Background and history

Trauma is by far the most common cause of lameness in the dry sow from point of weaning to point of farrowing. Environmental trauma to the coronary band area and to the sole or wall of the foot results in penetration of the sensitive tissues, infection and lameness. These foot conditions are called bush foot and foot rot. Trauma however, more commonly arises indirectly from other causes within the environment that create shear forces on the muscles, tendons, bones and bone structures. Such changes associated with cartilaginous structures are referred to as leg weakness or osteochondrosis.

The term Leg weakness" is also used sometimes to describe poor leg conformation or describe a clinical condition associated with lameness and stiffness. It arises due to abnormal changes in the articular cartilage and the growth (epiphyseal) plates. These plates are responsible for the growth of bones both in length and diameter. Whilst the exact mechanisms that cause these changes are not fully understood they arise due to the pressure and shear stresses that are placed upon these rapidly growing tissues. This pressure reduces the oxygen supply, causing abnormal growth and consistency of the cartilage. Damage to the cartilage tends to be progressive and irreversible. The damaged cartilage is replaced by fibrous tissue. This cartilage damage in turn produces shortening and bending of the bones near the joints and at the extremities of the long bones. Weak epiphyseal plates also have a tendency to fracture and cartilage covering the joint surfaces splits and forms fissures. It is important to appreciate that such changes in the cartilage take place in most if not all modern pigs from as early as two months of age. In some cases many of these can only be detected under the microscope. It is interesting to note that such changes can not be detected in the wild boar which takes up to two years to reach maturity. OCD is therefore a fact of life in modern pig production but its severity and its effects depend largely on the environment. OCD results from the many years of selecting animals for rapid growth, large muscle mass, and efficient feed conversion and therefore much greater weight on the growth plates whilst they are still immature, together with the stresses of intensive methods of production. Conversion of cartilage to bone involves the deposition of calcium and phosphorous and while the process of breaking down and reforming bone goes on throughout life, bone growth ceases when the sow is approximately 14 to 16 months of age.

It is not uncommon in breeding enterprises for 20 to 30% of boars and gilts to be culled after completing the performance test, due to leg weakness and leg deformities.

Sows with good leg conformation show angulation of the bones at the hip, knee and hock joints. The bones below the hock slope slightly forwards and the feet are well placed on the ground. Sows that are susceptible to leg weakness are straight legged with little angulation of the bones between the joints and the back tends to be arched. This alignment increases shear stresses on the growth plates.

Similar diseases

There are two, Mycoplasma hyosynoviae infection and erysipelas. In both of these the disease is usually sudden in onset sometimes with a raised body temperature and there is pain and swelling in the joints. A differentiating feature is the response to treatment. OCD does not respond to antibiotics whereas Mycoplasma hyosynoviae and erysipelas will respond within 24 to 36 hours, the former to lincomycin or tiamulin and the latter to penicillin.

Clinical signs

Acute disease

This is seen when there is a separation or fracture of the bones at the epiphyseal plate (epiphyseolysis) associated with sudden movement. The animal walks on three legs, the affected leg swinging freely. Crepitus or rubbing of the broken bones together can usually be felt. Sudden fractures can also occur in the knee and elbow joints, which are more common in the young growing pig. Fractures of the vertebrae in the spine occur particularly during lactation and immediately post weaning. In such cases the sow is in acute pain, often in a dog sitting position with the hind legs well forward. Animals housed in farrowing crates with slippery floors tend to slide the back legs forward and there is a risk of the hind muscles pulling away from their attachments to the pelvis (apophyseolysis). In such cases the sow will stand with assistance but it cannot pull the hind leg backwards. When it is placed on to the ground it just slides forward. Such animals should be culled immediately.

Chronic disease

The onset is gradual. The pig shows abnormal leg conformation and gait with or without stiffness and pain. The temperature remains normal and joints will not be swollen unless there are fractures.

Front legs

• These may be straight with the pig walking with a long step on its toes.
• The knees may be bent inwards or flexed which causes the pig to walk with short steps.
• The pasterns may be dropped. This is common in old sows due to shortened bones and slack tendons.
• The feet may be rotated or twisted.

Hind legs

• These are straight with a swinging action from the hips as the pig moves. Avoid selecting such females for breeding.
• The legs are tucked beneath the body.
• The hocks turn inwards and are close together.
• The pig walks with a goose stepping action.
• Likewise in old sows the pasterns may be dropped.

Abnormal gaits arise either from pain in the joints or abnormal movements in the hind legs from the hips which give a swaying motion. The pain is associated with damage to the sensitive membranes around the joints resulting from either splitting or erosion of the cartilage in the joints or movement of the growth plates. Some pigs however may show severe clinical signs yet on post mortem examinations the joints appear normal and vice versa. Joints may become inflamed (arthritis), particularly in the hip, knee and elbow. OCD may be seen within three months of gilts being introduced on to the farm, during their first pregnancy, in lactation or in the first 2 to 3 weeks post weaning.

