Section 4 | Medically Important Fungi & Common Pathogenic Parasites

Section 4 Contents

Part	Major Topics
Part A	Medical Mycology: introduction, classification and general characteristics of medically important fungi and yeasts
Part B	Dermatophytes
Part C	Aspergillus
Part D	Histoplasma
Part E	Candida
Part F	Cryptococcus
Part G	Medical Parasitology and Entamoeba histolytica
Part H	Giardia duodenalis
Part I	Plasmodium spp. and malaria
Part J	Wuchereria bancrofti
Part K	Leishmania spp.
Part L	Taenia spp.
Part M	Ascaris lumbricoides
Part N	Ancylostoma duodenale

PART A: Medically Important Fungi

1. Introduction to Medical Mycology

Medical mycology is the branch of microbiology concerned with fungi that cause disease in humans. Human fungal infections are collectively called mycoses. They range from superficial infections of the skin and hair to severe systemic infections involving the lungs, brain, blood and other internal organs.

Fungi are eukaryotic organisms. Unlike bacteria, their cells possess a true nucleus and membrane-bound organelles. The fungal cell wall contains mainly chitin, glucans and mannans, while the cell membrane contains ergosterol. These features are important because several antifungal drugs act by interfering with fungal membrane or cell wall synthesis.

2. General Characteristics of Fungi

Fungi are eukaryotic organisms.
They possess a true nucleus.
They lack chlorophyll and therefore do not perform photosynthesis.
They obtain nutrients by absorption.
The cell wall generally contains chitin and polysaccharides.
The cell membrane contains ergosterol.
They may be unicellular or multicellular.
Yeasts are generally unicellular.
Moulds consist of filamentous structures called hyphae.
A mass of hyphae is called a mycelium.
Many fungi reproduce by spores.
Some pathogenic fungi show thermal dimorphism.

3. Yeasts, Moulds and Dimorphic Fungi

Form	Characteristics	Examples
Yeast	Unicellular, usually oval or round, reproduces commonly by budding.	Candida, Cryptococcus
Mould	Multicellular filamentous fungus consisting of hyphae and mycelium.	Aspergillus, dermatophytes
Dimorphic fungi	Exist in different morphological forms depending on environmental conditions, especially temperature.	Histoplasma

4. Classification of Medically Important Fungi

For practical medical microbiology, fungi may be classified according to their morphology, route of infection and clinical disease.

Group	Examples	Major Disease
Dermatophytes	Trichophyton, Microsporum, Epidermophyton	Dermatophytosis
Yeasts	Candida, Cryptococcus	Candidiasis, cryptococcosis
Moulds	Aspergillus	Aspergillosis
Dimorphic fungi	Histoplasma	Histoplasmosis

5. Major Clinical Categories of Mycoses

Superficial Mycoses → Cutaneous Mycoses → Subcutaneous Mycoses → Systemic Mycoses → Opportunistic Mycoses

Superficial mycoses: limited mainly to superficial keratinized structures.
Cutaneous mycoses: involve skin, hair and nails.
Subcutaneous mycoses: involve deeper tissues following traumatic implantation.
Systemic mycoses: may disseminate from a primary pulmonary focus.
Opportunistic mycoses: occur particularly in immunocompromised individuals.

Exam Tip: In a 10-mark question on medical fungi, always mention the eukaryotic nature of fungi, cell wall, ergosterol, yeast/mould forms, dimorphism, reproduction and clinical classification before discussing individual pathogens.

1. Dermatophytes

Introduction

Dermatophytes are filamentous fungi that infect keratinized tissues such as skin, hair and nails. The infection is called dermatophytosis or commonly ringworm. Dermatophytes obtain nutrients from keratin and generally remain within keratinized tissues rather than invading living deeper tissues.

Classification

Genus	Important species	Common involvement
Trichophyton	T. rubrum, T. mentagrophytes	Skin, hair and nails
Microsporum	M. canis, M. audouinii	Skin and hair
Epidermophyton	E. floccosum	Skin and nails

Morphology

Filamentous fungi.
Hyphae are septate and hyaline.
Produce macroconidia and/or microconidia.
Different genera have characteristic conidial morphology.
They possess keratinolytic activity.

Biology and Transmission

Dermatophytes are transmitted by direct contact with infected humans or animals and indirectly through contaminated clothing, towels, floors and other fomites. Warm and humid conditions favour transmission and growth.

Epidemiological Classification

Anthropophilic: primarily infect humans.
Zoophilic: primarily associated with animals.
Geophilic: associated with soil.

Pathogenesis

Infection usually begins when fungal elements reach the skin. The fungi produce keratinases and other enzymes that allow them to utilize keratinized tissues. The inflammatory response contributes to redness, scaling and itching.

Clinical Manifestations

Tinea corporis – ringworm of the body.
Tinea capitis – scalp infection.
Tinea pedis – athlete's foot.
Tinea cruris – groin infection.
Tinea unguium – fungal infection of nails.
Tinea manuum – infection of hands.

