Septic arthritis is a bacterial infection of a joint. It is often combined with osteomyelitis, infection of the bone. Septic arthritis is commonly referred to as joint ill, navel ill, septic polyarthritis, septic epiphysitis and septic physitis. In the neonatal foal it is secondary to septicemia, which is a wide spread infection that is in the blood system of the animal. Septicemia is usually secondary to the foal not receiving enough good quality colostrum at birth. Colostrum is the antibody-rich first milk that gives the foal protection against bacteria in the environment. Infection can enter the foal's body through ingestion, inhalation or through the umbilical stump. About 50 percent of the foals with septic arthritis have umbilical abscesses.
In one study it was found that over 25 percent of septic foals have joint or bone infection. Anatomically, the blood vessels in the bones adjacent to joints and the growth plates, make a sharp turn. It is believed that the blood flow through these natural hairpin turns actually slows down, allowing the bacteria time to settle out and multiply in these regions.
Lameness or increased joint swelling (effusion) in a neonatal foal should be attributed to a septic arthritis/osteomyelitis process until proven otherwise. Many owners falsely believe that the mare must have stepped on the foal to make it lame but in fact the foal has a septic joint. This is important because septic joints need emergency treatment. (In humans, it is believed that septic joints must be treated within 24 hours to obtain complete recovery.)
Clinically, the foal may show multiple problems related to septicemia, such as shock, pneumonia, diarrhea and meningitis, or they may be normal with the exception of a hot, swollen joint. A single joint or multiple joints may be affected in a foal. The most common joint involved is the stifle followed by the hock, carpus, fetlock and shoulder.
Septic arthritis/ osteomyelitis in the foal has been described as four different types depending on where the infection is located. The S-type denotes synovial infection with no evidence of bony involvement. The synovia is the inside lining of the joint itself. This type is usually seen in young foals, a few days old, that have other systemic signs of illness. More than one joint may be affected. Joint distension is present and the animal is in extreme pain. The second type is the E-type or epiphysitis type, which involves osteomyelitis (bone infection) of the epiphysis, the bone that underlies the cartilage of a joint. The clinical signs are similar to the S-type, but the foals may be slightly older. The P-type occurs when there is osteomyelitis of the physis or growth plate of the bone. The P-type is usually seen in older foals and affects only one joint. A fourth type of infectious arthritis has been described as the T-type, which involves the small bones of the hock (tarsal bones).
Because septic arthritis is a painful condition, foals spend more time laying down. Pressure sores may develop over a bony prominence, such as the hip, hock or elbows. A pressure sore starts off as a thickening or leathery feel to the skin. This skin then sloughs off leaving a raw open wound. The presence of these sores should alert an owner that the foal is laying down more that he should and a source of pain should be investigated.
The short-term outcome of septic arthritis/osteomyelitis is dependent of the promptness with which treatment is initiated, the number of joints involved, the amount of bone destruction and the other body systems that may be involved. One study reported that approximately 60 percent of the foals with just the S-type septic joint survive the neonatal period. The presence of bone infection (osteomyelitis) decreases the short-term survival to less than 40 percent. The septic joint does not cause death of the foal, rather the owners often reject further treatment and elect euthanasia. Long-term full recovery is possible but some arthritis may remain.