Diagnosis

OCD is diagnosed on the clinical signs described. There are no serological or other tests and post-mortem examinations may be misleading because many pigs that are found to have joint lesions may not be lame.

Causes

• Environmental factors that cause the foot to slip on the floor.
• The design of slats can contribute to OCD. Some slats slope to the edges from the centre and are so smooth that when the animals stand, the feet slip into the gaps, causing repeated pressure on the growth plates.
• Full confinement of pregnant gilts when they are still growing can be a major contributing factor.
• Are the gilts mixed with sows at weaning? The modern hybrid gilt often suckles produces large litters and large amounts of milk which depletes her body calcium and phosphorous. The bones become weak and are therefore more prone to injury.
• High levels of vitamin A (in excess of 20,000 iu/kg) particularly in the younger growing pigs can interfere with the normal development of the growth plates.
• High stocking densities increase the incidence, particularly in the growing period and where animals are housed on solid concrete floors or slats.
• Trauma.
• Rapid weight gain.

Prevention

If OCD is causing a problem on your farm check through the list below and identify those points that are important in your system.

• Determine the time and place when OCD first becomes evident.
• Look carefully at surfaces that cause the foot to slip and result in increased pressure on the growth plates.
• If the problem is in gilts look carefully at the conformation of these animals and consider improving the selection procedure.
• Animals with a fine bone structure are more prone to leg weakness.
• Animals with heavy ham muscles and a disparity between anterior and posterior muscle masses, increases the horizontal pull on the growth plates. Such young gilts do not acclimatise well to stalls or tethers.
• If there is a problem in lactating gilts look at the floor surfaces in the farrowing crates. Are these slippery and causing pressure on weakened bones? The use of dry sand in these pens once or twice a week may assist in the prevention of the condition until the floor surfaces can be altered.
• The modern hybrid gilt often suckles ten or more pigs and produces a large amount of milk which is high in calcium and phosphorous. The bones are therefore more prone to calcium or phosphorus depletion and if the gilt is mixed at weaning time with older sows, damage and sudden fractures are likely to take place.
• The tethering and confinement of gilts during pregnancy can be a major contributing factor. Culling rates in such animals may rise towards 20% if the floor surfaces are very slippery.
• The design of slats can contribute to OCD. Some slats slope to the edges from the centre and are so smooth that when the animals stand, the feet slip into the gaps. This causes constant twisting and pressure on the growth plates and a high incidence of OCD may result.
• Gilts in the first pregnancy should not be housed on slippery floor surfaces.
• High levels of vitamin A (in excess of 20,000 iu/kg) particularly in the younger growing pigs can interfere with the normal growth of the epiphyseal plates. OCD lesions have been seen in piglets as young as 3 to 4 weeks of age where sows have been fed excessive levels of vitamin A.
• High lysine levels may also increase the incidence.
• High stocking densities, particularly in the growing period and where animals are housed on solid concrete floors or slats, will increase the incidence of OCD in the breeding stock. Such environments cause great stress on the legs and pens tend to be dirty and wet.
• Fat sprayed diets can make floor surfaces slippery.
• Pigs treated with porcine growth hormone tend to develop severe lesions of OCD at an early age.
• New concrete surfaces or slats during the first few weeks of use may develop slippery surfaces after contact with faeces and urine. They should be pressure washed with detergent on two or three occasions.
• Variations in nutrition within acceptable limits do not appear to influence OCD but conditions of osteomalacia and osteoporosis (soft and weak bones) occur where there is an imbalance of calcium and phosphorous in the diet or excessive withdrawal of these minerals from the bones. Soft and brittle bones in such deficiencies often fracture across the mid part of the long bones during lactation. Always check the calcium phosphorous ratios of the diet and add additional minerals during lactation, dicalcium bone phosphate could be used.
• Pain in the joints from OCD also gives rise to abnormal stresses on the muscles particularly where they are attached to the bone. Tearing of these attachments on the insides of the shoulders and legs and from the muscle masses in the pelvis is common causing severe pain and periostitis.
• Smooth concrete surfaces that have been finished with a steel float can predispose to OCD.
• The addition of 0.5% sodium bicarbonate/tonne to the ration has been shown to reduce the incidence.

Treatment

• There is no specific treatment for OCD, however, at an early stage the sow should be moved from its existing environment to a well-bedded pen where the foot can grip. If not, the lesions progress and ultimately arthritis and permanent lameness develop.
• Gilts that are confined in stalls or tethers, or group housed on floors that are wet and slippery should be moved as soon as clinical signs appear.