At this time of year it is important to monitor your foal’s limbs closely. We frequently see physitis in 3-9 month old foals, particularly the larger, rapidly-growing ones. Physitis is defined as inflammation of a physis, which is a growth plate. At the growth plates, cartilage is getting transformed into bone through a process called endochondral ossification, and the horse is getting taller. Damage to these growth plates can result in permanent damage to the foal’s limbs. This can be caused by excessive growth from nutrition and/or genetics, or from infection or trauma. Nutritional causes are common and often due to excessive feeding and mineral imbalances (mainly calcium and phosphorous). Physitis can also occur in a foal following an injury to the opposite leg due to the extra weight put on the “good leg.” The proposed mechanisms are inflammation due to excessive loading or weakened bone and/or cartilage, or a combination of these factors.
Figure 1. This foal has what we would consider good normal forelimb conformation at 3 months old. As different bones go through rapid growth at different times it is important to continually monitor your foal’s limbs through to 2 years of age. During this time angular limb deformities can develop (limbs become crooked). The white arrows show where the proximal growth plates
Common sites include the distal cannon bone (proximal fetlock), distal radius (proximal knee) and distal tibia (proximal hock) (Figure 1). Fetlock swelling is usually seen between 3-6 months of age, and the distal radius between 8 months and 2 years of age. The swelling is firm, round and can be painful and hot to touch. Affected joints get an hourglass appearance (Figure 2), and only in severe cases are the horses lame. Affected horses may walk more on the outside of their foot, as commonly the medial (inside) side of the growth plate is worse affected. The medial, inflamed growth plate then grows less and can cause a varus (“toe in”) conformation which is permanent without intervention.
Figure 2. The bell shape of the bone indicated by the arrow is abnormal and due to physitis (large white arrow). The smaller black arrow shows a lytic (black) area in the growth plate, this is abnormal and a sign of inflammation.
Treatment may include rest (confinement) and pain relief. It may take 2 weeks to 2 months to settle. Corrective trimming may also need to be done on the side of the physitis. Dietary modification of reduced energy intake and ensuring adequate levels of minerals in the right ratios is important. Most horses have a good prognosis if diagnosed early enough. Severe cases may require surgery and it can be a career limiting condition if an angular limb deformity (ALD) forms. See figures 3, 4, and 5.
Figure 3. This is the lateral fetlock image of the same joint Figure 2, showing the sclerosis (increased whiteness or density) around the lytic lesion, all are indications of an inflamed physis. This foal was treated with confinement and diet restrictions and was not lame at the time of diagnosis (3 months of age).
Figure 4 & 5. This is the same foal nearly two months later, you can see the lytic lesion has resolved and there is some sclerosis (increased density/whiteness) still present but less than the previous set of x-rays.