Anthrax: Clarifying the Cloudy

For most Americans, anthrax was a little-known entity until four years ago, when the disease gained worldwide attention with the bioterrorism infection of 22 U.S. postal workers. Some people thought at the time that anthrax was caused by a newly created, deadly bacterium, something out of a modern biologic horror story. Anthrax (caused by Bacillus anthracis), however, has been around for hundreds of years, and it probably has been responsible for killing thousands of animals and humans as far back as biblical times. It appears naturally today in livestock in certain regions of the United States.

The disease caused by anthrax was officially recognized in the late 1800s by Dr. Robert Koch, and anthrax is famous (in medical circles) for being the first disease to which a bacterial cause could be firmly linked. Anthrax was also the first bacterial disease for which a vaccine was made available.

Anthrax naturally occurs in grazing animals (cattle, sheep, and goats), as these species are the most susceptible to the bacteria, but virtually all mammals--including horses and humans--can contract this disease.

What is Anthrax?

Anthrax is a disease caused by the bacteria Bacillus anthracis or its spores. Although the disease has been sporadically occurring naturally for possibly thousands of years, it has gained renewed recognition in today's society as a potential bio-terrorism weapon. Anthrax is also known as malignant carbuncle, charbon, or woolsorter's disease, as workers who handled anthrax-contaminated raw wool were sometimes exposed and infected.

This bacteria is a bit unusual because of its spores; these highly resistant spores allow the bacteria to survive in the environment for years in virtually all climates. These spores prefer to live in soil. The occasional outbreaks of anthrax often occur following heavy rains, which can push long-hidden spores into new, more accessible grazing locations, or drought, when animals graze closer to the contaminated soil.

Anthrax is considered a zoonotic disease, which means it can be transmitted to humans via an infected animal or animal product. This is one of the reasons anthrax has many medical officials so concerned, as the disease could be intentionally introduced to the human population via infected animals (including horses). Although it's a rare disease in the United States, outbreaks in cattle occur every few years, usually in the central or south-central United States. Certain areas within the United States are considered endemic for the disease (see map on page 52).

Anthrax has occurred in most countries. Central and South America, the Middle East, and the Caribbean are considered at higher risk for naturally occurring disease.

2005 U.S. Anthrax map

Anthrax is considered endemic in parts of the United States, and, thus, animals in those areas can be more prone to contracting the disease.

Resistance is Futile

Anthrax causes a different disease in animals than it does in humans, which is likely due to the difference in how the disease is acquired. For example, cattle and horses usually ingest the bacterial spores as they are grazing; inhalation of the spores is unusual in these animals. Different species of animals are more susceptible to the disease than others, possibly because of their eating habits. For example, cattle, sheep, and goats are believed to be most susceptible and are also the most commonly affected. Unfortunately for these animals, often the first sign of disease is death. This bacterium ravages the body so quickly--sometimes within hours of ingestion--that infected animals usually die before treatment can begin.

Some animals, such as pigs, cats, and dogs, are fairly resistant and usually only develop signs of gastroenteritis.

Horses that have ingested spores usually have a slightly longer course of disease, up to 96 hours. Horses will usually develop a very high fever and show signs of colic and/ or diarrhea. They might also develop difficulty breathing (dyspnea) and can develop swelling on the underside of the neck and chest. Unless treated early in the course of the disease, anthrax is usually fatal in horses, but successful treatment can occur.

Treatment consists of supportive care, such as intravenous fluids, antibiotics, and anti-inflammatory agents. This particular bacterium is susceptible to many antibiotics; however, the reason it is so often fatal is because of two intrinsic characteristics. The bacterium has a special capsule that protects it from phagocytosis, which is the body's first (and one of the most effective) means of resistance to invading bacteria. The second factor is a toxin the bacterium produces once inside the host's body. This toxin is made up of several distinctly different proteins that act in a sequence that results in the animal's death.

Because of the severity of anthrax infection, this is a federally reportable disease. This means farms with suspected cases must be quarantined, and state and federal veterinarians notified.

