Anthrax
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Etiology/Bacteriology
Taxonomy
Domain: Bacteria
Phylum: Firmicutes
Class: Bacilli
Order: Bacillales
Family: Bacillaceae
Genus: Bacillus
Species: anthracis
Description
Bacillus anthracis is a Gram-positive facultative aerobic spore-forming bacterium found in the soil. B. anthracis derived its name from the Greek word for coal because this pathogen can cause black lesions on the victim's skin. The bacterium is non-motile and non-hemolytic on blood agar. B. anthracis is found mostly in spore-form in the environment, but when it has infected a host it will germinate and replicate in essentially all body tissues. When anthracis is in the spore form, it is resistant to most adverse environments and can survive for many decades.
Pathogenesis
Transmission
The most common way a human can contract anthrax is being in contact with infected animal products. Herbivore grazing animals can commonly contract anthrax because anthracis lives in the soil. A person may get anthrax by inhaling the spores from animal products, such as wool, have an open abrasion on the skin be exposed to the spores, or eating undercooked meat from an animal that was infected. More than 90% of anthrax cases are the cutaneous exposures. Anthrax cannot be spread person-to-person.
Infectious dose, incubation, and colonization
The infectious dose of B. anthracis is not entirely clear. Some suggest 100 spores will cause infection while some analysis have shown as few as 1-3 spores will cause infection. For inhalation anthrax, the infectious dose can be 8-50,000 spores.
The incubation time also depends on what time of anthrax is contracted. For inhalation anthrax, the incubation period is 2-5 days. Cutaneous anthrax will start to manifest symptoms within 2-3 days, with some cases being as short as 12 hours. Gastrointestinal anthrax is much more rare and the incubation time isn't known.
Colonization
Epidemiology
Virulence factors
Clinical features
Cutaneous
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The first sign of a cutaneous anthrax infection is a papule that eventually forms an ulcer with a black center. These lesions are painless. Fever rarely occurs with cutaneous anthrax infections.
Inhalation
If infected with inhalation anthrax, the first symptoms are flu-like and include fever, malaise, cough, and fatigue. After 48 hours, fever, tachypnoea, cyanosis, tachycardia, moist rales, and evidence of pleural effusion may be present. The pulse will be rapid and faint, the patient will become disoriented, and coma and death will soon follow. In half of the patients, meningitis will occur. Meningitis can occur from all three forms of anthrax and the mortality rate after contracting meningitis is near 100%.
Gastrointestinal
Gastrointenstinal anthrax presents itself initially with nausea, vomitting and anorexia, and fever. If left untreated, gastrointestinal anthrax will cause severe abdominal pain, haematemesis, bloody diarrhea, septicemia, and death. The initial symptoms our non-specific and can make it difficult to diagnose, resulting in a high mortality rate.
Diagnosis
To demonstrate that a patient is infected with B. anthracis, a methylene blue stain can be done as well as a blood agar culture. Anthrax will appear as a large Gram-positive rod after staining. A serologic diagnosis of anthrax can be made using a test specific for the protective antigen (PA) component of the toxin.
Cutaneous
Cutaneous anthrax must first be distringuished from plague. The patient also has an appropriate history of being around exposed animals. A specimen may be obtained from the ulcer and cultured on sheep blood agar and then Gram-stained to demonstrate the organism.
Gastrointestinal
To diagnose gastrointestinal anthax, it must be distinguished from dysentery. Obtaining a history of ingesting contaminated meat can be helpful. A stool sample negative for amebic cysts or trophs and for Shigella suggest the possibility of gastrointestinal anthrax.
Inhalation
For a patient that has inhalation anthrax, hemorrhagic mediastinitis with bloody pleural effusions will be present as well as a widened mediastinum when observed under a CT scan.
Treatment
For a potential exposure to anthrax, a person should remove all contaminated clothing and wash with soap and water. Medical responders need to have appropriate protection from infection which includes gloves, splash protection, and a full-face respirator. If anthrax is confirmed, antibiotics should be administered. The CDC recommends ciprofloxacin and doxycycline. If anthrax meningitis is suspected, doxycycline shouldn't be used because it does not penetrate the central nervous system very well. Pregnant or breastfeeding women can use amoxicillin. For inhalation anthrax, it is recommended to use multi-drug therapy, such as vancomycin, with the chosen antibiotic.
Prevention
Prevention methods for avoiding anthrax infection include: vaccinations, proper decontamination, and prophylactic treatments. Vaccinations exist for domestic animals and humans. Since anthrax is commonly acquired through contact with contaminated animal products, vaccination of animals is very important. Bovine anthrax vaccination is extremely effective, but usually has to be re-administered every year. Vaccination of humans is more common for bioterrorism cases, so American troops are vaccinated while it is uncommon for the general population to be vaccinated. If a human or animal has been suspected to die from anthrax, contact with the body should be avoided and the body and clothes should be burned.
Host Immune Response
References
Created by Danielle Vinnedge
1. Conway, Tyrrell. “Genus conway”. “Microbe Wiki” 2013. Volume 1. p. 1-2.
2. http://www.ncbi.nlm.nih.gov/pmc/articles/PMC1769905/
3. http://www.ncbi.nlm.nih.gov/pmc/articles/PMC2784286/
4. http://www.bt.cdc.gov/agent/anthrax/needtoknow.asp
5. http://www.health.state.mn.us/divs/idepc/diseases/anthrax/anslides.pdf
6. http://emedicine.medscape.com/article/212127-clinical