NR283 Ch. 6

Antiseptic

reduces the number of microorganisms on the skin.

Transmission of Infectious Agents

A chain of events occurs during the transmission of infecting organisms from one person to another (Fig. 6-11). The reservoir, or source of infection, may be a person with an obvious active infection in an acute stage, or a person who is asymptomatic and shows no clinical signs or symptoms. The latter may be in the early incubation stage of infection, or the person may be a carrier of the organism and never develop infection. Hepatitis B is an example of an infection that is often transmitted by unknown carriers or persons who have a subclinical form of infection that is very mild, with few or no manifestations. The mode of transmission from the reservoir to the new host may be: •Direct contact with no intermediary, such as touching an infectious lesion or sexual intercourse. Microbes may be in the blood, body secretions, or a lesion. Not all microorganisms can cross the blood-brain barrier or placental barriers. However, some microbes that can cross the placenta have serious effects on fetal development and health. Treponema pallidum, the cause of syphilis, can lead to multiple defects or death in the fetus, and Toxoplasma gondii, the cause of toxoplasmosis, results in many neurologic deficits. •Indirect contact involving an intermediary such as a contaminated hand or food, or a fomite, an inanimate object such as instruments or bed linen that carries organisms. In some cases, there are several stages in transmission. For example, shellfish can be contaminated by human feces in the water. The microorganisms in the shellfish are then ingested and cause infection in another human. •Droplet transmission (oral or respiratory) occurring when respiratory or salivary secretions containing pathogens such as tuberculosis bacteria are expelled from the body. The organisms from these secretions may be inhaled directly by another person close by or fall on nearby objects to be transmitted indirectly. •Aerosol transmission involving small particles from the respiratory tract that remain suspended in the air and travel on air currents, infecting any new host who inhales the particles. •Vector-borne, when an insect or animal serves as an intermediary host in a disease such as malaria. Hands are considered a major culprit in spreading infection from many sources, in health care facilities, the home, office, or school.

Principles of Infection

An infection occurs when a microbe or parasite is able to reproduce in or on the body's tissues. Infectious diseases may occur sporadically in single individuals, localized groups, and epidemics or worldwide pandemics.

Types of Microorganisms Bacteria

Bacteria are unicellular (single cell) organisms that do not require living tissue to survive. They vary in size, shape, and arrangement and are classified and named accordingly. The major groups of bacteria based on cellular shape are: •Bacilli, or rod-shaped organisms, which include vibrio (curved rods) and pleomorphic (variable or indistinct shape). •Spirals, which include spirochetes and spirilla, displaying a coiled shape or "wavy line" appearance. These two classifications of bacterial shape differ in that the spirochete contains a structure called an axial filament, whereas the spirilla have flagella. Both of these structures facilitate cell movement. •Cocci, or spherical forms. Bacterial cells can further be categorized by their characteristic groupings or arrangement. •Diplo- prefix, indicating pairs •Strep(to)- prefix, indicating chains •Staph(ylo)- prefix, indicating irregular, grapelike clusters •Tetrads- refers to cells grouped in a packet or square of four cells •Palisade- refers to cells lying together with the long sides parallel The basic structure of bacteria includes: 1.An outer rigid cell wall protects the microbe and provides a specific shape (Fig. 6-3). A bacterium has one of two types of cell walls, gram-positive or gram-negative, which differ in their chemical composition. This difference can be quickly determined in the laboratory using a Gram stain and provides a means of identification and classification for bacteria. This classification is useful in selecting appropriate antimicrobial therapy; for instance, penicillin acts on the cell wall of gram-positive bacteria. 2.A cell membrane is located inside the bacterial cell wall. This semipermeable membrane selectively controls movement of nutrients and other materials in and out of the cell. Metabolic processes also take place in the cell membrane. 3.An external capsule or slime layer is found on some, but not all, bacteria. This layer is found outside the cell wall and offers additional protection to the organism. It also interferes with the phagocytosis by macrophages and WBCs in the human body. 4.One or more rotating flagellae attached to the cell wall provide motility for some species. 5.Pili or fimbriae are tiny hairlike structures found on some bacteria, usually in the gram-negative class. Pili assist in attachment of the bacterium to tissue and also in transfer of genetic material (DNA) to another bacterium, thus leading to greater genetic variation. 6.Bacteria contain cytoplasm, which contains the chromosome (composed of one long strand of DNA), ribosomes and RNA, and plasmids, which are DNA fragments that are important in the exchange of genetic information with other bacteria. 7.Some bacteria secrete toxic substances, toxins, and enzymes. Toxins consist of two types, exotoxins and endotoxins. •Exotoxins are usually produced by gram-positive bacteria and diffuse through body fluids. They have a variety of effects, often interfering with nerve conduction, such as the neurotoxin from the tetanus bacillus. Other toxins termed enterotoxins may stimulate the vomiting center and cause gastrointestinal distress. Exotoxins stimulate antibody or antitoxin production, which after being processed to reduce the toxic effect, can be used as toxoids to induce an immune response. •Endotoxins are present in the cell wall of gram-negative organisms and are released after the bacterium dies. Endotoxins may cause fever and general weakness, or they may have serious effects on the circulatory system, causing increased capillary permeability, loss of vascular fluid, and endotoxic shock. •Enzymes are produced by some bacteria and are a source of damage to the host tissues or cells. For example, hemolysin is produced by bacteria called "hemolytic streptococcus." This enzyme destroys red blood cells, as seen on a culture medium containing red blood cells. 8.Several species can form spores, a latent form of the bacterium with a coating that is highly resistant to heat and other adverse conditions (Fig. 6-4). These bacteria can survive long periods in the spore state, but they cannot reproduce when in spore form. Later, when conditions improve, the bacteria resume a vegetative state and reproduce. Tetanus and botulism are two examples of dangerous infections caused by spores in the soil entering the body, where they return to the vegetative state and reproduce. Bacteria duplicate by a simple process called binary fission (see Fig. 6-4), a division of the cell to produce two daughter cells identical to the parent bacterium. The generation time or rate of replication varies from a few minutes to many hours, depending on the particular microbe. If binary fission occurs rapidly, a large colony of bacteria can develop very quickly, and this leads to the rapid onset of infection.

