Introduction and Epidemiology - Part 2 PDF

CHAIN OF INFECTION Transmission occurs when the agent leaves its reservoir or host through a portal of exit, is conveyed by some mode of transmission, and enters through an appropriate...

CHAIN OF INFECTION Transmission occurs when the agent leaves its reservoir or host through a portal of exit, is conveyed by some mode of transmission, and enters through an appropriate portal of entry to infect a susceptible host. This sequence is sometimes called the chain of infection. RESERVOIRS OF INFECTION The reservoir of an infectious agent is the habitat in which the agent normally lives, grows, and multiplies. Reservoirs include humans, animals, and the environment. The reservoir may or may not be the source from which an agent is transferred to a host. For example, the reservoir of Clostridium botulinum is soil, but the source of most botulism infections is improperly canned food containing C. botulinum spores. RESERVOIRS OF INFECTION - HUMAN RESERVOIRS Many common infectious diseases have human reservoirs. Diseases that are transmitted from person to person without intermediaries include the sexually transmitted diseases, measles, mumps, streptococcal infection, and many respiratory pathogens Human reservoirs may or may not show the effects of illness Asymptomatic or passive or healthy carriers - never experience symptoms despite being infected. Incubatory carriers - can transmit the agent during the incubation period before clinical illness begins. Convalescent carriers - recovered from illness but remain capable of transmitting to others. Chronic carriers are those who continue to harbor a pathogen for months or even years after their initial infection. RESERVOIRS OF INFECTION - ENVIRONMENTAL RESERVOIRS Plants, soil, and water in the environment are also reservoirs for some infectious agents. Many fungal agents, such as those that cause histoplasmosis, live and multiply in the soil. Outbreaks of Legionnaires disease are often traced to water supplies in cooling towers and evaporative condensers, reservoirs for the causative organism Legionella pneumophila. RESERVOIRS OF INFECTION - ANIMAL RESERVOIRS Humans are also subject to diseases that have animal reservoirs, diseases are transmitted from animal to animal, with humans as incidental hosts. Long recognized zoonotic diseases include: brucellosis (cows and pigs), anthrax (sheep), plague (rodents), trichinellosis/trichinosis (swine), tularemia (rabbits), and rabies (bats, raccoons, dogs, and other mammals). Zoonoses newly emergent in North America include West Nile encephalitis (birds), and monkeypox (prairie dogs). Many newly recognized infectious diseases in humans, including HIV/AIDS, Ebola infection and SARS, are thought to have emerged from animal hosts, although those hosts have not yet been identified. DISEASE RESERVOIRS AND EPIDEMICS Zoonosis refers to an infectious disease that is transmissible under natural conditions from vertebrate animals to humans. There are over 200 known types of zoonoses Zoonoses comprise a large percentage of new and existing diseases in humans Control of a zoonotic disease in the human population may not eliminate the disease as a potential public health problem Certain infectious diseases have complex life cycles involving an obligate transfer from a nonhuman host to humans followed by transfer back to the nonhuman host PORTALS OF ENTRY (GETTING IN) Portal of entry organisms can enter the body Mucous Membranes Skin The Parenteral Route ENTRY INTO HOST 1. Portals of Entry A. Mucous membranes (moist mucosa) -Most common route for most pathogens Colonize host tissues that are in contact with the external environment Pathogen develops tissue adherence mechanism & ability to overcome/withstand constant pressure of the host defense at the surface -Entry through mucous membranes: 1. Respiratory tract (most common) 2. Gastrointestinal tract 3. Urinary/genital tracts In total, there are about 400 square meters of mucous membrane surface area in the human body. PORTALS OF ENTRY Respiratory tract The easiest and the most frequent traveled portal of entry Breathing Small dust, moisture droplet Disease Colds, smallpox, pneumonia, tuberculosis, influenza, measles PORTALS OF ENTRY Gastrointestinal tract Through food, water and contaminated fingers Eliminated