What animal exposure causes anthrax, and what does the cutaneous lesion look like?

Cutaneous anthrax follows contact with animal wool, hair, or hides, where Bacillus anthracis spores survive. It is a painless ulcer with a coal-black necrotic center and marked non-pitting edema, and the Gram stain shows boxcar gram-positive rods. Inhaled spores cause woolsorters disease.

How do you tell cutaneous anthrax from a spider bite or a staph abscess?

The anthrax eschar is painless with a black necrotic center and huge non-pitting edema, and the slide shows boxcar gram-positive rods. A staph abscess is painful and purulent with gram-positive cocci in clusters. A spider bite is painful and does not grow boxcar rods. Painlessness plus hides plus boxcar rods is anthrax.

Which zoonotic bacteria come from cats, and how do bites differ from scratches?

A cat or dog bite that turns into rapid cellulitis with pus within a day is Pasteurella multocida. A cat scratch that causes tender regional lymphadenopathy is Bartonella henselae, the cause of cat-scratch disease.

What causes Q fever and why is there no rash or vector?

Q fever is Coxiella burnetii, inhaled as an aerosol from the birth fluids of parturient livestock such as sheep, goats, and cattle. There is no arthropod vector and no rash. It causes atypical pneumonia and can cause culture-negative endocarditis.

Animal Farm: The Zoonotic Bacteria You Catch From Animals (Medical Education)

Signature interactive

Walk the Barnyard

Each station is one exposure. In Explore, tap an animal to open its dossier. In Name That Bug, read the clue and tap the creature that matches. Labels sit outside the figure so nothing gets lost in the hay.

Tap any animal to open its dossier. The painless black eschar on the sheep is the one students miss most.

From the Attending

Stop memorizing nine random bugs. Memorize the exposure. The animal in the stem hands you the organism before you read a single lab value. Fur and hides means anthrax. Rabbit and tick means tularemia. Flea off a rodent means plague. Find the animal, name the bug.

The dossiers

The Livestock Lineup

One card per bug. Tap to flip for the gram stain, the buzzword, and why that exposure does it. Clinical photos are real and open full screen.

Tap a card to flip. Tap again to close.

Bacillus anthracis

Sheep, wool, hides · Anthrax

Bacillus anthracis

Gram: Gram-positive boxcar rods in chains, spore-forming.
Buzzword: Painless black eschar with a necrotic center and marked non-pitting edema. Inhaled spores cause woolsorters disease with a widened mediastinum.
Why: Spores survive for years in animal wool, hair, and hides.
Treat: Ciprofloxacin or doxycycline.

Cutaneous anthrax with black eschar and edema on the skin

📷 Cutaneous anthrax: black eschar · tap to expand

Gram stain of Bacillus anthracis showing boxcar gram-positive rods

📷 B. anthracis: boxcar gram-positive rods · tap to expand

Francisella tularensis

Rabbit, tick, deerfly · Tularemia

Francisella tularensis

Gram: Tiny gram-negative coccobacillus, intracellular. Needs cysteine-enriched media to grow.
Buzzword: Ulceroglandular disease, a tender ulcer with tender regional nodes. Famously low infectious dose.
Why: Skinning infected rabbits or a tick or deerfly bite.
Treat: Streptomycin or gentamicin.

📷 Tularemia: ulceroglandular ulcer · tap to expand

Yersinia pestis

Flea off a rodent · Plague

Yersinia pestis

Gram: Gram-negative rod with safety-pin bipolar staining on Giemsa or Wright stain.
Buzzword: Painful buboes in bubonic plague. Southwest United States, prairie dogs and rodents.
Why: Flea bite from a rodent reservoir.
Treat: Streptomycin or gentamicin.

📷 Plague: painful inguinal bubo · tap to expand

Brucella

Unpasteurized dairy · Brucellosis

Brucella

Gram: Gram-negative coccobacillus, intracellular, slow-growing.
Buzzword: Undulant fever that rises and falls, drenching night sweats, and granulomas. Farmers, vets, abattoir workers.
Why: Unpasteurized milk or cheese from cattle and goats.
Treat: Doxycycline plus rifampin.

📷 Brucella: gram-negative coccobacilli · tap to expand

Pasteurella multocida

Dog or cat bite · Cellulitis

Pasteurella multocida

Gram: Gram-negative coccobacillus, oral flora of cats and dogs.
Buzzword: Rapidly progressive cellulitis with pus within 24 hours of a bite.
Why: Inoculated by the bite itself.
Treat: Amoxicillin-clavulanate.

Bartonella henselae

Cat scratch · Cat-scratch disease

Bartonella henselae

Gram: Gram-negative rod, seen with Warthin-Starry silver stain.
Buzzword: Tender regional lymphadenopathy after a kitten scratch. Bacillary angiomatosis in HIV.
Why: Kitten scratch or flea-contaminated claws.
Treat: Azithromycin.

📷 Cat-scratch disease: inoculation papule · tap to expand

Leptospira interrogans

Water with rat urine · Weil disease

Leptospira interrogans

Gram: Hook-ended spirochete, too thin for Gram stain. Seen on darkfield.
Buzzword: Flu-like illness with calf myalgias and conjunctival suffusion. Severe form is Weil disease: jaundice plus acute kidney injury. Surfers and triathletes.
Why: Fresh water or mud with animal urine.
Treat: Penicillin or doxycycline.

