Tuberculosis, Systemic Endemic Mycoses & Zoonotic Infections

Every endemic mycosis vignette gives a geographic clue paired with a distinctive microscopic yeast morphology at 37°C: (1) Histoplasmosis (Histoplasma capsulatum) is endemic to the Ohio and Mississippi River Valleys (and Caribbean/Central America) in soil enriched with bat guano (spelunking in caves) or starling/chicken bird droppings (cleaning chicken coops). Crucially, Histoplasma is the ONLY endemic fungus that is SMALLER than a red blood cell (2–4 μm) and replicates INTRACELLULARLY inside alveolar and reticuloendothelial macrophages! Consequently, disseminated histoplasmosis in HIV/TNF-inhibitor patients presents with hepatosplenomegaly, pancytopenia, painful palatal/tongue mucosal ulcers, and bilateral adrenal insufficiency (while pulmonary disease mimics sarcoidosis/TB with calcified hilar nodes and erythema nodosum; diagnosed rapidly via Urine/Serum Histoplasma Antigen). (2) Blastomycosis (Blastomyces dermatitidis) overlaps the Ohio/Mississippi River Valleys, Great Lakes, and St. Lawrence River (rotting wood/beaver dams) and forms large yeasts (8–15 μm, same size as an RBC) with thick doubly refractive walls and pathognomonic BROAD-BASED BUDDING. Unlike Histoplasma, disseminated Blastomyces has classic tropism for Lung + SKIN (verrucous, crusted wart-like plaques with violaceous borders and microabscesses that mimic Squamous Cell Carcinoma!) + BONE (osteolytic osteomyelitis) + Prostate. (3) Coccidioidomycosis (Coccidioides immitis / posadasii — 'Valley Fever') is endemic to the arid Southwestern United States (Arizona, California's San Joaquin Valley, New Mexico, West Texas) following dust storms, earthquakes, or archaeological digs. At 25°C it forms barrel-shaped arthroconidia, and at 37°C in tissue it is the ONLY endemic fungus that does NOT bud—instead forming giant thick-walled SPHERULES (20–80 μm, larger than an RBC) packed with hundreds of endospores! It presents with pneumonia + arthralgias ('desert rheumatism') + Erythema Nodosum, and disseminates in immunocompromised hosts, Filipinos/African Americans, and 3rd-trimester pregnant women to the Meninges (basilar meningitis) and Bone. (4) Paracoccidioidomycosis (Paracoccidioides brasiliensis) occurs in Rural Latin/South America (male agricultural workers; estradiol inhibits mold-to-yeast transition in women!) and shows multiple peripheral buds radiating from a central yeast cell like a 'Captain's Wheel / Mickey Mouse ears'.

Across all four systemic dimorphic fungi (Histoplasma, Blastomyces, Coccidioides, Paracoccidioides), the treatment algorithm follows a single unified rule with one critical meningitis exception: Mild-to-Moderate Localized Pulmonary or Cutaneous Disease is treated with Oral Itraconazole (or Fluconazole/Itraconazole for Coccidioides), whereas Severe, Life-Threatening, or Disseminated Disease (and all disease in Pregnancy) is treated initially with Intravenous Liposomal Amphotericin B followed by step-down oral azole therapy. THE MENINGITIS EXCEPTION: For Coccidioidal Meningitis, intravenous Amphotericin B cannot cross the blood-brain barrier effectively, so Lifelong High-Dose Oral Fluconazole (which has >80% CSF penetration) is the treatment of choice! Contrast these endemic dimorphic fungi with two high-yield opportunistic/cutaneous molds: (1) Sporotrichosis (Sporothrix schenckii — 'Rose Gardener's Disease') is a thermally dimorphic fungus inoculated transcutaneously by rose thorns, sphagnum moss, or hay/splinters (or infected cats), forming a painless papule on the finger that ulcerates followed by ascending subcutaneous nodules along the proximal lymphatic drainage channels (lymphocutaneous sporotrichosis); histology shows cigar-shaped budding yeast and asteroid bodies, and treatment is Oral Itraconazole (or Saturated Solution of Potassium Iodide [SSKI]). (2) Aspergillus fumigatus is a monomorphic (NOT dimorphic!) mold with septate hyphae that branch at acute 45° angles, causing three distinct syndromes: Allergic Bronchopulmonary Aspergillosis (ABPA) in asthmatics/CF patients (migratory infiltrates, central bronchiectasis, eosinophilia, high IgE >1,000 IU/mL → treat with Corticosteroids + Itraconazole), Aspergilloma ('Fungus Ball') colonizing a pre-existing tuberculous upper-lobe cavity (presents with massive hemoptysis and a gravity-dependent mobile mass with an air-crescent sign on CT → surgical resection/embolization), and Invasive Aspergillosis in severely neutropenic/AML/transplant patients (angioinvasive necrotizing pneumonia with 'halo sign' on CT and positive serum Galactomannan assay → First-line therapy is Voriconazole, NOT fluconazole!).

