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damage to the underlying tissues that the barrier is compromised.”
On the other hand, secondary cellulitis develops when bacteria gain entry through a wound, surgical incision or another known route. Breaks in the surface caused by dermatitis, the technical name for inflammation of the outer layers of skin, can also allow bacteria in. “When the integrity of the skin is compromised, bacteria can gain entrance and replicate in the underlying tissue,” says Hammond. “It is impressive what can happen when that barrier is damaged.”
A variety of bacterial species have been implicated in cellulitis, but the infection is usually caused by Staphylococcus aureus and Streptococcus species. “These are common bacteria that are often found on skin,” Mudge says. Other bacterial species that may be involved include Enterococcus and Actinobacillus species. “Occasionally, cellulitis can be due to infection with Corynebacterium or Clostridium or a gram-negative bacterium such as Escherichia coli,” says Callie Fogle, DVM, of North Carolina State University.
Secondary cellulitis can develop anywhere on the body where a wound occurs. Primary cellulitis usually develops on a leg, and most often the hind legs. “The term ‘cellulitis’ is very general, referring to infection under the skin and sometimes involving the skin,” says Mudge. “But when we talk about cellulitis in horses we tend to think of the hind limb.”
Even when it initially seems mild, cellulitis is not an ailment to take lightly. The swelling can progress quickly, even within a few hours, to the point where fluid leaks from cracks in the overstretched skin.
Plus, if the infection is not controlled quickly, a number of serious complications can develop. For example, the bacteria may spread from the skin into the deeper tissues and structures of the leg. “A particularly aggressive or resistant bacteria may cause tissue necrosis or a more deep-seated infection, which in rare cases can affect the bone, tendon or synovial structures such as a joint or tendon sheath,” says Hammond.
Laminitis is also a possibility. “It’s usually a support-limb laminitis but it can also be laminitis in the affected leg,” says Fogle.
Systemic infections, such as sepsis , can also occur. “Horses can have further problems if the bacterial infection does not stay confined to that limb and goes throughout the body,” Fogle says. “The horse can become very sick from systemic infection. These are all risks with severe cellulitis, but are more likely in cases with a delay in the start of therapy.”