Bacillus anthracis: Anthrax and Its Forms

When you think of bacteria that have shaped human history, Bacillus anthracis deserves a place near the top of that list. This small but deadly microorganism causes anthrax, a disease that has affected humans and animals for millennia. Understanding this pathogen is essential for nursing students, as anthrax remains a significant concern in veterinary medicine, occupational health, and bioterrorism preparedness.

What is Bacillus anthracis? 🔗

Bacillus anthracis is a gram-positive, spore-forming, rod-shaped bacterium that causes anthrax in both humans and animals. The bacterium has two distinct forms: the vegetative (actively growing) form and the spore form. The spores are what make this organism particularly dangerous. These spores are extremely hardy, tolerating extremes of temperature, humidity, and ultraviolet light, and can survive in soil for decades without nutrients or water.

Under the microscope, Bacillus anthracis appears as large, rectangular rods arranged in chains, often described as having a “boxcar” or square-ended appearance. The organism is non-motile and produces a distinctive polypeptide capsule made of polyglutamate, which is unusual since most bacterial capsules are made of polysaccharides. When cultured on blood agar, it produces large, irregular-shaped colonies with a characteristic frosted glass appearance and shows no hemolysis.

How anthrax spreads to humans 🔗

Anthrax primarily affects herbivorous animals like cattle, sheep, and goats that become infected while grazing on contaminated soil or vegetation. Humans are incidental hosts who contract the disease through contact with infected animals or contaminated animal products. The bacteria are found naturally in soil worldwide, but certain regions, including parts of sub-Saharan Africa, Central and South Asia, southern and eastern Europe, and parts of Central and South America, have higher rates of endemic anthrax.

People typically contract anthrax through three main routes: handling infected animals or contaminated animal products (such as wool, hides, or hair), eating raw or undercooked meat from infected animals, or breathing in spores in occupational settings like wool mills, slaughterhouses, or tanneries. Human-to-human transmission does not occur, which means anthrax is not contagious between people.

Cutaneous anthrax: The most common form 🔗

Cutaneous anthrax accounts for more than 95% of human anthrax cases and develops when spores enter through broken skin. Within one to seven days after exposure, a small itchy bump appears at the infection site, resembling an insect bite. This papule progresses over several days into a fluid-filled vesicle, which then ruptures to form a painless ulcer.

The distinctive feature of cutaneous anthrax is the development of a black, necrotic center surrounded by significant edema (swelling). This characteristic black eschar is where the disease gets its name-“anthracis” comes from the Greek word for coal. The painlessness of the lesion is particularly notable and helps distinguish it from other skin infections. Without treatment, cutaneous anthrax can be fatal in up to 20% of cases, but with appropriate antibiotic therapy, almost all patients survive.

Clinical presentation 🔗

The lesion typically appears on exposed areas of the body-most commonly the head, neck, forearms, and hands. Surrounding the central black eschar, you’ll notice pronounced non-pitting edema that can extend quite far from the original site. Satellite vesicles may appear around the main lesion. Despite the dramatic appearance, patients often report minimal pain, which is a key diagnostic clue.

Inhalation anthrax: The deadliest form 🔗

Inhalation anthrax occurs when a person breathes in anthrax spores and is the most lethal form of the disease. After an incubation period of one to six days (though it can be up to two months), the disease begins with nonspecific symptoms including fever, malaise, fatigue, and dry cough. This initial phase can easily be mistaken for influenza or other respiratory infections.

The spores are engulfed by immune cells in the lungs and transported to lymph nodes in the chest, where they germinate, multiply, and begin producing deadly toxins. As the bacteria replicate in the mediastinal lymph nodes, they cause hemorrhagic lymphadenitis (inflamed, bleeding lymph nodes) and mediastinitis. The second stage of illness involves sudden deterioration with severe respiratory distress, high fever, and shock. Chest X-rays classically show a widened mediastinum due to massively enlarged lymph nodes, often with pleural effusions.

Up to 50% of patients with inhalation anthrax develop meningitis, presenting with severe headache, confusion, and rapid progression to coma. Even with aggressive treatment including antibiotics, antitoxins, and supportive care, mortality rates remain around 45%, making this the most feared form of anthrax.

Gastrointestinal anthrax: The rarest form in developed countries 🔗

Gastrointestinal anthrax results from consuming raw or undercooked meat from infected animals. This form is rare in the United States but occurs more frequently in regions where anthrax is endemic in livestock. The disease can affect the upper gastrointestinal tract (oropharyngeal anthrax) or the intestines (intestinal anthrax).