Fungi

Fungi are found everywhere, on animals, plants, humans, and foods. Growth of various types of fungi can be observed easily on cheese, fruit, or bread. They are often found on dead organic material such as plants. Fungal or mycotic infection results from single-celled yeasts or multicellular molds. These organisms are classified as eukaryotic and consist of single cells or chains of cells, which can form a variety of structures (see Fig. 6-1). Fungal growth is promoted by warmth and moisture. Fungi are frequently considered beneficial because they are important in the production of yogurt, beer, and other foods, as well as serving as a source of antibiotic drugs. The long filaments or strands of a fungus are hyphae, which intertwine to form a mass called the mycelium, the visible mass. Fungi reproduce by budding, extension of the hyphae, or producing various types of spores. Spores can spread easily through the air and are resistant to temperature change and chemicals. Only a few fungi are pathogenic, causing infection on the skin or mucous membranes. Infections such as tinea pedis (athlete's foot) result from the fungus invading the superficial layers of the skin. Tinea pedis infection is often transmitted in public pools, showers, or gymnasiums. Candida is a harmless fungus normally present on the skin (Fig. 6-7). However, it may cause infection in the oral cavity (see Fig. 17-5B), sometimes called thrush in infants, and is a common cause of vaginal infection. In immunodeficient individuals, Candida frequently becomes opportunistic, causing extensive chronic infection. Histoplasma is a fungus causing a lung infection that may become disseminated through the body in immunosuppressed patients. Histoplasmosis is transmitted by inhaling contaminated dust or soil particles.

Treatment and Antimicrobial Drugs

Guidelines for effective drug therapy include: 1.The drug should be taken at regular, evenly spaced intervals over 24 hours to maintain blood levels that are adequate to control and destroy the organisms. 2.Antimicrobial drugs should be taken until the prescribed medication is completely used (usually 5 to 10 days), even if the symptoms have subsided, to ensure that the infection is completely eradicated and prevent the development of resistant organisms. 3.It is important to follow directions for administration with respect to food or fluid intake because drugs may be inactivated or drug absorption impaired if consumed with certain foods. 4.It is best to identify the specific organism and choose the most effective antibiotic that has the least effect on resident flora and human tissue. 5.Because many individuals have drug allergies, obtaining a complete drug history is essential, keeping in mind that an allergy usually includes all members of the chemically related drug group. 6.In viral infections, antiviral agents do not destroy the virus but merely inhibit its reproduction, providing an opportunity for host defenses to remove the virus. Antibacterial agents (antibiotics) are not effective against viruses. Antibacterials block synthesis of a bacterial cell wall or interfere with bacterial metabolism, but because viruses lack these components, antibacterials have no effect on them. Antibacterial drugs may be given in viral infection to reduce the risk of secondary bacterial infection in particularly vulnerable clients, but this is not a common practice. Use of an antimicrobial drug for a viral illness such as the common cold usually makes the person feel worse without any benefit.