Specimens

Skin scrapings from active margin of lesion.
Hair pluckings.
Nail clippings and subungual debris.

Laboratory Diagnosis

Direct Microscopy

Skin, hair or nail specimens are treated with potassium hydroxide (KOH). KOH dissolves keratin and cellular material while fungal hyphae remain visible.

Clinical lesion → Skin/Hair/Nail specimen → KOH mount → Septate hyphae → Suggestive of dermatophyte infection

Culture

Specimens can be inoculated onto suitable fungal media such as Sabouraud dextrose agar. Colony morphology, microscopic morphology and specialized identification methods are used for species identification.

Microscopic Identification

Lactophenol cotton blue preparations may demonstrate characteristic macroconidia and microconidia.

Treatment Principles

Topical antifungal therapy is useful for many localized infections.
Systemic therapy may be required for scalp and nail infections.
Keep affected areas clean and dry.
Avoid sharing towels, clothing and personal articles.

Prevention and Control

Maintain personal hygiene.
Keep skin dry.
Properly treat infected persons and animals.
Avoid sharing combs, towels and clothing.
Maintain hygienic footwear.

Key diagnostic point: For suspected dermatophytosis, the most important basic laboratory approach is examination of an appropriately collected skin, hair or nail specimen by KOH preparation followed by fungal culture when required.

2. Aspergillus

Introduction

Aspergillus is a genus of filamentous moulds widely distributed in soil, decaying vegetation, dust and air. Human infection is called aspergillosis. Aspergillus fumigatus is an important human pathogen, although other species including A. flavus and A. niger can also cause disease.

Classification

Species	Clinical importance
Aspergillus fumigatus	Major cause of invasive and allergic aspergillosis
Aspergillus flavus	Invasive infection and aflatoxin production
Aspergillus niger	Otomycosis and pulmonary disease

Morphology

Septate, hyaline hyphae.
Hyphae commonly show dichotomous branching.
Conidiophores terminate in a vesicle.
Phialides produce chains of conidia.
Conidia are important for airborne transmission.

Transmission

Humans usually acquire Aspergillus by inhaling airborne conidia. Direct person-to-person transmission is not the usual route.

Pathogenesis

In healthy individuals, inhaled conidia are usually removed by innate defence mechanisms. In susceptible individuals, especially those with neutropenia, prolonged immunosuppression or severe underlying disease, germination and tissue invasion can occur.

Clinical Forms

Form	Description
Allergic aspergillosis	Hypersensitivity-related disease, especially involving airways.
Aspergilloma	Fungal ball developing within a pre-existing cavity.
Chronic pulmonary aspergillosis	Chronic pulmonary disease in susceptible patients.
Invasive aspergillosis	Angioinvasive infection occurring mainly in severely immunocompromised patients.

Specimens

Sputum.
Bronchoalveolar lavage.
Tissue biopsy.
Other specimens according to site of infection.

Laboratory Diagnosis

Microscopy

KOH preparation or histopathological examination may demonstrate septate hyphae with characteristic acute-angle branching.

Culture

Growth on fungal culture media is evaluated by colony morphology and microscopic structures.

Antigen Detection

Galactomannan detection can be useful in appropriate clinical settings, particularly in suspected invasive aspergillosis.

Molecular Diagnosis

PCR-based methods may assist diagnosis in selected clinical and laboratory settings.

Complications

Hemoptysis.
Pulmonary tissue destruction.
Vascular invasion.
Dissemination to other organs.

Treatment Principles

Treatment depends on clinical form and host factors. Significant invasive disease generally requires systemic antifungal treatment and management of the underlying immunological risk.

Prevention

Reduction of exposure to heavily contaminated dust in high-risk patients.
Appropriate hospital infection-control practices.
Early diagnosis in high-risk individuals.

3. Histoplasma

Introduction

Histoplasma is a medically important dimorphic fungus associated with histoplasmosis. Infection usually occurs after inhalation of fungal propagules from contaminated environmental material.

Morphology and Dimorphism

Histoplasma demonstrates temperature-dependent morphological forms. The mould form is associated with environmental growth, whereas the yeast form develops in tissues at body temperature.

Environmental mould form ↓ Inhalation of infectious propagules ↓ Lungs ↓ Conversion to yeast form ↓ Macrophage-associated multiplication ↓ Possible dissemination

Pathogenesis

After inhalation, fungal elements reach the lungs and may be taken up by macrophages. In some individuals the infection remains localized, while others may develop progressive or disseminated disease, particularly when cellular immunity is impaired.

Clinical Manifestations

Asymptomatic infection.
Acute pulmonary histoplasmosis.
Chronic pulmonary disease.
Disseminated histoplasmosis.

Laboratory Diagnosis

Microscopic examination of appropriate clinical material.
Histopathology demonstrating intracellular yeast forms.
Culture of clinical specimens.
Antigen detection in selected cases.
Serological tests in appropriate clinical settings.
Molecular methods where available.

