Leg weakness (osteochondrosis)
Age affected: Growers/finishers, sows, boars.
Causes: Genetic; rapid growth; lack of exercise; nutritional imbalance.
Effects: Difficulty rising or walking, swaying, misshapen limbs.
Osteochondrosis is a generalised skeletal disease with leads to leg weakness. The changes have been demonstrated in piglets 1 day of age and may be congenital. Osteochondrosis becomes more obvious with age, although developing and healing lesions may occur at any one time. Areas of cartilage remain in bone developing from the cartilaginous physis or growth plate separating epiphyseal (end and metaphyseal (shaft) bone and at the epiphyseal-articular surface. These cartilaginous areas lead to structural weakness and fracture or distortion of the bone, separation of the cartilage from the underlying bone and to clefts in the epiphyseal plate. The cause appears to be a defect in the chondrocytes (cartilage cells) which do not mature normally. The matrix around them may prevent blood vessel formation and the formation of normal bone. Such lesions may eventually form bone. Growth rate and heredity factors appear to affect their development, especially at the elbow, but diet or flooring type appear to have no effect. These changes lead to abnormal growth and change in the shape of bones and joints, including the vertebrae. Erosion of the articular cartilage and painful osteoarthritis of the joint develop. Epiphyseolysis can result when the head of the femur is shed in rapidly growing animals.
Mode of transmission
The condition is not transmissible from pig to pig in a group, but any hereditary elements can be vertically-transmitted. Common environmental factors (nutrition, flooring, degree of exercise will affect successive batches.
Present in up to 80% of pigs, particularly heavy, rapidly growing in meaty breeds such as Dutch Landrace or Swedish Landrace, or Duroc kept on hard flooring. Clinical signs include shortening of the step, knock and buck knees and swaying of the forelimbs. In the hind limb the feet may be positioned too far forward, and pigs stand with their legs apart and show weakness at the pastern and swaying of the hindquarters. Changes increase with age. Epiphyseolysis may lead to sudden hind limb lameness and pain at the hip but not necessarily at the stifle or hock. The affected leg is shorter than the unaffected one, but the thickness of the muscle mass makes identification difficult. Animals may be unable to rise, adopt a dog-sitting position and resent movement. Epiphyseolysis occurs commonly in young gilts when first delivered or at service. Osteochondrosis is a common cause of leg weakness and lameness in young breeding stock and may progress to arthrosis (degeneration of the joints) and deformation of affected bones or resolve completely by 6-9 months of age. Forelimb weakness is obvious in some Duroc boars. Kyphosis (curvature of the spine) may also develop as animals attempt to spare affected limbs.
Clinical signs of bowed legs and swaying gait suggest the presence of osteochondrosis. Epiphyseolysis should be suspected where gilts are delivered lame or where they become lame suddenly at or around service. Osteochondrosis can be confirmed by radiography from 63 days of age. Confirmation at slaughter or post-mortem is more common.
Osteochondrosis is mainly noticed when lameness is being investigated and during routine investigation of carcase joints during poet-mortem. Deformation of the joint surfaces may be seen with collapse of the underlying bone. Thickening of the cartilage with separation from underlying bone is the most prominent feature of the pathology and in joints, flakes of cartilage may become 'joint mice' and produce synovitis (inflammation of the joints). Deformation may be found at affected points in long bones and they may even fracture especially at the neck of the femur. Epiphyseolysis can be detected post-mortem by disarticulating the hip. The femur ends in a severed neck and is sometimes surrounded by clots of blood or bruised muscle. The joint cavity remains filled with the head of the femur. Osteochondrosis must be separated from inflammatory joint conditions such as erysipelas and mycoplasmal arthritis. Joint ill is usually confined to smaller pigs. Epiphyseolysis may be confused with fracture of the femur especially pathological fracture in hypocalcaemic gilts.
Treatment and prevention
There is no treatment although lameness may resolve after moving to non-slip softer floors or deep bedding. The production effects resulting from lame boars and gilts can be overcome by use of artificial insemination. Where epiphyseolysis is present, animals should be killed immediately without moving them. The condition cannot be prevented completely, but its severity can be reduced. It has been suggested that the calcium and phosphorus levels of high nutrient density diets should be increased in order to supply adequate levels of these minerals in rapidly-growing pig. The growth rate of breeding stock can be reduced. Exercise may be beneficial and gilts intended for breeding may be kept in straw yards. There is evidence that the condition may be under genetic control. It is much less common in large White than in Landrace pigs and genetic studies with Duroc boars suggest that it can be selected against. This selection may simply reduce the stress on joints provided by large hams or other features of conformation. In all cases where leg weakness or osteochondrosis are a problem, the condition of the floors should be improved, non-slip surfaces used and the possibility of fighting or other violent exercise minimised.