STEPTRACKERMICROBIOLOGY + DERMATOLOGY | MYCOBACTERIA | HIGH-YIELD REVIEW

Nontuberculous Mycobacteria & Leprosy

Environmental mycobacteria that infect AIDS patients, fish tanks and children's necks — and the leprosy bacillus whose two forms mirror Th1 vs Th2 immunity.

FOCUS ON
MAC (M avium-intracellulare): disseminated in AIDS with CD4 < 50 → fever, diarrhea, hepatosplenomegaly, ↑ ALP; prophylaxis (azithromycin) only if not on ART. Also cavitary lung disease in older women ("Lady Windermere"). M marinum: fish tanks/swimming pools → nodules up the arm. M scrofulaceum: cervical lymphadenitis in children. M leprae (Hansen): grows in cool areas, armadillos; tuberculoid (Th1, few bacilli, few hypoesthetic patches) vs lepromatous (Th2, many bacilli, leonine face, glove-and-stocking neuropathy).
ORGANISM · DISEASE
SOURCE · HOST
CLINICAL PICTURE
CLUE · TREATMENT
MAC — disseminated
Soil, water; AIDS with CD4 < 50
Fever, night sweats, weight loss, diarrhea, hepatosplenomegaly, anemia, ↑ ALP
Blood culture · clarithromycin/azithromycin + ethambutol (± rifabutin)
MAC — pulmonary
Structural lung disease, thin elderly women
Chronic cough, nodular bronchiectasis (middle lobe, lingula), cavities
"Lady Windermere syndrome"
MAC — prophylaxis
CD4 < 50 and not starting ART right away
—
Azithromycin weekly; not needed if ART started promptly
M kansasii
Water
TB-like cavitary pulmonary disease
Rifampin, isoniazid, ethambutol
M marinum
Fish tanks, pools; grows at cooler temperatures
Papule/ulcer on hand → nodules along lymphatics
"Fish tank granuloma" · clarithromycin, doxycycline
M scrofulaceum
Soil, water
Unilateral cervical lymphadenitis in children (scrofula)
Excision
Rapid growers
M abscessus, fortuitum, chelonae
Infections after tattoos, pedicures, cosmetic surgery, catheters; CF lungs
Very drug-resistant
M leprae — basics
Acid-fast, can't be cultured in vitro; grows at cool body temperatures (skin, nerves, nose, testes); armadillos in US
Long incubation (years)
Grown in mouse footpad / armadillo
Tuberculoid leprosy
Th1 response (IL-2, IFN-γ); strong cell-mediated immunity
Few well-demarcated, hypopigmented, hairless, anesthetic patches; thickened nerves; few bacilli
Lepromin skin test ⁺ · dapsone + rifampin
Lepromatous leprosy
Th2 response (IL-4, IL-10); poor cell-mediated immunity
Diffuse nodules, leonine facies, loss of eyebrows, nasal collapse, glove-and-stocking anesthesia, testicular atrophy; many bacilli
Lepromin ⁻ · dapsone + rifampin + clofazimine; can be fatal
Erythema nodosum leprosum
Immune-complex reaction during therapy
Painful nodules, fever, neuritis
Thalidomide, steroids

Mechanism — leprosy is a spectrum of T-cell responses

Strong Th1IFN-γ activates macrophages
→
TuberculoidBacilli contained, nerve damage local
Th2 dominanceIL-4, IL-10
→
LepromatousBacilli multiply everywhere, foamy macrophages

Borderline forms sit between; reversal reactions occur as immunity shifts toward Th1.

Tuberculoid vs lepromatous

FEATURE
TUBERCULOID
LEPROMATOUS
T-cell response
Th1
Th2
Bacilli
Few
Many
Skin
Few anesthetic patches
Diffuse nodules, leonine face
Lepromin test
+
−
Contagious
Low
Higher

How it's tested

COMMON CONFUSIONMAC prophylaxis threshold is CD4 < 50 (azithromycin); Pneumocystis is < 200 and Toxoplasma < 100.
THE TRAPDapsone causes hemolysis in G6PD deficiency and methemoglobinemia — check G6PD before leprosy or PCP treatment.
YOUR APPROACHMycobacterium + host (AIDS, child, aquarium hobbyist, travel/armadillo) + site (blood, lung, skin, node, nerve) → species.

If you see… think…

AIDS, CD4 30, fever, diarrhea, high ALP, acid-fast blood culture → disseminated MAC
Elderly thin woman with middle-lobe bronchiectasis → MAC (Lady Windermere)
Nodules spreading up the arm of an aquarium owner → M marinum
Child with unilateral chronic neck node, AFB ⁺ → M scrofulaceum
Skin infection after a pedicure footbath → rapid-growing mycobacteria
Anesthetic hypopigmented patch with thick nerve → tuberculoid leprosy
Leonine facies, lost eyebrows, glove-and-stocking numbness → lepromatous leprosy
US reservoir of M leprae → armadillos
Leprosy drug causing hemolysis in G6PD deficiency → dapsone
Painful nodules during leprosy treatment → ENL → thalidomide