MSD Manual

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Pet Owner Version

Actinobacillosis in Horses


Geoffrey W. Smith


Reviewed/Revised May 2019 | Modified Oct 2022

Actinobacillosis is caused by bacteria in the genus Actinobacillus. Several different forms of disease occur, depending on the particular species of Actinobacillus involved and the type of animal infected. Soft tissue infections are common, and lymph node involvement is frequently a step in the spread of the disease throughout the animal’s entire body. Bony tissue close to muscles or other infected tissue may also be infected.

Actinobacillus equuli infects both foals and adult horses. Foals become infected through a contaminated umbilical cord or by inhaling or ingesting the bacteria. Signs of infection usually start with diarrhea and then progress with inflammation of the brain and spinal cord, pneumonia, inflammation of the kidneys, or infected joints. The chance of infection in foals can be reduced with good sanitation in the birthing environment and by ensuring that the foal receives colostrum, the first milk containing antibodies from the nursing mother shortly after birth. Infections in adult horses can result in abortion, blood poisoning, and inflammation of the kidneys or heart.

Other infectious agents can cause similar signs, so a bacterial culture should be performed to specifically identify the cause of illness. With this information, the correct antibacterial drugs can be selected for treatment.

Actinobacillus lignieresii primarily causes large abscesses of the tongue, a condition often called wooden tongue. The tongue becomes hard and swollen, leading to drooling and difficulty eating. This infection occurs most frequently in cattle, but is also seen in horses. Treatment may include surgical removal of the contaminated tissue, potassium iodide given by mouth, or antibacterial drugs.

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