40Human Diseases Caused by Fungi and Protists

Prescott’s Microbiology, 9th Edition

40Human Diseases Caused by Fungi and Protists

CHAPTER OVERVIEW

This chapter discusses some of the more important fungal and protist diseases of humans. The clinical manifestations, diagnosis, epidemiology, pathogenesis, and treatment of selected diseases are presented.

LEARNING OUTCOMES

After reading this chapter you should be able to:

describe the system used to characterize fungal diseases
explain why protist infection is becoming a greater challenge
report the common fungal diseases spread by airborne transmission
identify typical signs and symptoms of common airborne fungal diseases
correlate airborne fungal infection and disease severity with fungal virulence factors
report the common protist diseases spread by arthropod transmission
identify typical signs and symptoms of arthropod-borne protist diseases
correlate arthropod-borne protist infection and disease severity with protist virulence factors
report the common fungal and protist diseases spread by direct contact
identify typical signs and symptoms of fungal and protist diseases spread by direct contact
correlate direct contact fungal infection and disease severity with fungal virulence factors
correlate direct contact protist infection and disease severity with protist virulence factors
report the common food-borne and waterborne fungal and protist diseases
identify typical signs and symptoms of common food-borne and waterborne fungal and protist diseases
correlate vehicle-borne protist infection and disease severity with protist virulence factors
explain in specific terms why access to clean water is considered a basic human right
predict the impact to the host resulting when normal fungal microbiota are introduced to a new host environment
distinguish growth and reproduction challenges experienced by normal fungal microbiota and ubiquitous environmental fungi when attempting to infect a human
cite specific host conditions that promote fungal infection

CHAPTER OUTLINE

I.Pathogenic Fungi and Protists

A.Medical mycology—discipline that deals with fungi that cause human disease; fungal diseases are called mycoses

B.Systemic mycoses are caused by dimorphic fungi (except for Cryptococcus neoformans, which has only a yeast form); usually acquired by inhalation of spores from soil; infection begins as lung lesions, becomes chronic, and disseminates through the bloodstream to other organs

C.Protists are transmitted by arthropod vectors or by food and water vehicles; fewer than 20 protist genera cause human disease; however, malaria, trypanosomes, and amoeba account for hundreds of millions of cases

D.Immunosuppressed populations are at increased risk for infections of both fungal and protist pathogens

II.Airborne Diseases

A.Blastomycosis—Blastomyces dermatitidis

1.Occurs in three clinical forms: cutaneous, pulmonary, and disseminated

2.Diagnosis is aided by serological tests; antifungal agents are effective; surgery may be necessary to drain large abscesses; no prevention or control measures

B.Coccidiomycosis—Coccidioides immitis

1.Acquired by inhalation of spores

2.Usually an asymptomatic or mild respiratory infection that spontaneously resolves in a few weeks; occasionally progresses to chronic pulmonary disease

3.Diagnosis is by culturing; serological tests also are available; treatment with several antifungal agents; prevention involves reduction of exposure to dust in endemic areas

C.Cryptococcosis—Cryptococcus neoformans or C. gattii

1.Aged, dried pigeon droppings are a source of infection; fungus enters by the respiratory tract

2.Minor transitory pulmonary infection that can disseminate and cause meningitis; more serious and chronic disease in HIV/AIDS patients and the immunocompromised

3.Diagnosis is by microscopic examination of specimens and immunological procedures; treatment includes amphotericin B and/or fluconazole; no prevention or control measures

D.Histoplasmosis—Histoplasma capsulatum

1.A facultative fungus that grows intracellularly; can be found as a budding yeast and in a mold form

2.Found worldwide in soils; spores are easily spread by air currents and inhaled; the spores are most prevalent where bird droppings have accumulated

3.Inhaled microsporidia are transformed into budding yeasts within alveolar spaces and are eventually observed within the cells of the monocyte-macrophage system; symptoms are usually those of mild respiratory involvement; it rarely disseminates

4.Diagnosis by immunological tests and culture; most effective treatment is amphotericin B, ketoconazole, or intraconazole; prevention and control by using protective clothing and masks and by soil decontamination where feasible

III.Arthropod-Borne Diseases

A.Malaria—Plasmodium falcipurum, P. malariae, P. vivax, P. ovale

1.Transmitted to humans by bite of an infected female Anopheles mosquito; sporozoites in blood move to and reproduce in the liver (asexual schizogony), producing merozoites;these enter erythrocytes and form large trophozoites that undergo schizogony and the schizonts produce merozoites; the cycle repeats every 48 to 72 hours

2.Periodic sudden release of merozoites, toxins, cell debris from the infected erythrocytes; and TNF- and interleukin-1 from macrophages trigger the characteristic attack of chills and fever; anemia can result, and the spleen and liver often hypertrophy

3.Diagnosis is by microscopic examination of blood smears; serological tests are also available; treatment is by chloroquine or related drugs; no effective vaccine is available and prevention is through bed netting and insecticides

B.Leishmaniasis—Leishmaniatropica, L. mexicana, L. braziliensis, L. donovani

1.Caused by flagellated protists (hemoflagellates) transmitted by sandflies from canines and rodents

