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A hydrocyst is a fluid-filled cyst that occurs in the subcutaneous space, usually in the brisket area of a deer. It has been seen in the shoulder and hindquarter areas as well. The cyst develops in response to a traumatic injury at the site (usually on the ventral aspect of the body) with a resultant accumulation and encapsulation of fluid. Hydrocysts (breast blisters) have also been observed on the keel of wild turkeys.
Clinical signs observed with a hydrocyst are locomotor problems if the cyst becomes large enough (hydrocysts range in size from three inches in length and width to 30 inches in length and 12 inches in width) and hair loss between the brisket and front legs due to the constant rubbing.
A hydrocyst oftentimes has a scar present on the wall of the cyst capsule indicating the site of the original traumatic injury. The cyst can contain a gallon or more of serosanguineous (yellow-tinged) or blood-tinged clear liquid and fibrinous inflammatory products (similar in appearance to egg drop soup).
A hydrocyst is usually small in size and is not a significant mortality factor to the individual animal or the population. A severely affected animal may have some difficulty moving, thereby making it more susceptible to predation or harvesting by a hunter. Most of the reports in Michigan of a hydrocyst in a deer have occurred in fall harvested animals. The meat of a deer with a hydrocyst is safe to eat.