Arranged by Sarah Mohssen

SectionIII– Parasitology By Nada Sajet

(hand-tomouth).

Nematodes,

Tissue

Trichinella spp. have a

number of

animal reservoir hosts,

including

bears, walruses, pigs, rodents,

and

other animals. Dog and cat

hookworms cause cutaneous

larva

migrans (CLM), and the dog

and cat

ascarid, Toxocara spp., causes

visceral and ocular larva

migrans

(VLM, OLM). These

infections can be

serious and cause severe

disease if

not treated.

Trichinella organisms are

acquired by

ingestion of raw or poorly

cooked

infected meat.

CLM is caused by skin

penetration of

infective larvae from the soil;

children

should avoid sandboxes where

dogs and

cats are known to defecate.

Larval

migration is limited to the

skin.

VLM and OLM are caused by

accidental

ingestion of Toxocara spp.

eggs from

contaminated soil; larval

migration

occurs throughout the body,

including

the eyes.

Adequate cooking of

infected

meat; awareness of

possibility

of contaminated soils for

dog

and cat hookworms

and/or

ascarids; covering of all

sandboxes where pets

have

access to defecation and

children play.

Nematodes,

filarial

Wuchereria bancrofti, Loa

loa, and

Onchocerca volvulus have no

animal

reservoirs and are found only

in

humans, whereas Brugia spp.

can

also be found in cats and

monkeys.

Dracunculus medinensis can

infect

dogs, cats, and monkeys and

Filarial nematodes are

transmitted through

the bite of a blood-sucking

arthropod

(midges, mosquitoes, flies).

Dracunculus

infections are acquired through

ingestion of water

contaminated with

small crustaceans, Cyclops

spp., which

contain infective larvae.

Vector control;

protection of

well-water sources

491

 Arranged by Sarah Mohssen

SectionIII– Parasitology By Nada Sajet

also

humans.

Cestodes,

Intestinal

The human serves as the

definitive host

for beef (Taenia saginata) and

pork

(Taenia solium) tapeworms;

cows/

camels and pigs serve as

intermediate hosts, respectively.

Humans also serve as the

intermediate host for T. solium

(cysticercosis). Diphyllobothrium

latum adult tapeworms can be

found

in a number of wild animals, the

most important being dogs, bears,

seals, and walruses, which serve

as

reservoir hosts; humans are the

definitive host. Hymenolepis

nana

(dwarf tapeworm) can occur in

rodents; humans can serve as

both

intermediate and definitive hosts,

with development from the egg to

adult worm occurring in the

human

intestine.

Human infection with the adult

worm

occurs through ingestion of raw or

poorly cooked meat (beef, camel,

pork)

containing the intermediate forms,

the

cysticerci. Humans become the

accidental intermediate host when

eggs

from an adult T. solium tapeworm

are

ingested. The cysticerci develop

in the

muscle and tissues of the human

rather

than the pig. Infection with the

adult D.

latum tapeworm occurs through

ingestion of poorly cooked

freshwater

fish containing the sparganum or

plerocercoid larval form. Infection

with

H. nana is primarily acquired

through

accidental ingestion of eggs from

an

adult tapeworm.

Adequate cooking of

infected

meat; treatment of patients

harboring adult tapeworms

(accidental ingestion of

eggs

can lead to infection

Cestodes,

Tissue

dult worms are found in a variety

of

animals; the human becomes the

accidental intermediate host after

ingestion of eggs from the adult

worms. Reservoir hosts include

dogs,

cats, and rodents

Ingestion of certain tapeworm

eggs or

accidental contact with certain

larval

forms can lead to tissue infection

with

Taenia solium, Echinococcus spp.,

and

several others.

Preventive measures

involve

increased attention to

personal

hygiene and sanitation

measures.

Trematodes,

intestinal

Fish-eating wild and domestic

animals

Ingestion of water chestnut and

caltrop

Avoiding eating raw water

plants

492

 Arranged by Sarah Mohssen

SectionIII– Parasitology By Nada Sajet

serve as reservoir hosts. The

definitive host of Fasciolopsis

buski

is the pig.

(raw, peeled with the teeth) is the

source of infection; metacercariae

are

encysted on the plant material. Pig

feces are used to fertilize various

water

plant crops.

that may contain encysted

larval forms of the flukes;

adequate waste disposal of

farm animal feces (pigs).