Patients with oropharyngeal involvement develop ulcers in the posterior throat, difficulty swallowing, neck swelling, and enlarged lymph nodes. Those with intestinal anthrax experience fever, nausea, vomiting, and diarrhea that can progress to bloody diarrhea and hematemesis (vomiting blood). The terminal ileum and cecum are most frequently affected. Without treatment, mortality ranges from 25% to 60%, but with proper antibiotic therapy, about 60% of patients survive.

Laboratory investigation and diagnosis 🔗

Confirming anthrax requires testing for the bacteria in blood, skin lesion swabs, cerebrospinal fluid, or respiratory secretions. Samples should be collected before starting antibiotics whenever possible, though treatment should never be delayed for sample collection.

Microscopic examination 🔗

Gram staining reveals large, gram-positive, rectangular rods arranged in chains. For cutaneous lesions, the preferred diagnostic method is staining ulcer exudate with polychrome methylene blue or Giemsa stain. The M’Fadyean stain (polychrome methylene blue) can demonstrate the capsule, showing blue-black bacilli surrounded by pink capsular material. However, the capsule may not be visible in samples from decomposed specimens.

Culture methods 🔗

Bacillus anthracis grows well on sheep blood agar, producing characteristic colonies with a frosted glass appearance. The colonies are non-hemolytic, exceptionally tenacious when teased with a loop, and may show extensive tailing. For environmental samples that may contain multiple bacterial species, selective media like PLET (Polymyxin-Lysozyme-EDTA-Thallous acetate) agar is used to isolate Bacillus anthracis from other organisms.

Molecular testing 🔗

Polymerase chain reaction (PCR) assays detect specific genetic markers of Bacillus anthracis with 100% sensitivity and specificity. These tests can identify the organism in clinical specimens even after antibiotic treatment has begun, making them invaluable when culture is no longer possible. The Laboratory Response Network (LRN) PCR assay detects chromosomal targets as well as genes on the plasmids required for full virulence.

Serological testing 🔗

Antibody testing using ELISA can detect protective antigen (PA) antibodies in patient serum. A fourfold rise in antibody titers between acute and convalescent sera collected two to four weeks apart confirms diagnosis in unvaccinated individuals. This method is particularly useful for retrospective diagnosis when direct detection methods are negative.

Why Bacillus anthracis is so dangerous 🔗

The lethality of Bacillus anthracis comes not just from the bacteria themselves but from the potent toxins they produce. These toxins cause cell damage and death, leading to the characteristic tissue destruction seen in anthrax infections. The pathological findings in infected tissues are often disproportionate to the number of bacteria present, emphasizing the devastating effect of the toxins.

The ability to form spores that survive for decades in the environment makes eradication nearly impossible in endemic areas. Once soil becomes contaminated, it can remain a source of infection indefinitely. This environmental persistence, combined with the potential for rapid progression to fatal disease, makes anthrax a continuing concern in both natural outbreaks and as a potential bioterrorism agent.

Treatment and prevention 🔗

Early antibiotic treatment is critical for survival. Ciprofloxacin and doxycycline are the first-line antibiotics for anthrax treatment and post-exposure prophylaxis. For severe cases, particularly inhalation anthrax, a multi-drug regimen is required, combining a bactericidal agent with a protein synthesis inhibitor. Patients with meningitis require a three-drug combination therapy.

An anthrax vaccine exists and is recommended for people at high risk of exposure, including certain veterinarians, laboratory workers handling the organism, and military personnel. Following potential exposure, post-exposure prophylaxis involves 60 days of antibiotics to prevent disease from ungerminated spores that may remain dormant in the body.

What do you think? How would you approach patient education about anthrax risk for someone working in the livestock industry? What infection control measures would be most critical when caring for a patient with suspected cutaneous anthrax?

References

1. https://www.cdc.gov/anthrax/hcp/clinical-overview/index.html
2. https://centerforhealthsecurity.org/sites/default/files/2023-11/anthrax.pdf
3. https://www.cdc.gov/anthrax/about/index.html
4. https://www.cdc.gov/mmwr/volumes/72/rr/rr7206a1.htm
5. https://www.ncbi.nlm.nih.gov/books/NBK507773/
6. https://pmc.ncbi.nlm.nih.gov/articles/PMC2730312/