Other Agents of Disease Helminths

Helminths or worms are not microorganisms, but are often included with microbes because they are parasites and cause infections in humans throughout the world. They are multicellular, eukaryotic organisms that are divided into many subgroups, depending on their physical characteristics. They may be very small, barely visible, or up to 1 meter in length. Their life cycle consists of at least three stages, ovum (egg), larva, and adult. The ova or larvae may be ingested in contaminated food or water, or may enter through the skin or be transmitted by infected insects. They are often found in the intestine but can inhabit the lung or blood vessels during parts of their life cycle. Helminths are usually diagnosed by observation of ova or eggs in stool specimens North America include pinworms (Fig. 6-9), hookworms, tapeworms (Fig. 6-10), and Ascaris or giant roundworms. When large numbers of worms are present in the body, systemic effects may develop, such as severe anemia.

Patterns of Infection

Infections have varied patterns as defined by their characteristics and/or location. •Local infections—organism enters the body and remains confined to a specific location •Focal infections—pathogen spreads from a local infection to other tissues •Systemic infections—infection spreads to several sites and tissue fluids, typically through the circulatory system •Septicemia—caused by multiplication of pathogens in the blood. This condition is the cause of sepsis, a toxic inflammatory condition arising from the spread of microbes •Bacteremia—presence of bacteria in the blood •Toxemia—presence of toxins in the blood •Viremia—presence of viruses in the blood •Mixed Infections—several infectious agents concurrently establish themselves at the same site •Acute infections—appear rapidly with severe symptoms but are short lived •Chronic infections—less severe symptoms than acute but persist for a long period •Primary infections—initial infection followed by complications caused by another microbe •Secondary infections—follows a primary infection and is caused by a microbe other than that causing the primary infection. Opportunistic pathogens are often the cause of a secondary infection. •Subclinical infections—does not cause and apparent signs or symptoms, although it may persist over long periods of time.

Physiology of Infection Onset and Development

Infectious agents can be present in the body for some time before any clinical signs are apparent. The microorganisms must gain entry to the body, choose a hospitable site, establish a colony, and begin reproducing (Fig. 6-13). Only if the host defenses are insufficient to destroy all the pathogens during this process will infection be established. The incubation period refers to the time between entry of the organism into the body and appearance of clinical signs of the disease. Incubation periods vary considerably, depending on the characteristics of the organism, and may last days or months. The prodromal period, which is more evident in some infections than others, follows. This is the time when the infected person may feel fatigued, lose appetite, or have a headache, and usually senses that "I am coming down with something." Next comes the acute period, when the infectious disease develops fully, and the clinical manifestations. reach a peak. The onset of a specific infection may be insidious with a prolonged prodromal period, or sudden or acute with the clinical signs appearing quickly with severe manifestations.

Example of Infection: Influenza (Flu)

Influenza is a viral infection that may affect both the upper and the lower respiratory tracts. Annually on average 10% to 20% of the population is affected in North America. Although the influenza infection itself may be mild, it is frequently complicated by secondary bacterial infections such as pneumonia. The mortality rate from complications can be high, particularly in those older than 65 years and those with chronic cardiovascular or respiratory disease. The influenza viruses are classified as RNA viruses of the myxovirus group. There are three subgroups of the influenza virus—type A, the most prevalent pathogen, type B, and type C. Types A and B cause epidemics and pandemics that tend to occur in cycles. The influenza virus, particularly type A, is difficult to control because it undergoes frequent mutations leading to antigenic shifts or variations. se older than 65 may have some immunity to the virus from earlier outbreaks of similar viruses. The H1N1 influenza virus is genetically and antigenically similar to the virus that caused the Spanish flu pandemic in 1918. It contains genetic material from avian, pig, and human influenza types and is expected to mutate rapidly. The designation H1N1 refers to the specific type of antigens on the viral capsule. The vaccine may be administered as an intranasal spray (live vaccine) or intramuscular injection (inactivated or killed). It is now recommended that all individuals be immunized annually between November and February. For many health care providers immunization is a condition of employment. The vaccine that remains effective from 2 to 4 months reduces the severity of the infection in cases in which it does not provide total prevention. The influenza virus was first isolated and identified in 1933. It is transmitted directly by respiratory droplet or indirectly by contact with a contaminated object. The virus can survive at room temperature as long as 2 weeks. Influenza usually has a sudden, acute onset with fever and chills, marked malaise, headache, general muscle aching, sore throat, unproductive or dry cough, and nasal congestion. The infection is often self-limiting, although fatigue may persist for several weeks afterward. Continued fever or other signs usually indicate complications, such as the development of bacterial pneumonia. Certain antiviral drugs such as oseltamivir (Tamiflu) if given promptly, may reduce the symptoms in some cases. The CDC recommended the use of oseltamivir (Tamiflu) or zanamivir (Relenza) for the 2008 to 2009 flu season.

