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.
Animal 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 animal=s body through bloody discharges from the
natural openings of the body after death, 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. Spores may also be present in bonemeal, protein concentrates,
excreta, and tissue and body fluids of infected carcasses.
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 in Animals
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
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.