Anthrax: Etiology, Clinical Signs, Diagnosis, Treatment, Prevention, and
Control
Etiology
Bacillus anthracis, a large
gram-positive rod, causes anthrax in humans and animals.
Vegetative growth in culture media is
characterized by chain formation of tightly packed rods (“Medusa-head”
colonies), whereas in vivo growth differs by having short chains, rounded ends
to the rods, and well-formed capsules that may surround several cells. The
organism is a spore-former but usually only develops spores when growing
aerobically at 15.0 to 40.0° C rather than in vivo. The spores are extremely
hardy and may survive in dry alkaline soils that contain high nitrogen levels
for decades or more. Therefore discharges or tissue from fatal cases
contaminate soils and allow the organism to remain in certain geographic
pockets where the disease occurs sporadically. Rain or wet conditions coupled
with temperatures greater than 15.5° C foster germination and vegetative growth
that subsequently result in sporulation as dryness returns to contaminated
soil. Most clinical cases occur during the grazing season. Cattle exposed to
contaminated ground may ingest the spores either directly from the soil or from
plants grown on contaminated soil. The spores then become vegetative in the
host. Abrasions of the oral mucosa or digestive tract may allow an edematous
localized infection, which then seeds lymphatics and eventually results in
bacteremia. Localized infections may occur subsequent to skin wounds and have
been called “malignant carbuncle” in people. Inhalation of spores is a less
common means of infection but can occur in people (“woolsorter’s
disease”) or animals and often is fatal. Animal byproducts such as hides,
slaughterhouse material, and bone meals from endemic areas may harbor the
organism or spores and represent dangers to people and animals exposed to these
tissues. Insects that feed on blood also may transmit the infection. Virulence
of B. anthracis is caused by a polyglutamic acid capsule that
deters phagocytosis and lysis and a potent toxin consisting of an edema factor,
a protective antigen, and a lethal factor. These three factors working together
frequently form a lethal combination because they kill phagocytic cells, damage
capillaries, and interfere with clotting of blood. A vicious cycle of capillary
permeability, thrombosis, and tissue edema evolves.
Clinical Signs
Peracute B. anthracis infection
in cattle may result in death so rapid as to be thought to be sudden and thus
confused with other causes of sudden death such as lightning, fatal internal
hemorrhage, clostridial myositis, bloat, or metabolic conditions. Blood-tinged
or dark reddish-black sanguineous discharges from body orifices are common in
cattle dying from acute anthrax and may lead to confusion with death resulting
from caudal vena caval thrombosis, bleeding abomasal ulcers, jejuna hemorrhage
syndrome, arsenic poisoning, or peracute salmonellosis.
Acute anthrax causes fever, complete loss of
appetite and production, depression, and evidence of blood in most body
secretions, including feces, urine, milk, and nasal discharge. Tachycardia,
dyspnea, and possible neurologic signs also are present. Unless treated with
intense therapy very early in the course of the disease, affected cattle become
recumbent within 1 to 2 days and die. Whenever anthrax is suspected based on
signs (or lack thereof) and sanguineous discharges from body orifices,
necropsy should not be performed until other tests have been
performed to rule out the disease.
Localized wound infections with B.
anthracis are possible in cattle but uncommon and diffi cult to
diagnose. A history of anthrax on the farm or within the locale may add a
heightened index of suspicion for peracute or acute fatal cases.
Diagnosis
Blood collected from the jugular vein, mammary
vein, or ear vein may provide material for cytologic examination and culture
when the carcass is fresh. It is no longer necessary to send an ear from the
carcass, and in fact such procedures may merely increase the risk of human
exposure.
Carcasses that are rotten or more than 12 hours
old may be overgrown by clostridial organisms that confuse attempts at
cytologic diagnosis.
Blood samples or smears should be examined at a
qualified laboratory to ensure correct interpretation. Other blood tests,
including an FA technique and ELISA, are available at some laboratories.
Inoculation of collected material into guinea pigs has been used to diagnose
anthrax in the past, but collected material may contain other opportunistic
pathogens, thereby confusing the diagnosis.
Although necropsy of possible anthrax cases is
not recommended, it frequently is performed because other diseases may need to
be ruled out. Prosectors should wear gloves, gowns, and masks whenever anthrax
has been considered in the differential diagnosis of a dead cow. Splenic
enlargement, widespread serosal hemorrhages, sanguineous or serosanguineous
body cavity fluids, and dark red or black body orifice discharges are the major
necropsy findings. A diagnosis of anthrax requires notifi cation of regulatory
veterinarians to aid in quarantine management and
carcass disposal.
Treatment
Treatment seldom is possible because of the
acute or peracute course of illness. Penicillin and tetracycline in high levels
should be effective in early cases or the less common localized infections.
Prevention and Control
Prevention and control can be accomplished by
the following:
1. Avoid infected pastures.
2. Spore vaccines—the Sterne strain of rough,
nonencapsulated B. anthracis was derived through growth of
virulent B. anthracis on 50% serum agar in 10% to 30% CO2. The
resulting avirulent organism is used in the spore form as a live vaccine for
cattle, sheep, and goats. The vaccine is recommended once yearly before pasture
season. Use of any vaccine in dairy cattle may require regulatory approval,
although there is no evidence that milk contains spores following vaccination
of lactating cows.
3. Complete disposal of infected carcasses is
done by burning or burial at least 6 feet into the ground and covering the
carcass with quicklime. Regulatory veterinarians should be consulted regarding
appropriate disinfection techniques.
Younger veterinarians may benefit from
consultation with neighboring older colleagues to learn whether and where
anthrax has been diagnosed previously in their service territory.
Recently awareness of the zoonotic potential of
this infection has been highlighted by discussion of the “weaponization” of the
agent as a terrorist threat. Human cases of anthrax in many parts of the world
have become uncommon because of the success of control measures and lack of
human exposure to infected livestock.
Genetically modified organisms and alternative
exposure methods besides livestock have created new concerns for human and
animal health. One offshoot is the increase in research to find alternative
vaccination strategies, such as DNA vaccines, which may prove useful for animal
disease control in the future.