Treatment Principles

Management depends on the clinical form and severity. Mild localized disease may differ substantially from severe disseminated infection, which requires systemic antifungal therapy.

Important Exam Point

Histoplasma is an important example of a thermally dimorphic fungus. The association of tissue yeast forms with macrophages is an important diagnostic and pathogenesis point.

4. Candida

Introduction

Candida species are yeast-like fungi that may form part of the normal microbiota of humans but can cause opportunistic infections when local or systemic host defence is disturbed. Candida albicans is an important human pathogen.

Characteristics

Yeast-like organisms.
Gram-positive budding yeast cells are commonly observed.
C. albicans can produce pseudohyphae and true hyphae.
Some strains produce germ tubes under appropriate conditions.

Virulence Factors

Adhesion to host cells.
Yeast-to-hypha transition.
Biofilm formation.
Secreted hydrolytic enzymes.
Ability to survive changes in host environment.

Risk Factors

Broad-spectrum antibiotic therapy.
Diabetes mellitus.
Immunosuppression.
Indwelling vascular catheters.
Major surgery.
Prematurity.
Prolonged hospitalization.

Clinical Manifestations

Site	Clinical disease
Mouth	Oral candidiasis / thrush
Vagina	Vulvovaginal candidiasis
Skin folds	Candidal intertrigo
Bloodstream	Candidemia
Deep organs	Invasive candidiasis

Specimens

Oral swab.
Vaginal specimen.
Blood for suspected candidemia.
Urine or other specimens depending on clinical site.
Tissue or sterile-site material in invasive disease.

Laboratory Diagnosis

Microscopy

Gram staining may demonstrate budding yeast cells and pseudohyphae. KOH preparations can also demonstrate fungal elements in suitable specimens.

Culture

Candida can grow on routine fungal media. Colony appearance and microscopic morphology are used as part of identification.

Germ Tube Test

The germ tube test is a traditional presumptive test for C. albicans and closely related species. A positive result should be interpreted with other identification findings rather than used as the sole basis for final species identification.

Modern Identification

Chromogenic media.
Biochemical identification systems.
MALDI-TOF MS where available.
Molecular identification.

Complications

Candidemia.
Endocarditis.
Endophthalmitis.
Disseminated infection.
Deep organ involvement.

Treatment Principles

Treatment depends on site, severity, species and antifungal susceptibility. Topical treatment may be sufficient for some superficial infections, whereas invasive candidiasis requires systemic therapy.

Prevention

Judicious antibiotic use.
Good catheter care.
Control of predisposing conditions.
Appropriate infection-control practices.

5. Cryptococcus

Introduction

Cryptococcus is an encapsulated yeast that can cause cryptococcosis. Cryptococcus neoformans is particularly important in immunocompromised individuals and may cause pulmonary and central nervous system disease.

Morphology

Round to oval budding yeast.
Prominent polysaccharide capsule.
Usually narrow-based budding.
Capsule is an important virulence factor.

Transmission

Infection generally follows inhalation of environmental fungal particles. The lungs are commonly the initial site before possible dissemination.

Pathogenesis

The capsule interferes with effective host immune responses and helps the organism survive within the host. In susceptible individuals, organisms can disseminate from the lungs to the central nervous system.

Clinical Manifestations

Pulmonary cryptococcosis.
Cryptococcal meningitis.
Disseminated cryptococcosis.

Specimens

Cerebrospinal fluid.
Blood.
Sputum or respiratory specimens.
Tissue where appropriate.

Laboratory Diagnosis

India Ink

India ink preparation may demonstrate encapsulated yeast as clear halos against a dark background. It is a classical demonstration method.

Cryptococcal Antigen

Cryptococcal antigen detection in CSF or serum is an important diagnostic method, particularly for suspected cryptococcal meningitis.

Culture

Culture of appropriate specimens can establish the diagnosis and allow further identification.

Molecular Tests

PCR-based assays may be used in selected diagnostic laboratories.

Complications

Meningitis.
Raised intracranial pressure.
Neurological complications.
Disseminated disease.

Remember: Cryptococcus is classically associated with an encapsulated yeast and cryptococcal meningitis, especially in immunocompromised individuals.

PART B: Common Pathogenic Parasites

Introduction to Medical Parasitology

Medical parasitology is the branch of microbiology and biology concerned with parasites that infect humans, their hosts, vectors, life cycles, pathogenesis, diagnosis, treatment and prevention.

Parasites may be broadly grouped into protozoa and helminths. Protozoa are unicellular eukaryotic organisms, whereas helminths are multicellular parasitic worms.

Group	Examples
Protozoa	Entamoeba, Giardia, Plasmodium, Leishmania
Nematodes	Wuchereria, Ascaris, Ancylostoma
Cestodes	Taenia

Important Terms

Definitive host: host in which the adult parasite or sexual stage occurs.
Intermediate host: host in which larval or asexual development occurs.
Vector: living organism involved in transmission of an infectious agent.
Infective stage: stage capable of initiating infection in a susceptible host.
Diagnostic stage: stage detected during laboratory diagnosis.

