here is a copy of an article about it:
Clostridial myositis
Serious bacterial infections following IM injections are caused by Clostridial species. In the survey of 500 equine practitioners, 28 (5.6%) had encountered a case of clostridial infection at an IM injection site. These occur in association with non-antibiotic injections only. There is no relationship to the infection and the technique used to prepare the skin and fill the syringe.
Signs develop over 2-5 days post injection and progress rapidly within 5-10 days. Swelling and heat at the injection site are followed by widespread crepitation from gas formation, and skin will become cold and necrotic. Early, aggressive surgical debridement and antibiotic therapy are essential for any hope of survival. Unfortunately, prognosis is very poor once signs develop.