Topic 6: Mycology, Parasitology & Immunology Basics
Introduction
This chapter covers three distinct but equally important areas for FMGE: medical mycology (fungal infections), parasitology (protozoa and helminths), and basic immunology as applied to infectious diseases. Fungal and parasitic infections are collectively called opportunistic infections and are particularly relevant in immunocompromised patients. Immunological principles underpin vaccine responses, hypersensitivity reactions, and serological diagnosis.
Medical Mycology
Fungi are eukaryotic organisms with a rigid cell wall (containing chitin, not peptidoglycan). They can be unicellular (yeasts) or multicellular (moulds/dimorphic).
Classification of Medically Important Fungi
| Type | Characteristics | Examples |
|---|---|---|
| Yeast | Unicellular, round/oval, budding | Candida albicans, Cryptococcus neoformans |
| Mould | Multicellular, filamentous (hyphae) | Aspergillus, Mucor, Rhizopus |
| Dimorphic | Yeast at 37°C, mould at 25°C | Histoplasma capsulatum, Blastomyces, Paracoccidioides |
Superficial and Cutaneous Mycoses
- Pityriasis versicolor (Tinea versicolor) — Malassezia furfur; hypopigmented or hyperpigmented macules on trunk; “spaghetti and meatballs” appearance on KOH; treated with topical selenium sulfide or azoles
- Tinea (dermatophytoses) — Trichophyton, Microsporum, Epidermophyton; keratins degraded by fungal keratinases; all cause ringworm lesions with active scaly border and central clearing
| Infection | Site |
|---|---|
| Tinea corporis | Body |
| Tinea cruris | Groin (jock itch) |
| Tinea pedis | Foot (athlete’s foot) |
| Tinea capitis | Scalp (black dot ringworm) |
| Tinea unguium | Nails (onychomycosis) |
Opportunistic Fungal Infections
- Candida albicans — budding yeast with pseudohyphae and germ tubes; causes oral thrush (white plaques), vulvovaginitis, esophagitis, disseminated candidiasis; treated with fluconazole (uncomplicated) or amphotericin B (invasive)
- Cryptococcus neoformans — encapsulated yeast (India ink shows clear halo); acquired from pigeon droppings; causes cryptococcal meningitis in AIDS; antigen detection in CSF (CrAg) is diagnostic; treated with amphotericin B + flucytosine, then fluconazole
- Aspergillus fumigatus — septate hyphae branching at 45° angles; causes allergic bronchopulmonary aspergillosis (ABPA), aspergilloma (“fungus ball” in pre-existing cavity), invasive aspergillosis in neutropenia; treated with voriconazole
Endemic Mycoses (Dimorphic Fungi)
- Histoplasma capsulatum — Ohio/Mississippi River valleys; bird/bat guano; intracellular yeast within macrophages; GMS stain shows small (2–4 µm) yeast with narrow-based budding; culture on Sabouraud agar; treated with itraconazole or amphotericin B
- Paracoccidioides brasiliensis — Latin America; “mulberry-like” yeast with “pilot’s wheel” (multiple budding); treated with itraconazole
- Blastomyces dermatitidis — Great Lakes/North America; broad-based budding yeast; treated with itraconazole or amphotericin B
Parasitology — Protozoa
Intestinal Protozoa
- Entamoeba histolytica — amebic dysentery; four-nucleated cyst (mature cyst); hematophagous trophozoite (engulfed RBCs in cytoplasm); liver abscess (right lobe); transmitted via contaminated water; diagnosis by stool examination (cyst/trophozoite); treated with metronidazole (trophozoites) + iodoquinoline or paromomycin (cysts)
- Giardia lamblia — “trophozoites have parachute-like appearance; cysts with 4 nuclei; causes malabsorption, frothy stools; transmitted via contaminated water; treated with metronidazole or tinidazole
- Cryptosporidium — sporozoites form holes in brush border of enterocytes; watery diarrhea in AIDS (CD4 <100); acid-fast stain (modified Kinyoun) shows red oocysts against green background; treated with nitazoxanide
Blood and Tissue Protozoa
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Plasmodium vivax and falciparum — malaria; transmitted by female Anopheles mosquito; cyclic fever patterns (tertian: P. vivax/P. falciparum every 48h; quartan: P. malariae every 72h)
- P. vivax: Enlarged spleen; hypnozoites in liver → relapse; needs primaquine (to eliminate liver hypnozoites)
- P. falciparum: Highest mortality; crescent-shaped gametocytes; Blackwater fever (hemolysis + hemoglobinuria); ** cerebral malaria** (sequestered RBCs in brain capillaries); severe and resistant; needs IV artesunate