Trematodes,

liver, lung

Cats, dogs, and wild fish-eating

mammals can serve as reservoir

hosts for Opisthorchis spp.,

Clonorchis sinensis, and

Paragonimus spp. Fasciola

hepatica

is normally a parasite of sheep,

and

F. gigantica is a parasite of cattle;

humans are accidental hosts.

Infection occurs through ingestion

of raw

or poorly cooked fish, crabs,

crayfish,

and certain plants in or on which

metacercariae are encysted.

Infection

with Fasciola spp. is not easily

acquired

(the parasite is not that well

adapted to

the human host).

Thorough cooking of

potentially

infected fish, crabs,

crayfish;

avoiding eating raw water

plants that may contain

encysted metacercariae.

Trematodes,

Blood

Schistosoma mansoni and S.

haematobium appear to be

restricted

to the human host; S. japonicum

can

be found in cattle, deer, dogs, and

rodents. The worms mature in the

blood vessels, and eggs make

their

way outside the body in stool

and/or

urine. The freshwater snail is a

mandatory part of the life cycle

(contains developmental forms of

schistosome).

Infection occurs through skin

penetration

by infected cercariae released

from a

freshwater snail containing the

intermediate stages of the

schistosome

life cycle. Cercariae can be

released

from the snail intermediate host

singly

or in groups

Protection from potentially

contaminated water sources;

awareness of mode of

transmission; proper

handling

of human waste containing

eggs (continued infection of

snail intermediate hosts).

492

Arranged by Sarah Mohssen

Section IV - Medical Mycology By Dr. Kareem Lilo

Introduction

Mycology is the study of fungi. The name “fungi” is derived from

“mykos” meaning mushroom. The fungi are eukaryotic organisms

and they differ from the bacteria, which are prokaryotic organisms,

in many ways . The fungi possess rigid cell walls, which possess two characteristic cell structures:

chitin and ergosterol.

Chitin: The fungi consist primarily of chitin, unlike peptidoglycan

present in cell wall of bacteria. Hence, fungi are not sensitive to action of penicillin and other

antibiotics that inhibit peptidoglycan synthesis. Chitin is a polysaccharide consisting of long chains

of N-acetylglucosamine. In addition to chitin, the fungal cell wall also contains mannan and other

polysaccharides. Of these, beta-glucan is most important, because it is the target of antifungal drug

caspofungin.

Ergosterol: The cell membrane of fungus contains ergosterol, unlike human cell membrane which

contains cholesterol. The antifungal agents, such as amphotericin B, fluconazole, and ketoconazole

have selective action on the fungi due to this basic difference in membrane sterols.

Fungal infections are mycoses . Most pathogenic fungi are exogenous, their natural habitats being

water, soil, and organic debris. Th e mycoses with the highest incidence— candidiasis and

dermatophytosis—are caused by fungi that are part of the normal human microbiota and highly

adapted

to survival on the human host. For convenience, mycoses may be classifi ed as superfi cial,

cutaneous, subcutaneous, or systemic, invading the internal organs ( Table 4-1 ). Th e systemic

mycoses may be caused by endemic fungi, which are usually primary pathogens, or by ubiquitous,

oft en secondary opportunistic pathogens.

493

Arranged by Sarah Mohssen

Section IV - Medical Mycology By Dr. Kareem Lilo

Taxonomical Classification

The fungi are placed in the phylum Thallophyta. There are four classes of fungi: Zygomycetes,

Ascomycetes, Basidiomycetes, and Deuteromycetes or Fungi Imperfecti .

Pathogenesis of Fungal Infection

Most fungi are obligate aerobes or facultative anaerobes, but none are obligate anaerobes. The natural

habitat of most fungi is environment, because all these fungi require a preformed organic source of

carbon, hence their constant association with decaying matter. C. albicans is exception and is an

important fungus, which is a part of the normal human flora. The effects of fungi on humans can be

grouped in three major ways as follows: (a) colonization and

disease, (b) hypersensitive diseases, and (c) diseases caused by

mycotoxins or fungal toxins.

