Anthrax

Anthrax, a disease of mammals and humans, is

caused by a spore-forming bacterium called Bacillus

anthracis. Anthrax has an almost worldwide distribution

and is a zoonotic disease, meaning it may spread

from animals to humans. All mammals appear to be

susceptible to anthrax to some degree, but ruminants

such as cattle, sheep, and goats are the most susceptible and commonly affected, followed by horses,

and then swine.

The U.S. Department of Agricultures (USDA)

main diagnostics laboratory in Ames, Iowa, the

National Veterinary Services Laboratories (NVSL),

maintains small quantities of anthrax to use as reference material in making confirmatory anthrax diagnoses in animals. USDAs Animal and Plant Health

Inspection Service (APHIS) maintains that laboratory

as part of fulfilling its mission to protect American

agriculture.

Disease Epidemiology

Anthrax is endemic to the United States, occurring sporadically throughout the country as environmental conditions allow. The Del Rio, Texas, region

has reported ongoing outbreaks of anthrax in deer

and livestock this summer. The most recent outbreak

there occurred on Sept. 21, 2001. Other recent outbreaks include an outbreak in cattle and horses in

Minnesota in June-July 2000; in cattle, horses, and

bison in North Dakota in August 2000; and in cattle in

Nebraska in January 2001.

During their vegetative stage, cells of the anthrax

agent multiply in the lymph nodes of susceptible animals, including humans. When cells of B. anthracis

escape from the animals body and are exposed to

oxygen, they form spores. These spores are highly

resistant to heat, cold, chemical disinfectants, and

long dry periods. B. anthracis spores are reported to

survive for years in the environment. Environmental

persistence may be related to a number of factors,

including high levels of soil nitrogen and organic 

content, alkaline soil (a pH level higher than 6.0), and

ambient temperatures higher than 60 degrees

Fahrenheit.

The anthrax organism may be spread within an

area by streams, insects, wild animals and birds, and

contamination from wastes of infected animals.

Anthrax may be perpetuated in nature by hosts such

as a wildlife reservoir, which in turn spills over into

the livestock population. Animals are usually infected

by ingesting soilborne spores, such as in contaminated

food or water. Spores can be picked up directly from

the soil through grazing or from feed grown on infected soil. When periods of drought cause livestock to

forage much closer to the ground, animals may

ingest spores in soil they accidentally eat along with

forage. After flooding, the concentration of spores

caught in standing water increases when preexisting

or transitory ponds begin to evaporate.

Although rare, it is possible for animals to inhale

dust harboring anthrax spores. Bites from flies and

other insects that may harbor vegetative anthrax have

also been reported to be vehicles for mechanical

transmission.

Clinical Signs

Disease occurs when spores enter the body, germinate, multiply, and release toxins. The incubation 

period of natural infection in animals is typically 3 to 7

days with a range of 1 to 14 days, or more.

In cattle and sheep, the course of illness may last

about 1 to 2 hours. Clinical signs, such as fever up to

107 degrees Fahrenheit, muscle tremors, respiratory

distress, and convulsions, often go unnoticed. After

death, there may be bloody discharges from the 

natural openings of the body, rapid bloating, a lack of

rigor mortis, and the presence of unclotted blood.

This failure of blood to clot is due to a toxin released

by B. anthracis.

Anthrax in horses and related animals is acute

and can last up to 96 hours. Clinical manifestations

depend upon how the infection occurred. If due to

ingestion of spores, as in cattle, septicemia, fever,

colic, and enteritis are prominent. Anthrax due to

insect bite introduction (mechanical transmission) is

characterized by localized hot, painful, edematous,

and subcutaneous swellings at the bite location that

spread to the throat, lower neck, floor of the thorax,

abdomen, prepuce, and mammary glands. These

horses may have a high fever and dyspnea due to

swelling of the throat or colic due to intestinal 

involvement.

Swine, dogs, and cats usually show a characteristic swelling of the neck secondary to regional lymph

node involvement, which causes dysphagia and 

dyspnea following ingestion of the bacteria. An intestinal form of anthrax with severe enteritis sometimes

occurs in these species. Many carnivores apparently

have a natural resistance, and recovery is not

uncommon.

Factsheet

Veterinary Services November 2001

APHIS
The U.S. Department of Agriculture (USDA) prohibits discrimination

in all its programs and activities on the basis of race, color, national

origin, sex, religion, age, disability, political beliefs, sexual orientation, or marital or family status. (Not all prohibited bases apply to

all programs.) Persons with disabilities who require alternative

means for communication of program information (Braille, large

print, audiotape, etc.) should contact USDA’s TARGET Center at

To file a complaint of discrimination, write USDA, Director, Office of

Civil Rights, Room 326–W, Whitten Building, 1400 Independence

Avenue, SW, Washington, DC 20250–9410 or call (202) 720–5964

(voice and TDD). USDA is an equal opportunity provider and

employer.

United States Department of Agriculture • Animal and Plant Health Inspection Service • Safeguarding American Agriculture