2025 – PAGE 305 – INFECTIOUS DISEASES

GRAM-NEGATIVE ORGANISMS

MNEMONIC: If it ends in –ELLA, it’s probably a gram-negative organism! Brucella, Shigella, Bordetella, Salmonella, etc.

RICKETTSIA RICKETTSII and ROCKY MOUNTAIN SPOTTED FEVER (RMSF)

Rickettsia rickettsii causes ROCKY MOUNTAIN SPOTTED FEVER (RMSF). It is the most common tick-borne illness. Look for hematologic and neurologic signs, such as thrombocytopenia, petechiae, HEADACHE, lethargy, and altered mentation. Also look for muscle pain. The spotted rash starts at the palms, wrists, and soles but then spreads to the trunk within hours. Diagnose first by clinical findings, but then confirm with a biopsy of a lesion and direct immunofluorescence. Treat with DOXYCYCLINE.

PEARLS: Start treatment based on your clinical suspicion BEFORE sending the biopsy. Do NOT delay treatment if there is NOT a history of a tick bite. Treat patients of ALL AGES with DOXYCYCLINE because the benefits outweigh the risks. Also, it’s possible that an EKG may show conduction defects (low-yield).

PEARLS: Ehrlichia chaffeensis can have a very similar presentation but would have associated liver dysfunction and possible leukopenia and/or lymphopenia. It’s also known as Human “Monocytic” Ehrlichiosis even though there is no monocytosis. Fortunately, the treatment is the SAME.

MNEMONIC: To remember the direction of the rash, imagine ROCKY Balboa initially running around the PERIPHERY of Philadelphia, and then running up a MOUNTAIN of stairs in the CENTER of the city.

MNEMONIC: (video) – www.pbrlinks.com/RICKETTSIA1

ENTEROBACTER

Enterobacter is usually hospital-acquired, resulting in urinary or respiratory tract infections. It’s a bad bug. Try treating with cephalosporins, though it’s frequently an ESBL organism requiring a CARBAPENEM (imipenem).

(DOUBLE TAKE) BARTONELLA HENSELAE

Bartonella henselae causes CAT SCRATCH DISEASE. Look for DRAINING and TENDER lymphadenopathy and possibly a history of cat (or dog) exposure. It resolves on its own, so there’s no need to treat unless the patient is immunocompromised or has severe symptoms including hepatosplenomegaly. If so, treat with non-penicillin drugs, like macrolides, cephalosporins, doxycycline, or trimethoprim-sulfamethoxazole.

MNEMONIC: Imagine BART Simpson’s cat being a TENDER kitty that’s fine when it’s LEFT ALONE but SCRATCHES you when you try to pet it. The solution is to LEAVE IT ALONE!

• KEY: TENDER = tender lymphadenopathy, LEAVE IT ALONE = no treatment needed.

CITROBACTER FREUNDII

If you read about a BRAIN ABSCESS on the pediatric exam and see Citrobacter freundii in the answer choices, pick it and move on! It can be a hospital-acquired infection, even in neonates.

MNEMONIC: freundii looks like Freud. Sigmund Freud a famous neurologist who contributed heavily to modern psychology. If you can connect freundii with Freud, then you can remember that freundii is related to some kind of infection in the brain (like an abscess)!