Mycoplasma synoviae in Chickens: Infectious Synovitis, Lameness, and Eggshell Problems

Mycoplasma synoviae is a contagious bacterial disease of poultry. In chickens, it is best known for causing infectious synovitis, which means inflammation around joints and tendons that can lead to lameness, swelling, and birds sitting more than usual. Some flocks also show mild respiratory signs, while others look normal until stress or another infection brings the problem to the surface.

This organism spreads both vertically through infected eggs and horizontally between birds through respiratory secretions, close contact, and contaminated shoes, tools, crates, or other equipment. Once it gets into a flock, it can move quietly for days to months before obvious signs appear.

One frustrating part of this disease is that infected chickens often become long-term carriers. That means a bird may improve clinically but still carry the organism and continue to pose a risk to the rest of the flock. In layers, Mycoplasma synoviae has also been associated with reduced egg production and distinctive shell defects, especially thin, rough, translucent shells with an abnormal pointed end.

For backyard flocks, this can be confusing because lameness and shell problems have many possible causes. Your vet can help sort out whether Mycoplasma synoviae is likely, whether another disease is involved too, and what level of care makes sense for your birds and your goals.

Mycoplasma synoviae is caused by infection with the bacterium Mycoplasma synoviae. Chickens can pick it up from infected breeder stock through the egg, from direct contact with infected birds, or from contaminated people, clothing, cages, and equipment moving between groups. Human activity and fomites are important ways the organism spreads between farms and flocks.

Stress often makes disease expression worse. Poor air quality, crowding, transport, weather swings, and concurrent infections can all make a previously quiet infection become more obvious. In some birds, respiratory infection stays mild. In others, the organism localizes in joints and tendon sheaths, leading to infectious synovitis with swelling and pain.

In laying hens, Mycoplasma synoviae has also been linked to eggshell apex abnormalities. These eggs may have a rough shell surface near the pointed end, thinning, translucency, and a higher risk of cracks or breaks. Research suggests the oviduct can be involved in birds producing these abnormal eggs, and shell problems may be more noticeable when other respiratory infections, such as infectious bronchitis, are also present.

Because chickens can remain infected for life, a flock may seem to improve and then show problems again later. That carrier state is one reason flock history, source of birds, and recent additions are so important when your vet is trying to identify the cause.

Diagnosis starts with a good flock history and physical exam. Your vet will want to know the age of the birds, whether they are layers or meat birds, where they came from, whether new birds were added recently, and whether you are seeing lameness, respiratory signs, or shell changes. Joint swelling and flock patterns can raise suspicion, but they do not confirm the diagnosis on their own.

Testing is usually needed because other problems can look similar, including trauma, skeletal issues, viral tenosynovitis, staphylococcal joint infections, infectious bronchitis, Newcastle disease, and Mycoplasma gallisepticum. Common tests include PCR on choanal, tracheal, sinus, air sac, or joint samples, plus blood tests such as plate agglutination or ELISA to look for antibodies. PCR is widely used because Mycoplasma organisms can be difficult to culture.

If a hen is laying abnormal eggs, your vet may also evaluate the rest of the flock, review production changes, and consider other causes of shell defects. In some cases, necropsy and laboratory submission of a recently deceased bird can provide the clearest answers, especially when more than one disease may be involved.

For backyard chickens in the US, a practical diagnostic cost range is often $190-$650 for an exam and basic testing, while more extensive workups with radiographs, necropsy, multiple PCR panels, or repeated flock sampling may run $650-$1,500+. Your vet can tailor testing to your goals, whether that is confirming infection in one pet chicken or making a flock-level management plan.

Prevention starts with buying birds and hatching eggs from reputable, tested sources. In the US, the National Poultry Improvement Plan includes Mycoplasma synoviae in its disease-control and certification framework for poultry breeding stock, which can help reduce risk when you are choosing where new birds come from. Ask direct questions about testing history before bringing home chicks, started pullets, or hatching eggs.

Strong biosecurity matters every day, not only during an outbreak. Quarantine new birds before mixing them with your flock. Avoid sharing crates, feeders, waterers, and equipment with other poultry keepers unless items are thoroughly cleaned and disinfected. Change footwear or use dedicated coop shoes, wash hands after handling birds, and limit unnecessary visitors in poultry areas.

Good flock management also lowers the chance that hidden infection will flare into obvious disease. Keep bedding dry, reduce crowding, support ventilation, and provide clean feed and water off the ground when possible. Stress reduction will not prevent every case, but it can make a meaningful difference in how severely disease shows up.

There is no vaccine listed for backyard chicken mycoplasma prevention in VCA's current client guidance, so prevention relies heavily on sourcing, quarantine, and biosecurity. If your flock has had confirmed or suspected Mycoplasma synoviae before, talk with your vet before adding any new birds. A closed flock is often the safest long-term option.