Anthrax - Clinical

Signs and Diagnosis

Anthrax, a disease of mammals, including humans, is

caused by a spore-forming bacterium called Bacillus

anthracis. This is one of three information sheets on

anthrax. The other information sheets include a general overview and discussions of safety aspects in

dealing with anthrax.

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 or more days. Anthrax

occurs in several forms, largely defined by the length

of the clinical course of disease. In cattle and sheep,

the peracute course of illness may last only 1 to 2

hours. The very first indication of problems may be

sudden death of the animal. 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 blood does not clot.

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

by B. anthracis. The acute form of anthrax in ruminants may have a course of 24 to 48 hours. Affected

animals have a high fever, complete anorexia, diarrhea, severe depression, and listlessness. There

may be local edema of the tongue with accumulation

of edematous fluid in the throat, sternum, perineum,

or flanks. Pregnant cows may abort. Milk production

may drop severely and what milk there is may be yellow or blood stained.

In horses and related animals, the acute course

of illness is more common and can last up to 96

hours. Clinical manifestations depend upon the route

of infection. If anthrax spores were ingested, septicemia, fever, colic, and enteritis are usually prominent. Anthrax due to introduction by insect bite

(mechanical transmission) is characterized by localized hot, painful, edematous, and subcutaneous

swellings at the bite location that spread to the throat,

lower neck, ventral thorax, abdomen, prepuce, and

mammary glands. Infected horses may have a high

fever and dyspnea due to swelling of the throat or

colic due to intestinal involvement.

In swine, dogs, and cats, the sub-acute or chronic course of illness is the most common. The route of

infection is usually ingestion of contaminated meat or

other food. Clinical manifestations often include a

characteristic swelling of the neck secondary to

regional lymph node involvement, which causes dysphagia and dyspnea. An intestinal form, with severe

enteritis sometimes occurs in these species. Many

carnivores apparently have a natural resistance, and

recovery is not uncommon.

Diagnosis 

Sudden death in an animal without prior signs

should lead to a suspicion of anthrax. If anthrax is

suspected as the cause of death, the carcass should

NOT be opened because exposure of the vegetative

anthrax bacteria in body fluids to oxygen induces

spore formation. The spores will contaminate the

environment and present a health risk to personnel

and other nearby animals. In the unopened carcass,

the putrefactive processes destroy the bacilli.

Diagnosis can be confirmed by collecting a postmortem blood sample aseptically from a peripheral

vein (e.g., the jugular vein) and examining a blood

smear for the presence of the bacillus capsule using

McFadyean’s polychrome methylene blue stain or by

culturing the bacilli. When a postmortem blood sample cannot be obtained from a peripheral vein, blood

can be obtained from a small incision in the ear using

a cotton swab. The swab should be allowed to air dry

before shipping in order to encourage sporulation and

kill other bacteria. B. anthracis competes poorly with

putrefactive bacteria and may not be seen in smears

after 2 or 3 days; culture is then necessary for confirmation of diagnosis but growth is increasingly unlikely

in blood held for more than 3 days. Potential culture

specimens can also be obtained from hemorrhagic

nasal, buccal, or anal exudates or from materials

contaminated with the exudates.

For More Information

For more information, contact 

USDA, APHIS, Veterinary Services

Emergency Programs

4700 River Road, Unit 41, 

Riverdale, MD 20737-1231

Telephone (301) 734-8073 

Fax (301) 734-7817

or visit our Web site at www.aphis.usda.gov/vs/

For Public Health Information

For more information on anthrax regarding public 

health, contact 

Centers for Disease Control and Prevention 

1600 Clifton Road

Atlanta, GA 30333

Telephone (404) 639-3311

or visit their Web site at www.cdc.gov/

Factsheet

Veterinary Services February 2002

APHIS
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employer.

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