A single bite from the black-legged deer tick Ixodes scapularis in the Northeastern and Upper Midwestern United States (Connecticut, Massachusetts, New York, Wisconsin, Minnesota; requires ≥36–48 hours of tick attachment for spirochete transmission!) transmits three distinct pathogens: (1) Lyme Disease (Borrelia burgdorferi spirochete) evolves in 3 stages: Stage 1 (Early Localized, days–weeks) = expanding Erythema Migrans ('bull's-eye' rash ≥5 cm) + flu-like malaise (diagnose clinically and treat immediately—serology is often false-negative in Stage 1!); Stage 2 (Early Disseminated, weeks–months) = multiple erythema migrans lesions, unilateral or bilateral Facial Nerve (CN VII) Palsy, lymphocytic meningitis, and AV Nodal Heart Block (Lyme Carditis); Stage 3 (Late Disseminated, months–years) = monoarticular/oligoarticular Arthritis of the large joints (especially the Knee) and subacute encephalopathy/polyneuropathy. Treat Lyme disease with Oral Doxycycline (10–21 days; Amoxicillin or Cefuroxime in children <8 years or pregnant women, though short courses of doxycycline are now considered safe in young children by AAP), except for High-Grade AV Block or Parenchymal Neuroborreliosis/Meningitis, which require Intravenous Ceftriaxone! (2) Babesiosis (Babesia microti) is an intraerythrocytic protozoan causing fever, hemolytic anemia (low haptoglobin, high LDH, jaundice), hemoglobinuria, and thrombocytopenia (severe in asplenic patients!), showing pathognomonic intraerythrocytic ring forms and 'Maltese Cross' tetrads on Giemsa thin blood smear (treat with Atovaquone + Azithromycin). (3) Human Granulocytic Anaplasmosis (Anaplasma phagocytophilum) (and Ehrlichiosis [Ehrlichia chaffeensis] transmitted by the Amblyomma americanum Lone Star tick in the South-Central US) are obligate intracellular bacteria that infect neutrophils (Anaplasma) or monocytes (Ehrlichia), presenting as a 'rashless RMSF' with fever, headache, myalgias, LEUKOPENIA, THROMBOCYTOPENIA, elevated AST/ALT, and mulberry-like intracytoplasmic inclusions ('Morulae') inside WBCs on buffy-coat smear (treat immediately with Doxycycline). Contrast these with Rocky Mountain Spotted Fever (RMSF — Rickettsia rickettsii), transmitted by the Dermacentor dog/wood tick (most common in North Carolina, Oklahoma, Arkansas, Tennessee, Missouri): RMSF invades vascular endothelial cells, presenting with fever, severe headache, and a maculopapular-to-petechial rash that starts on the WRISTS AND ANKLES on Day 2–5 and spreads CENTRIPETALLY to the trunk AND involves the PALMS AND SOLES—start Doxycycline IMMEDIATELY in ALL patients (including children and pregnant women!) upon clinical suspicion without waiting for serology!

