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39 Human Diseases Caused by Fungi and Protozoa

 

CHAPTER OVERVIEW

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

 

CHAPTER OBJECTIVES

After reading this chapter you should be able to:

! discuss the five groupsCsuperficial, cutaneous, subcutaneous, systemic, and opportunisticCof diseases caused by fungi (mycoses) including Pneumocystis carinii pneumonia

! discuss some of the more important diseases caused by protozoans (including amebiasis, giardiasis, malaria, the hemoflagellate diseases, toxoplasmosis, and trichomoniasis)

 

CHAPTER OUTLINE

I. Fungal DiseasesCmycoses

A. Superficial mycoses

1. Most occur in the tropics

2. Limited to the outer surface of the hair and the skin

3. Referred to as piedras or tineas because of the hard nodules that are formed

4. Treatment involves removal of skin scales and infected hairs

5. Prevention is by good personal hygiene

B. Cutaneous mycosesCdermatophytoses, ringworms, tineas

1. Occur worldwide; most common fungal diseases

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

3. Diagnosed by microscopic examination of skin biopsies and by culture on Sabouraud=s glucose agar

4. TreatmentCtopical ointments, oral griseofulvin, or oral itraconazole (sporanox)

5. Different diseases are distinguished according to the causative agent and the area of the body affected (e.g., tinea barbaeCbeard hair; tinea pedisCathlete=s foot)

6. Includes such common diseases as ringworm, jock itch, and athlete=s foot

C. Subcutaneous mycoses

1. Introduced in soil-contaminated puncture wounds

2. Develop slowly over a period of years

3. Nodule develops and then ulcerates; organisms spread along lymphatic channels; more nodules then develop at other locations

4. Diagnosis is by culture of the infected tissue

5. TreatmentC5-flucytosine, iodides, amphotericin B, and surgical excision

6. Examples include chromomycosis, maduromycosis, sporotrichosis

D. Systemic mycoses

1. Caused by dimorphic fungi, except for Cryptococcus neoformans which has only a yeast form

2. Usually acquired by inhalation of spores from soil

3. Begins as lung lesions, becomes chronic, and disseminates through the bloodstream to other organs

4. BlastomycosisCBlastomyces dermatitidis; antibiotics are used; surgery may be necessary to drain large abscesses

5. CoccidiomycosisCCoccidioides immitis; usually an asymptomatic or mild respiratory infection that spontaneously resolves in a few weeks; occasionally progresses to chronic pulmonary disease; antibiotics are used for treatment

6. CryptococcosisCCryptococcus neoformans; minor transitory pulmonary infection that can disseminate and cause meningitis if it reaches the central nervous system; found in approximately 15% of AIDS patients; treatment involves amphotericin B and flucytosine or intraconazole

7. HistoplasmosisCHistoplasma capsulatum; facultative intracellular parasite that causes disease of the reticuloendothelial system; symptoms are usually those of mild respiratory involvement; occasional dissemination; occupational hazard for spelunkers and bat guano miners; treatment is amphotericin B, ketoconazole, or intraconazole

E. Opportunistic mycosesCnormally harmless, but can cause disease in a compromised host

1. AspergillosisCAspergillus fumigatus or A. flavus; pulmonary involvement with potential systemic dissemination; in severely compromised individuals, invasive aspergillosis may occur and fill the lungs with mycelia (aspergillomas); treatment involves treating the underlying illness to strengthen resistance and the use of intraconazole

2. CandidiasisCCandida albicans; usually involves the skin and mucous membranes of people whose normal microbiota has been disturbed by prolonged antibiotic therapy

a. Oral candidiasis (thrush)Cmouth; common in newborns

b. ParonychiaCsubcutaneous tissues of the digits

c. OnychomycosisCsubcutaneous tissues of the nails

d. Intertriginous candidiasisCwarm, moist areas such as axillae, groin, and skin folds

e. Diaper candidiasisCcan occur if diapers are not changed frequently

f. Candidal vaginitisCoccurs when lactobacilli are depleted by diabetes, antibiotics, oral contraceptives, pregnancy, or other factors

g. BalanitisCsexually transmitted to males; infection of the glans penis (primarily in uncircumcised males)

h. Treatment involves topical agents; oral antibiotics are used for systemic candidiasis

