Perosis in Chickens: Slipped Tendon and Hock Deformity

Perosis is a developmental leg problem in chickens, especially young, growing birds. Pet parents may hear it called slipped tendon, but that phrase does not tell the whole story. The condition usually starts with abnormal development around the hock joint and nearby leg bones. As the joint changes shape, the tendon at the back of the leg can move out of its normal groove.

Common changes include hock swelling, flattening or enlargement of the joint, twisting of the lower leg, bowed legs, and difficulty standing. In more advanced cases, the tendon slips from its condyles and the bird may sit more, limp, or become unable to walk normally.

Perosis is usually considered a nutrition-related orthopedic problem, not a contagious disease. In chickens, it is classically associated with manganese or choline deficiency, although other nutrient problems can contribute and some other conditions can look similar. Because several disorders can cause lameness and hock swelling, your vet may need to sort out whether this is true perosis, trauma, infection, or another leg disorder.

The outlook depends a lot on timing. If your vet identifies the issue early and the diet is corrected quickly, some birds improve. If the deformity is severe or has been present for a while, the leg may not return to normal and long-term mobility can remain limited.

The classic causes of perosis in chickens are manganese deficiency and choline deficiency in the diet. Merck Veterinary Manual describes perosis as a classic sign of choline deficiency in chicks and also lists manganese deficiency as a major cause of enlargement and malformation of the hock joint, twisting of the leg bones, and tendon slippage. In practical terms, this usually means the ration is unbalanced, homemade feed is incomplete, feed was mixed incorrectly, or birds are eating too many treats and not enough complete poultry feed.

Other nutrient problems can contribute or create a similar picture. Merck notes that perosis is not a single-nutrient, one-cause condition and can also be seen with some other vitamin deficiencies. High calcium or phosphorus intake can worsen manganese deficiency by reducing manganese absorption. That means even well-meaning supplementation can backfire if the overall diet is not balanced for the bird's age and purpose.

Young, rapidly growing birds are at the highest risk because their bones and joints are developing quickly. Breeder nutrition matters too. Poor breeder diets can affect hatchability and chick skeletal development, so some chicks may start life at a disadvantage.

Not every swollen hock is perosis. Your vet may also consider trauma, infectious synovitis, other developmental leg disorders, and less common metabolic bone problems. That is why a careful history, feed review, and physical exam matter before making assumptions.

Your vet usually starts with a hands-on exam and a detailed feeding history. Bring the exact feed name, form, label, supplements, treats, and the age of the bird. In many backyard cases, the diet history is one of the most important clues. Your vet will look at the hock shape, leg alignment, ability to stand, and whether the tendon feels displaced.

Because several conditions can mimic perosis, diagnosis is often about combining the exam with context. Your vet may recommend radiographs to look for bone deformity, fractures, or other orthopedic disease. If the hock is very swollen, warm, or painful, your vet may also consider infectious causes such as synovitis rather than assuming the problem is nutritional.

In flock situations, your vet may assess more than one bird and review the ration for the whole group. That can be especially helpful when multiple chicks show poor growth or leg changes. Feed analysis is not always needed, but it can be useful when the history is unclear, the feed may have been stored poorly, or a homemade ration is being used.

Early diagnosis matters because mild cases may improve when the underlying diet issue is corrected and supportive care is started. Once the bones and joint surfaces are badly deformed, treatment may focus more on comfort, mobility, and quality of life than full correction.

Prevention starts with feeding a complete, age-appropriate poultry ration as the main diet. This is the most reliable way to provide the manganese, choline, and other nutrients growing chickens need. Homemade diets, heavy treat feeding, and unbalanced supplementation are common ways birds drift into nutrient problems. If you keep multiple age groups, make sure chicks are not relying on feed meant for adult layers or other species.

Store feed properly and replace stale or damp feed promptly. Even a well-formulated ration cannot help much if birds are not eating enough of it or if the feed has been diluted by scratch grains and table foods. If you use supplements, ask your vet before adding them. Extra calcium or phosphorus can interfere with manganese balance, so more is not always safer.

Watch young birds closely during growth spurts. Early hock puffiness, awkward gait, or a chick sitting more than its flockmates can be the first sign that something is off. Prompt flock review may prevent more birds from developing permanent deformities.

If you breed chickens, breeder nutrition matters too. Poor parent-flock nutrition can affect hatchability and chick skeletal development. Regular preventive visits with your vet are helpful for backyard flocks, especially if you mix your own feed, raise fast-growing birds, or have had leg problems before.