Anthrax in Horses: Disease Considerations and Insight
Guy Sheppard, DVM and Melanie Landis, DVM
Bacillus anthracis is a spore-forming, facultative anaerobic, gram positive rod that is the causative agent of anthrax. Spore formation occurs when the vegetative form of the bacteria, found within the animal, is exposed to air upon the animal’s death. The spores then settle into the ground under and around the carcass. The spores are highly resistant to environmental elements and can remain viable for decades within the soil. B. anthracis is found worldwide, appearing to survive best in dry, sandy, slightly alkaline and calcium-rich soils.1 Disease outbreaks are frequently linked to soil disturbances, such as heavy rains followed by dry, windy conditions.In Texas, the organism is considered endemic in many of the southwestern regions of the state.
Anthrax is typically considered a disease of herbivores, especially ruminants, but is capable of infecting almost any warm-blooded species. Animals may become infected through the ingestion or inhalation of the spores. It is theorized that horses may be more unlikely to be infected than cattle, as horses typically graze towards the tips of the grass versus further down at ground level where the spores are more heavily concentrated. Furthermore, both biting and non-biting flies have been implicated in transmission and disease outbreaks.1
Frequently, death may be the first and only clinical sign observed and the carcass is commonly found bleeding from the various body orifices. Clinical signs seen prior to death may include edematous swellings associated with the neck, breast, thorax, abdomen, and/or flank, elevated body temperature, and staggering and convulsions. Generally a common finding associated with B. anthracis infection is that blood collected at or near the time of death typically fails to clot.
TVMDL has confirmed two cases of anthrax to date this year in horses that reside in anthrax endemic areas of Texas. Attending veterinarians have reported some success in treating infected horses. Of note, the blood from these horses did clot in a red top tube, and bacterial culture identification of Bacillus anthracis was made using whole blood from an accompanying EDTA tube. The Sterne strain vaccine is licensed for use in horses located in endemic areas, but the American Association of Equine Practitioners (AAEP) advises against use of the vaccine in pregnant mares and reports adverse vaccine reactions in young and miniature horses.
For information about this article, contact Dr. Guy Sheppard or Dr. Melanie Landis, veterinary diagnosticians at the College Station laboratory. To learn more about anthrax testing at TVMDL, contact the College Station laboratory at 1.888.646.5623 or Amarillo laboratory at 1.888.646.5624 or visit the TVMDL website at tvmdl.tamu.edu.
- Anthrax in humans and animals (4th); World Health Organization; 2008.