Anthrax is a rapidly fatal disease caused by Bacillus anthracis bacteria. This bacterium can infect horses, other livestock species, and humans. Anthrax cases occur globally every year, including sporadic outbreaks in North America.

B. anthracis primarily exists as spores within soil. Disturbance of the soil can make spores accessible to livestock, resulting in infection. Horses typically become infected after inhaling or ingesting the spores. Once inside the body, the bacteria produce toxins that cause tissue damage and organ failure.

Anthrax infection typically causes death within 2-3 days, even with treatment. Symptoms in horses include colic, fever, difficulty breathing, and swelling of the tissues. If treatment is pursued, it typically involves high doses of antibiotics and supportive care.

Due to the high fatality rate, outbreak management is critical for preventing the spread of this disease. Management strategies include proper disposal of carcasses, quarantine of the affected property, and decontamination of the environment.

A vaccination against anthrax is available for horses, but it is only recommended in specific circumstances, such as during an outbreak or in areas previously contaminated by the bacteria.

Anthrax Transmission and Infection

The cause of anthrax is the bacterium Bacillus anthracis, which are spore-forming bacteria commonly found in soil. [1] The spores of B. anthracis are incredibly hardy and have low nutrient requirements, allowing the bacteria to survive in soil for decades. [1]

When horses inhale or ingest B. anthracis spores from the environment, they are at risk of developing anthrax. [1] Biting flies can also transmit anthrax to horses if they contaminate an open wound, carrying the bacteria with them. [1]

Once the spores of B. anthracis enter the body, the bacteria germinate (develop into the replicating form). [2] These activated bacteria then replicate within lymph nodes, the sites in the body where lymphatic fluid from the immune system is filtered and recirculated. [1]

Lymphatic fluid passing through the lymph nodes carries the bacteria into the blood stream, which distributes the bacteria throughout the body. [1]

Disease Progression

B. anthracis bacteria produce toxins that affect tissues within the body. [1] The first toxin, lethal toxin, triggers cell death in tissues it interacts with, ultimately causing organ failure and shock. [2][3] The release of lethal toxin by B. anthracis is the ultimate cause of death in animals affected by anthrax. [1]

The second toxin, edema toxin, causes cells in tissues to rupture and release large amounts of fluid. [3] It also prevents proper functioning of macrophages, one of the body’s main immune cells, resulting in an uncontrolled infection. [3] Production of edema toxin causes some of the symptoms associated with anthrax, such as fluid leaking from the nose. [1]

Anthrax cannot spread directly between sick animals. [1] The infectious form of the bacteria, the spores, only develop when exposed to oxygen and suitable growing environments, such as soil. [1]

Risk Factors

Anthrax is uncommon in horses compared to other species such as cattle, bison, and pigs. [1]

The most common source of anthrax spores is contaminated soil. [2] Grazing livestock species can consume up to 5-6 pounds of soil per day on their forage. [4] Hay, grain, and other feed products grown in contaminated soils may also contain anthrax spores. [4]

Most outbreaks of anthrax occur after a disturbance that brings new soil to the surface. [2]

Potential disturbances include: [1][2][4][5]

  • Dry weather foll