Anthrax in humans has three distinct forms: Cutaneous (skin), pulmonary, or intestinal (GI). Humans are thought to be moderately resistant to the bacteria, and if otherwise healthy, need quite a large exposure to become infected. The cutaneous form is the most common naturally occurring form of the disease. It is very treatable and rarely fatal with proper medical care.

With the cutaneous form, Bacillus anthracis spores present on contaminated animal hair or skin can enter open wounds and cause very characteristic skin lesions. The lesions initially look like insect bites, then rapidly become lesions with necrotic (black) centers.

The inhalation form is the most dangerous--and most often fatal--form of the disease for humans. As the name suggests, the Bacillus anthracis spores are inhaled and cause a very serious pneumonia and/ or pleuropneumonia, which is often fatal. The GI form is contracted by eating contaminated meat that was not cooked properly.

Cursed Fields?

Natural infection occurs in horses, and most other animals, from eating feed or grass that has been contaminated with Bacillus anthracis spores. The spores have been found within some species of biting flies, and it is thought these insects can transmit the disease through bites. The spores of this bacterium are very resistant within the environment, and they can remain intact and infective in the soil for years. Some farms with a history of anthrax can have recurring problems years apart, hence the term "cursed fields" when referring to these endemic properties.


The most accurate way to diagnose anthrax is to grow the organism from a blood sample. Even in deceased animals, a blood culture can provide a diagnosis when a suspicious death has occurred. Therefore, a full necropsy does not have to be performed, which could spread contagious spores and put the veterinarian at risk.


With such a scary disease, what's a horse owner to do?

The vast majority of horse owners do not have to lose sleep worrying about the threat of anthrax. For areas that are endemic, there is a commercially available vaccine usually used in ruminants, but which is licensed for use in horses. However, the use of this product in horses has been associated with unpleasant reactions such as swelling and pain at the vaccination site. As a result, unless there is a known history of disease on a farm, vaccination is not performed. As always, discuss the need for anthrax vaccine with your veterinarian.

When exposed animals die of anthrax, contamination of the soil will occur if burial is used to dispose of the animal. Therefore, burning is recommended to dispose of the carcass and prevent dissemination of the spores.

Unnatural Infection

Bioterrorism is not a new idea. Unfortunately, people with a goal of killing on a large scale have tried using anthrax recently in our country, and in the past in other parts of the world. Soldiers during previous wars have infected horses bound for cavalry duty with anthrax-laced sugar cubes. Whether this was to eliminate the horses, people, or both, history is unclear. However, unless the world learns to embrace peace and tolerance, it is likely anthrax will be used again--possibly in animals--in an attempt to infect humans.

Luckily, this is a very inefficient way to transmit the disease to people; unluckily, the animals will once again suffer needlessly as humans struggle to co-exist.



Cieslak, T.J.; Eitzen, E.M. Clinical and Epidemiologic Principles of Anthrax. In Emerging Infectious Diseases, 1999 July-Aug; 5 (4), 552-5.
Pipkin, A. Anthrax. In Large Animal Internal Medicine, ed. Bradford P. Smith. Mosby, Phildelphia, 1996, 1246-1248.
Acha, P.N.; Szyfres, B. Anthrax. In Zoonoses and Communicable Diseases Common to Man and Animals, 2nd Edition. Pan American Health Organization, Washington, D.C. 1989, 10-15.
Burrows Textbook of Microbiology, 22nd Edition. Bob A. Freman, Editor. WB Saunders, Philadelphia, 1985, 564-570.


Animal Plant Health Inspection Service, Veterinary Services, Anthrax General Information and Vaccination,
Knowles, Ralph C., DVM. Anthrax Affects Everyone. The Horse, November 2001, 26.
Texas Animal Health Commission, Asking About Anthrax,

About the Author

Christina S. Cable, DVM, Dipl. ACVS

Christina S. Cable, DVM, Dipl. ACVS, owns Early Winter Equine in Lansing, New York. The practice focuses on primary care of mares and foals and performance horse problems.

Stay on top of the most recent Horse Health news with FREE weekly newsletters from Learn More