Control of Transmission and Infection

Isolation of infected persons is rarely carried out on a large scale, and there are fewer diseases that must be reported to government bodies. It is not feasible to test every client or patient for the presence of infection before initiating care. Universal precautions provide the basic guidelines by which all blood, body fluids, and wastes are considered "infected" in any client regardless of the client's apparent condition. To break the cycle and minimize the risk of infection: •The reservoir or sources of infection must be located and removed. Sources and contacts must be identified in some situations, especially when asymptomatic carriers may be involved, or when travelers may be infected: •Contaminated food or water or carrier food handlers should be identified to prevent continued transmission or epidemics of infectious disease. •In some cases, infection can be transmitted before clinical signs are evident in the infected person, and this permits widespread contamination if the incubation period is prolonged. •Infected travelers should refrain from travel to prevent spreading infectious diseases into new areas, and travelers who become ill should seek prompt health care and share their specific travel history with health care workers. •The portal of exit (secretions, e.g., blood, saliva, urine) of microbes from the reservoir should be blocked. •Knowledge of the mode (droplet, fecal-oral) or modes of transmission of specific infections is essential to block transmission. Precautions must be undertaken in a prescribed manner; for example, the use of appropriate condoms following recommended guidelines is essential to prevent the spread of sexually transmitted disease during intimate sexual activity. Using disposable equipment, proper sterilization and cleaning, good ventilation, and frequent handwashing are some ways to reduce transmission: •Portals of entry and exit should be blocked by covering the nose and mouth with a mask and placing barriers over breaks in the skin or mucous membranes. •Reduce host susceptibility (increase host resistance) by maintaining immunizations and boosters according to guidelines. Proper nutrition to maintain skin and mucous membranes is also essential in reducing host susceptibility. Additional techniques to reduce transmission include: 1.Adequate cleaning of surroundings and clothing. 2.Sterilization of fomites by exposure to heat using several methods, such as autoclaving. Time, packaging, and temperature are critical to success. Moist heat is preferable, because it penetrates more efficiently and can destroy microbes at lower temperatures. Incineration (burning) and autoclaving are also effective methods of destroying microbes in waste. 3.Disinfectants are chemical solutions that are known to destroy microorganisms or their toxins on inanimate objects. The literature on these solutions must be carefully checked to determine the limitations of the specific chemicals as well as the instructions for use. 4.Antiseptics are chemicals applied to the body that do not usually cause tissue damage, such as isopropyl alcohol-70%, which is the active ingredient in hand sanitizers.

