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What are Actinobacillosis?

The disease Actinobacillosis specifies a collection of diseases resulting from a gram-negative coccobacilli and a part of the genus Actinobacillus. The bacteria are found in the oral cavity, GI and reproductive tracts of horses. The genus includes more than 22 bacterial species, four of which are often linked to diseases in animals. Depending upon the species of bacteria and the animal that is infected, the following may be seen:

  • Soft tissue infection
  • Involvement of the lymph nodes of the animal (occurs as the disease is spreading)
  • Infection of the bony tissue that is near the muscles or tissue that has been impacted

Horses are the natural host for the species Actinobacillus equuli.

Actinobacillosis includes several diseases that result from different bacteria from the genus Actinobacillus. Symptoms will vary based on the specific bacteria causing the infection.

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Symptoms of Actinobacillosis in Horses

Symptoms will vary based upon the species of bacteria that has infected your horse. For example, in Actinobacillus equuli the following symptoms may be seen:

  • Diarrhea
  • Brain and spinal cord swelling
  • Pneumonia
  • Kidney inflammation
  • Bacterial arthritis
  • Abortion of fetus in pregnant adult horse
  • Blood poisoning

Horses that are infected with Actinobacillus lignieresii may experience the following symptoms:

  • Tongue lesions and swollen tongue
  • Over-salivation
  • Tongue hard to the touch
  • Difficulty swallowing
  • Ulcers of the soft tissues that are involved with the necks, head and limbs

Horses may experience peritonitis, which is the swelling of the membrane that lines the stomach. 


There are a few forms of the disease Actinobacillosis and the form is dependent upon the species of the bacteria and the particular animal. Forms of the condition include:

  • Actinobacillus equuli - Can infect foals and full-grown horses; a foal may acquire the infection from the umbilical cord or through inhaling or consuming the bacteria
  • Actinobacillus lignieresii - More often seen in cattle but also affects horses; infected horses will have ulcers of their tongue (known as wooden tongue)
  • Actinobacillus arthritidis (formerly known as Bisgaard taxon 9) - seen in horses with joint inflammation and bacteria in the blood

Causes of Actinobacillosis in Horses

The bacteria in the genus Actinobacillus cause Actinobacillosis. In the case of Actinobacillosis, the bacteria exist within the horse’s upper GI tract and disease occurs when an open wound allows the bacteria to access soft tissue nearby. Local infection can then be spread amongst other tissues by the lymphatic system.

Diagnosis of Actinobacillosis in Horses

Should you notice symptoms in your horse, you will want to schedule a visit with your veterinarian. Your veterinarian will conduct a full physical examination and will ask you what symptoms you have noticed, when you first noticed them, and any changes in the symptoms that you have seen. Based on the results of the examination, your veterinarian may seek to isolate the bacteria in order to determine which bacteria is affecting your horse and how to best treat it. Should your veterinarian suspect infection with Actinobacillus lignieresii (often a hard pale tongue with unusual growths will be observed), he will recommend a biopsy of a lesion on your horse, in order to study the bacteria and make a diagnosis. Blood testing is not reliable for diagnosis and the results of hematologic and chemistry tests are usually normal.

Treatment of Actinobacillosis in Horses

Once your veterinarian has diagnosed your horse, treatment will be prescribed based on the type of Actinobacillus infection. 

Should your horse be infected with Actinobacillus equuli, your veterinarian may recommend treatment with one of the following: chloramphenicol, gentamicin, or third-generation cephalosporins. His recommendation will be based up the infection itself and how to best treat it. Other options include B-Lactam antibiotics and potentiated sulfonamides, though in some cases the horse’s infection is resistant to these antibiotics. 

Should your horse be infected with Actinobacillus lignieresii, your veterinarian may recommend surgery to excise any infected tissue, sodium iodide (70mg/kg, 10%-20% solution administered once and then one or two more times at seven to ten days), or antibacterial medication may be recommended, particularly in severe cases or those that are not responding to sodium iodide.

Recovery of Actinobacillosis in Horses

If your horse has been infected with a form of Actinobacillus, it is important to follow the recommendations of your veterinarian in order to facilitate his recovery. If your horse is pregnant while infected, your veterinarian can help reduce the chance of the newborn foal being infected through a sanitized birth environment as well as making sure that the foal consumes milk that includes the antibodies from his mother right after he is born. 

The prognosis for adult horses with Actinobacillus is good with treatment. Adults usually respond within 24-48 hours of supportive therapy and medication, with improvement being seen. Foals with the infection often have a more difficult time recovering, particularly when experiencing septicemia.