Mycoplasma Arthritis in Sheep: Chronic Joint Disease and Lameness

Mycoplasma arthritis in sheep is an infectious form of arthritis caused by Mycoplasma bacteria, most notably Mycoplasma agalactiae in small ruminants. These organisms can invade the joints and trigger inflammation of the synovial lining, pain, swelling, and progressive damage to cartilage. In some flocks, arthritis is the main problem. In others, it appears alongside mastitis, eye disease, or respiratory signs.

One challenge is that "mycoplasma arthritis" is a practical clinical label, but sheep with swollen joints and lameness can have several different infectious diseases that look similar. Merck notes that contagious agalactia in sheep and goats commonly involves arthritis, and that diagnosis depends heavily on laboratory testing, especially in chronic herds. Merck also notes that differentiating chlamydial arthritis from mycoplasmal arthritis requires microbiologic investigation.

For pet parents and producers, the big concern is chronicity. Some sheep improve with treatment and supportive care, while others are left with lasting stiffness, enlarged joints, reduced mobility, and poor thrift. Because infected animals may remain carriers, flock-level management is often just as important as care for the individual sheep.

See your vet promptly if a sheep has hot or swollen joints, refuses to bear weight, becomes weak, or if several animals in the group develop lameness at the same time.

The underlying cause is infection with Mycoplasma organisms, especially Mycoplasma agalactiae in sheep and goats. Merck describes contagious agalactia as a worldwide disease of dairy and meat sheep and goats in which mastitis, conjunctivitis, and arthritis are the typical signs. Sporadic cases have been reported in the United States, and infected animals can remain lifelong carriers even after treatment.

Mycoplasmas usually spread through close contact with infected animals and contaminated milk, secretions, or equipment. Once in the body, the organism may circulate briefly and then localize in tissues such as joints, mammary glands, eyes, or lungs. In practical terms, flock stress, commingling, transport, poor biosecurity, and introduction of carrier animals can all increase risk.

Not every sheep with swollen joints has mycoplasma arthritis. Important look-alikes include chlamydial polyarthritis, Erysipelothrix-associated polyarthritis in lambs, footrot or other hoof disease, trauma, and less commonly systemic infections that seed the joints. That is why your vet may recommend testing instead of treating based on appearance alone.

Because mycoplasmas lack a cell wall, they behave differently from many routine bacteria. This affects both testing and treatment choices, and it is one reason culture can be difficult and why antimicrobial selection should be guided by your vet whenever possible.

Diagnosis starts with a careful history and exam. Your vet will want to know the sheep's age, how many animals are affected, whether signs are acute or chronic, and whether there have also been cases of pneumonia, mastitis, or conjunctivitis. A hands-on lameness exam helps identify which joints are involved and whether the problem is truly articular or coming from the hoof, tendon, bone, or soft tissues.

Definitive diagnosis usually requires laboratory testing. Depending on the case, your vet may collect joint fluid, milk, eye swabs, respiratory samples, or tissues from a necropsy case. Merck notes that in contagious agalactia, diagnosis depends on lab testing, especially in chronically infected herds, and that useful postmortem samples can include joint fluid, lung tissue, and mammary tissue. PCR and specialized culture are often more helpful than routine bacterial culture when mycoplasma is suspected.

Imaging can also help. Radiographs may show soft tissue swelling and joint effusion early, with more permanent bony or cartilage changes in chronic disease. Bloodwork is not specific, but it may help assess inflammation, dehydration, or concurrent illness.

Because several infectious causes of sheep polyarthritis overlap clinically, your vet may diagnose this as a flock problem rather than an isolated lame sheep. In those situations, testing a few representative animals, or performing necropsy on a freshly deceased or euthanized case, may be the most efficient way to guide treatment and prevention.

Prevention focuses on biosecurity, early detection, and flock management. Do not bring new sheep directly into the main group without a quarantine period and a health review with your vet, especially if they come from herds with respiratory disease, mastitis, eye problems, or unexplained lameness. Because mycoplasma infections can persist in carriers, apparently normal animals may still introduce disease.

Good hygiene matters. Clean and disinfect lambing, milking, feeding, and handling equipment between groups when possible. Reduce overcrowding, improve ventilation, and lower stress during transport, weaning, and regrouping. If a sheep develops swollen joints or other suspicious signs, isolate it promptly and have your vet assess the flock pattern rather than waiting for multiple cases.

Work with your vet on a flock health plan if you have recurring lameness or mixed signs such as arthritis plus mastitis or conjunctivitis. In chronic herd situations, testing representative animals and making culling decisions may be more effective than repeated treatment of relapsing individuals. Merck notes that animals infected with contagious agalactia organisms can remain lifelong carriers, which is why prevention often depends on management as much as medication.

There is no one-size-fits-all prevention program. The best plan depends on whether you manage a small hobby flock, dairy sheep, seedstock, or commercial lambs. Your vet can help tailor quarantine, testing, treatment, and culling decisions to your goals and local disease risks.