through faeces and transmitted to other hosts Inhospitable environment for microorganism Can m/o still survive? Hydrochloric acid and enzymes in stomach Bile and enzymes in the small intestine Preferred entry point for Hepatitis A virus, the parasite Giardia, the bacterium Vibrio cholerae (dysentery), Salmonella Helicobacter pylori- risk factor for the gastroduodenal ulcers Common disease derived from microbes that enter in this way Poliomyelitis, hepatitis A, typhoid fever, amoebic dysentery, giardiasis, shigellosis (bacillary dysentery), and cholera PORTALS OF ENTRY Urinary/genital tracts Are more common in women Anatomical relationship between the anus and urethra, much closer Sexually transmitted infections (STIs) Penetrates an unbroken mucous Genital warts and HIV Chlamydia Herpes Syphilis Gonorrhea Non-STI Enters through cuts, abrasion Sexual activity- tiny tears in the tissue As a portal of exit B. SKIN (KERATINIZED CUTANEOUS MEMBRANE) - Unless broken, skin is usually an impermeable barrier to microbes - Many are harmless - Can infect opening on the skin - Hair follicles and sweat glands ducts - E.g. larvae of the hookworm bore through intact skin - Some fungi grow on the keratin in skin/infect the skin - Few can colonize surface C. PARENTERAL ROUTE A break in the barrier Allow the enter of pathogen into the tissues beneath the skin. Penetrate skin: injections, bites, cuts, surgery, etc. Eg. Plasmodium, malaria, tetanus, gangrene Deposit organisms directly into deeper tissues Preferred portal of entry to cause disease C. PARENTERAL ROUTE 2 layers-epidermis and dermis Epidermis dead cells, no access of blood Dermis blood vessel, wound can cause systemic infection Gain access to the epidermis, result in localized infections Enterthe dermis can cause systemic problems PREFERRED PORTALS OF ENTRY After microbes enter human body, they do not necessarily cause disease. Pathogenic when entry are done through preferred portals of entry. Example: Salmonella enterica serovar Typhi (also known as Salmonella typhi) - intestinal infections streptococci - respiratory tract infection Many can use multiple entry points PORTAL OF EXIT (GETTING OUT) The Portal of Exit refers to any route that the pathogen can leave the reservoir. This depends entirely on the characteristics of the reservoir. In humans, the main portals of exit include: Alimentary: vomiting, diarrhea, saliva Genitourinary: sexual contact Respiratory: secretions from coughing, sneezing, or talking Skin: open wounds PORTAL OF EXIT (GETTING OUT) The portal of exit usually corresponds to the site where the pathogen is localized. For example, influenza viruses and Mycobacterium tuberculosis exit the respiratory tract Schistosomes through urine Vibrio cholerae through feces Sarcoptes scabiei in scabies skin lesions Enterovirus 70, a cause of hemorrhagic conjunctivitis, in conjunctival secretions. Some bloodborne agents can exit by crossing the placenta from mother to fetus (rubella, syphilis, toxoplasmosis), while others exit through cuts or needles in the skin (hepatitis B) or blood-sucking arthropods (malaria). INFECTIOUS DISEASE TRANSMISSION Epidemiologists follow transmission of a disease by correlating geographic, climatic, social, and demographic data Used to identify possible modes of transmission For example, a disease limited to a tropical area may suggest something about its vector (e.g., malaria) INFECTIOUS DISEASE TRANSMISSION Pathogens can be classified by their mechanism of transmission, but all mechanisms have the following stages in common: Escape from host Travel Entry into new host Pathogen transmission can be direct or indirect MODES OF DISEASE TRANSMISSION An infectious agent may be transmitted from its natural reservoir to a susceptible host in different ways. Direct (Infected individual transmits a disease directly to a susceptible host without the assistance of an intermediary) Direct contact (e.g., flu, common cold, STDs, ringworm) Droplet spread Indirect (Occurs when transmission is facilitated by a living or nonliving agent) Airborne Vehicle-borne Vector-borne (mechanical or biologic) MODES OF DISEASE TRANSMISSION - DIRECT Direct contact Skin-to-skin contact, kissing, and sexual intercourse Also refers to contact with soil or vegetation harboring infectious organisms. E.g. infectious mononucleosis (“kissing disease”) and gonorrhea are spread from person to person by direct contact. Hookworm is spread by direct contact with contaminated soil. MODES OF DISEASE TRANSMISSION - DIRECT Droplet spread refers to spray with relatively large, short-range aerosols produced by sneezing, coughing, or even talking. Classified as direct because transmission is by direct spray over a few feet, before the droplets fall to the ground. Pertussis and meningococcal infection are examples of diseases transmitted from an infectious patient to a susceptible host by droplet spread. MODES OF DISEASE TRANSMISSION - INDIRECT Airborne transmission occurs when infectious agents are carried by dust or droplet nuclei suspended in air. Airborne dust includes material that has settled on surfaces and become resuspended by air currents as well as infectious particles blown from the soil by the wind. Droplet nuclei are dried residue of less than 5 microns in size, may remain suspended in the air for long periods of time, may be blown over great distances. Measles, for example, has occurred in children who came into a physician’s office after a child with measles had left, because the measles virus remained suspended in the air MODES OF DISEASE TRANSMISSION - INDIRECT Vehicles may indirectly transmit an infectious agent include food, water, biologic products (blood), and fomites (inanimate objects such as handkerchiefs, bedding, or surgical scalpels). A vehicle may passively carry a pathogen — as food or water may carry hepatitis A virus. Alternatively, the vehicle may provide an environment in which the agent grows, multiplies, or produces toxin — as improperly canned foods provide an environment that supports production of botulinum toxin by Clostridium botulinum. MODES OF DISEASE TRANSMISSION - INDIRECT Vectors Vectors such as mosquitoes, fleas, and ticks may carry an infectious agent through purely mechanical means or may support growth or changes in the agent. Examples of mechanical transmission are flies carrying Shigella on their appendages and fleas carrying Yersinia pestis, the causative agent of plague, in their gut. In contrast, in biologic transmission, the causative agent of malaria or guinea worm disease undergoes maturation in an intermediate host before it can be transmitted to humans. HOST Susceptibility of a host depends on genetic or constitutional factors, specific immunity, and nonspecific factors that affect an individual’s ability to resist infection or to limit pathogenicity. An individual’s genetic makeup may either increase or decrease susceptibility. E.g. persons with sickle cell trait seem to be at least partially protected from a particular type of malaria. HOST – CONT’D Specific immunity - protective antibodies that are directed against a specific agent. May develop in response to infection, vaccine, or toxoid (toxin that has been deactivated but retains its capacity to stimulate production of toxin antibodies) or may be acquired by transplacental transfer from mother to fetus or by injection of antitoxin or immune globulin. Nonspecific factors that defend against infection include the skin, mucous membranes, gastric acidity, cilia in the respiratory tract, the cough reflex, and nonspecific immune response. Factors that may increase susceptibility to infection by disrupting host defenses include malnutrition, alcoholism, and disease or therapy that impairs the nonspecific immune response. IMPLICATIONS FOR PUBLIC HEALTH Knowledge of the portals of exit and entry and modes of transmission provides a basis for determining appropriate control measures. In general, control measures are usually directed against the segment in the infection chain that is most susceptible to intervention, unless practical issues dictate otherwise. For some diseases, the most appropriate intervention may be directed at controlling or eliminating the agent at its source. Antibiotic treatments for communicable disease An asymptomatic but infected person may be treated both to clear the infection and to reduce the risk of transmission to others Soil may be decontaminated or covered to prevent escape of the agent. IMPLICATIONS FOR PUBLIC HEALTH – CONT’D Some interventions are directed at the mode of transmission. Isolation of someone with infection, or counseling persons to avoid the specific type of contact