Porth's Essentials of Pathophysiology, 4e

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Mechanisms of Infectious Disease

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the characteristic microscopic appearance of the asexual fruiting structures and spores. Like the bacterial pathogens of humans, fungi can produce disease in the human host only if they can grow at the temperature of the infected body site. For example, a number of fungal pathogens called dermatophytes are incapable of growing at core body temperature (37°C), and the infection is limited to the cooler cutaneous sur- faces. Diseases caused by these organisms, including ringworm, athlete’s foot, and jock itch, are collectively called superficial mycoses. Systemic mycoses are seri- ous fungal infections of deep tissues and, by definition, are caused by organisms capable of growth at 37°C. Yeasts such as Candida albicans are commensal flora of the skin, mucous membranes, and gastrointestinal tract and are capable of growth at a wider range of tempera- tures. Intact immune mechanisms and competition for nutrients provided by the bacterial flora normally keep colonizing fungi in check. Alterations in either of these components by disease states or antibiotic therapy can upset the balance, permitting fungal overgrowth and setting the stage for opportunistic infections. Parasites In a strict sense, any organism that derives benefits from its biologic relationship with another organism is a parasite. In the study of clinical microbiology, how- ever, the term parasite has evolved to designate mem- bers of the animal kingdom that infect and cause disease in other animals and includes protozoa, helminths, and arthropods. The protozoa are unicellular animals with a complete complement of eukaryotic cellular machinery, includ- ing a well-defined nucleus and organelles. Reproduction may be sexual or asexual, and life cycles may be simple or complicated, with several maturation stages requir- ing more than one host for completion. Most are sapro- phytes, but a few have adapted to the accommodations of the human environment and produce a variety of diseases, including malaria, amebic dysentery, and giar- diasis. Protozoan infections can be passed directly from host to host through sexual contact, indirectly through contaminated water or food, or by way of an arthropod vector. Direct or indirect transmission results from the ingestion of highly resistant cysts or spores that are shed in the feces of an infected host. When the cysts reach the intestine, they mature into vegetative forms called tro- phozoites , which are capable of asexual reproduction or cyst formation. Most trophozoites are motile by means of flagella, cilia, or ameboid motion. The helminths are a collection of wormlike parasites that include the nematodes or roundworms, cestodes or tapeworms, and trematodes or flukes. The helminths reproduce sexually within the definitive host, and some require an intermediate host for the development and maturation of offspring. Humans can serve as the defini- tive or intermediate host or, in certain diseases such as trichinosis, as both. Transmission of helminth diseases occurs primarily through the ingestion of fertilized eggs (ova) or the penetration of infectious larval stages

through the skin—directly or with the aid of an arthro- pod vector. Helminth infections can involve many organ systems and sites, including the liver and lung, urinary and intestinal tracts, circulatory and central nervous systems, and muscle. Although most helminth diseases have been eradicated from the United States, they are still a major health concern of developing nations. The parasitic arthropods of humans and animals include the vectors of infectious diseases (e.g., ticks, mosquitoes, biting flies) and the ectoparasites. The ecto- parasites infest external body surfaces and cause local- ized tissue damage or inflammation secondary to the bite or burrowing action of the arthropod. The most promi- nent human ectoparasites are mites (scabies), chiggers, lice (head, body, and pubic), and fleas. Transmission of ectoparasites occurs directly by contact with imma- ture or mature forms of the arthropod or its eggs found on the infested host or the host’s clothing, bedding, or grooming articles such as combs and brushes. Many of the ectoparasites are vectors of other infectious diseases, including endemic typhus and bubonic plague. ■■ Throughout life, humans are continuously and harmlessly exposed to and colonized by a multitude of microscopic organisms.This relationship is kept in check by the intact defense mechanisms of the host (e.g., mucosal and cutaneous barriers, normal immune function) and the innocuous nature of most environmental microorganisms. ■■ The agents of infectious disease represent a diversity of microorganisms that are usually not visible to the human eye.The term infection describes the presence and injurious multiplication of an infectious agent within a human host, whereas colonization describes the act of establishing a presence, a step required in the multifaceted process of infection. ■■ Microorganisms can be separated into eukaryotes (fungi and parasites), organisms containing a membrane-bound nucleus; and prokaryotes (bacteria), organisms in which the nucleus is not separated. Both eukaryotes and prokaryotes are organisms because they contain all the biologic equipment necessary for replication and metabolism. ■■ Viruses, which are the smallest pathogens, have no organized cellular structure, but consist of a protein coat surrounding a nucleic acid core of DNA or RNA. Unlike eukaryotes and prokaryotes, viruses are incapable of replication outside of a living cell. SUMMARY CONCEPTS

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