2.Can be mucocutaneous, cutaneous, or visceral; symptoms vary with the particular etiological organism involved

3.Treated with pentavalent antimonial compounds; recovery usually confers permanent immunity; vector and reservoir control and epidemiological surveillance are the best options for control

C.Trypanosomiasis—Trypanosoma brucei, T. cruzi

1.T. brucei, a hemoflagellate, causes African trypanosomiasis; transmitted by tsetse flies; causes interstitial inflammation and necrosis of the lymph nodes, brain, and heart; causes sleeping sickness (uncontrollable lethargy)

2.T. cruzi causes American trypanosomiasis (Chagas' disease); transmitted when bite of triatomid (kissing) bug is contaminated with insect feces; symptoms are similar to those caused by T. brucei

3.Trypanosomiasis is diagnosed by microscopic examination of blood and by serological tests; treatment with antibiotics is specific for type of trypanosome and stage of disease; vaccines are not useful because the parasite can change its protein coat to avoid the immune response

IV.Direct Contact Diseases

A.Fungal Pathogens

1.Most occur in the tropics

2.The fungi that cause the disease are limited to the outer surface of the hair and the skin

a.Piedras are infections of hair shaft that result in formation of a hard nodule

b.Tineas are infections of the outer layer of skin, nails, and hair

3.Treatment involves removal of skin scales and infected hairs; prevention is by good personal hygiene

4.Occur worldwide; most common fungal diseases are cutaneous mycoses such as dermatomycoses, ringworms, tineas

5.Three genera, Epidermophyton, Microsporum, and Trichophyton, are involved

6.Diagnosed by microscopic examination of skin biopsies and by culture on Sabouraud's glucose agar

7.Treatment—topical ointments, oral griseofulvin, or oral itraconazole (sporanox)

8.Different diseases are distinguished according to the causative agent and the area of the body affected (tinea barbae—beard hair, tinea capitis—scalp hair, tinea corporis—any part of skin, tinea cruris—groin, tinea pedis—athlete's foot, tinea mannum—hands, tinea unguium—nail bed)

9.The fungi that cause subcutaneous mycosis are typically saprophytes in soil; they gain entry by puncture wounds

10.Disease develops slowly over a period of years, during which time nodule develops and then ulcerates; organisms spread along lymphatic channels, producing more nodules at other locations

11.Treatment is with 5-fluorocytosine, iodides, amphotericin B, and surgical excision; diagnosis is by culture of the infected tissue

12.Examples include chromoblastomycosis, maduromycosis, and sporotrichosis

a. Chromoblastomycosis is caused by the black molds Phialophora verrucosa and Fonseca-

pedrasoiand most common in tropical environments

b. Maduromycosis can lead to serious deformities such as Madura foot mycetoma

c. Sporotrichosis is the most common mycosis in the United States caused by infection with

Sporothrix schenckiicontracted when handling contaminated plant debris

B.Protist Pathogens

1. Trichomoniasis—Trichomonas vaginalis; a sexually transmitted disease; host accumulates

leukocytes at the site of infection; in females, this leads to a yellow purulent discharge and

itching; in males, most infections are asymptomatic; treatment is with metronidazole

2. Diagnosis in females can be made by direct examination of the protist in discharge.

V.Food-Borne and Waterborne Diseases

A.Amebiasis (amebic dysentery)—Entamoeba histolytica

1.Ingested cysts excyst in the intestine and proteolytically destroy the epithelial lining of the large intestine

2.Disease severity ranges from asymptomatic to fulminating dysentery, exhaustive diarrhea, and abscesses of the liver, lungs, and brain

3.Diagnosis is based on finding trophozoites in fresh, warm stools and cysts in ordinary stools; serological testing also should be done; treatment with several drugs is possible; prevention and control involves avoiding contaminated water; hyperchlorination or iodination can destroy waterborne cysts

B.Amebic meningoencephalitis and keratitis—caused by Naegleria and Acanthamoebae; facultative parasites that cause granulomatous amoebic encephalitis and keratitis (particularly among wearers of soft contact lenses); found in freshwater and soil; diagnosis is by microscopic examination of clinical specimens; most are resistant to common antimicrobial agents

C.Cryptosporidiosis—Cryptosporidium parvum

1.Found in the intestines of many birds and mammals, which shed oocysts into the environment in fecal material; when oocysts are ingested, they excyst in the small intestine; the released sporozoites parasitize intestinal epithelial cells

2.Major symptom of infection is diarrhea; diagnosis is by microscopic examination of feces; treatment with nitazoxanide and rehydration; patients will usually recover, but the disease can be fatal in late stage AIDS patients

D.Cyclosporiasis—Cyclospora cayetanensis

1.Mainly in tropical regions; linked to contaminated produce

2.Frequent, explosive diarrhea with fever, fatigue, and weight loss; infects intestines and shed cyst oocysts in feces; these must differentiate into sporozoites to be infectious