associated with transmission. Vehicle-borne transmission may be interrupted by elimination or decontamination of the vehicle. To prevent fecal-oral transmission, efforts often focus on rearranging the environment to reduce the risk of contamination in the future and on changing behaviors, such as promoting handwashing. For airborne diseases, strategies may be directed at modifying ventilation or air pressure, and filtering or treating the air. To interrupt vector-borne transmission, measures may be directed toward controlling the vector population, such as spraying to reduce the mosquito population IMPLICATIONS FOR PUBLIC HEALTH – CONT’D Some strategies that protect portals of entry are simple and effective bed nets are used to protect sleeping persons from being bitten by mosquitoes that may transmit malaria. A dentist’s mask and gloves are intended to protect the dentist from a patient’s blood, secretions, and droplets, as well to protect the patient from the dentist. Wearing of long pants and sleeves and use of insect repellent are recommended to reduce the risk of Lyme disease and West Nile virus infection, which are transmitted by the bite of ticks and mosquitoes. Some interventions aim to increase a host’s defenses Vaccinations promote development of specific antibodies that protect against infection. Prophylactic use of antimalarial drugs, recommended for visitors to malaria-endemic areas, prevent infection from taking root. IMPLICATIONS FOR PUBLIC HEALTH – CONT’D Some interventions attempt to prevent a pathogen from encountering a susceptible host. Herd immunity suggests that if a high enough proportion of individuals in a population are resistant to an agent, then those few who are susceptible will be protected by the resistant majority HERD IMMUNITY Herd immunity is the resistance of a group to infection due to immunity of a high proportion of the group (Figure 32.6) If a high proportion of individuals are immune to an infection then the whole population will be protected Immunized people protect nonimmunized people because the pathogen cannot be passed on and the cycle of infectivity is broken Diseases such as influenza tend to occur in cycles FIGURE 32.6 Susceptible B A Infected Immune C © 2012 Pearson Education, Inc. HERD IMMUNITY In theory, herd immunity means that not everyone in a community needs to be resistant (immune) to prevent disease spread and occurrence of an outbreak. In practice, herd immunity has not prevented outbreaks of measles and rubella in populations with immunization levels as high as 85% to 90%. One problem is that, in highly immunized populations, the relatively few susceptible persons are often clustered in subgroups defined by socioeconomic or cultural factors. If the pathogen is introduced into one of these subgroups, an outbreak may occur. THE HOST COMMUNITY Coevolution of a host and its parasite is common Virulence of the parasite in host-to-host transmission diminishes and resistance of the host increases (e.g., myxoma virus introduced to control rabbits in Australia) A host-to-host pathogen that kills its host before it can infect another host may become extinct If a pathogen does not rely on host-to-host transmission it may remain extremely virulent (e.g., E. coli in hospitals) FIGURE 32.5 100 100 Rabbit mortality Rabbit mortality (%) 80 Virus virulence (%) 60 90 40 Virus virulence 20 80 0 0 1 2 3 4 5 6 Years © 2012 Pearson Education, Inc. HEALTHCARE-ASSOCIATED INFECTIONS Healthcare-associated infections (HAIs) Was referred to as “nosocomial” or “hospital” infection. An infection occurring in a patient during the process of care in a hospital or other health-care facility which was not present or incubating at the time of admission. This includes infections acquired in the health-care facility but appearing after discharge. Preventing HAIs is a top priority for CDC and its partners in public health and health care HEALTHCARE-ASSOCIATED INFECTIONS Result from an infectious agent Other Blood- acquired at a healthcare facility 17% stream (Figure 32.9) infections There are about 1,700,000 HAIs 14% per year in the U.S. Surgical site infections Some are acquired from other 22% patients, but others are caused by Urinary tract pathogens that are selected for and infections maintained by the hospital Respiratory 32% tract environment