Syphilis (Treponema pallidum spirochete) progresses through three clinical stages: (1) Primary Syphilis (3 weeks post-exposure) presents with a SOLITARY, PAINLESS, indurated genital ulcer ('Hard Chancre') with a clean base and non-tender bilateral inguinal lymphadenopathy (diagnosed by Darkfield Microscopy or PCR, as serology may still be negative; heals spontaneously in 3–6 weeks; contrast with Haemophilus ducreyi [Chancroid] which causes a PAINFUL ulcer with ragged borders and suppurative inguinal nodes, and Klebsiella granulomatis [Granuloma inguinale] which causes painless beefy-red ulcers with Donovan bodies). (2) Secondary Syphilis (6–12 weeks later) reflects spirochetemia, presenting with a generalized non-pruritic maculopapular copper-colored rash involving the PALMS AND SOLES, flat broad-based whitish-gray wart-like plaques on moist intertriginous mucosa (Condylomata Lata, teeming with spirochetes; contrast with HPV 6/11 Condylomata acuminata which are cauliflower-like!), patchy 'moth-eaten' alopecia, and generalized lymphadenopathy. (3) Tertiary Syphilis (years later) causes destructive granulomatous Gummas of skin/bone, Cardiovascular Syphilitic Aortitis (obliterative endarteritis of the vasa vasorum of the ascending thoracic aorta → aortic root dilation and Aortic Regurgitation with 'tree-bark' intima), and Neurosyphilis (Tabes Dorsalis demyelination of dorsal columns/roots causing sensory ataxia, positive Romberg, lancinating leg pains, Charcot joints, and Argyll Robertson Pupils—bilateral small pupils that accommodate [constrict to near targets] but DO NOT react to light ['Prostitute's Pupil']). Screen with a non-treponemal test (RPR or VDRL, which measures anticardiolipin antibody titers that decrease after cure; false-positive in SLE/antiphospholipid syndrome) and confirm with a specific treponemal test (FTA-ABS or TP-PA, which remains positive for life). Treat Primary, Secondary, and Early Latent Syphilis with a single intramuscular dose of Benzathine Penicillin G (2.4 million units IM), and treat Neurosyphilis, Ocular Syphilis, or Pregnant patients (desensitize if penicillin-allergic!) with IV Aqueous Crystalline Penicillin G (warn patients of the self-limiting 24-hour Jarisch-Herxheimer Reaction—fever, chills, myalgias, and tachycardia within hours of penicillin lysis of spirochetes, managed with NSAIDs/acetaminophen). Finally, memorize the rapid animal-reservoir zoonoses: Brucella → unpasteurized goat/sheep cheese/milk or livestock, undulant fever, night sweats, osteomyelitis (treat: Doxycycline + Rifampin/Gentamicin); Francisella tularensis (Tularemia) → skinning wild rabbits or Dermacentor tick bite, ulceroglandular painful skin ulcer + regional lymphadenopathy (treat: Streptomycin/Gentamicin or Doxycycline); Pasteurella multocida → rapid cellulitis within <24 hours of a Cat or Dog bite (treat: Amoxicillin-Clavulanate); Coxiella burnetii (Q Fever) → inhaling barnyard placental aerosols from birthing sheep/goats/cattle, presenting with pneumonia + hepatitis + culture-negative endocarditis WITHOUT a rash (treat: Doxycycline); and Leptospira interrogans → freshwater swimming/kayaking in water contaminated with animal (rodent) urine, presenting with fever, myalgias (calves), and conjunctival suffusion (or Weil disease: jaundice + renal failure + hemorrhage).

Screening for Latent Tuberculosis Infection (LTBI) relies on either the Mantoux purified protein derivative (PPD) intradermal skin test or an Interferon-Gamma Release Assay (IGRA). When reading a PPD at 48–72 hours, clinicians measure only palpable induration (never erythema) along the transverse axis of the forearm. Because PPD shares antigens with Mycobacterium bovis, prior BCG vaccination produces false-positive PPD results; in BCG-vaccinated patients, IGRA is the preferred screening modality because its synthetic antigens (ESAT-6 and CFP-10) are absent from all BCG strains and most non-tuberculous mycobacteria.