For parasite questions, always identify the infective stage, diagnostic stage, host/vector, route of transmission and specimen used for diagnosis. These are frequently tested points.

6. Entamoeba histolytica

Introduction

Entamoeba histolytica is an intestinal protozoan parasite responsible for amoebiasis. It primarily affects the large intestine and may also cause extraintestinal disease, particularly amoebic liver abscess.

Classification

Feature	Information
Group	Protozoan parasite
Site	Large intestine; may disseminate to liver and other organs
Important stages	Trophozoite and cyst

Morphology

Trophozoite

Motile feeding stage.
Actively divides by binary fission.
May contain ingested red blood cells.
Found mainly in intestinal lesions and fresh diarrhoeal stool.

Cyst

Resistant stage.
Important for transmission.
Mature cyst typically contains four nuclei.
Survives outside the host better than trophozoites.

Life Cycle

Mature cyst ingestion ↓ Excystation in intestine ↓ Trophozoites released ↓ Colonization of large intestine ↓ Multiplication ↓ Encystation ↓ Cysts passed in stool

Transmission

Transmission is primarily fecal-oral through ingestion of mature cysts in contaminated water, food or through contaminated hands.

Pathogenesis

Trophozoites may attach to colonic epithelium and produce tissue damage. The resulting ulcers can cause intestinal symptoms and, in invasive disease, trophozoites may enter the portal circulation and reach the liver.

Clinical Manifestations

Asymptomatic intestinal infection.
Amoebic dysentery.
Abdominal pain and diarrhoea.
Tenesmus.
Amoebic colitis.
Amoebic liver abscess.

Specimen

Fresh stool for trophozoite detection.
Formed stool for cyst detection.
Multiple stool specimens may improve detection.
Abscess material may be examined in extraintestinal disease, with appropriate clinical interpretation.

Laboratory Diagnosis

Direct Microscopy

Saline and iodine wet preparations may demonstrate trophozoites or cysts. Finding trophozoites containing ingested RBCs strongly supports invasive E. histolytica infection.

Concentration Methods

Concentration procedures improve detection of cysts in stool.

Antigen Detection

Species-specific antigen tests can help distinguish E. histolytica from morphologically similar nonpathogenic Entamoeba species.

Molecular Diagnosis

PCR and other nucleic-acid amplification methods can provide species-level identification where available.

Serology

Serological tests are particularly useful in extraintestinal amoebiasis, including liver abscess.

Prevention

Safe drinking water.
Proper sewage disposal.
Hand hygiene.
Proper washing of fruits and vegetables.
Detection and treatment of infected individuals.

7. Giardia duodenalis

Introduction

Giardia duodenalis, also known as Giardia lamblia or Giardia intestinalis, is an intestinal protozoan parasite causing giardiasis. It commonly affects the small intestine.

Morphology

Trophozoite

Pear-shaped.
Bilaterally symmetrical.
Two nuclei.
Multiple flagella.
Ventral adhesive disc.

Cyst

Oval or ellipsoidal.
Resistant environmental stage.
Mature cyst contains four nuclei.
Important infective stage.

Life Cycle

Ingestion of cyst ↓ Excystation in small intestine ↓ Trophozoites attach to intestinal mucosa ↓ Multiplication by binary fission ↓ Encystation ↓ Cysts passed in feces

Transmission

Transmission occurs mainly through ingestion of cysts in contaminated water or food and through fecal-oral contact.

Pathogenesis

Trophozoites attach to the intestinal surface and interfere with normal intestinal absorption and digestion. This may lead to malabsorption, particularly of fats.

Clinical Manifestations

Watery or foul-smelling diarrhoea.
Abdominal cramps.
Bloating.
Flatulence.
Steatorrhoea.
Weight loss in prolonged infection.
Malabsorption.

Laboratory Diagnosis

Stool wet mount.
Concentration techniques.
Permanent stains where required.
Stool antigen detection.
Nucleic-acid amplification tests where available.

Prevention

Safe water supply.
Sanitation.
Hand hygiene.
Protection of recreational water from fecal contamination.

Exam point: Giardia has a characteristic two-nucleus trophozoite with a ventral adhesive disc and a four-nucleate mature cyst.

8. Plasmodium spp. — Malaria

Introduction

Malaria is a major parasitic disease caused by protozoa of the genus Plasmodium and transmitted principally through the bite of infected female Anopheles mosquitoes.

Important Human Plasmodium Species

Species	Important feature
P. falciparum	Major cause of severe and potentially fatal malaria
P. vivax	Common malaria species; can produce relapses from dormant liver stages
P. malariae	Associated with quartan malaria
P. ovale	Can produce relapsing malaria through dormant liver stages
P. knowlesi	Important zoonotic malaria parasite in parts of Southeast Asia

Hosts

The human host supports asexual multiplication, whereas the female Anopheles mosquito serves as the definitive host because sexual development occurs in the mosquito.

