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 followed by heavy rain and flooding, which washes away topsoil
  • Dirt excavation for landscaping or construction
  • Areas of water run-off or erosion
  • Tilling soil for agricultural land use
  • Overgrazing allowing for wind erosion of pastureland

Certain soil types, such as limestone, may support B. anthracis in its replicative form. [5] Outbreaks are more common in areas with higher soil pH, higher nitrogen or calcium content, and high levels of organic matter. [5]

Contaminated water is another potential exposure route for anthrax. [4] Horses drinking water from ditches or dugouts with nearby soil erosion may acquire the infection. [4] Water troughs may also pose an infection risk if scavenging animals, such as vultures, feed on an anthrax-contaminated carcass then use the trough for bathing. [4]

Distribution

Anthrax spores can be found on every inhabited continent. [6] However, it is most common in sub-Saharan Africa, North America, Australia, and Asia. [6]

Sporadic outbreaks occur throughout Canada and the United States, particularly in: [4][7]

  • Alberta
  • Saskatchewan
  • Manitoba
  • Northwest Territories
  • North and South Dakota
  • Minnesota
  • Texas
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Symptoms

Horses typically develop an acute (sudden onset) form of anthrax 3-7 days after exposure to B. anthracis spores. [1]

Symptoms of acute anthrax include: [1]

  • Colic
  • Fever
  • Difficulty breathing
  • Swelling of the lower neck, thorax, and abdomen
  • Swelling of the prepuce or mammary gland

Sudden death with no previous symptoms can also occur. [1]

The symptoms of anthrax can appear similar to other diseases, such as: [8]

Diagnosis

Since horses die rapidly after the onset of symptoms, most cases of anthrax are only diagnosed after the death of the horse. [1]

To diagnose anthrax, veterinarians can examine a blood sample to identify square, “train car” shaped bacteria characteristic of B. anthracis. [1] Tests that identify bacterial DNA are also available for blood or body fluid samples. [1]

Veterinarians must be extremely cautious when examining the carcass of a horse suspected to have anthrax. [1] Opening the carcass exposes the bacteria to air, which triggers spore formation. [1] The large amount of spores contained within a carcass can rapidly contaminate the environment, leading to future cases of anthrax if animals are exposed to the contaminated area. [1]

For this reason, veterinarians generally do not perform a necropsy (post-mortem dissection) on sudden death cases where anthrax is a possibility. [1]

Signs that a carcass may have died from anthrax include: [1]

  • No rigor mortis after death
  • Profuse bleeding from the body orifices
  • Minimal blood clotting
  • Multiple small pinpoint bleeding sites on the gums and mucous membranes

Treatment

There are no known treatments for anthrax, and most horses die 2-4 days after developing symptoms. [1] If treatment is attempted, it typically involves: [1][4]

  • High doses of intravenous antibiotics
  • IV fluids
  • Intranasal oxygen
  • Anti-inflammatories

In some cases, hyperimmune serum may be available for treatment. [4] This is a product derived from blood gathered from horses previously vaccinated for anthrax. [4] The high levels of antibodies against anthrax present in hyperimmune serum can target the bacteria and resolve the infection. [8]

Potentially exposed horses who are not showing signs may benefit from antibiotic treatment to prevent the onset of disease. [1][4] Other recommendations for these horses include: [1][9]

  • Washing the horses to remove all soil contamination
  • Moving them to a new, uncontaminated area
  • Vaccination

Outbreak Management

Proper management of an anthrax outbreak is crucial for preventing environmental contamination and future cases of disease. [4] The main principles of outbreak management are quarantine, vaccination, and decontamination. [4]

Quarantine

The property where an outbreak occurs must be quarantined to prevent the spread of disease. [4] Although anthrax does not spread directly from animal to animal, preventing animals from leaving the property reduces the risk of contamination of new areas if that animal succumbs to the infection. [4]

Additionally, preventing new animals from entering the contaminated property can prevent additional cases of disease. [4]

The duration of quarantine necessary depends on government regulations but is typically until 21 days after the last animal death. [4]

Vaccination

During an outbreak, at-risk animals may benefit from vaccination to stimulate their immune system against B. anthracis. [4] Horses can be vaccinated as soon as an outbreak is identified, with a follow up vaccination 2-3 weeks later. [4]

Some government bodies also recommend vaccination of all livestock within a 5 mile (8 km) radius of the contaminated property. [4]

For future prevention, horses living on the contaminated property should be vaccinated annually for at least 5 years after the conclusion of the outbreak. [4] This vaccination schedule can prevent future outbreaks in the event of further soil erosion or wildlife anthrax cases re-contaminating the area. [4]

Carcass Disposal

The goal of carcass disposal is to prevent spores from forming and contaminating the environment further. [1] The carcass itself, as well as any soil, bedding, or objects contacted by the animal prior to death must be disposed of. [2]

Veterinarians must work closely with government officials to determine an appropriate disposal plan. [1][2]

Disposal options that are effective in eliminating B. anthracis include: [1]