Any positive PPD or IGRA mandates a chest radiograph to exclude active disease before starting LTBI monotherapy. Active reactivation TB presents with fever, drenching night sweats, weight loss, hemoptysis, and upper-lobe cavitary infiltrates. Definitive workup requires three sputum specimens collected 8–24 hours apart (at least one early morning) for acid-fast bacilli (AFB) smear microscopy, nucleic acid amplification testing (Xpert MTB/RIF)—which rapidly detects both M. tuberculosis DNA and rpoB rifampin resistance—and Lowenstein-Jensen or liquid mycobacterial culture (gold standard). Extrapulmonary manifestations include Pott disease (thoracolumbar vertebral osteomyelitis), basilar meningitis (high CSF protein, lymphocytic pleocytosis, low glucose), and constrictive pericarditis.

Once active TB is excluded, CDC-preferred LTBI regimens emphasize shorter rifamycin-based courses due to superior completion and lower hepatotoxicity than 9 months of daily Isoniazid (9H): 3HP (3 months of once-weekly Isoniazid plus Rifapentine), 4R (4 months of daily Rifampin), or 3HR (3 months of daily Isoniazid plus Rifampin). Active TB requires a 2-month intensive phase of all four RIPE drugs followed by a 4-month continuation phase of Isoniazid plus Rifampin. By contrast, Mycobacterium avium-intracellulare complex (MAC) causes disseminated disease in HIV patients with CD4⁺ counts <50 cells/mm³ and is treated with azithromycin (or clarithromycin) plus ethambutol.

Aspergillus fumigatus clinical presentation depends entirely on host immune status. In hyper-reactive atopic hosts with asthma or cystic fibrosis, Allergic Bronchopulmonary Aspergillosis (ABPA) causes central bronchiectasis, fleeting pulmonary infiltrates, peripheral eosinophilia, and markedly elevated total serum IgE (>1,000 IU/mL); treatment is systemic corticosteroids plus itraconazole. In immunocompetent patients with pre-existing pulmonary cavities (prior tuberculosis or sarcoidosis), a non-invasive Aspergilloma (mycetoma) forms a gravity-dependent intracavitary fungus ball that shifts position on decubitus CT and presents with life-threatening hemoptysis. In severely neutropenic patients, angioinvasive Invasive Pulmonary Aspergillosis causes hemorrhagic infarction (CT halo sign and air-crescent sign) with positive serum galactomannan antigen, requiring immediate IV voriconazole.

Fungi of the order Mucorales (Rhizopus, Mucor, Lichtheimia) possess ketone reductase enzymes and thrive in the hyperglycemic, acidotic environment of Diabetic Ketoacidosis (DKA), where acidosis releases free iron from serum transferrin. Inhaled spores germinate in the paranasal sinuses, invade blood vessels causing thrombosis and ischemic necrosis (black palatal or nasal eschar), and extend directly through the cribriform plate and orbital apex into the cavernous sinus (causing proptosis, ophthalmoplegia via CN III, IV, V₁, V₂, VI palsies, and internal carotid artery thrombosis). Survival requires immediate reversal of acidosis, aggressive surgical debridement of all necrotic tissue, and high-dose IV liposomal Amphotericin B.

When evaluating bone marrow or bronchoalveolar lavage specimens from immunosuppressed patients residing in the Ohio and Mississippi River valleys, Histoplasma capsulatum appears as tiny (2–4 μm) narrow-based budding yeasts packed inside macrophage cytoplasm—smaller than an erythrocyte—and is rapidly diagnosed via urine or serum Histoplasma polysaccharide antigen detection. Mild-to-moderate endemic mycoses (Histoplasma, Blastomyces) are treated with oral itraconazole, whereas severe pulmonary, central nervous system, or disseminated infections require lipid Amphotericin B.

Primary syphilis presents 3 weeks post-exposure with a painless, indurated chancre at the inoculation site. Without treatment, spirochetemia leads to Secondary syphilis (6–12 weeks later), characterized by constitutional symptoms, generalized nontender lymphadenopathy, a diffuse maculopapular coppery rash involving the palms and soles, patchy moth-eaten alopecia, snail-track oral mucous patches, and condylomata lata (broad-based, flat-topped, gray moist plaques on the perineum teeming with spirochetes—contrast with HPV condylomata acuminata, which are cauliflower-like). Years later, Tertiary syphilis manifests as: (1) Gummas (chronic necrotizing granulomas of skin, bone, and liver); (2) Cardiovascular syphilis (obliterative endarteritis of the vasa vasorum causing ascending thoracic aortic aneurysm with tree-bark intimal wrinkling and aortic regurgitation); and (3) Neurosyphilis.