Mode of Action

Most antibacterial drugs may act in one of four ways: 1.Interference with bacterial cell wall synthesis is a bactericidal mechanism and is seen in drugs such as penicillin (see Fig. 6-3). Large doses of such drugs are usually safe in humans because human cells lack cell walls and are not directly affected by the drug. 2.A second mechanism is to increase the permeability of the bacterial cell membrane, allowing leakage of bacterial cell contents; this mechanism is exemplified by polymyxin. 3.Some drugs, such as tetracycline, interfere with protein synthesis and cell reproduction. These have significant effects on the developing fetus and young child. 4.Another group, including the sulfonamides, interferes with the synthesis of essential metabolites. The common problems with antibacterial drugs are allergic reactions, both mild and severe, and digestive tract discomfort. Penicillin and its related compounds may cause anaphylaxis. Digestive tract discomfort may result from irritation of the stomach or the change in the intestinal resident flora caused by the antibacterial action, often leading to diarrhea. Secondary infections, particularly fungal, may develop as the balance of resident flora is disturbed. One drug, cefotaxime, is a third-generation cephalosporin and is related to the penicillin family. It has been developed to be more active against gram negative microbes and multi-drug resistant organisms. This drug can pass through the blood-brain barrier, thus it is more effective in treating some forms of meningitis as well. Antiviral agents decrease the reproduction of viruses inside the host cell but cannot destroy the virus. They control but do not cure infection. In some cases the drugs are effective only against actively replicating viruses, not against those in the latent stage. These drugs may interfere with attachment of the virus to the host cell, with the shedding of the protein coat, with the action of enzymes such as reverse transcriptase required for synthesis of DNA and RNA or with protein synthesis. The drugs may be virus specific; for example, acyclovir is effective against herpes simplex viruses. Antiviral drugs tend to have significant adverse effects on the host because they alter viral interaction within the host cell. Antifungal agents may interfere with mitosis in fungi (e.g., griseofulvin), or they may increase fungal membrane permeability; amphotericin B may be administered intravenously for systemic infections. Antiprotozoal agents have a similar characteristic to the antifungal agents in that the targets are eukaryotic cells and can be toxic to human cells. Many pathogenic protozoa also have several stages in their life cycles that require treatment with different agents at different stages. Antihelminthic agents have a variety of modes of action. These agents share the same drawback as the antifungal and antiprotozoal agents as they are attacking eukaryotic organisms. Some are designed to suppress a metabolic process that is more important to the helminth than the host while others inhibit the movement of the worm and/or prevent it from remaining in the specific organ.

Microorganisms

Microbiology refers to the study of microorganisms or microbes, very small living forms that are visible only with a microscope. Microorganisms include bacteria, fungi, protozoa, and viruses. Bacteria are classified as prokaryotic cells because they are very simple in structure—lacking even a nuclear membrane—but they function metabolically and reproduce. They also have a complex cell wall structure. By comparison, eukaryotic cells are nucleated cells found in higher plants and animals, including humans. They lack cell walls (except in plants) but their DNA is enclosed in a nuclear membrane and the cell membrane has a complex structure. Pathogens are the disease-causing microbes often referred to as "germs." Infectious diseases result from invasion of the body by microbes and multiplication of these microbes, followed by damage to the body. The need for oxygen, carbohydrates, a specific pH or temperature, or a living host depends on the needs of the particular microbe.

Virulence and Pathogenicity of Microorganisms

Pathogenicity refers to the capacity of microbes to cause disease. Nonpathogens can become pathogens. When a member of the resident flora is introduced into another area of the body, it may become an opportunistic pathogen. For example, if Escherichia coli from the colon enter the urinary tract, they will cause infection. (This microbe is the most common cause of cystitis.) Virulence is the degree of pathogenicity of a specific microbe, based on: •Invasive qualities, allowing it to directly damage host cells and tissues and spread •Toxic qualities, including production of enzymes, exotoxins, and endotoxins that damage host cells or interfere with a host function such as nerve conduction •Adherence to tissue by pili, fimbriae, capsules, or specific membrane receptor sites. Certain organisms tend to establish infection in particular areas of the body, considered hospitable to that microbe; for example, streptococci are common in respiratory and ear infections. •Ability to avoid host defenses (e.g., the presence of a capsule or mutation with altered antigenicity). Microorganisms undergo frequent mutation. Slight changes in the organism may occur spontaneously or in response to environmental conditions, including the presence of drugs. When bacteria or viruses mutate, antibodies that matched the earlier form are no longer effective, so the individual is no longer protected. Vaccines or drugs are unlikely to be effective against the new form. This is why a new influenza vaccine must be developed and administered each year. Virulence is often expressed in the case fatality rate, the percentage of deaths occurring in the number of persons who develop the disease. In parasitic infections host resistance and the ability of a microbe to cause disease often coexist in a delicate balance.

Other Agents of Disease Prions

Prions are protein-like agents that are transmitted by consumption of contaminated tissues such as muscle or the use of donor tissues contaminated with the protein. There is a great deal that is not known about prion disorders and some researchers question whether prions are actually the agent of diseases. A prion is an abnormal molecule that is transmissible in tissues or blood of animals or humans. It induces proteins within the brain of the recipient to undergo abnormal folding and change of shape. This renders the protein molecule non-functional and causes degenerative disease of the nervous system. Prion diseases in humans include Creutzfeldt-Jakob disease and variant Creutzfeldt-Jakob disease. these are rapidly progressive and fatal. It is thought that variant Creutzfeldt-Jakob disease is caused by consumption of meat that has been contaminated with nervous tissue from an infected animal such as beef cattle. In areas where bovine spongiform encephalopathy (BSE), the animal prion infection, is prevalent, consumption of ground meats, sausages, or offal should be avoided.

