Brain Abscess Syndrome

This fact sheet was updated on 11/4/20.

Other names: Cranial Abscessation Syndrome, BAS


Brain abscess syndrome is a naturally occurring disease described primarily in white-tailed deer in which abscesses (enclosed pockets of pus) form within the brain. Several species of bacteria including, Actinomyces, Staphylococcus and Streptococcus, can be responsible for these abscesses, but the most common cause is Trueperella pyogenes.


Brain abscess syndrome occurs sporadically in North American white-tailed deer and results in occasional deaths. This disease has a marked gender and age bias, with over 80% of cases occurring in adult male bucks. However, it does not seem to significantly impact deer populations. For example, in the Southeastern part of the states, brain abscess syndrome accounts for less than 10% of naturally occurring deaths in adult males.

Species Affected

While T. pyogenes can cause abscesses to form in many animal species, particularly ungulates (hooved mammals), brain abscesses due to these bacteria are most common in white–tailed and other deer. The bacteria also commonly cause abscesses in other parts of the body in deer, moose, pronghorn, and wild sheep. While it is rare, humans can contract T. pyogenes if they come in contact with infected deer, though the bacteria is often misidentified. 


Brain abscess syndrome probably occurs throughout North America in most of the white-tailed deer population range. In the United States, these abscesses have been observed in white-tailed deer in Florida, Georgia, Idaho, Kentucky, Maryland, Michigan, Minnesota, Missouri, North Carolina, New Jersey, New York, Pennsylvania, South Carolina, Texas, Virginia, Washington, Wisconsin, West Virginia, and Wyoming. Northern Maryland in particular, sees a high incidence of BAS, with an annual mortality of around 35% in adult male white-tailed deer. This disease has also been observed in white-tailed deer in the Canadian Provinces of Alberta, British Columbia, Ontario, and Saskatchewan. Some studies suggest that brain abscesses may be less common in arid regions due to the bacteria’s inability to survive in dry conditions. 


T. pyogenes commonly lives on the skin and gums of deer and causes infection when it enters an open wound. This bacterium is also present in the circulation of all ruminants. Brain abscesses develop in deer when the bacteria enter wounds in the velvet of a buck’s antler, a broken antler, or the pedicle (base of the antler) of a recently shed or damaged antler. After the bacteria find and colonize a wound it can damage the bone of the skull and travel into the brain where the abscess forms and enlarges. Some brain abscess can be very large, involving up to 30% of the brain. Most cases of brain abscess syndrome occur September through April in adult antlered males because of normal buck breeding behavior such as antler rubbing, sparring, and shedding, which creates a greater occurrence of open wounds on or near the antler. Older males are affected more frequently. Females and immature males can also develop brain abscesses, though it is less common.

Clinical Signs

Deer with brain abscess syndrome exhibit neurological signs such as loss of coordination, apparent blindness, lack of fear, aggression, weakness, paralysis, depression, and emaciation. Swollen eyes, broken antlers, swollen joints, and lameness may also be observed. Pus may be present in the eye sockets and pedicles. Irregular antler development may also be observed in affected individuals and the bony structures of the skull can be eroded or pitted.


Diagnosis is based on clinical signs and presence of abscesses in the brain at necropsy. The bacteria must be isolated and identified in the laboratory in order to determine the species responsible for abscess formation. 


Brain abscesses occur sporadically in wild deer. The disease is well advanced when the external signs are present, so treatment is usually not attempted or effective. 


The bacteria responsible for brain abscess formation occur naturally on the skin of deer, and infection occurs as a result of natural breeding behavior or other trauma, meaning prevention and control is not feasible. Deer with BAS are not safe for human consumption because of the possibility of whole-body dissemination of the bacteria.

Suggested Reading

  • Baumann, C. D., W. R. Davidson, D. E. Roscoe, and K. Beheler-Amass. 2001. Intracranial abscessation in white-tailed deer of North America. Journal of Wildlife Diseases 37: 661-670.
  • Karns, G. R., et al 2009. Intracranial abscessation as a natural mortality factor for adult male white-tailed deer (Odocoileus virginianus) in Kent County, Maryland, USA. Journal of Wildlife Disease 45: 196-200.
  • Wobeser, G. 2001. Actinomyces and Arcanobacterium Infections. Pages 487-488 in E. S. Williams and I. K. Barker, editors. Infectious diseases of wild mammals. Iowa State University Press, Ames, Iowa, USA.


Abcess in brain of deerAn abscess in the brain of a deer. Photo courtesy of the Pennsylvania Game Commission