Generalised swelling in your horse’s legs is a result of an increase in fluid in the subcutaneous tissues. This will either be due to inflammation or as a result of decreased drainage in the venous and lymphatic systems.
The most common cause of swollen legs during wet periods is due to decreased movement. The venous and lymphatic systems rely on movement and muscular contraction to help move fluid up the legs against gravity. When your horse is more sedentary during wet weather there is a build-up of fluid in the venous system and a decrease in lymphatic drainage resulting in excess fluid in the subcutaneous tissues of the leg. The swellings should be mild, with no heat, and are typically non-painful. This can be likened to people getting swollen ankles when flying long distances. If your horse has had an injury to the lower limbs previously this can also affect the efficiency of the lymphatic system and your horse may be more susceptible to swollen legs.
There are several medical causes of swollen legs. ‘Greasy heel’ can be instigated or exacerbated by wet weather and will result in inflammation (cellulitis) of the lower limbs and resultant swelling. This swelling is typically hot and can be painful.
Vasculitis is inflammation of the blood vessels in your horse’s legs and will result in an increase in fluid leakage from blood vessels. This condition is not directly related to wet weather but can be secondary to greasy heel or systemic infections.