3. Pneumocystis carinii pneumonia is caused by a eucaryotic protist

a. It was once considered a protozoan parasite but recent comparisons of rRNA and DNA sequences from several genes have shown it to be more closely related to fungi

b. Disease occurs almost exclusively in immunocompromised hosts including more than 80% of AIDS patients

c. It is transmitted by inhalation of infected material and remains localized in the lungs, even in fatal cases

d. Definitive diagnosis involves demonstrating the presence of the organism in infected lung material or PCR analysis

e. Treatment is by oxygen therapy and combination drug therapy

II. Protozoan Diseases

A. Amoebiasis (amebic dysentery)CEntamoeba histolytica; ingested cysts excyst in the intestine and proteolytically destroy the epithelial lining of the large intestine; disease severity may be asymptomatic to fulminating dysentery, exhaustive diarrhea, and abscesses of the liver, lungs, and brain; treatment with several antibiotics is possible; prevention involves avoiding contaminated water; hyperchlorination or iodination can destroy waterborne cysts

B. CryptosporidiosisCCryptosporidium parvum; found in the intestines of many birds and mammals; oocysts are shed into the environment in fecal material; they are then ingested and excyst in the small intestine; the released sporozoites parasitize intestinal epithelial cells, causing diarrhea; treatment is supportive; patients will usually recover, but the disease can be fatal in late stage AIDS patients

C. Freshwater amoebaeCNaegleria and Acanthamoebae; facultative parasites that cause primary amebic meningoencephalitis and keratitis (particularly among wearers of soft contact lenses); found in fresh water and soil

D. GiardiasisCGiardia lamblia; most common cause of waterborne epidemic diarrheal disease; common in wilderness areas because many animal carriers shed cysts into otherwise Aclean@ water; varies in severity; asymptomatic carriers are common; may be chronic or acute; treatment is usually metronidazole (Flagyl); prevention involves avoiding contaminated water and the use of slow sand filters in the processing of drinking water

E. MalariaCPlasmodium species; transmitted by bite of an infected female Anopheles mosquito; reproduces in the liver and also penetrates erythrocytes; a periodic sudden release of merozoites, toxins, cell debris from the infected erythrocytes and TNF-I and interleukin-1 from macrophages trigger the characteristic attack of chills and fever; anemia can result, and the spleen and liver often hypertrophy; treatment is by chloroquine or related drugs

F. Hemoflagellate diseases

1. LeishmaniasisCcaused by flagellated protozoans; transmitted by sandflies; can be mucocutaneous, cutaneous, or visceral; symptoms vary with the particular etiological organism involved; treated with pentavalent antimonial compounds; recovery usually confers permanent immunity; vector and reservoir control and epidemiological surveillance are the best options for control

2. TrypanosomiasisCtransmitted by tsetse flies; causes interstitial inflammation and necrosis of the lymph nodes, brain, and heart; causes sleeping sickness (uncontrollable lethargy) and Chagas= disease, which is currently untreatable; vaccines are not useful because the parasite can change its coat to avoid the immune response

G. ToxoplasmosisCToxoplasma gondii, fecal-oral transmission from infected animals; also transmitted by ingestion of undercooked meat and by congenital transfer, blood transfusion, or tissue transplant; most cases are asymptomatic or resemble mononucleosis; can be fatal in immunocompromised individuals; a major cause of death in AIDS patients; treatment is with antibiotics

H. TrichomoniasisCTrichomonas vaginalis; 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; one of the most common sexually transmitted diseases

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