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The Common Cold

Definition The common cold is a clinical syndrome characterized by upper respiratory signs and symptoms (e.g., rhinorrhea, sneezing, nasal obstruction), which are frequently accompanied by sore throat, cough, and general malaise. The common cold is among the most prevalent of illness syndromes and is very costly in aggregate, especially considering the resulting absenteeism from school and work. Common cold is both a clinical and a cultural…

Antiviral Therapy (Non-HIV)

Although some viral infections are self-limited, others can cause significant morbidity and mortality. Effective therapy is available for many of these infections. This chapter reviews currently available antiviral agents for the treatment of infections caused by viruses other than human immunodeficiency virus (HIV). Not all agents discussed are licensed in all countries. Currently available agents can be classified into those that inhibit viral replication at the…

Nematode Infections

Definition Nematode infections are highly prevalent, affecting millions of people worldwide. Nematodes are complex, nonsegmented roundworms with specialized organs that include a protective outer coating or cuticle, a complete and functional gastrointestinal tract, and muscular, nervous, and reproductive systems. Most species are free-living in soil or water, but a few parasitize humans. The Pathogens Nematodes of medical importance can be categorized into those that primarily affect…

Trematode Infections

Schistosomiasis Definition Schistosomiasis, which is classified as a neglected tropical disease, is a parasitic infection caused by the trematode worms (blood flukes) of the genus Schistosoma. It has been reported in 78 countries worldwide, with 290 million people infected and nearly 800 million at high risk for infection. Mortality from schistosomiasis is estimated at 13,000 deaths per year, and the burden of disease at 3.3 million…

Cestodes

Definition The Pathogens Cestode parasites are members of the animal kingdom, subphylum Cestoda. The organisms are characterized by several life cycle stages, which typically develop in distinct hosts. The adult stage is the tapeworm, which is acquired by ingestion of uncooked tissues that harbor larval forms. After ingestion, the larvae excyst, and the scolex attaches to the intestines. Segments, termed proglottids , develop at the base…

Babesiosis and Other Protozoan Diseases

Babesiosis Babesiosis is a tick-borne, malaria-like disease caused by sporozoan parasites of the genus Babesia that infect red blood cells. Epidemiology With few exceptions, Babesia spp are transmitted by ixodid ticks. Thus, wherever humans are intensely exposed to hard-bodied ticks, babesiosis should be part of the differential diagnosis for a patient presenting with fever and hematologic abnormalities. Three worldwide epidemiologic patterns are apparent. The first involves…

Amebiasis

Definition Amebiasis is due to infection with the enteric protozoan parasite Entamoeba histolytica . Amebiasis can cause asymptomatic colonization, diarrhea, dysentery, and colitis. It also can spread extraintestinally to cause liver and rarely brain abscess ( Fig. 323-1 ). The Pathogen E. histolytica has a low infectious dose (<100 organisms), is resistant to chlorine, and is environmentally stable. Certain genotypes of E. histolytica appear to be…

Giardiasis

Definition Giardia lamblia ( Giardia duodenalis, Giardia intestinalis ) is a ubiquitous, small intestinal protozoan parasite of humans and other mammals. It is the most common parasitic infection of the gastrointestinal tract in the United States as well as worldwide, and it is responsible for outbreaks of diarrhea and sporadic endemic disease. , The Pathogen Giardia has a simple life cycle. The trophozoite, which is 9…

Cryptosporidiosis

Definition Cryptosporidiosis is the disease in humans and animals caused by protozoal parasites of the genus Cryptosporidium (phylum Apicomplexa). Cryptosporidium spp are major waterborne parasites worldwide, and more than 20 species have been documented to infect humans. The Pathogen The family Cryptosporidiidae has a hidden sporocyst and undergoes monoxenous completion of its cycle in one host, where it can cause predominantly intestinal, cloacal, and gastric infections.…

Toxoplasmosis

Definition Toxoplasmosis refers to symptomatic disease, whereas T. gondii infection refers to latent, asymptomatic disease. Toxoplasma gondii is a ubiquitous protozoan parasite. Although felids are T. gondii ’s only definitive host (i.e., where parasitic sexual reproduction occurs), the parasite’s intermediate hosts include most warm blooded animals, from birds to humans. In both the definitive and intermediate hosts, T. gondii causes an acute infection followed by the…