Protozoa

Protozoa are eukaryotic or more complex organisms. They are unicellular, motile, and lack a cell wall, but occur in a number of shapes, sometimes within the life cycle of a single type. Many live independently, some live on dead organic matter, and others are parasites living in or on another living host. As in other microbial classifications, protozoa are divided into a number of subcategories. The pathogens are usually parasites. Some diseases caused by protozoan infection include trichomoniasis, malaria, and amebic dysentery. Trichomonas vaginalis is distinguished by its flagella (Fig. 6-8A). It causes a sexually transmitted infection of the reproductive tracts of men and women, attaching to the mucous membranes and causing inflammation. The causative agents for malaria, the Plasmodium species, belong to a group of nonmotile protozoa called sporozoa. Plasmodium vivax is found in temperate climates such as the southern United States (Fig. 6-8B). Clinically these microbes are found in the red blood cells, where they undergo several stages in their life cycle. The red blood cells become large and eventually rupture and release new microbes and toxins into the blood, causing acute illness. Transmitted by a blood-sucking insect, the female Anopheles mosquito. One form of malarial parasite, Plasmodium falciparum, is extremely virulent and has become resistant to almost all antimalarial drugs. It is expected that global warming will put more of the world's population at risk of malaria in the future as the Anopheles mosquito extends its range and infects non immune individuals. The amebas are a motile group of protozoa, moving by extending part of their cytoplasm and flowing forward (ameboid movement). They engulf food in the same manner. The important pathogen in this group is Entamoeba histolytica, a parasite in the large intestine that causes amebic dysentery, a severe form of diarrhea, and liver abscesses if it penetrates into the portal circulation. These organisms exist in two forms. One form is actively pathogenic and is termed the trophozoite. Trophozoites secrete proteolytic enzymes, which break down the intestinal mucosa, causing flask-shaped ulcers. Trophozoites may invade blood vessels and spread to other organs, such as the liver. The organism also forms cysts, which are resistant to environmental conditions and are excreted in feces. Entamoeba histolytica infection is spread by the fecal-oral route.

Host Resistance

The healthy individual is quite resistant to infection. With some infections, such as tuberculosis, host resistance is a primary factor in determining the risk of active infection following exposure (Box 6-1). interferons are proteins produced by human host cells in response to viral invasion of the cell. These interferons then influence the activity of nearby host cells, increasing their resistance to viral invasion and interfering with viral replication. Interferons also stimulate the immune system and are used in cancer treatment for this reason. Unfortunately, they have not proved to be as beneficial in the widespread treatment of cancer or other immune-based diseases as expected. Factors that decrease host resistance include: •Age (infants and the elderly) •Genetic susceptibility •Immunodeficiency of any type •Malnutrition •Chronic disease, including cardiovascular disease, cancer, and diabetes •Severe physical or emotional stress •Inflammation or trauma affecting the integrity of the skin or mucosa, including burns, lack of protective secretions, bladder catheters, or other invasive procedures. Sometimes infection occurs easily because of a very small break in the skin or mucous membrane, or in an area of inflammation. •Impaired inflammatory response, for example, due to long-term glucocorticoid medication •Severe or multiple infections are very common in homeless individuals, in whom multiple factors decrease host resistance.

Signs and Symptoms of Infection Local Signs

The local signs of infection are usually those of inflammation: pain or tenderness, swelling, redness, and warmth (Fig. 6-14). If the infection is caused by bacteria, a purulent exudate, or pus, is usually present, whereas a viral infection results in serous, clear exudates. The color and other characteristics of the exudates and tissue may help to identify the microorganism. Other local signs depend on the site of infection. For instance, in the respiratory tract, local signs probably include coughing or sneezing and difficulty in breathing. In the digestive tract, local signs might include vomiting or diarrhea.