Colonization and Disease

Most fungal infections are mild and self-limited. Intact skin is an effective host defense against

certain fungi. But if the skin is broken, organisms, the fungi enter through that broken skin and

initiate the infection. Fatty acid content, pH, epithelial turnover, and normal bacterial flora of the skin

contribute to host resistance against fungi. For example, the mucous membrane of the nasopharynx

Table(4-1)The Major Mycoses and Causative Fungi

494

Arranged by Sarah Mohssen

Section IV - Medical Mycology By Dr. Kareem Lilo

traps inhaled fungal spores. Cell-mediated immunity is much important in conferring protection

against fungi. Suppression of cell-mediated immunity can lead to reactivation and dissemination of

asymptomatic

fungal infection and to diseases caused by opportunistic fungi. The humoral immunity is mediated

by production of IgG and IgM antibody. But their role in protection from fungal disease is uncertain.

Fungal infection that occurs in the immunocompromised hosts is called as opportunistic mycosis.

Hypersensitivity Diseases

Humans are continually exposed to air-borne fungal spores and other fungal elements present in the

environment. These spores can be antigenic stimulants and depending on individual’s

immunological status may induce a state of hypersensitivity by production of immunoglobulins or

sensitized lymphocytes. Rhinitis, bronchial asthma, alveolitis, and various forms of atopy are the

clinical manifestations of hypersensitive pneumonitis. The clinical manifestations of the

hypersensitivity disease are seen only in sensitized person, after repeated exposure to the fungus,

fungal metabolites, or other cross-reactive materials. Allergies to the fungal spores are manifested

primarily by an asthmatic reaction including rapid bronchial constriction mediated by IgE,

eosinophilia, and positive hypersensitivity skin test reaction. These are caused due to immediate

hypersensitivity reactions of the host to fungal spores.

Diseases Caused by Fungal Toxins

Mycotoxicosis is caused by ingested fungal toxins. Mycotoxicosis

caused by eating amanita mushroom is the best example of mycotoxicosis. This group of fungi

produces five toxins. Of these, amanitine and phalloidin are the two most potent hepatotoxins.The

toxicity of amanitine is due to its ability to inhibit cellular RNA polymerase, which prevents mRNA

synthesis. Aflatoxin is another fungal toxin produced by Aspergillus flavus that causes disease in

humans. Aflatoxin-B causes a mutation in the P53 tumor-suppressor gene, resulting in a loss of P53

protein, thereby in a resultant loss of growth control in the hepatocytes. Hence, it causes damage to

liver, and it induces tumor in liver in animals and is associated with hepatic carcinomas in humans.

Laboratory Diagnosis

Laboratory diagnosis of fungal infections depends on: (a) direct microscopy, (b) culture, (c)

serological tests, (d ) nonculture methods, and (e) molecular methods.

Direct Microscopy

495

Arranged by Sarah Mohssen

Section IV - Medical Mycology By Dr. Kareem Lilo

Direct microscopic examination depends on demonstration of characteristic asexual spores, hyphae,

or yeast in various clinical

specimens by light microscopy. The commonly used clinical specimens are sputum, lung biopsy

material, and skin scrapings.

The specimen is either treated with 10% KOH or stained

with special fungal stains. Use of 10% KOH dissolves tissue

material, leaving the alkali-resistant fungi intact. Calcofluor dye is a fluorescent dye that combines

with fungal cell wall and is useful in identification of fungi in tissue specimens. Methenamine silver

stain is useful for demonstration of fungi in tissues. India ink preparation of cerebrospinal fluid (CSF)

is a useful method for demonstration of white capsule of C. neoformans in CSF. Gram staining is also

useful to demonstrate Gram-positive Candida species in the specimen.

The disadvantages of microscopy are that it shows low sensitivity and requires an experienced

microscopist for specific identification.

Culture

Fungal culture is a frequently used method for confirming

the diagnosis of fungal infection. SDA is the most commonly

used medium for fungal culture. Other media include CHROM

agar, blood agar, etc. The low pH of the medium and addition

of chloramphenicol and cycloheximide to the medium inhibit the growth of bacteria in the specimen

and thereby facilitate the appearance of slow-growing fungi. Fungal colony is identified by rapidity

of growth, color, and morphology of the colony at the obverse and pigmentation at the reverse.

Microscopy of the fungal colony is carried out in lactophenol cotton blue (LPCB) mount to study the

morphology of hyphae, spores, and other structures. The appearance of the mycelium and the nature

of the asexual spores are very much helpful to identify the fungus.

