Three CDC Category A bioterrorism agents: Bacillus anthracis, Yersinia pestis, Francisella tularensis — gram-negative or gram-positive zoonoses with potential for aerosol weaponization and rapid mortality.
Also known as: Bacillus anthracis, Yersinia pestis, Francisella tularensis, bioterrorism, Category A agents, cutaneous anthrax, bubonic plague, ulceroglandular tularemia
Overview
A trio of CDC Category A select agents notable for their potential as bioterrorism weapons due to ease of dissemination, high mortality, and public health impact: anthrax (Bacillus anthracis, gram-positive spore-forming rod), plague (Yersinia pestis, gram-negative bipolar 'safety pin' coccobacillus), and tularemia (Francisella tularensis, small gram-negative coccobacillus).
Epidemiology
Anthrax: natural cases in livestock workers (cutaneous most common); inhalational from 2001 US mailings. Plague: endemic in rural Southwest US (~7 cases/year) via prairie dogs and ground squirrels with flea vectors. Tularemia: ~200 US cases/year, focal in Arkansas, Missouri, Oklahoma, often after rabbit hunting or tick bite.
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Question 1Infectious DiseaseMedium
A 30-year-old man from rural New Mexico develops abrupt high fever, chills, and headache, and within a day a single exquisitely tender, swollen lymph node in the right groin. He had been camping and recalls flea bites; a ground-squirrel die-off was reported nearby. A gram-negative rod with bipolar 'safety-pin' staining is seen on aspirate. Which of the following is the most likely diagnosis?
ABubonic plague
BLymphogranuloma venereum
CUlceroglandular tularemia
DCat-scratch disease
EFlea-borne typhus
Reveal answer & full explanation
Correct answer: A — Bubonic plague
ABubonic plague✓
BLymphogranuloma venereum
CUlceroglandular tularemia
DCat-scratch disease
EFlea-borne typhus
Why Bubonic plague is correct
Bubonic plague is caused by Yersinia pestis, a CDC Category A bioterrorism agent, and is endemic to the rural southwestern United States, where fleas transmit it from ground squirrels, prairie dogs, and other rodents.
The hallmark is abrupt fever with a rapidly enlarging, exquisitely painful regional lymph node (bubo); the organism is a gram-negative rod with bipolar 'safety-pin' staining and can progress to lethal septicemic or aerosolizable pneumonic forms.
Why the others are wrong
Cat-scratch disease — follows a kitten scratch and causes a milder, more indolent regional lymphadenopathy from Bartonella henselae.
Ulceroglandular tularemia — also a Category A rodent/tick zoonosis, but presents with a skin ulcer at the inoculation site plus adenopathy, not an isolated painful bubo with safety-pin bacilli.
Lymphogranuloma venereum — is a sexually transmitted Chlamydia trachomatis infection causing inguinal buboes without the acute rodent/flea exposure and toxic onset.
Flea-borne typhus — flea bites, abrupt fever, and headache fit flea-borne typhus from Rickettsia typhi, but it causes a nonspecific febrile illness, sometimes with a maculopapular truncal rash, rather than an exquisitely tender regional bubo, and the obligate intracellular rickettsiae are not seen as bipolar safety-pin rods on a node aspirate.
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Anthrax: contact with animal hides, wool, livestock; IV drug use (heroin contamination in Europe); bioterrorism exposure
Plague: rodent and flea exposure in endemic Southwest; sick prairie dogs; outdoor cats
Tularemia: rabbit and hare hunting (dressing carcasses), tick and deerfly bites, lawn mowing over carcasses, laboratory exposure
Laboratory work with any of these organisms (all three are highly infectious by aerosol)
Pathophysiology
Anthrax spores germinate in macrophages, producing edema toxin (adenylyl cyclase) and lethal toxin causing massive edema and tissue necrosis. Yersinia pestis is transmitted via flea bite, replicates in lymph nodes (buboes), and can progress to septicemic and pneumonic forms. Francisella tularensis is a facultative intracellular pathogen with ID50 as low as 10 organisms; replicates within macrophages and produces granulomatous inflammation in regional lymph nodes.