New Issues Affecting Infections and Transmission

The severe acute respiratory syndrome (SARS) epidemic in the Toronto area was well established before information about cases occurring in travelers from Southeast Asia was received. In some cases the incubation period is so short that it is difficult to prevent an epidemic even if health statistics are collected, for example, in cholera infections. In such situations the focus must be on preventing the spread of infection to the wider community. Increased global travel, changing environments and global weather patterns, and changes in food and water supplies are some of the factors leading to altered disease patterns. They also update the list of notifiable diseases, approximately 60 diseases that must be reported to public health agencies. The CDC reports are published in Morbidity and Mortality Weekly Report. The United Nations (UN) has also assumed a role in a number of global issues related to infectious diseases such as AIDS, tuberculosis, and malaria. The other issue to be addressed is the increasing number of microbes that are resistant to several drug groups, thus making infection control much more difficult. The multi-drug resistant microbes include strains of Mycobacterium tuberculosis, Plasmodium falciparum, Streptococcus pneumoniae, Haemophilus influenzae, Staphylococcus aureus, and Neisseria gonorrhoeae. Currently there is much more emphasis on reduced use of antibacterial drugs to treat minor infections or as prophylactics to lessen the problem.

Viruses

There are several types of viruses, many of which include numerous subtypes. Table 6-1 lists some types of viruses and common pathogens causing disease in humans. A virus is a very small obligate intracellular parasite that requires a living host cell for replication. The need of viruses for living tissue complicates any laboratory procedure to grow or test viruses. When it is extracellular, a virus particle is called a virion. It consists of a protein coat, or capsid, and a core of either DNA or RNA (see Fig. 6-1B). The protein coat comes in many shapes and sizes and undergoes change relatively quickly in the evolution of the virions. The nucleic acid content and its form provide methods of classification of viruses. A retrovirus such as HIV, the human immunodeficiency virus, contains RNA only, plus an enzyme to convert RNA into DNA, a process activated when the virus enters the host cell. Most viruses contain DNA. Some viruses have an additional outer protective envelope. When a virus infects a person, it attaches to a host cell, and the viral genetic material enters the cell. Viral DNA or RNA takes over control of the host cell, using the host's capacity for cell metabolism to synthesize protein, producing many new viral components. Some viruses remain in a latent stage; they enter host cells and replicate very slowly or not at all until sometime later. Viruses can also insert their capsid proteins into the cell membrane of the host cells; these cells are then recognized as viral invaders and are attacked by the body's immune system. Frequently one type of virus exists in many similar forms or strains, and viruses tend to mutate, or change slightly, during replication (e.g., the cold or influenza viruses). Some viruses such as the influenza virus are composed of nucleic acids from differing viral strains in animals and humans. Influenza H1N1 has components from both swine influenza and human influenza; these mixtures can change rapidly, leading to new combinations.

Chlamydiae, Rickettsiae, and Mycoplasmas

These three groups of microorganisms have some similarities to both bacteria and viruses (Table 6-2). They replicate by binary fission, but they lack some basic component; therefore they require the presence of living cells for reproduction. •Chlamydiae are considered very primitive forms related to bacteria that lack many enzymes for metabolic processes. They exist in two forms. One, the elementary body (EB) is infectious, possessing a cell wall and the ability to bind to epithelial cells. The other form, the reticulate body (RB) is noninfectious, but uses the host cell to make ATP and reproduce as an obligate intracellular organism (Fig. 6-6A). After large numbers of new microbes are produced inside the host cells, the new RBs change into EBs, rupturing the host cells' membranes and dispersing to infect more cells. Chlamydial infection is a common sexually transmitted disease that causes pelvic inflammatory disease and sterility in women. Infants born to infected mothers may develop eye infections or pneumonia. •Rickettsiae are tiny gram-negative bacteria that live inside a host cell (obligate intracellular parasites). They are transmitted by insect vectors, such as lice or ticks, and cause diseases such as typhus fever and Rocky Mountain spotted fever. They attack blood vessel walls, causing a typical rash and small hemorrhages. •Mycoplasmal infection is a common cause of pneumonia. These microbes lack cell walls—therefore are not affected by many antimicrobial drugs—and they can appear in many shapes. They are the smallest cellular microbe.

Mutation

a change in the genetic makeup (DNA) of a cell, which will be inherited.

Disinfectant

a chemical that may destroy or inhibit the growth and reproduction of microorganisms.

Leukopenia

a decreased number of leukocytes in the blood.

Neutropenia

a deficit of neutrophils in the blood.

Lymphadenopathy

a disease affecting the lymph nodes.

Epidemic

a disease occurring in higher numbers than usual in a certain population within a given time period.

Endemic

a disease that is always present in a specific region.

Pathogen

a disease-causing microorganism.