Serological Tests

Demonstration of the antibodies in patient’s serum or CSF is useful for diagnosis of fungal infections,

especially in systemic fungal infections. A significant rise of antibody titer in a paired sera sample

confirms the diagnosis. The complement fixation test was the earliest test used in fungal serology and

is still used in the diagnosis of suspected cases of histoplasmosis, blastomycosis, or coccidiomycosis.

Recently, newer tests like ELISA (enzyme-linked immunosorbent assay), Western blot, and

radioimmunoassays are increasingly used for serodiagnosis of fungal infections.

Nonculture Methods

These methods include (a) detection of fungal antigen, (b) detection of fungal cell wall markers, and

(c) detection of fungal metabolites.

496

Arranged by Sarah Mohssen

Section IV - Medical Mycology By Dr. Kareem Lilo

Antigen detection: It is useful in immunocompromised hosts where antibody detection is not as

sensitive. Detection of fungal antigen in serum, CSF, and urine is increasingly used for diagnosis of

many fungal infections. Demonstration of antigen indicates recent or active infection. Latex

agglutination test is a frequently used test to demonstrate polysaccharide capsular antigen of C.

neoformans in CSF for diagnosis of cryptococcal meningitis. False-positive reactions due to

Trichosporon beigelli and Capnocytophaga canimorsus are known.

Detection of fungal cell wall markers: Mannan is a highly immunogenic component of the candidal

cell wall. Mannan antigen detection, therefore, is most widely used method in the diagnosis of

candidiasis. Galactomannan is a heat-stable heteropolysaccharide found in the cell walls of all

Aspergillus species. Production of the

galactomannan antigen is proportional to fungal load in tissue,

hence is being used as the prognostic marker for diagnosis

of invasive aspergillosis. A sandwich ELISA using rat monoclonal

antibody EB-A2 against galactomannan antigen is being

currently used in Europe for diagnosis of invasive aspergillosis.

Most pathogenic fungi have 1, 3-beta-D-glucan in their cell walls and minute quantities are secreted

into the circulation during the life cycle. Detection of this antigen can also be used as an indicator of

invasive fungal infections. Detection of 1, 3-beta-D-glucan is based on its ability to activate a

coagulation cascade within amebocytes derived from the hemolymph of horseshoe crabs. This uses a

different cascade than endotoxin to cause coagulation, hence is specific for fungi. The test does not

detect certain species, such as C. neoformans and Zygomycetes.

Detection of fungal metabolites: Detection of distinctive fungal metabolites is another approach for

the diagnosis of fungal infections. Gas liquid chromatography is being used to quantify arabinitol for

diagnosis of C. albicans infections.

Antifungal Drugs

A few drugs are available for therapy of systemic fungal infection, unlike a large number of

antibiotics available to treat bacterial infections. The drugs used to treat bacterial disease have no

effect on fungal diseases. Amphotericin B and various azoles are the most effective antifungal drugs.

They act on the ergosterol of fungal cell membrane that is not found in bacterial or human cell

membrane. Similarly, caspofungin inhibits synthesis of beta-glucan, which is found only in fungal

membrane but not in bacterial or human cell membrane. Table 4-2 summarizes the common

antifungal agents and their primary sites of activity.

Table ( 4-2 )Antifungal agents and primary sites of activity

497

Arranged by Sarah Mohssen

Section IV - Medical Mycology By Dr. Kareem Lilo

Superficial ,Cutaneous, and Subcutaneous Mycoses

Fungal infections, depending on the tissues that are initially colonized, can be classified into three

major groups as follows:

Superficial mycoses: These are surface infections of the skin,

affecting the outermost layers of skin, hair, and mucosa.

Cutaneous mycoses: These are infections of the skin involving

the epidermis and its integuments, the hair, and nails.

Subcutaneous mycoses: These are infections of the dermis, subcutaneous tissue, muscle, and fascia

Superficial Mycoses

Superficial mycosis caused by different fungi is restricted to the outer most layers of the skin and

hair. The condition usually causes cosmetic problem, which can be easily diagnosed and treated. It

includes four important conditions:

(a) pityriasis versicolor, (b) tinea nigra, (c) black piedra, and

498

Arranged by Sarah Mohssen

Section IV - Medical Mycology By Dr. Kareem Lilo

(d) white piedra

Pityriasis Versicolor

Pityriasis versicolor or tinea versicolor is a superficial infection

of the skin caused by Malassezia furfur (Pityrosporum orbiculare).