- Diagnosis: Giemsa-stained thick and thin blood films; rapid antigen detection tests (RDT)
- Treatment: Chloroquine (P. vivax in India where sensitive); ACT (artemisinin-based combination therapy) for P. falciparum; primaquine for P. vivax hypnozoites
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Trypanosoma cruzi (Chagas disease) — Reduviid bug (“kissing bug”) bite; amastigotes in tissue; ** Romana’s sign** (periorbital swelling); transmitted by Triatoma bug feces/rubbing into bite wound; endemic in South America; treated with nifurtimox or benznidazole
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Leishmania donovani — visceral leishmaniasis (kala-azar); sandfly (Phlebotomus) bite; intracellular amastigotes within macrophages; LD bodies (Leishman-Donovan bodies); hepatosplenomegaly, pancytopenia, weight loss; diagnosis by bone marrow/splenic aspirate (AMastigote culture); treated with amphotericin B liposomal (first line in India)
Parasitology — Helminths
| Helminth | Transmission | Key Feature | Treatment |
|---|---|---|---|
| Ascaris lumbricoides | Fecal-oral eggs | Large roundworm; Löffler’s filaments in lungs; high burden → malnutrition | Albendazole |
| Enterobius vermicularis | Fecal-oral eggs | Scotch tape test; perianal pruritus; pinworm | Albendazole |
| Ancylostoma duodenale (Hookworm) | Barefoot walking | Iron deficiency anemia; cutting plates in duodenum | Albendazole |
| Strongyloides stercoralis | Barefoot walking | Threadworm; autoinfection; can cause hyperinfection in immunocompromised | Ivermectin |
| Wuchereria bancrofti | Mosquito bite | Lymphatic filariasis; elephantiasis; nocturnal periodicity (mosquitoes bite at night); microfilariae in peripheral blood smear at night; DEC (diethylcarbamazine) | |
| Taenia saginata (beef) / T. solium (pork) | Undercooked beef/pork | Proglottids in stool; scolex with 4 suckers (T. saginata) or rostellum with hooklets (T. solium); T. solium can cause cysticercosis (larval form in tissues including brain — neurocysticercosis) | Praziquantel |
| Echinococcus granulosus | Dog/sheep contact | Hydatid cyst (liver, lung); cyst with daughter cysts; Casoni’s test; surgical removal + albendazole |
Basic Immunology for FMGE
Types of Immunity
- Innate immunity — first line of defense: skin, mucous membranes, phagocytes (neutrophils, macrophages), NK cells, complement, interferons
- Adaptive immunity — specific; involves lymphocytes (T and B cells); humoral (B cells → antibodies) and cell-mediated (T cells)
Antibody Functions
| Antibody | Function | Location |
|---|---|---|
| IgG | Main serum antibody; crosses placenta; opsonization; secondary response | Blood, tissues |
| IgM | First antibody produced; pentamer; complement activation; does NOT cross placenta | Blood |
| IgA | Secretory (sIgA) in secretions; mucosal protection | Secretions, saliva, tears |
| IgE | Allergic reactions; parasitic infections; basophil/mast cell activation | Bound to mast cells |
| IgD | B cell receptor function | B cell surface |
Hypersensitivity Reactions
| Type | Mechanism | Examples |
|---|---|---|
| Type I (Immediate/Anaphylactic) | IgE-mediated; mast cell degranulation | Anaphylaxis, asthma, allergic rhinitis |
| Type II (Cytotoxic) | IgG/IgM against cell surface antigens | Hemolytic anemia, Goodpasture’s, Graves’ |
| Type III (Immune Complex) | Antigen-antibody complex deposition | Serum sickness, post-streptococcal GN, SLE |
| Type IV (Delayed/Cell-mediated) | T-cell mediated | TB skin test (PPD), contact dermatitis, transplant rejection |
Vaccines (Key Types)
- Live attenuated — MMR, oral polio (Salk/Sabin), rotavirus, yellow fever, BCG, varicella
- Inactivated/killed — injectable polio (IPV), rabies, hepatitis A, Japanese encephalitis
- Toxoid — tetanus (TT), diphtheria (DT)
- Polysaccharide/conjugate — pneumococcal (PCV13/PPSV23), Hib, meningococcal, hepatitis B (recombinant HBsAg — conjugate)
⚡ Exam tip: Fungal elements in KOH preparation: Pseudohyphae + budding yeast + germ tubes = Candida albicans. Encapsulated yeast with India ink = Cryptococcus neoformans.
⚡ Exam tip: Malaria with crescent/gammetocyte forms in blood smear = Plasmodium falciparum (dangerous; cerebral malaria risk).
⚡ Exam tip: A patient with cysticercosis (calcified lesions in brain) from eating undercooked pork = Taenia solium. Treat neurocysticercosis with albendazole + steroids.