Anthrax in Horses

Updated Jun. 17, 2024
horse competing

What is Anthrax?

Most people have heard of anthrax; it has been used as a biological weapon and a scare tactic in terrorism attacks during the early 2000s. Anthrax, caused by the bacterium Bacillus anthracis, is an infectious disease, and can prove fatal for horses (or humans, for that matter). There are legal ramifications surrounding anthrax, and when diagnosed, the veterinarian is obligated by law to report it to the appropriate government agency.

Symptoms and Types

The signs of anthrax infection depend on how the animal was infected. Horses most commonly are infected by either ingestion of anthrax spores, or via the skin from an insect bite.

When spores have been ingested:

  • Depression
  • Fever
  • Chills
  • Colic
  • Severe bloody diarrhea/enteritis
  • Death 

Infection from insect bite:

  • Enlarged lymph nodes
  • Swelling in the throat
  • Difficulty breathing
  • Large, painful, swollen area at site of bite


The spore-forming bacterium Bacillus anthracis is the causative agent of anthrax.  Horses most commonly become infected with this bacterium through ingesting spores in the soil. These spores are extremely resistant to heat, cold, and desiccation, and can survive in the environment for decades. Biting insects can also spread anthrax.


In order to diagnose anthrax, your veterinarian may take a blood sample from your horse. In most cases, anthrax is diagnosed upon the horse's death, as the bacteria produce lethal toxins that often work very fast. If anthrax is the suspected cause of death of an animal, that carcass should not be moved or opened up until the proper authorities have been notified. Anthrax is a disease that should be reported to the state veterinarian. If anthrax is diagnosed on your farm, the farm will be placed under quarantine and the remaining animals may be vaccinated against it.



Anthrax is treatable with the proper antibiotic therapy. However cases of anthrax are often undiagnosed until after the death of the horse. Bacillus anthracis is usually susceptible to common antibiotics such as penicillin and oxytetracyline, although the disease must be caught early and treated vigorously in order for treatment to be successful.


There is a vaccine approved for use in livestock in the U.S. for anthrax, but horse owners should not use this vaccine unless their animals are located in an endemic area. Luckily, encountering anthrax spores in the U.S. is rare, with most recent outbreaks occurring mostly sporadically in the western part of the country and the Dakotas.


Featured Image:


Anna O'Brien, DVM


Anna O'Brien, DVM


Anna O’Brien, DVM is a large animal veterinarian. A 2008 graduate of Purdue University, she currently works in Maryland, just outside of...

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