infections 15% Figure 32.9 © 2012 Pearson Education, Inc. HEALTHCARE-ASSOCIATED INFECTIONS Infectious diseases are spread in hospitals for several reasons: Patients have low resistance to infectious disease Healthcare facilities treat infectious disease patients Multiple patients in the same room Healthcare personnel move from patient to patient Healthcare procedures may breach the skin and introduce infection HEALTHCARE-ASSOCIATED INFECTIONS Infectious diseases are spread in hospitals for several reasons (cont’d): Newborn infants are susceptible to infection Surgical procedures expose organs to contamination Certain drugs increase a patient’s susceptibility to infection Use of antibiotics has selected for antibiotic-resistant organisms IMPACT OF HAIS More serious illness (Increased morbidity). Patient stay longer in a health-care facility. Long-term disability. Excess deaths. High additional financial burden. High personal costs on patients and their families….and Demoralising for staff & patients SOURCE OF INFECTION HAIs may be caused by infectious agents from: Endogenous sources are body sites, such as the skin, nose, mouth, gastrointestinal (GI) tract, or vagina that are normally inhabited by microorganisms. Exogenous sources are those external to the patient, such as patient care personnel, visitors, patient care equipment, medical devices, or the health care environment MODE OF TRANSMISSION 1) Contact Direct contact actual contact with an infected person Indirect contact contact with contaminated surfaces touched by the infected person, or where droplets of body fluid have landed; Spread on unwashed hands) 2) Airborne - "aerosols" tiny infected particles from an infected person released when they cough or sneeze which can be breathed in…Example : Pulmonary Tuberculosis 3) Consuming contaminated food/water or swallowing of microorganisms carried on the hands 4) Blood exposures TYPES OF HEALTHCARE-ASSOCIATED INFECTION Central line-associated bloodstream infection (CLABSI) Catheter-associated urinary tract infection (CAUTI) Surgical site infections (SSI) Ventilator-associated pneumonia (VAP) CENTRAL LINE-ASSOCIATED BLOODSTREAM INFECTION (CLABSI) – INTRO. CLABSIs occur when germs enter the bloodstream through a central line. A central line is a catheter (tube) that often place in a large vein in the neck, chest or groin to give medication or fluids or to collect blood for medical tests. Different from peripheral IVs (shorter IVs placed in the hand, arm or foot for vein access). Access a major vein close to the heart. Can remain in place for weeks or months, and are much more likely to be a source of serious infection. Commonly used in intensive care units. CENTRAL LINE-ASSOCIATED BLOODSTREAM INFECTION (CLABSI) – CAUSES, SIGNS AND SYMPTOMS CLABSIs occur when germs (usually bacteria or fungi) enter the bloodstream through the central line. People with CLABSIs may experience: Fever Red skin and soreness around the central line CENTRAL LINE-ASSOCIATED BLOODSTREAM INFECTION (CLABSI) – PREVENTION Healthcare providers should follow recommended infection control guidelines to reduce the risk of CLABSIs. Patients should: Speak up. Talk to your healthcare providers about any questions or worries. Ask what they're doing to protect you. Ask a healthcare provider about your options and the pros and cons of a central line, if one is needed. If so, ask them to help you understand the need for it and how long it will be in place. Avoid touching the tubing and where it exits the skin as much as possible. CENTRAL LINE-ASSOCIATED BLOODSTREAM INFECTION (CLABSI) – TREATMENTS Treatment for this type of infection is with antibiotics (i.e. vancomycin). Sometimes the catheter needs to be removed and replaced. CATHETER-ASSOCIATED URINARY TRACT INFECTION (CAUTI) – INTRO. A catheter-associated urinary tract infection (CAUTI) occurs when germs enter the urinary tract through a urinary catheter and cause infection. CAUTIs are one of the most common types of healthcare-associated infections (HAIs). Most CAUTIs can be treated with antibiotics and/or removal or change of the catheter. The healthcare provider will determine the best treatment for each patient. CATHETER-ASSOCIATED URINARY TRACT INFECTION (CAUTI) – SIGNS AND SYMPTOMS Burning or pain in the lower abdomen (i.e., below the stomach) Fever Burning