Last revised February 22, 2006
Anthrax is caused by Bacillus anthracis, a spore-forming bacterium. Spores give B. anthracis its ability to survive in the soil for years to decades. The name is derived from the Greek word for coal, anthrakis, because the cutaneous form of the disease causes black, coal-like skin lesions.
Anthrax in animals may be peracute, acute or subacute, or chronic. The peracute form most often affects cattle, sheep, and goats at the start of an outbreak. The acute or subacute form is common in cattle, sheep, and horses. Chronic anthrax is most often seen in less susceptible species such as swine, but it has also been reported as developing in cattle, horses, dogs, and cats. Route of infection in animals is most often ingestion, rather than inhalation or inoculation via skin lesions. Initial suspicions of anthrax may be raised when livestock are found dead, bloated, and without rigor mortis. Blood may be evident at body orifices.
Cattle, sheep, and goats¾Clinical signs of peracute anthrax in cattle, sheep, and goats include staggering, trembling, breathing difficulty, convulsions, and death. Progression of the disease is rapid and premonitory signs may go unnoticed; often animals are found dead, bloated, and without rigor mortis. Blood may fail to clot because of a toxin released by B. anthracis. Acute anthrax manifests itself in high fevers (up to 107 F), excitement, increased heart rate, deepening of respiration, followed by depression, incoordination, cessation of rumination, reduction in milk production, discolored milk (blood-tinged or deep yellow), bloody discharges, respiratory distress, convulsions, abortion, and death within 48 to 72 hours. Subcutaneous swelling and edema, usually involving the ventral aspect of the neck (brisket), thorax, shoulders, perineum and flank, are characteristic of chronic anthrax infection.
Swine¾Relatively resistant, but when affected, usually develop localized edema of the throat. Pigs so affected may die from suffocation. Acute anthrax, manifested as sudden death with few clinical signs, and a chronic gastrointestinal form of anthrax with clinical signs of enteritis, diarrhea, or constipation have also been reported as developing in this species. Some affected pigs may recover after a few days of illness, whereas others are affected by fatal bacteremias.
Dogs and cats¾Both species appear to be relatively resistant to anthrax and recovery is not uncommon. Although reports of the disease developing in dogs and cats are rare, consumption of contaminated meat or hides appears to be the most common source of infection and the gastrointestinal form of the disease is most typical. Inhalational or cutaneous anthrax has not been reported for these species. Because cats groom themselves regularly, contamination of their fur with spores could also result in infection by ingestion. Initial signs are most often related to pharyngitis and gastroenteritis. Ulcerative lesions may be observed in the oral cavity and throat. Progression is indicated by edema and swelling of the lips, face, head, and neck (secondary to regional lymphadenopathy). The spleen, liver, and kidneys may become palpably enlarged, and septicemia may contribute to shock, renal failure, and respiratory distress. Sudden death with few clinical signs has also been reported. The incubation period for natural infection is believed to be 3 to 7 days, but periods of 1 to 14 days have been reported.
In humans, anthrax may take one of three forms: cutaneous, inhalational, or gastrointestinal. Cutaneous anthrax accounts for about 95% of all natural infections and develops when B. anthracis enters the skin through existing cuts or abrasions. The incubation period ranges from 12 hours to 12 days, and the arms, hands, face, and neck are most often affected. Lesions may initially resemble an itchy insect bite. Within 48 to 72 hours, however, vesiculization and ulceration occur, producing a 1- to 3-centimeter lesion with a characteristic black necrotic center. Adjacent lymph nodes may be swollen and painful. Without antibiotic treatment, the death rate from cutaneous anthrax is approximately 20%; if appropriately treated, death is rare.
Gastrointestinal anthrax results from infection of the upper (oropharyngeal form) or lower (intestinal form) gastrointestinal tract after consumption of contaminated meat. The oropharyngeal form is characterized by formation of an oral or esophageal ulcer, followed by swelling of the regional lymph nodes, edema, and sepsis. Lesions in the intestinal form are found most often in the terminal ileum or cecum. Initial clinical signs include nausea, inappetence, vomiting, and fever, followed by abdominal pain, vomiting of blood, and diarrhea. Mortality for intestinal anthrax is estimated at 25 to 75% of those affected. No cases of gastrointestinal anthrax have been reported in the United States during the 20th or 21st centuries.
Inhalational anthrax may initially present as a flu-like illness with fever, headache, myalgia, a nonproductive cough, and mild chest pain. After 1 to 3 days, a short period of improvement may follow, after which the patient rapidly deteriorates. The second stage of the disease is characterized by high fever, respiratory distress, and shock. Pleural effusion and a widened mediastinum may be evident on chest radiographs. Historically, mortality for treated individuals has approached 95% if treatment is not begun within 48 hours of the onset of clinical signs. Only 18 cases of inhalational anthrax were reported in the United States during the 20th century. Prior to cases diagnosed in Connecticut, Florida, Washington, New York and New Jersey in 2001, the last case of inhalational anthrax in the United States was diagnosed in 1976. Another case has apparently occurred in 2006 in a person who may have been occupationally exposed from handling unprocessed animal hides.