Toxin

a substance that can harm the body or interfere with its function; poisonous.

Leukocytosis

an above-normal number of leukocytes (WBCs) in the blood.

Nosocomial

an infection acquired while hospitalized.

Parasite

an organism that lives on or in another living organism.

Other Agents of Disease Algae

are eukaryotic microorganisms widespread in fresh and marine waters; they are a main component of plankton and are usually not a concern for human disease. Medical concerns involving algae include: human consumption of marine animals that have fed on algae and accumulated toxins produced by the algae and recent disorders attributed to the algae Pfiesteria piscicida.

Nosocomial infections

are infections that occur in health care facilities, including hospitals, nursing homes, doctors' offices, and dental offices. The CDC estimates that 10% to 15% of patients acquire an infection in the hospital. Reasons for these infections include the presence of many microorganisms in these settings, patients with contagious diseases, overcrowding, use of contaminated instruments, immunocompromised and weakened patients, the chain of transmission through staff, diagnostic procedures, and equipment, therapeutic aids, and food trays. Also, many microbes in health. Common nosocomial infections include urinary tract infections (the highest number), pneumonia, diarrhea, and surgical wound infection. Most infections in health care facilities are spread by direct contact between persons or contaminated objects. Recently there have been several outbreaks of infection in hospitals by a more dangerous strain of the bacterium, Clostridium difficile (c-diff), particularly in intensive care units where most individuals are taking antimicrobial drugs. The resulting disruption of normal flora allows C. difficile to multiply and cause severe diarrhea and many deaths. Methicillin-resistant Staphylococcus aureus (MRSA) infections are also increasingly seen as a source of nosocomial infection that is very challenging to treat (this is in the community, not just the hospital).

Methods of Diagnosis

culture & staining blood cultures Frequently drug sensitivity tests, such as the Kirby Bauer method (Disc Diffusion Method) and the Minimum Inhibitory Concentration (MIC) method (Fig. 6-15), are also instituted. •Blood tests, particularly variations in the numbers of leukocytes, are another general indicator of infection. With bacterial infections, leukocytosis, or an increase in white blood cells, is common, whereas viral infections often cause leukopenia, a reduction in the number of leukocytes in the blood. Changes in the distribution of types of leukocytes occur as well (differential count), depending on the organism, for example, monocytosis or neutropenia. Neutrophils tend to increase with acute infections, but lymphocytes and monocytes increase with chronic infection. C-reactive protein and erythrocyte sedimentation rate are usually elevated and are a general indicator of inflammation. •In addition, radiologic examination may be used to identify the site of the infection and may assist in the identification of the agent. For example, lung congestion localized in one lobe (consolidation) usually indicates pneumococcal pneumonia.

Culture

growth of microorganisms on a specific nutritious medium in a laboratory.

Pili

hair-like appendages on some bacteria for adhesion to tissue and transfer of DNA.

Systemic signs

include signs and symptoms common to significant infections in any area of the body. Fever, fatigue and weakness, headache, and nausea are all commonly associated with infection. The characteristics of fever (pyrexia) may vary with the causative organism. The body temperature may be very high or spiking and may be accompanied by chills (see Chapter 5), or it may be elevated only slightly. In some viral infections the temperature is subnormal. With severe infection the nervous system may be affected, resulting in confusion or disorientation, seizures (convulsions), or loss of consciousness.

Classification

•Antibiotic is an older term and can be misleading. Antibiotics are drugs derived from organisms, such as penicillin from mold. Now many drugs are synthetic. •Antimicrobials may be classified by the type of microbe against which the drug is active, such as antibacterials, antivirals, and antifungals. These drugs are unique to the type of organism and are not interchangeable. •Bactericidal refers to drugs that destroy organisms, whereas bacteriostatic applies to drugs that decrease the microbe's rate of reproduction and rely on the host's defenses to destroy the organisms. •Broad spectrum refers to antibacterials that are effective against both gram-negative and gram-positive organisms; narrow spectrum agents act against either gram-negative or gram-positive organisms, but not both. Narrow spectrum drugs are often preferred because they are less likely to upset the balance of resident flora in the body, which may result in an overgrowth of one organism and secondary or superinfection. For example, after a prolonged course of tetracycline, clients may develop a fungal (Candida) infection in the mouth, and women may develop vaginal candidiasis. •The terms first-generation and second-generation drug now appear in texts, first generation referring to the original drug class, second generation referring to a later, improved version of the same drug group.