while peeing Peeing more frequently than usual CATHETER-ASSOCIATED URINARY TRACT INFECTION (CAUTI) – CAUSES AND RISK FACTORS A CAUTI occurs when germs (usually bacteria) enter the body through a urinary catheter and cause infection. The infection can happen in any part of the urinary tract (e.g., kidneys, ureters, bladder, and urethra). The most important risk factor for developing a CAUTI is prolonged use of a urinary catheter. CATHETER-ASSOCIATED URINARY TRACT INFECTION (CAUTI) – REDUCING RISK Patients with a urinary catheter should: Understand why they need it and frequently ask healthcare providers if the catheter is still needed. If the patient has a long-term catheter, they must clean their hands before and after touching the catheter. Check the position of the urine bag; it should always be below the level of the bladder. Patients with a urinary catheter should not: Tug or pull on the tubing. Twist or kink the catheter tubing. Healthcare workers and facilities can prevent CAUTIs and protect patients with proper infection control processes. SURGICAL SITE INFECTIONS (SSI) – INTRO A surgical site infection (SSI) is an infection in the part of the body where a surgery took place. SSIs can generally be treated with antibiotics but may require additional medical care. Can occur in: Skin Tissue Organs Implanted material (i.e. hip replacement) Treatment generally includes antibiotics, but the type of treatment depends on the germ causing the infection. Sometimes, patients need another surgery to treat SSIs. SURGICAL SITE INFECTIONS (SSI) – SIGNS AND SYMPTOMS Redness and pain around the area where you had surgery. Cloudy fluid draining from your surgical wound. Fever. Other signs and symptoms may also occur. SURGICAL SITE INFECTIONS (SSI) – REDUCING RISK Before surgery: Tell your healthcare provider about other medical problems you may have. Health problems such as allergies, diabetes and obesity could affect your surgery and your treatment. Quit smoking. Patients who smoke get more infections. Talk to your healthcare provider about how you can quit before your surgery. Do not shave near where you will have surgery. Shaving with a razor can irritate your skin and make it easier to develop an infection. SURGICAL SITE INFECTIONS (SSI) – REDUCING RISK During surgery: Speak up if someone tries to shave you with a razor in the area where you will have surgery. Ask why you need to be shaved in the area and talk with your surgeon if you have any concerns. SURGICAL SITE INFECTIONS (SSI) – REDUCING RISK After surgery: If you do not see your healthcare provider clean their hands, please ask them to do so. Family and friends should not touch the surgical wound or dressings. Family and friends should clean their hands with soap and water or an alcohol-based hand rub before and after visiting you. If you do not see them clean their hands, ask them to clean their hands. Always clean your hands before and after caring for your wound. SURGICAL SITE INFECTIONS (SSI) – REDUCING RISK Before you leave the hospital: Make sure you understand how to care for your wound. Make sure you know who to contact if you have questions or problems. Once you are home: If you have any symptoms of an infection, such as redness and pain at the surgery site, drainage, or fever, call your healthcare provider immediately. VENTILATOR-ASSOCIATED PNEUMONIA (VAP) – INTRO. Ventilator-associated pneumonia (VAP) a lung infection that develops in a person who is on a ventilator. VAP can be treated with antibiotics. Happens when bacteria get into a patient's lungs and cause an infection. VENTILATOR-ASSOCIATED PNEUMONIA (VAP) – REDUCING RISK Quit smoking. Patients who smoke get more infections. Seek information about how to quit before surgery. Ask healthcare providers to clean their hands before touching the patient or equipment. Ask about raising the head of the bed. Ask when the patient can try breathing without a ventilator. Ask how often healthcare providers clean the patient's mouth. VENTILATOR-ASSOCIATED PNEUMONIA (VAP) – TREATMENT AND RECOVERY Treatment generally includes antibiotics, but the type of antibiotic depends on the bacteria causing the infection. The healthcare provider will decide which antibiotic is best. VENTILATOR-ASSOCIATED PNEUMONIA (VAP) – TREATMENT VENTILATOR-ASSOCIATED PNEUMONIA (VAP) – TREATMENT