What Are the Symptoms of Mycoplasma in Chickens?

Mycoplasma infections in chickens most commonly show up as respiratory illness: sneezing, nasal discharge, rattling or gurgling breathing, and swollen sinuses around the face and eyes. The two species that cause the most trouble, Mycoplasma gallisepticum (MG) and Mycoplasma synoviae (MS), produce overlapping but distinct symptom profiles, and the disease frequently becomes chronic rather than clearing on its own. What makes mycoplasma tricky is that infected birds can look perfectly healthy for weeks or months before stress, cold weather, or a secondary infection triggers visible signs.

The Two Species That Matter Most

Four mycoplasma species are recognized as economically important poultry pathogens, but two dominate the conversation for chicken keepers. MG is the primary cause of chronic respiratory disease in chickens and infectious sinusitis in turkeys, while MS is best known for causing joint inflammation (synovitis) and airway disease in chickens and turkeys.¹ Both are listed by the World Organisation for Animal Health as significant respiratory pathogens. The remaining two species, M. meleagridis and M. iowae, are primarily turkey problems and rarely concern chicken owners.²

MG tends to hit the respiratory tract hardest, while MS has a particular affinity for joints and tendons on top of respiratory involvement. In practice, many flocks carry both species simultaneously, which complicates the picture. Knowing which species is involved matters because the symptoms you see, the damage to egg production, and the long-term outlook all differ.

Respiratory Signs

The hallmark of MG infection is what veterinarians call chronic respiratory disease. You will hear sneezing, coughing, and a wet, rattling sound when affected birds breathe. Nasal discharge ranges from clear and watery early on to thick and mucoid as the infection progresses. Some birds develop open-mouth breathing or tail-bobbing, both signs of labored respiration. These symptoms can appear within days of infection and persist for weeks.³

What happens at the tissue level is a sustained inflammatory response. Within 24 hours of infection, the immune system ramps up in the trachea, and by about three days post-infection the inflammatory response peaks, with thousands of genes involved in immune signaling becoming active.⁴ The result is swelling of the mucous membranes, loss of the tiny cilia that normally sweep debris out of the airway, and excess mucus production. This creates an environment ripe for secondary bacterial infections, especially E. coli, which often turns a mild mycoplasma case into a severe one.

MS also causes respiratory disease, though it tends to be milder than MG when it stays confined to the airways. Airsacculitis, an inflammation of the thin air sacs that extend throughout a chicken’s body, is a common finding with both species. Moderately pathogenic MS strains may produce lesions only in the upper respiratory tract when birds are infected through the natural route (via the eyes or nose), while highly pathogenic strains can spread to the joints and internal organs.⁵

Swollen Faces, Puffy Eyes, and Sinus Problems

One of the more dramatic visible signs of mycoplasma, especially MG, is facial swelling. The infraorbital sinuses, located just below the eyes, fill with thick exudate and balloon outward. This gives affected chickens a distinctly puffy or lopsided look. In severe cases, the swelling can be so pronounced that one or both eyes become partially or completely obscured.⁶ Conjunctivitis, with reddened, watery, or crusty eyes, commonly accompanies the sinus inflammation. Affected birds may rub their faces on their wings or on perches in an attempt to relieve the discomfort.

These facial symptoms are often what first alerts backyard flock owners to a problem, since chickens are good at hiding general malaise but cannot hide a visibly swollen face. The swelling can be unilateral or bilateral. Turkeys tend to develop even more exaggerated sinus swelling than chickens.

Lameness and Swollen Joints

MS earns its species name, “synoviae,” from its tendency to infect synovial tissue, the lining of joints and tendon sheaths. Affected chickens develop swollen hock joints (the prominent backward-bending joint in the leg), walk with a stilted or reluctant gait, and in severe cases become unable to stand or walk at all. The joints may feel warm and fluid-filled to the touch.

Research has shown that specific characteristics of the MS organism influence how reliably it triggers joint disease. Strains with certain surface properties induced acute synovitis in chicken leg joints far more frequently than strains lacking those properties.⁷ At the microscopic level, the infection causes the synovial cells lining the joint to multiply abnormally, and the organism has been found living inside fat cells and connective tissue cells near the joint.⁸

MG can also contribute to lameness, though it does so less directly. Experimental infections with both MG and MS produced varying degrees of joint swelling, bone deformity, and abnormal cartilage development. Lameness in these birds appeared to result from a combination of joint inflammation and structural changes to the growing bones, sometimes including visible bowing and shortening of a leg bone.⁹ For backyard keepers, a chicken sitting down frequently, reluctant to move, or favoring one leg deserves a closer look, especially if respiratory signs are also present in the flock.

Egg Production Drops and Shell Problems

Mycoplasma infections hit the bottom line of any laying flock. MG-infected commercial layer flocks have been documented to produce roughly a dozen fewer eggs per hen in their first laying cycle compared to uninfected flocks.¹⁰ In backyard flocks, where conditions are less controlled, the drop can be equally stark though harder to quantify precisely.