Life Cycle

Infected female Anopheles bite ↓ Sporozoites enter blood ↓ Liver invasion ↓ Hepatic schizogony ↓ Merozoites released ↓ RBC invasion ↓ Ring → trophozoite → schizont ↓ RBC rupture ↓ Repeated erythrocytic cycles ↓ Gametocyte formation ↓ Mosquito ingests gametocytes ↓ Sexual development in mosquito ↓ Sporozoites reach salivary glands

Pathogenesis

Clinical disease is primarily associated with the erythrocytic stage. Rupture of infected RBCs releases parasite products and stimulates inflammatory responses. P. falciparum can cause extensive microvascular sequestration and severe organ dysfunction.

Clinical Manifestations

Fever.
Chills and rigors.
Sweating.
Headache.
Weakness.
Anaemia.
Splenomegaly.

Severe Malaria

Cerebral malaria.
Severe anaemia.
Hypoglycaemia.
Acute kidney injury.
Respiratory distress.
Shock.
Metabolic complications.

Laboratory Diagnosis

Thick Blood Film

The thick smear concentrates blood elements and is useful for parasite detection and estimating parasite density.

Thin Blood Film

The thin film preserves RBC morphology and is particularly useful for species identification.

Rapid Diagnostic Tests

Antigen-based rapid diagnostic tests can detect malaria parasite antigens and are particularly useful where microscopy is unavailable or rapid testing is needed.

Molecular Diagnosis

PCR and related molecular methods provide high sensitivity and can be useful for species identification and confirmation.

Prevention and Control

Insecticide-treated mosquito nets.
Vector control.
Reduction of mosquito breeding sites.
Early diagnosis and treatment.
Appropriate chemoprevention for eligible populations.
Use of available malaria vaccines according to national programmes.

Very important for 10 marks: Draw the complete Plasmodium life cycle and clearly distinguish the human asexual phase from the mosquito sexual phase.

9. Wuchereria bancrofti

Introduction

Wuchereria bancrofti is a filarial nematode responsible for lymphatic filariasis. Adult worms live mainly in lymphatic vessels and nodes, while microfilariae circulate in peripheral blood.

Vector

Transmission occurs through mosquitoes. Depending on geographical location, several mosquito genera may serve as vectors.

Life Cycle

Mosquito feeds on infected person ↓ Microfilariae ingested ↓ Development inside mosquito ↓ Infective L3 larvae ↓ Mosquito bites human ↓ L3 larvae enter skin ↓ Migration to lymphatics ↓ Adult worms develop ↓ Microfilariae produced ↓ Microfilariae circulate in blood

Clinical Manifestations

Asymptomatic infection.
Acute lymphangitis.
Lymphadenitis.
Lymphoedema.
Hydrocele.
Elephantiasis in chronic disease.

Specimen

Peripheral blood is the major diagnostic specimen for detecting microfilariae.

Laboratory Diagnosis

Peripheral blood microscopy.
Concentration techniques.
Special stains may improve visualization.
Antigen detection.
Molecular assays in specialized settings.

Important Biological Feature

Microfilariae may demonstrate periodicity in peripheral blood. Sampling time therefore needs to be considered when microscopy is used.

Prevention and Control

Mass drug administration programmes where indicated.
Mosquito control.
Use of bed nets and personal protection.
Management of chronic lymphoedema.

10. Leishmania spp.

Introduction

Leishmania species are intracellular protozoan parasites transmitted by infected sandflies. They cause a group of diseases collectively known as leishmaniasis.

Major Clinical Forms

Form	Main site
Cutaneous leishmaniasis	Skin
Mucocutaneous leishmaniasis	Mucosal tissues and skin
Visceral leishmaniasis	Reticuloendothelial system, especially spleen, liver and bone marrow

Morphological Forms

Amastigote

The amastigote is the intracellular form found mainly within macrophages of the mammalian host. It is small, round or oval and contains a nucleus and kinetoplast.

Promastigote

The promastigote is the flagellated form found in the sandfly vector and in culture.

Life Cycle

Infected sandfly bites human ↓ Promastigotes enter tissue ↓ Phagocytosis by macrophages ↓ Conversion to amastigotes ↓ Intracellular multiplication ↓ Infected macrophages taken up by sandfly ↓ Development into promastigotes ↓ Multiplication and migration to vector proboscis

Pathogenesis

Leishmania survives within macrophages and interacts extensively with cell-mediated immunity. The clinical form depends on the species, parasite characteristics and host immune response.

Clinical Features

Cutaneous Disease

Papule at inoculation site.
Progression to ulcer.
Chronic skin lesion.

Visceral Disease

Prolonged fever.
Splenomegaly.
Hepatomegaly.
Pancytopenia.
Weight loss.
Hypergammaglobulinemia.