M. furfur requires fatty acids for growth, hence is cultured on

the Sabouraud’s dextrose agar (SDA) overlayed with a layer of

olive oil. On incubation at 37°C, the fungus produces creamy

colonies within 5–7 days. The fungus is found inparts of the body rich in sebaceous glands. The

lesions of pityriasis versicolor are found most commonly on the upper tissue, arms, and abdomen.

Laboratory diagnosis of the condition is usually made by demonstration of both budding yeast cell and

hyphae in KOH preparation of skin scrapings. Characteristic “spaghetti and meatballs” appearance of

fungus is demonstrated in the microscopy of KOH preparation of the skin. Culture is not carried out

routinely for diagnosis. Topical miconazole is treatment of choice.

Tinea Nigra

Tinea nigra is an infection of keratinized layer of skin caused

by Exophiala werneckii or Cladosporium werneckii. C. werneckii is a

dimorphic fungus that produces melanin. The fungus on the SDA grows as yeast with many cells in

various stages of cell division

producing typical two-celled oval structure, on primary isolation from clinical specimen. On

prolonged incubation, elongated hyphae develop and in older cultures, mycelia and conidia are

predominantly found. A well-demarcated brown-black macular lesion, which

appears as brownish spot of the skin, is typical manifestation

of the condition. These brownish to black lesions are most

commonly seen on palms and soles.

Laboratory diagnosis of tinea nigra is made by microscopy of the KOH preparation of skin scrapings

collected from the affected part. Typical darkly pigmented yeast-like cells and hyper fragmented

hyphae are demonstrated. Culture of the skin scraping on the SDA confirms the diagnosis.

Black Piedra

Black piedra is a superficial infection of the hair caused by Piedraia

hortae, a dematiaceous fungus. . Culture of specimens

on SDA shows slow-growing brown to reddish black mycelium,

which is considered asexual or anamorphic stage of the fungus.

499

Arranged by Sarah Mohssen

Section IV - Medical Mycology By Dr. Kareem Lilo

Infection of shaft of hairs of beard and scalp is the major clinical feature of black piedra. Laboratory

diagnosis of the condition is made by demonstration of nodules containing asci with spindle-shaped

ascospores in 10% KOH mount of the hair.

White Piedra

White piedra is an infection of the hair caused by yeast-like organism Trichosporon beigelli. The

fungus can be grown on SDA and other media containing cycloheximide. On SDA, it forms greencolored colonies, which subsequently become yellowish gray and wrinkled. Microscopic examination

of the colony shows septate hyphae that break rapidly to form arthroconidia. The latter subsequently

become round and develop to Blastoconidia The development of a soft, pasty,

cream-colored growth along infected hair shaft characterizes the condition. The initial growth of T.

beigelli occurs beneath the

epidermis of hair. The infected hair shaft consists of mycelium

that rapidly fragments to arthroconidia.

Laboratory diagnosis of the condition is made by demonstration of fragmented hyphae that develop

into arthroconidia or produce blastoconidia in 10% KOH mount of hair. Culture of the fungus from

clinical specimen confirms the diagnosis.

Features of the organisms causing superficial mycoses are

summarized in (Table 4-4).

Table (4-3) Features of the organisms causing superficial mycoses

Cutaneous Mycoses

Dermatophytoses or cutaneous mycoses are diseases of the skin,

hair, and nail. These infections are caused by a homogeneous

group of closely related fungi known as dermatophytes. These dermatophytes infect only superficial

keratinized structures, such as skin, hair, and nail, but not deeper tissues.

Dermatophytes

500

Arranged by Sarah Mohssen

Section IV - Medical Mycology By Dr. Kareem Lilo

The most important dermatophytes that cause infection in

humans are classified into the following three genera:

(i) Trichophyton—causes infection of hair, skin, and nail.

(ii) Microsporum—causes infection of hair and skin.

(iii) Epidermophyton—causes infection of skin and nails, but

not hair.

The dermatophytes on the basis of their natural habitat and host preferences can be classified into

following groups:

(i) Anthropophilic species: These dermatophytes are typically adapted to live on human host. They

are transmitted from human to human through fallen hairs, desquamated epithelium, combs, hair

brushes, towels, etc. Examples are Trichophyton rubrum, icrosporum audouinii, and Epidermophyton

floccosum.