  • Incineration
  • Rendering with decontamination of all wastewater
  • Deep burial to prevent scavenging

Although deep burial is an option for disposal, it is not preferred as future disturbances to the soil can expose animals to spores. [1] Carcasses should be buried at least 2 meters (6.6 ft) deep. [2]

Decontamination

Decontamination of anthrax spores is extremely difficult. [2][5] The spores are resistant to many disinfectants, including phenols. [2] Effective disinfection strategies include: [4]

  • Autoclaving
  • Hydrogen peroxide
  • Formaldehyde or glutaraldehyde
  • Peracetic acid
  • Chlorine solutions

Environmental decontamination involves rapid application of disinfectant to an area. [4] Typically, high concentrations of disinfectant are necessary, such as 5-10% formaldehyde. [4] Complete removal of vegetation in the area through herbicides and burning may be necessary. [5]

Prevention

Prevention of anthrax primarily involves vaccination. [1] A live attenuated vaccine based on the Sterne strain of B. anthracis is available for livestock use. [1][3] This strain does not form a bacterial capsule, making it easier for the immune system to target the bacteria for removal. [3]

Currently, vaccination is only recommended as a preventative measure for horses pastured in areas known to have endemic (commonly found) anthrax. [9] The anthrax vaccine can have significant effects in horses, including injection site reactions, severe tissue swelling, and death. [1][10]

Adult horses receive the vaccine annually, ideally 4 weeks prior to pasture exposure. [9] A second vaccination 2-3 weeks after the first dose may be beneficial in heavily contaminated areas. [9]

Pregnant mares should not be vaccinated against anthrax. [9]

Human Exposure

Anthrax can affect humans, potentially causing death. [1] Due to the resilience of the bacterial spores, the low dose necessary for infection, and the high fatality rate, anthrax has been identified as a potential bioterrorism agent in many countries. [5]

In 2001, there was an intentional release of anthrax spores by mail in the United States, resulting in the death of 5 people and infection of 17 others. [5]

Humans can develop three forms of anthrax infection: [8]

  • Skin nodules
  • Intestinal anthrax
  • Hemorrhagic pneumonia

Exposure typically results from handling contaminated hides or wool, allowing spores to contaminate skin wounds. [5] Inhalation or ingesting contaminated meat are other possible routes of exposure. [8]

Prevention

People who handle carcasses, hides, or hair products from animals have the highest risk of anthrax exposure. [1] This includes farmers, butchers, wool shearers, and veterinarians. [8]

Anyone handling animal products that may be contaminated anthrax must wear personal protective equipment, including: [1][5]

  • Gloves
  • Boots or disposable shoe covers
  • Protective coveralls
  • Eye protection
  • Respiratory protection such as HEPA filters

Many doctors recommend prolonged antibiotic treatment for all human individuals potentially exposed to anthrax. [1] In some cases, treatment protocols can last up to 60 days after exposure. [8]

Summary

Anthrax is a fatal disease caused by the spore-forming bacterium Bacillus anthracis.

  • Anthrax can affect many species, including horses and humans
  • Anthrax is found worldwide, and there are cases reported in North America each year
  • Horses become infected after inhaling or ingesting bacterial spores
  • Symptoms include colic, fever, difficulty breathing, and severe swelling
  • Most horses die within 2-3 days after symptoms develop, even with treatment
  • Outbreaks are managed through proper carcass disposal, quarantine, and decontamination
  • Vaccines are available for horses living in high-risk areas

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References

  1. Sellon, D. C., & Long, M. T., Eds. Equine Infectious Diseases. Second edition. Saunders/Elsevier, St. Louis, Missouri. 2013.
  2. Mair, T. R., & Hutchinson, R. E., Eds. Infectious Diseases of the Horse. Equine Veterinary Journal. 2009.
  3. Alam, Md. E. et al. Review of Anthrax: A Disease of Farm Animals. Journal of Advanced Veterinary and Animal Research. 2022.
  4. Ndiva Mongoh, M. et al. A Review of Management Practices for the Control of Anthrax in Animals: The 2005 Anthrax Epizootic in North Dakota – Case Study. Zoonoses and Public Health. 2008.
  5. Sharp, R. J., & Roberts, A. G. Anthrax: The Challenges for Decontamination. Journal of Chemical Technology & Biotechnology. 2006.
  6. Carlson, C. J. et al. The Global Distribution of Bacillus Anthracis and Associated Anthrax Risk to Humans, Livestock and Wildlife. Nature Microbiology. Nature Publishing Group. 2019.
  7. Epp, T. et al. Case-Control Study Investigating an Anthrax Outbreak in Saskatchewan, Canada — Summer 2006. The Canadian Veterinary Journal. 2010.
  8. Fasanella, A. et al. Anthrax Undervalued Zoonosis. Veterinary Microbiology. 2010.
  9. Anthrax Vaccination Guidelines. American Association of Equine Practitioners.
  10. Wobeser, B. K. Anthrax Vaccine Associated Deaths in Miniature Horses. The Canadian Veterinary Journal. 2015.View Summary