Standard bacteriologic culture of blood is the most widely available and useful diagnostic test. Bacterial growth is usually evident within 6 to 24 hours. If the diagnostic laboratory has been alerted to the possibility of anthrax, additional biochemical and microbiologic tests should provide a definitive diagnosis within an additional 12 to 24 hours. B. anthracis may also be isolated from skin lesions (in the case of cutaneous anthrax) or respiratory secretions. Careful microscopic examination of stained smears of blood, vesicular fluid, or edema fluid may reveal the presence of B. anthracis. Edema fluid may be the first sample choice for suspected chronic infections, because B. anthracis usually only becomes bacteremic shortly before death. Antibodies may be identified in the blood of infected individuals. Chest radiographs of patients suffering from respiratory ailments may also be diagnostic.
Necropsies should not be performed on animals that are suspected to have died from anthrax, because decomposition of intact carcasses progresses rapidly and the vegetative (nonspore) form of B. anthracis is destroyed in a short time. Anthrax bacilli form spores when exposed to free oxygen and necropsy may increase exposure risk for healthy animals and people. Instead, blood should be obtained aseptically from a peripheral vessel and forwarded to a laboratory for diagnosis. Smears may be prepared by using a swab to obtain blood from a small incision made in the ear or by means of a syringe from an accessible vein. The smear is dried, fixed, and stained with polychrome methylene blue.
Animal vaccination programs have reduced animal mortality from this disease drastically and, as cases of animal disease have decreased, human cases resulting from animal exposure have decreased as well. An approved vaccine is not available for dogs or cats. Animal vaccines have not been approved for and should not be administered to humans. A human vaccine is available but not readily accessible, and population-wide vaccination in the United States has not been recommended because risk has been considered to be low. In countries where anthrax is common, humans should avoid unnecessary contact with livestock and contaminated animal products and should not consume meat that has not been properly inspected and cooked. Postexposure prophylaxis may be achieved through long-term (60 days) oral administration of ciprofloxacin, doxycycline, or amoxicillin. Strains identified as involved in recent exposures in the United States are susceptible to doxycyline and it has recently been recommended as the drug of choice for prophylaxis for people. In addition to receiving antibiotics, it has been recommended that exposed persons be immunized.
Because the course of the disease is so rapid, prompt administration of appropriate antibiotics is essential. Most naturally occurring anthrax strains are susceptible to penicillins, and penicillins have been considered the first line of defense against anthrax. Doxycycline is considered to be a suitable alternative. When natural anthrax affects large animals (e.g., cattle, sheep, goats, swine, and horses), antibiotics of choice include penicillin and oxytetracycline. For small animals, amoxicillin, doxycycline, and enrofloxacin have been recommended; however, their effectiveness is not well documented. Because antibiotic-resistant strains can be readily isolated in laboratories, experts have suggested that ciprofloxacin may be the drug of choice when terrorism is suspected as the source of B. anthracis, at least until antibiotic susceptibilities can be determined. The strain of B. anthracis isolated from malicious exposures in the United States in 2001 was shown to be susceptible to doxycycline, ciprofloxacin, and several other antibiotics.
For animals and humans, anthrax is a reportable disease in the United States. Local and state health departments, federal animal health officials, and the CDC’s National Center for Infectious Diseases, Meningitis and Special Pathogens Branch should immediately be notified of any suspected cases. In addition, diagnostic laboratories should be informed that anthrax is a possible diagnosis when specimens are submitted, to ensure that safe processing protocols are followed.
Management of anthrax in livestock includes quarantine of the affected herd, removal of the herd from the contaminated pasture (if possible), vaccination of healthy livestock, treatment of livestock with clinical signs of disease, disposal of contaminated carcasses (preferably by burning), and incineration of bedding and other material found near the carcass. Because antibiotic treatment has been shown to interfere with response to the Sterne vaccine in animals, animals receiving concurrent antibiotic treatment should be revaccinated after the antibiotic regimen has been completed. Only one case of human-to-human transmission has been reported, therefore, experts believe that standard barrier isolation precautions are sufficient for healthcare workers who are in contact with anthrax patients.
Decontamination protocols are situation-dependent and are based on anticipated aerosolization risk, spore survival, and environmental conditions.
Use of anthrax as a biological weapon
Inhalation anthrax resulting from aerosolization of B. anthracis is of most concern when considering the use of anthrax as a biological weapon. Cutaneous anthrax caused by such an exposure is expected to be easier to identify and treat, with less resultant mortality. A complete risk assessment for direct contamination of food or water with anthrax spores is not available.
Reprinted with permission from the American Veterinary Medical Association