Porth's Essentials of Pathophysiology, 4e

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Infection and Immunity

U N I T 4

Portal of Entry Microbes can enter the host by direct contact, ingestion, and inhalation. The portal of entry does not dictate the site of infection. Ingested pathogens may penetrate the intestinal mucosa, disseminate through the circulatory system, and cause diseases in other organs such as the lung or liver. Whatever the mechanisms of entry, the transmission of infectious agents is directly related to the number of infectious agents absorbed by the host. Penetration. Any disruption in the integrity of the body’s surface barrier—skin or mucous membranes— is a potential site for invasion of microorganisms. The break may be the result of an accidental injury causing abrasions, burns, or penetrating wounds; medical pro- cedures such as surgery or catheterization; or a primary infectious process such as chickenpox or impetigo that produces surface lesions. Direct inoculation from intra- venous drug use or an animal or arthropod bite also can occur. Direct Contact. Some pathogens are transmitted directly from infected tissue or secretions to exposed, intact mucous membranes. This is especially true of certain sexually transmitted infections (STIs) such as gonorrhea, syphilis, chlamydia, and genital herpes, for which exposure of uninfected membranes to pathogens occurs during intimate contact (see Chapter 41). The transmission of STIs is not limited to sexual con- tact. Vertical transmission of these agents, from mother to child, can occur across the placenta or during birth when the mucous membranes of the child come in con- tact with infected vaginal secretions of the mother. When an infectious disease is transmitted from mother to child during gestation or birth, it is classified as a congeni- tal infection. The most frequently observed congenital infections include toxoplasmosis (caused by the para- site Toxoplasma gondii ), syphilis, rubella, cytomegalo- virus infection, and herpes simplex virus infections (the so-called TORCH infections, discussed in Chapter 6); varicella-zoster (chickenpox); parvovirus B19; group B streptococci ( Streptococcus agalactiae ); and HIV. Of these, cytomegalovirus is by far the most common cause of congenital infection in the United States, affecting nearly 1% of all newborns. However, with more than 500,000 babies contracting HIV from their mother and with a 25% to 30% chance of vertical transmission without appropriate antiretroviral treatment, HIV is rapidly gaining in stature as a congenitally transmitted infection (see Chapter 16). Ingestion. The entry of pathogenic microorganisms or their toxins through the oral cavity and gastrointesti- nal tract represents one of the more efficient means of disease transmission in humans. Many bacterial, viral, and parasitic infections, including cholera, typhoid fever, dysentery (amebic and bacillary), food poisoning, traveler’s diarrhea, cryptosporidiosis, and hepatitis A, are initiated through the ingestion of contaminated food and water. This mechanism of transmission necessitates

SUMMARY CONCEPTS (continued)

Epidemiology of Infectious Diseases Epidemiologists working in infectious disease study the factors, events, and circumstances that influence the trans- mission of infectious diseases among humans. The ulti- mate goal of epidemiologists is to devise strategies that interrupt or eliminate the spread of infectious agents. To accomplish this, they classify infectious diseases accord- ing to incidence, portal of entry and source, symptoms, disease course, site of infection, and virulence factors so that potential outbreaks may be predicted and averted or appropriately treated. Mechanisms ofTransmission The outcomes of infections depend on the ability of microbes to breach host barriers and colonize and damage host tissues. Host barriers to infection prevent the microbes from entering the body and assist innate and adaptive immune defenses in eliminating the agent (see Chapter 15). ■■ Parasites (protozoa, helminths, and arthropods) are members of the animal kingdom that infect or colonize other animals, which then transmit them to humans. In some cases, they directly infect the human host. ■■ Bacteria are autonomously replicating unicellular organisms known as prokaryotes because they lack an organized nucleus. Most bacteria produce a cell wall that is produced only by prokaryotes and is therefore an attractive target for antibacterial therapy. ■■ The Rickettsiaceae, Anaplasmataceae, Chlamydiaceae, and Coxiella are obligate intracellular pathogens, like the viruses, but produce a rigid peptidoglycan cell wall, reproduce asexually by cellular division, and contain RNA and DNA, similar to the bacteria. Chlamydial diseases include sexually transmitted genital infections, neonatal and adult conjunctivitis, and infant pneumonia. ■■ Fungi are free-living, eukaryotic saprophytes, some of which are members of the normal human microflora. Fortunately, few fungi are capable of causing diseases in humans, and most of these are incidental, self-limited infections of skin and subcutaneous tissue. Despite their normally harmless nature, fungi can cause life-threatening opportunistic diseases when host defense capabilities have been disabled.