Clinical presentation
Symptoms
Cutaneous anthrax (>95% of natural cases): painless papule → vesicle → painless black eschar with surrounding non-pitting edema
Cellulitis or brown recluse bite — Painful, unlike the characteristic painless black eschar of cutaneous anthrax
Lymphogranuloma venereum or cat-scratch disease — Regional adenopathy without the rapid systemic toxicity of plague
Community-acquired pneumonia — Inhalational anthrax and pneumonic plague mimic CAP early; CXR widened mediastinum (anthrax) and bloody sputum (plague) distinguish
Streptococcal lymphadenitis — Ulceroglandular tularemia has a punched-out ulcer at inoculation site
Diagnostic workup
Diagnostic criteria
Compatible exposure + clinical syndrome + microbiologic confirmation by culture, PCR, or serology. Public health notification is mandatory and immediate.
Labs
ALERT LAB before sending — all three are biosafety hazards
Anthrax: blood cultures (large gram-positive rods in chains, capsule visible by India ink), PCR, ELISA for protective antigen
Plague: Wayson or Wright-Giemsa stain of bubo aspirate showing bipolar 'safety pin' coccobacilli; blood, sputum cultures; F1 antigen ELISA, PCR
Tularemia: serology (microagglutination) and PCR; cultures positive but rarely attempted owing to lab biosafety
CBC: leukocytosis with left shift
Coagulation panel: DIC in plague
Imaging
Anthrax: chest CT showing widened mediastinum and hemorrhagic mediastinitis
Plague: chest x-ray for pneumonic plague (patchy infiltrates with rapid progression)
CT of involved nodes in tularemia
Treatment
First-line
Anthrax (cutaneous, no systemic): ciprofloxacin or doxycycline 7-10 days for naturally acquired disease; extend to 60 days only if concurrent aerosol/spore exposure is suspected (bioterrorism)
Anthrax (systemic / inhalational): combination therapy with two bactericidal agents from different classes (e.g., ciprofloxacin + meropenem) plus a protein synthesis inhibitor (doxycycline or minocycline preferred over clindamycin or linezolid) until meningitis is excluded, then one bactericidal agent plus a protein synthesis inhibitor; add anthrax antitoxin (raxibacumab or obiltoxaximab)
Plague: an aminoglycoside (streptomycin or gentamicin) or a fluoroquinolone (ciprofloxacin, levofloxacin) is first-line, and doxycycline is also first-line for uncomplicated bubonic plague; use two drug classes for severe pneumonic/septicemic disease or after a bioterrorism release until susceptibilities are known; droplet precautions until at least 48 h of therapy with clinical improvement
Tularemia (severe): streptomycin or gentamicin IV; doxycycline or ciprofloxacin for milder disease and PEP
Anthrax PEP
Ciprofloxacin or doxycycline for 60 days
Three-dose anthrax vaccine series concurrently
Plague PEP
Doxycycline or ciprofloxacin for 7 days for close contacts of pneumonic cases
Tularemia PEP
Doxycycline or ciprofloxacin for 14 days after high-risk lab exposure
Second-line / adjunct
Anthrax antitoxin (raxibacumab, obiltoxaximab) for systemic disease
Drainage of buboes only if pointing — gentle aspiration with BSL precautions
Mandatory public health and CDC notification for all three
Lab-acquired infection — all three are aerosol-transmissible
PANCE pearls
Painless ulcer with surrounding edema = cutaneous anthrax; treat early to prevent systemic spread.
Doxycycline and ciprofloxacin are the rule-of-thumb oral agents across all three when injectables unavailable.
Pneumonic plague is the only one of the three that spreads person-to-person — droplet precautions, treat contacts.
Tularemia has the lowest infectious dose of common bacterial pathogens (~10 organisms by aerosol).
Always alert the laboratory before sending cultures — risk of laboratory-acquired infection is high.
CDC Category A agents (easily spread, high mortality) are anthrax, botulism, plague, smallpox, tularemia, and viral hemorrhagic fevers; Category B includes Q fever, brucellosis, ricin, and staphylococcal enterotoxin B; Category C covers emerging pathogens such as Nipah virus and hantavirus.
CDC MMWR — CDC Use of Anthrax Vaccine in the United States — Recommendations of the ACIP
WHO — WHO Anthrax in Humans and Animals, 4th edition
CDC MMWR 2021 — Nelson CA, et al. Antimicrobial Treatment and Prophylaxis of Plague: Recommendations for Naturally Acquired Infections and Bioterrorism Response. MMWR Recomm Rep 2021;70(No. RR-3):1-27
CDC MMWR 2023 — Bower WA, et al. CDC Guidelines for the Prevention and Treatment of Anthrax, 2023. MMWR Recomm Rep 2023;72(No. RR-6):1-47
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