Laboratory Diagnosis

Microscopic examination of lesion material.
Tissue aspirates.
Bone marrow examination for visceral disease where appropriate.
Culture.
Serological tests for visceral disease.
PCR and other molecular assays.

Prevention

Sandfly control.
Personal protective measures.
Environmental control.
Early diagnosis and treatment.
Reservoir control where relevant.

11. Taenia spp.

Introduction

Taenia are cestode parasites that infect humans and other animals. Two major human Taenia species are Taenia solium and Taenia saginata.

Comparison of T. solium and T. saginata

Feature	T. solium	T. saginata
Common name	Pork tapeworm	Beef tapeworm
Intermediate host	Pig	Cattle
Adult habitat	Human intestine	Human intestine
Important special disease	Cysticercosis	Usually intestinal taeniasis

Morphology

Long, ribbon-like, segmented worms.
Body consists of scolex, neck and proglottids.
Scolex attaches to intestinal mucosa.
Gravid proglottids contain numerous eggs.

Life Cycle

Eggs/proglottids passed in human feces ↓ Environment contaminated ↓ Intermediate host ingests eggs ↓ Oncospheres penetrate intestine ↓ Larval cysts develop in tissues ↓ Human consumes infected meat ↓ Larva develops into adult worm in intestine

T. solium and Cysticercosis

Humans may become accidental intermediate hosts of T. solium after ingestion of eggs. Larval cysticerci may develop in tissues, including the central nervous system, producing neurocysticercosis.

Clinical Manifestations

Abdominal discomfort.
Nausea.
Altered bowel habits.
Passage of proglottids.
Weight loss in some patients.
Neurocysticercosis due to T. solium larval infection.

Laboratory Diagnosis

Stool examination for eggs or proglottids.
Species identification by examination of proglottid or scolex features.
Molecular methods when required.
Imaging and serology for suspected cysticercosis.

Prevention

Proper cooking of pork and beef.
Sanitary disposal of human feces.
Identification and treatment of infected individuals.
Meat inspection.
Improved sanitation and pig management.

12. Ascaris lumbricoides

Introduction

Ascaris lumbricoides is a large intestinal nematode of humans and is associated with soil-transmitted helminth infection.

Morphology

Large cylindrical worms.
Sexes are separate.
Female is generally larger than male.
Male has a curved posterior end.
Eggs have a characteristic thick shell.

Life Cycle

Eggs passed in feces ↓ Embryonation in soil ↓ Infective embryonated egg ↓ Human ingestion ↓ Larvae hatch in intestine ↓ Intestinal wall penetration ↓ Liver → heart → lungs ↓ Larvae enter alveoli ↓ Ascend respiratory tract ↓ Swallowed ↓ Adult worms develop in small intestine

Pathogenesis

Larval migration through the lungs can produce respiratory symptoms. Adult worms in the intestine may cause nutritional effects or mechanical complications when present in large numbers.

Clinical Manifestations

Abdominal discomfort.
Malnutrition in heavy infection.
Cough during pulmonary migration.
Eosinophilia.
Intestinal obstruction in heavy infection.
Biliary or pancreatic complications in some cases.

Specimen

Stool is the principal specimen for detecting eggs.

Laboratory Diagnosis

Direct stool microscopy.
Concentration techniques.
Identification of characteristic eggs.

Prevention

Sanitary disposal of feces.
Hand washing.
Washing vegetables and fruits.
Wearing footwear and maintaining sanitation.
Periodic deworming programmes where indicated.

13. Ancylostoma duodenale

Introduction

Ancylostoma duodenale is a human hookworm responsible for intestinal hookworm infection. Adult worms attach to the intestinal mucosa and consume blood, making chronic blood loss an important feature of infection.

Morphology

Small cylindrical nematode.
Adult worms possess a well-developed buccal capsule.
A. duodenale possesses teeth in the buccal capsule.
Eggs are oval and thin-shelled.

Life Cycle

Eggs passed in stool ↓ Larvae develop in soil ↓ Filariform L3 becomes infective ↓ Larvae penetrate human skin ↓ Blood circulation ↓ Heart → lungs ↓ Alveoli → trachea ↓ Swallowed ↓ Small intestine ↓ Adult worms ↓ Egg production

Transmission

The major route is skin penetration by infective filariform larvae in contaminated soil.

Pathogenesis

Adult worms attach to intestinal mucosa and feed on blood. Continued blood loss can produce iron deficiency and anaemia, particularly in heavy infection.

Clinical Manifestations

Itching and skin irritation at penetration site.
Respiratory symptoms during larval migration.
Abdominal discomfort.
Iron-deficiency anaemia.
Weakness and fatigue.
Protein loss in heavy infection.

Laboratory Diagnosis

Stool examination demonstrates characteristic hookworm eggs. Because eggs of several hookworm species are morphologically similar, species-level identification may require larval culture or molecular methods.

Prevention and Control

Use of footwear.
Proper sanitation.
Prevention of soil contamination with feces.
Periodic deworming programmes where indicated.
Diagnosis and treatment of infected individuals.