3.Diagnosis is by observation of oocysts in feces; treatment is with trimethoprim and sulfamethoxazole and fluid replacement; prevention is through avoidance of contaminated food and water

E.Giardiasis—Giardia intestinalis

1.Most common cause of waterborne epidemic diarrheal disease; commonly found in daycare facilities with diapered children

2.Transmission is usually by cyst-contaminated water supplies, and disease is common in wilderness areas where animal carriers shed cysts into otherwise "clean" water

3.Disease varies in severity; asymptomatic carriers are common; may be chronic or acute

4.Diagnosis is by identification of trophozoites; immunological tests also are available; treatment is with quinacrine or metronidazole; prevention involves avoiding contaminated water and the use of slow-sand filters in the processing of drinking water

F.Toxoplasmosis—Toxoplasma gondii

1.Fecal-oral transmission from infected animals; also transmitted by ingestion of undercooked meat and by congenital transfer, blood transfusion, or tissue transplant

2.Most cases are asymptomatic; can be fatal in immunocompromised individuals; leads to severe congenital effects for pregnant women; associated with exposure to cat feces containing oocytes

3.Acute disease is characterized by lymphadenopathy, enlargement of reticular cells, pulmonary necrosis, myocarditis, hepatitis, and retinitis; a major cause of death in AIDS patients

4.Diagnosis is by serological tests; chemotherapeutic agents are available for treatment; prevention and control require minimizing exposure by not eating raw meat and eggs, washing hands after working in soil, cleaning cat litter boxes daily, keeping cats indoors, and feeding cats commercial food

5. Treatment with a combination of pyrimethamine and sulfadiazine in order to curtail some

3000 congenital infections yearly in the United States

VI.Opportunistic Diseases

1.Opportunistic organisms are normally harmless but can cause disease in a immunologically compromised host

2.Aspergillosis—Aspergillus fumigatus or A. flavus

a.Portal of entry is respiratory tract; inhalation can lead to several types of pulmonary aspergillosis; the fungus can spread to other tissues and organs; in immunocompromised patients, invasive aspergillosis (mycelia in lungs) may occur

b.Diagnosis depends on examination of specimens or isolation and characterization of fungus or immunological methods; treated with voriconazole and intraconazole

3.Candidiasis—Candida albicans

a.C. albicans is part of normal microbiota and can be transmitted sexually

b.Exhibits a diverse spectrum of disease:

i.Oropharyngeal candidiasis (thrush)—mouth; common in newborns

ii.Paronychia—subcutaneous tissues of the digits

iii.Onychomycosis—subcutaneous tissues of the nails

iv.Intertriginous candidiasis—warm, moist areas such as axillae, groin, and skin folds (e.g., diaper candidiasis, candidal vaginitis, and balanitis)

c.Diagnosis is difficult; no satisfactory treatment; cutaneous lesions can be treated with topical agents; oral antibiotics are used for systemic candidiasis

4.Microsporidosis—obligate intracellular Microspora

a.Infect a wide range of animal hosts that act as reservoirs; highly resistant spores (with characteristic polar tube structure) survive long periods in the environment; mainly seen in AIDS patients

b.Spores inject intraspore contents (sporoplasm) into host cells using polar body; the sporoplasm multiplies asexually and generates more spores; symptoms include hepatitis, pneumonia, skin lesions, weight loss, diarrhea, and wasting syndrome

c.Diagnosis is based on stained microsporidia, if possible with electron microscopy, and via PCR; treatment is not well defined

5.Pneumocystis pneumonia

a.Caused by a fungus (Pneumocystis jeroveci) that was once thought to be a protozoan

b.Disease occurs almost exclusively in immunocompromised hosts including more than 80% of AIDS patients; the fungus remains localized in the lungs, even in fatal cases

c.Definitive diagnosis involves demonstrating the presence of the organism in infected lung material or PCR analysis; treatment is by oxygen therapy and combination drug therapy; prevention and control is through prophylaxis with drugs in susceptible persons

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Prescott’s Microbiology, 9th Edition

CRITICAL THINKING

1.One of the apparent paradoxes associated with giardiasis is that outbreaks occur more frequently in the RockyMountain and New England states where the raw water is considered to be of fairly high quality, whereas outbreaks are less frequent in the South, Southwest, and Midwest, where quality of raw water is actually considered much poorer. Explain. (Consider the types of treatment used in the two types of areas.)

2.Describe the life cycle of Plasmodium vivax and discuss how it is related to the symptoms associated with malaria.

3. Speculate why there are fewer chemotherapeutic agents available to fight fungus and parasitic protist

infections than bacterial infections. Consider that both fungi and protists are eukaryotic like their host.

4. Travel guides often suggest to drink bottled water when traveling to distant rural areas or to boil water

prior to drinking in these areas, but local inhabitants do not. Comment on this recommendation.

CONCEPT MAPPING CHALLENGE

Construct a concept map using the following words and your own linking terms.

Fungus Protozoan Parasite Water Skin Eukaryote Opportunist Spores Intestine Mycosis

Cyst Tinea Flagella Malaria Amoeba Trypanosome Giardia