Leishmaniasis

Definition Leishmaniasis, which is caused by protozoan parasites of the genus Leishmania, is generally transmitted between mammalian hosts by female phlebotomine sandflies. The parasite exists as an extracellular flagellated form, the promastigote, in the gut of the female sandfly, and as an intracellular form, the amastigote, in the phagolysosomal compartment of macrophages in the mammalian host. The disease in humans affects either the skin/mucosae or internal…

Chagas Disease

Definition Chagas disease, or American trypanosomiasis, is caused by the protozoan parasite Trypanosoma cruzi . The terms Chagas disease , American trypanosomiasis , and T. cruzi infection are synonyms. The Pathogen Several dozen species are included in the genus Trypanosoma , but only the African trypanosome Trypanosoma brucei (subspecies T. b. gambiense [West African] and T. b. rhodesiense [East African]) ( Chapter 317 ) and the…

African Sleeping Sickness

Definition Human African trypanosomiasis, commonly known as sleeping sickness, is a vector-borne parasitic disease transmitted to humans and animals by the bite of the tsetse fly (genus Glossina ). Infection is caused by protozoa of the genus Trypanosoma and species brucei . In humans, two forms of illness are caused by two distinct subspecies that are morphologically identical but differ in their geographic range and clinical…

Malaria

Definition Malaria is caused by infection with protozoan parasites of the genus Plasmodium, all of which are transmitted by bites of infected anopheline mosquitoes. Malaria is typically characterized by an acute febrile illness, with parasites infecting large numbers of erythrocytes, and classically entails recurrent episodes of fever and chills. Malaria causes hundreds of millions of illnesses each year throughout most of the tropics. Severe disease can…

Antiparasitic Therapy

Although substantial progress has been made in reducing the impact of protozoal and helminthic infections, parasitic diseases remain a major cause of morbidity and mortality worldwide, particularly in impoverished tropical regions. In the United States, parasitic diseases are most often encountered among immigrants and international travelers to endemic areas and among a growing number of persons with compromised immunity due to human immunodeficiency virus/acquired immunodeficiency syndrome…

Mycetoma and Dematiaceous Fungal Infections

Definition Dematiaceous fungi represent a large group of fungal organisms characterized by the presence of abundant melanin in the cell wall. This melanin gives rise to a brown-black coloration on artificial culture media and in histopathologic specimens. A related term, phaeohyphomycosis, refers broadly to infection by these pigmented fungi. The two terms are often used interchangeably. Dematiaceous fungal infections generally fall into three broad categories: mycetoma…

Pneumocystis Pneumonia

Definition Pneumocystis jirovecii is a fungus that causes pneumonia almost exclusively in immunodeficient patients. Pneumocystis pneumonia (PCP) has been the most common life-threatening opportunistic infection in persons infected with human immunodeficiency virus (HIV), especially those with acquired immunodeficiency syndrome (AIDS). Although the frequency of PCP has decreased in HIV-infected patients, first in association with the widespread use of anti- Pneumocystis prophylaxis and later with the introduction…

Mucormycosis

Definition Mucormycosis is the unifying term used to describe infections caused by fungi belonging to the order Mucorales. Mucorales typically cause aggressive, acute-onset, frequently fatal or disfiguring, angioinvasive infections, especially in immunosuppressed hosts. Epidemiology Mucorales fungi are distributed worldwide and found in decaying organic substrates. The true incidence of mucormycosis is not known and probably is underestimated because of difficulties in antemortem diagnosis. The most common…

Aspergillosis

Definition Aspergillosis is a disease caused by one or more of the species of the genus Aspergillus . Sporelike structures called conidia are aerosolized from the mold form of the organism growing in the environment. When conidia reach tissue, they germinate to form invasive filaments called hyphae . The Pathogens The most common species infecting humans are Aspergillus fumigatus, Aspergillus flavus, Aspergillus terreus , and Aspergillus…

Candidiasis

Definition Candidiasis encompasses a wide variety of clinical syndromes caused by yeasts of the genus Candida . Of the species that cause infection in humans, Candida albicans is the most common; Candida glabrata, Candida parapsilosis , and Candida tropicalis are responsible for most of the remaining infections, but Candida auris has emerged as a major pathogen in some parts of the world. Organisms such as Candida…