Mycoplasma gallisepticum in Chickens: Chronic Respiratory Disease Explained

Mycoplasma gallisepticum (MG) is a contagious bacterial pathogen that causes chronic respiratory disease in chickens. It affects the upper and lower respiratory tract and can also contribute to air sac disease, poor growth, and lower egg production. In many flocks, the illness spreads slowly, affects many birds, and causes more sickness than death.

One reason MG is frustrating is that infected chickens can become long-term carriers. A bird may look better after treatment or supportive care but still carry the organism and spread it to flockmates later, especially during stress. Stressors such as crowding, poor ventilation, transport, heat, cold, or other infections can make signs flare up again.

MG often becomes more serious when it occurs alongside other respiratory problems. Coinfections with viruses or bacteria can worsen coughing, nasal discharge, breathing effort, and flock losses. That is why a lab-confirmed diagnosis matters. Several poultry diseases can look similar at home.

For pet parents with backyard chickens, the goal is usually not only helping the sick bird feel better, but also protecting the rest of the flock. Your vet can help you balance testing, treatment options, isolation, and long-term flock planning based on your birds' role as pets, layers, or breeding stock.

MG is caused by the bacterium Mycoplasma gallisepticum. Unlike many bacteria, mycoplasmas do not have a normal cell wall. That matters because some antibiotics that target cell walls, such as penicillins, are not effective against MG. Your vet chooses treatment based on the bird, the flock, food-animal rules, and likely coinfections.

The infection spreads in two main ways: horizontal transmission and vertical transmission. Horizontal spread happens when chickens have direct contact with respiratory secretions, contaminated dust, equipment, crates, feeders, waterers, clothing, or hands. Vertical spread means infected breeder hens can pass the organism through eggs to chicks.

Backyard flocks often become exposed when new birds are added without quarantine, birds are purchased from unknown sources, or wild birds and shared equipment bring pathogens onto the property. Stress also plays a major role. Poor ventilation, crowding, transport, weather swings, parasites, and other respiratory infections can all make disease more likely or more severe.

In many cases, MG is not acting alone. Coinfections with organisms such as E. coli or respiratory viruses can intensify inflammation and make a mild upper respiratory infection turn into a more serious flock problem. That is one reason your vet may recommend testing instead of treating based on symptoms alone.

Your vet starts with the basics: flock history, age of the birds, source of the birds, recent additions, egg production changes, housing conditions, and a hands-on exam. They may ask how many birds are affected, whether signs started suddenly or gradually, and whether there has been contact with other poultry or wild birds.

Because MG can look like several other respiratory diseases, diagnosis usually needs lab support. Real-time PCR testing on swabs is commonly used and can help identify MG quickly. Your vet may collect samples from the choanal cleft, trachea, conjunctiva, sinuses, or air sacs, depending on the case. Culture can also be used, though it is more specialized. Serology such as agglutination or ELISA may help with flock surveillance, and confirmatory testing may be needed because false positives can occur.

If birds have died or are very sick, your vet may recommend necropsy and additional testing to look for coinfections or rule out reportable diseases. This can be especially important when there is severe breathing trouble, neurologic signs, sudden deaths, or a fast-moving outbreak.

For pet parents, the most practical question is often whether to test one bird or the flock. Your vet can help decide based on your goals, the number of birds affected, whether the flock is closed or breeding, and whether you need a diagnosis to guide isolation, treatment, or future flock decisions.

Prevention starts with biosecurity and sourcing. The most effective step is buying chicks or started birds from reputable MG-monitored or NPIP-participating sources whenever possible. Avoid bringing home birds from mixed-source swaps or auctions unless you are prepared for strict quarantine and testing.

Quarantine all new birds in a separate airspace before they join your flock. A practical quarantine period is often at least 30 days, and longer may be reasonable if there are any respiratory signs. During that time, use separate shoes, feeders, waterers, and hand hygiene. Handle your home flock first and quarantined birds last.

Good daily management also matters. Keep the coop well ventilated, dry, and not overcrowded. Reduce dust, clean waterers and feeders regularly, and avoid ammonia buildup from wet litter. Stress reduction helps too. Sudden weather exposure, poor nutrition, parasite burdens, and repeated flock mixing can all make respiratory disease more likely.

Limit contact with wild birds and shared poultry equipment. Remove attractants that bring wild birds close to the coop, store feed securely, and avoid borrowing crates or tools unless they are thoroughly cleaned and disinfected. If a chicken develops respiratory signs, isolate it promptly and contact your vet. Early action can reduce spread and help protect the rest of the flock.