Late neurosyphilis comprises meningovascular stroke (middle cerebral artery arteritis) and parenchymal syndromes. Tabes dorsalis results from demyelination of the posterior columns and dorsal nerve roots, producing loss of proprioception and vibratory sense, lancinating lightning pains, absent deep tendon reflexes, broad-based sensory ataxia with a positive Romberg sign, and Charcot neuropathic joints. Physical exam reveals the classic Argyll Robertson pupil (prostitute's pupil: bilateral small pupils that constrict to accommodation/convergence but fail to constrict to direct or consensual light due to pretectal midbrain lesions). Neurosyphilis requires high-dose IV Aqueous Crystalline Penicillin G for 10–14 days.

Syphilis screening utilizes quantitative nontreponemal tests (RPR, VDRL) that detect anticardiolipin-lecithin-cholesterol antibodies (reagin); false-positives occur in SLE (antiphospholipid antibodies), pregnancy, viral infections, and leprosy. Positive nontreponemal screens are confirmed with specific treponemal tests (FTA-ABS, TP-PA), which turn positive earlier and remain positive for life regardless of treatment. Treatment success is monitored by a 4-fold decline in RPR/VDRL titer (e.g., 1:32 to 1:8) within 6–12 months. Within hours of penicillin administration, lysis of spirochetes releases endotoxin-like lipoproteins, causing the self-limited Jarisch-Herxheimer reaction (fever, rigors, tachycardia, flushing, myalgias), managed solely with NSAIDs/antipyretics.

While P. vivax and P. ovale produce benign tertian fever (48-hour cycle) and P. malariae produces quartan fever (72-hour cycle, associated with membranoproliferative glomerulonephritis), Plasmodium falciparum infects erythrocytes of all ages, achieving parasitemia levels >5–10% with irregular fever spikes. Because widespread pfCRT efflux pump mutations confer chloroquine resistance in almost all P. falciparum-endemic regions, uncomplicated falciparum malaria is treated with oral artemether-lumefantrine or atovaquone-proguanil, whereas severe or cerebral malaria requires IV artesunate. For travelers to chloroquine-resistant zones, daily atovaquone-proguanil, daily doxycycline, or weekly mefloquine (safe in pregnancy, contraindicated in psychiatric/seizure history) is prescribed.

Both Babesia microti and Plasmodium falciparum produce intraerythrocytic ring forms with hemolytic anemia, jaundice, and hemoglobinuria. However, Babesiosis occurs in patients without foreign travel who visited coastal New England or the Upper Midwest (often co-infected with Borrelia burgdorferi or Anaplasma phagocytophilum), demonstrates the pathognomonic Maltese cross tetrad of budding merozoites on Giemsa smear, lacks brownish hemozoin pigment deposits, and shows extracellular ring forms. Standard antimalarials like chloroquine or primaquine are ineffective; treat mild-to-moderate babesiosis with atovaquone plus azithromycin.

Marked peripheral eosinophilia in a returning traveler or immigrant strongly points toward tissue-invasive helminths. Taenia solium (pork tapeworm) causes two distinct syndromes depending on the stage ingested: eating undercooked pork containing larval cysticerci causes asymptomatic adult intestinal tapeworms, whereas ingesting food or water contaminated with human feces containing T. solium eggs causes Neurocysticercosis (multiple calcified and cystic parenchymal brain lesions presenting with adult-onset seizures; treat with albendazole plus dexamethasone). Schistosoma haematobium (freshwater snails in Egypt/Africa; terminal spine on egg) causes painless hematuria, bladder wall calcification, and squamous cell carcinoma of the bladder (treat with praziquantel). Echinococcus granulosus (dog feces/sheep herding) causes unilocular hepatic hydatid cysts with eggshell calcifications; never aspirate prior to albendazole/hypertonic saline instillation because cyst fluid spillage triggers fatal anaphylactic shock.