Darkfield image of Leptospira spirochetes with hooked ends

📷 Leptospira: hook-ended spirochetes · tap to expand

Coxiella burnetii

Birth fluids, no vector · Q fever

Coxiella burnetii

Gram: Tiny intracellular gram-negative. Highly infectious aerosol, environmentally hardy spore-like form.
Buzzword: No vector, no rash. Atypical pneumonia, and culture-negative endocarditis.
Why: Inhaled birth fluids from parturient sheep, goats, and cattle.
Treat: Doxycycline.

Erysipelothrix rhusiopathiae

Fish, swine, butcher · Erysipeloid

Erysipelothrix rhusiopathiae

Gram: Gram-positive rod.
Buzzword: Erysipeloid, a violaceous, well-demarcated, painful hand lesion in fish handlers and butchers.
Why: Skin inoculation handling fish, meat, or swine.
Treat: Penicillin.

The discriminator

The Black Eschar Lock

This is the one students miss. A painless black eschar with a necrotic center, animal fur or hides, and boxcar gram-positive rods is anthrax. Not the vector-borne bugs. Not Staph.

📷 Real photo: cutaneous anthrax · tap to expand

Your attending drops a chart on the desk and points at the skin photo.

AttendingDrum maker. Works with imported goat hides. Big black sore on his arm. He swears a spider bit him. What is the one feature that ends the argument?

StudentThe spider story?

AttendingForget the spider. Press on the eschar. It does NOT hurt. A spider bite hurts. A staph abscess hurts. This is PAINLESS. Painless plus necrotic plus animal hides is one organism.

StudentBacillus anthracis.

Painless black eschar plus fur or hides plus boxcar gram-positive rods is cutaneous anthrax. Every time.

Run the Fork

A black or ulcerated lesion walks in. Answer each fork before it reveals.

Step 1. There is a skin lesion turning black. You press the eschar. Is the lesion painful or painless, and which way does that point?

Painlessness is the fork. Anthrax edema and lethal toxins kill tissue without the throbbing pain of a Staph abscess. A painful, pus-filled lesion with gram-positive cocci in grape clusters is Staph, never a black eschar.

Step 2. The lesion is painless. The history mentions a job. Which exposure locks in anthrax over tularemia and plague?

Fur and hides is anthrax. Rabbit and tick is tularemia, whose ulcer is tender with tender nodes. Flea and rodent is plague, which gives a painful bubo, not a painless eschar. The exposure sorts the three zoonoses.

Step 3. The Gram stain of the lesion seals it. What confirms Bacillus anthracis?

Boxcar gram-positive rods name anthrax. Cysteine-dependent gram-negative coccobacilli are Francisella, tularemia. Safety-pin bipolar gram-negative rods are Yersinia pestis, plague. The morphology is the final lock.

From the Attending

Three forks, one answer. Painless. Hides. Boxcar rods. Miss the painlessness and you chase a spider or a Staph abscess and you are wrong. Know your clues.

Decision tool

Sort the Barn by Gram Stain

The slide narrows it fast. Tap a bug, then tap the bin where it belongs. Anthrax and Erysipelothrix are the only gram-positives in this barn.

Gram-positive rods

Gram-negative coccobacilli

Gram-negative rods

Spirochete (no Gram)

The Whole Barn on One Card

Self-test: the disease column is blurred. Tap the table to reveal.

Animal or exposure	Bug	Gram stain	Buzzword	Disease
Wool, hair, hides	Bacillus anthracis	Gram-positive boxcar rods, spores	Painless black eschar; woolsorters disease	Anthrax
Rabbit, tick, deerfly	Francisella tularensis	Gram-negative coccobacillus, needs cysteine	Ulcer plus tender nodes; tiny dose	Tularemia
Flea off a rodent	Yersinia pestis	Gram-negative rod, safety-pin bipolar	Painful bubo	Plague
Unpasteurized dairy	Brucella	Gram-negative coccobacillus, intracellular	Undulant fever, night sweats	Brucellosis
Dog or cat bite	Pasteurella multocida	Gram-negative coccobacillus	Cellulitis with pus inside 24 hours	Bite cellulitis
Cat scratch	Bartonella henselae	Gram-negative rod, silver stain	Tender regional nodes	Cat-scratch disease
Water with animal urine	Leptospira interrogans	Hook-ended spirochete, darkfield	Calf myalgias, conjunctival suffusion, Weil	Leptospirosis
Livestock birth fluids	Coxiella burnetii	Tiny intracellular gram-negative	No vector, no rash; culture-negative endocarditis	Q fever
Fish, swine, butcher	Erysipelothrix rhusiopathiae	Gram-positive rod	Violaceous painful hand lesion	Erysipeloid

🧬 Gram Stain Chart 🦠 Staph vs Strep

Clinical walkthrough

Rounds: Board-Style Vignettes

Eight board-style vignettes. Shuffled, never-repeat, with full explanations for every choice. Cross out and highlight as you read, then walk the reasoning chain.

Animal Farm: The Bugs You Catch From Animals

Walk the Barnyard

The Livestock Lineup

Bacillus anthracis

Francisella tularensis

Yersinia pestis

Brucella

Pasteurella multocida

Bartonella henselae

Leptospira interrogans

Coxiella burnetii

Erysipelothrix rhusiopathiae

The Black Eschar Lock

Run the Fork

Sort the Barn by Gram Stain

The Whole Barn on One Card

Rounds: Board-Style Vignettes