Important distinction: Ascaris infection is commonly acquired by ingestion of embryonated eggs, whereas Ancylostoma infection is classically acquired when infective larvae penetrate the skin.

Important Diagnostic Specimens and Methods

Parasite	Major specimen	Important diagnostic method
Entamoeba histolytica	Stool	Microscopy, antigen detection, PCR
Giardia	Stool	Microscopy, antigen detection, molecular tests
Plasmodium	Peripheral blood	Thick/thin smear, RDT, PCR
Wuchereria bancrofti	Peripheral blood	Microfilariae, antigen detection
Leishmania	Lesion/tissue/bone marrow depending on disease	Microscopy, culture, serology, PCR
Taenia	Stool	Eggs/proglottids; imaging and serology for cysticercosis
Ascaris	Stool	Egg detection
Ancylostoma	Stool	Egg detection

General Laboratory Approach to Fungal and Parasitic Infections

Clinical history ↓ Correct specimen selection ↓ Proper collection and transport ↓ Direct examination ↓ Culture / concentration where appropriate ↓ Antigen / antibody testing where appropriate ↓ Molecular identification where available ↓ Interpretation with clinical findings ↓ Final laboratory report

Principles of Good Specimen Collection

Collect the specimen from the correct anatomical site.
Collect before antimicrobial therapy whenever possible.
Use sterile containers when required.
Avoid contamination.
Provide adequate quantity.
Transport rapidly under appropriate conditions.
Clearly label the specimen.
Provide relevant clinical information to the laboratory.

High-Yield Comparison: Fungi vs Parasites

Feature	Fungi	Parasites
Cellular organization	Eukaryotic	Protozoa are unicellular; helminths are multicellular
Major forms	Yeast, mould, dimorphic forms	Trophozoites, cysts, eggs, larvae, adult worms etc.
Common specimen	Skin, sputum, blood, CSF, tissue	Stool, blood, tissue, body fluids
Microscopy	KOH, Gram stain, special stains, histology	Wet mount, concentration, permanent stains, blood films
Culture	Important for many fungi	Limited compared with fungal culture; specialized culture for selected parasites
Antigen testing	Useful for selected fungal diseases	Useful for selected protozoan and helminth infections
Molecular methods	PCR/sequence-based methods	PCR and other NAATs for selected parasites

2-Mark Questions

1. Define medical mycology.

2. What is a mycosis?

3. Differentiate yeast and mould.

4. What is fungal dimorphism?

5. Name three dermatophyte genera.

6. What is dermatophytosis?

7. Mention two important species of Aspergillus.

8. What is an aspergilloma?

9. Mention one important dimorphic fungus.

10. What is candidiasis?

11. What is the germ tube test?

12. Mention two virulence factors of Candida.

13. What is the capsule of Cryptococcus?

14. What is India ink preparation?

15. Define parasitology.

16. Define definitive host.

17. Define intermediate host.

18. What is an infective stage?

19. Name two intestinal protozoan parasites.

20. Mention the infective stage of Giardia.

21. Name four important human Plasmodium species.

22. What is cerebral malaria?

23. Name the vector of malaria.

24. What is a thick blood film?

25. What is a thin blood film?

26. Name the parasite causing lymphatic filariasis.

27. What is elephantiasis?

28. Name the vector of leishmaniasis.

29. What is an amastigote?

30. What is a promastigote?

31. Name two medically important Taenia species.

32. What is cysticercosis?

33. What is the infective stage of Ascaris for humans?

34. What is hookworm infection?

35. How does Ancylostoma larvae enter the human body?

5-Mark Questions

Describe the general characteristics and classification of medically important fungi.
Differentiate yeast, mould and dimorphic fungi with suitable examples.
Describe the morphology, transmission and laboratory diagnosis of dermatophytes.
Write a short note on Aspergillus and aspergillosis.
Describe the dimorphism and pathogenesis of Histoplasma.
Describe Candida and the laboratory diagnosis of candidiasis.
Write a note on Cryptococcus and cryptococcal meningitis.
Describe the morphology and life cycle of Entamoeba histolytica.
Describe Giardia morphology, life cycle and diagnosis.
Write a short note on malaria diagnosis.
Describe the life cycle of Wuchereria bancrofti.
Write a note on visceral leishmaniasis.
Differentiate Taenia solium and Taenia saginata.
Describe the life cycle of Ascaris lumbricoides.
Describe the pathogenesis and diagnosis of Ancylostoma duodenale infection.