MS causes a distinctive eggshell defect known as eggshell apex abnormalities. These are rough, thin, or wrinkled patches concentrated at the pointed end of the egg. In experimental infections, about a quarter of eggs showed these shell defects, and some birds laid soft-shelled eggs entirely. Mean daily egg production dropped by roughly 11% in infected groups compared to uninfected controls.¹¹ The shell abnormalities are particularly telling because they can occur even when the hens show no respiratory or joint symptoms, making them a valuable early clue that MS is circulating in a flock.

The egg damage worsens when MS coincides with other respiratory pathogens. Hens infected with infectious bronchitis virus before MS exposure produced abnormal eggs more frequently than hens with MS alone.¹² For anyone noticing a sudden cluster of weird-looking eggs, especially combined with even mild respiratory signs, MS should be on the shortlist of suspects.

Why Symptoms Come and Go

One of the most frustrating aspects of mycoplasma in chickens is its tendency toward chronic, subclinical infection. Birds can carry MG or MS for their entire lives without ever showing obvious signs. Stress from molting, extreme temperatures, overcrowding, transport, or the onset of lay can push a latent infection into active disease. A survey of small flock owners dealing with mycoplasmosis found that only about 39% reported full clinical recovery in their birds, suggesting that most infected flocks deal with ongoing or recurring illness.¹³

This carrier state is what makes mycoplasma so persistent in flocks. A hen that recovered from sneezing last fall may look perfectly fine through the winter, then flare up again in spring when she starts laying heavily. Meanwhile, she has been shedding the organism and infecting flockmates. The same survey found that introducing live birds was a common event before outbreaks, reinforcing the risk of bringing new, potentially infected chickens into an existing flock without quarantine or testing.

When Secondary Infections Pile On

Mycoplasma rarely works alone in the real world. The damage it does to the respiratory lining opens the door for secondary bacterial infections, and the combination is far worse than either pathogen by itself. Co-infection of MG with E. coli, one of the most common scenarios, causes more extensive immune damage and more severe tissue lesions than either pathogen alone, leading to widespread loss of airway cilia and heavy mucus accumulation in the lungs.¹⁴ This MG-plus-E. coli combination is actually the classic model of chronic respiratory disease as it plays out on farms.

The picture gets even grimmer when viral pathogens enter the mix. In experimental work examining what happens when MG, E. coli, and H9N2 avian influenza virus all infect the same birds, the triple-infection group had a mortality rate of 70%, far exceeding what any single pathogen produced. These birds also shed influenza virus in higher quantities and for a longer duration than birds infected with the virus alone.¹⁵ The practical takeaway is that a flock with underlying mycoplasma is more vulnerable to every other respiratory challenge that comes along. A cold snap, a dusty coop, or exposure to a new virus can turn a smoldering mycoplasma problem into a crisis.

How Chickens Catch It

Mycoplasma spreads primarily through direct contact between birds: respiratory secretions, shared water, and airborne droplets over short distances. Horizontal transmission, bird-to-bird spread within and between flocks, is the main route. Research on MS in indigenous chickens found that vertical transmission (from hen to chick through the egg) was actually rare: none of over 300 embryo samples tested positive, and only two out of nearly a thousand day-old chicks from MS-positive parent flocks were infected.¹⁶

That said, vertical transmission of MG through hatching eggs has been documented more reliably in other contexts and remains a concern for breeders. The safest assumption is that any new bird could be a carrier. In regions with high infection pressure, seroprevalence can be strikingly high. A study of poultry farms in central Ethiopia found over 70% of sampled birds tested positive for MG antibodies, with commercial farms showing higher rates than small-scale operations. Poor biosecurity was strongly associated with infection, with the odds of MG positivity being dramatically higher on farms with weak biosecurity practices.¹⁷

Getting a Diagnosis

Symptoms alone cannot confirm mycoplasma because several other poultry diseases, including infectious coryza, infectious bronchitis, and infectious laryngotracheitis, cause similar respiratory signs. Testing is the only way to know for sure. Veterinarians and poultry labs use a combination of blood-based (serological) tests and molecular tests. Serological tests look for antibodies in the bird’s blood, indicating exposure, while PCR-based molecular tests detect the organism’s genetic material directly from tracheal swabs or tissue samples.¹⁸

For backyard flock owners, the most accessible option is usually a blood test through a state poultry lab or veterinary diagnostic lab. In the United States, many state extension services offer MG and MS testing at low cost. A positive antibody test means the bird has been exposed but does not tell you whether the organism is actively multiplying at that moment. PCR testing is more definitive for active infection but requires swab samples and a lab equipped for molecular work. Since mycoplasma lacks a cell wall and cannot be cultured on standard bacterial media, growing it in the lab is slow and specialized.

