.