10-Mark Long Questions

Discuss medically important fungi in detail, including their general characteristics, classification, morphology, reproduction, pathogenicity, clinical importance and laboratory diagnosis.
Describe dermatophytes in detail under the headings of classification, morphology, transmission, epidemiology, pathogenesis, clinical forms, specimen collection, laboratory diagnosis, treatment, prevention and control.
Describe Aspergillus and aspergillosis in detail, including morphology, biology, transmission, pathogenesis, clinical forms, laboratory diagnosis and prevention.
Discuss Candida as an opportunistic pathogen with special reference to virulence factors, clinical manifestations and laboratory diagnosis.
Describe Cryptococcus and cryptococcosis with emphasis on morphology, capsule, pathogenesis, meningitis and laboratory diagnosis.
Describe Entamoeba histolytica in detail, including morphology, life cycle, transmission, pathogenesis, clinical manifestations and laboratory diagnosis.
Discuss Giardia duodenalis under morphology, life cycle, transmission, pathogenesis, clinical manifestations, diagnosis and prevention.
Describe the complete life cycle of Plasmodium in humans and mosquitoes and discuss the laboratory diagnosis and prevention of malaria.
Discuss Wuchereria bancrofti infection with reference to morphology, life cycle, vector, pathogenesis, clinical manifestations, diagnosis and control.
Describe Leishmania infection in detail, including amastigote and promastigote forms, life cycle, vector, clinical forms and laboratory diagnosis.
Discuss Taenia solium and Taenia saginata with special reference to morphology, life cycle, disease, diagnosis and prevention.
Describe Ascaris lumbricoides with special reference to morphology, life cycle, pathogenesis, clinical manifestations, diagnosis and control.
Discuss Ancylostoma duodenale infection with reference to morphology, life cycle, transmission, pathogenesis, diagnosis, treatment and prevention.

Diagrams You Should Practice for Examination

General structure of a fungal cell.
Yeast and mould comparison.
Dermatophyte hyphae and conidia.
Aspergillus conidial head.
Histoplasma dimorphism.
Life cycle of Entamoeba histolytica.
Giardia trophozoite.
Complete Plasmodium life cycle.
Wuchereria bancrofti life cycle.
Leishmania life cycle.
Taenia solium life cycle.
Ascaris life cycle.
Ancylostoma life cycle.

Diagram rule: For a 10-mark answer, a well-labelled life-cycle diagram should not replace written explanation. Draw the diagram and explain each major stage underneath it.

Important Viva Questions

Why are fungi called eukaryotic microorganisms?
What is the major sterol of fungal cell membranes?
What is the difference between yeast and mould?
What is fungal dimorphism?
Name three dermatophyte genera.
Why is KOH used for fungal examination?
What is the characteristic branching pattern of Aspergillus?
What is the importance of galactomannan?
What is the major virulence factor of Cryptococcus?
What is the germ tube test used for?
What is an opportunistic fungal infection?
What is a trophozoite?
What is a cyst?
What is the infective stage of Giardia?
Which parasite causes malaria?
Which mosquito transmits malaria?
Why are both thick and thin blood films used?
What is the definitive host in malaria?
What is a microfilaria?
What is the vector of lymphatic filariasis?
What are amastigote and promastigote forms?
What is the vector of Leishmania?
What is cysticercosis?
How is Ascaris transmitted?
How does hookworm enter the human body?

Last-Minute Revision Table

Organism	Remember This First
Dermatophytes	Keratinized tissues + KOH + dermatophytosis
Aspergillus	Septate hyphae + acute-angle branching + airborne conidia
Histoplasma	Dimorphic fungus + intracellular yeast
Candida	Yeast + pseudohyphae + germ tube + opportunistic infection
Cryptococcus	Encapsulated yeast + meningitis + cryptococcal antigen
Entamoeba	Cyst transmission + trophozoite tissue invasion + liver abscess
Giardia	Two nuclei + adhesive disc + malabsorption
Plasmodium	Anopheles mosquito + RBC stages + thick/thin blood film
Wuchereria	Filarial worm + mosquito + lymphatic disease
Leishmania	Amastigote in macrophages + promastigote in vector
Taenia	Tapeworm + proglottids + T. solium cysticercosis
Ascaris	Embryonated egg ingestion + lung migration
Ancylostoma	Skin penetration by larvae + intestinal blood loss + anaemia

Section 4: Final Examination Summary

Section 4 covers two major areas of medical microbiology: medical mycology and medical parasitology. For fungi, the most important concepts are fungal morphology, yeast and mould forms, dimorphism, pathogenic mechanisms, specimen selection, direct microscopy, culture, antigen detection and molecular diagnosis.

For parasites, examination answers should emphasize morphology, host-vector relationships, infective and diagnostic stages, complete life cycles, transmission, epidemiology, pathogenesis, clinical disease, specimen collection and appropriate laboratory diagnosis.

A strong long answer should not simply list symptoms. It should connect the organism's biological characteristics with its transmission, pathogenesis, clinical disease and laboratory diagnosis. Where a life cycle is important, the stages should be drawn and explained in sequence.

High-yield preparation strategy: Prepare every organism under the same sequence: Introduction → Classification → Morphology → Biology/Life Cycle → Transmission → Epidemiology → Pathogenesis → Clinical Manifestations → Complications → Specimen → Laboratory Diagnosis → Interpretation → Treatment Principles → Prevention and Control.