Treatment Challenges

Mycoplasma can be treated with antibiotics, but it cannot be cured in the traditional sense. Because the organism embeds itself in host tissues and persists in carrier birds, antibiotics reduce symptoms and bacterial load but rarely eliminate the infection entirely. The absence of a cell wall means that commonly used antibiotics like penicillins and cephalosporins are completely useless against mycoplasma, since those drugs work by disrupting cell wall construction.¹⁹

The antibiotics that do work against mycoplasma include tetracyclines, macrolides (like tylosin and tilmicosin), fluoroquinolones (like enrofloxacin), and pleuromutilins (like tiamulin). However, resistance is a growing problem. Sensitivity testing of field isolates from Egyptian flocks showed variable resistance patterns, and there is evidence that even disinfectant misuse on farms can contribute to antibiotic resistance in MG through cross-resistance mechanisms.²⁰ For backyard keepers, it is worth knowing that many effective mycoplasma antibiotics require a veterinary prescription and carry egg or meat withdrawal periods, meaning you cannot eat the eggs or process the bird for some time after treatment.

Prevention and Vaccination

Because treatment is limited, prevention is the stronger strategy. The foundation of mycoplasma control is biosecurity: sourcing birds from tested, mycoplasma-free flocks, quarantining new additions, operating on an all-in-all-out basis when possible, and limiting contact with wild birds and other poultry. An effective monitoring program using regular serological testing rounds out the approach.²¹

Vaccines exist for MG, and commercial layer operations commonly use them, but the picture is complicated. Current vaccines provide incomplete protection, and some live vaccines can themselves cause mild disease or production dips. Research on commercial layers found that vaccinating pullets with the temperature-sensitive ts11 MG vaccine before lay, followed by a booster with the F-strain vaccine after peak production, maintained performance without significant suppression of egg output.²² Still, the limited protection and occasional adverse reactions from existing vaccines remain a concern.²³

No widely available commercial MS vaccine exists for chickens yet, though research into next-generation approaches is active. Experimental recombinant vaccines using modified Salmonella as a delivery vehicle have shown promising results against both MG and MS in lab settings, with survival rates of 90 to 95% after challenge and reduced bacterial shedding.²⁴ These are still experimental, but they point toward a future where a single vaccine could protect against both major poultry mycoplasma species.

Distinguishing Mycoplasma From Other Respiratory Diseases

Several other infections mimic mycoplasma symptoms closely enough to fool even experienced poultry keepers. Infectious coryza, caused by Avibacterium paragallinarum, produces a very similar picture of facial swelling, nasal discharge, and reduced egg production, but it tends to come on faster and cause a more foul-smelling discharge. Infectious laryngotracheitis causes gasping and bloody mucus, which is less typical of mycoplasma. Infectious bronchitis virus targets the respiratory tract and the reproductive system, causing respiratory distress and shell abnormalities that overlap heavily with what MS does to eggs.

The overlap is not just academic. On real farms, these pathogens often circulate together. A hen with a runny nose and a drop in egg quality could have mycoplasma, or infectious bronchitis, or both, or something else entirely. This is why testing matters. Treating blindly with tylosin when the actual problem is viral laryngotracheitis wastes money and delays appropriate management.

What Backyard Flock Owners Should Watch For

If you keep a small flock, the symptoms to flag are subtle at first. A single hen sneezing occasionally, a bit of eye bubbling (frothy discharge at the corner of the eye), slightly reduced appetite, or one bird that seems less active than usual. These early signs are easy to write off, especially in cold weather when a little nasal discharge might seem normal. The disease tends to creep through a flock over days to weeks rather than hitting everyone at once.

A few practical guidelines are worth keeping in mind. First, never add new birds without a quarantine period and, ideally, testing. The survey data consistently points to introducing live birds as a major source of outbreaks.²⁵ Second, once mycoplasma is confirmed in a flock, you should assume it will be there permanently. Depopulation and thorough cleaning is the only reliable way to truly eliminate it from a premises, and for most backyard keepers that is not realistic. Third, maintaining good ventilation, dry litter, and low dust levels in the coop reduces respiratory stress and may keep a latent infection from flaring. Finally, be cautious about selling or giving away birds from a positive flock without disclosing their status, as you would be spreading the organism to someone else’s birds.

Breed Susceptibility and Flock Size

Not all chickens respond to mycoplasma equally. Research has found significant differences in MG prevalence between breeds, with some commercial layer breeds showing higher positivity rates than dual-purpose or indigenous breeds.²⁶ Larger commercial operations also tend to see higher infection pressure than small-scale farms, likely because of higher bird density and more frequent movement of people and equipment between houses. Age plays a role too: older birds have had more time and opportunity for exposure, and the stress of prolonged production may reduce immune resilience.

For backyard keepers with mixed-breed flocks, this means you may notice that some birds in the same coop develop obvious symptoms while others seem unaffected. That variation is real and reflects differences in individual immune response and genetic susceptibility, not necessarily differences in exposure. All birds in a positive flock should be considered carriers regardless of whether they look sick.