Chapter 14. Principles of Disease and Epidemiology

• Unbroken skin and mucous membranes are good barriers to disease.

• Furthermore specialized cells and proteins (antibodies) can destroy harmful microbes.

• We are still susceptible to disease-causing organisms (pathogens).

– Caused by an organisms special abilities: Toxins, endospores, enzymes or capsules.

• Infectious disease mortality decreased most of the 20^th Century until it increase 58% from 1980-92.

• Pathology, Infection and Disease.

– Pathology is the scientific study of disease (pathos = suffering).

• Pathology is first concerned with etiology (cause) and then parthogenesis (how the disease develops).

• Infection is an invasion or colonization of the body by pathogenic organisms and disease is a change from a state of health as a result of an infection.

• Infection can be finding an organism associated with a part of the body it does not normally inhabit (Ex: E. coli in the urinary tract)

• Normal Microbiota.

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– A newborn’s first contact with microbes is with lactobacilli from the mother.

• Predominant organism in the newborn’s intestine.

• Other organisms are added by ingestion on food or other contact.

• A typical human has 1 X 10¹³ cells and 1 X 10¹⁴ bacteria.

• Normal microbiota (flora) are those organisms that take up residence and (colonize) but do not cause disease.

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• Microbes are localized to certain regions of the body not everywhere (Fig. 14.2)

– Relationship between normal microbiota and the host.

• Normal microbes can be beneficial by preventing colonization by harmful organisms, this is called microbial antagonism.

– Occurs through nutrient competition, producing harmful substances or changing conditions (pH or Oxygen).

– Ex: Candida albicans is inhibited by the normal microbiota of the vagina, due to the fact they maintain the pH of the vagina around pH of 3.5-4.5.

» Antibiotics, excessive douching or deodorants can change the pH towards neutral eliminating this protective effect.

– Ex: Clostridium difficile in the large intestine is inhibited by the normal flora.

» Can become a problem if normal microbiota is killed by antibiotics.

» Responsible for most GI infections as a result of antibiotics (mild à death)

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– Some of these relationships are mutualistic (benefiting both), while others are commensal (benefiting one harming neither).

– Opportunistic Microorganisms.

• Some organisms that are normally mutualistic can become harmful under the proper conditions (Ex: E. coli).

– These organisms are considered opportunistic pathogens, they normally do not cause disease in their normal habitat but can under certain conditions.

» Host’s health may also be involved.

• Etiology of Infectious Disease.

– Some diseases have a well known etiology (polio and TB) others are unknown (some cancers and Alzheimer’s).

• Not all diseases are caused by microbes some are genetic.

• Here we will only discuss infectious disorders.

– Koch’s Postulates.

• German Physician who played a major role in indicating that microorganisms cause specific diseases.

• Found that Bacillus anthracis was always present in the blood of animals that had the disease (anthrax).

• Koch’s Postulates:

– 1) Same pathogen found in every case of the disease.

– 2) Pathogen must by isolated from the diseased host and grown in pure culture.

– 3) Cultured pathogen must cause the disease when inoculated in a healthy host.

– 4)

– Exceptions to Koch’s postulates.

• Some organisms cannot be cultured by normal means.

– Ex: Mycobacterium leprae and viral pathogens cannot be normally cultured.

– Other culturing methods must be allowed so the postulates have been modified.

» Culturing in chick embryos.

– Some symptoms in humans can be caused by a multitude of organisms.

» Ex: Nephritis (kidney inflammation) can involve one of several organisms all give the same symptoms.

» Other diseases with poorly identified etiologies include pneumonia and meningitis

– Some organisms can cause several disease conditions.

» Ex: Streptococcus pyogenes can cause sore throat, scarlet fever skin infections etc.

– Ethical considerations also impose exceptions on the postulates (Ex: infecting humans).

• Classifying Infectious Diseases.

– Patients may have symptoms (subjective changes in body function not apparent to an observer) and/or signs (objective changes that can be observed and measured) of disease.

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– Diseases can be described by their behavior within the host.

• Communicable diseases spreads from a host directly or indirectly (chicken pox and measles).

– Contagious are diseases like chicken pox and measles that easily spread from host to host.

• Noncommunicable are organisms that do not spread from one host to another (Ex: Clostridium tetani).

– The Occurrence of Disease.

• In order to understand occurrence you must know incidence (number of people in a population with a disease over a certain time period) and prevalence (number of people in a population with a disease over a certain time period, regardless of when it first appeared).

– Incidence is indicator of the spread of a disease.

– Prevalence is an indicator of how serious and how long a disease affects a population.

• A sporadic disease is one that occurs occasionally.

• An endemic disease is a disease constantly present in a population

• A disease epidemic is many people catching a disease in a relatively short period of time.

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– Severity or duration of disease.

• Another way of describing a disease is by its severity or duration.

• An acute disease (Ex: Influenza) is usually one that has a rapid onset and short duration, while a chronic disease develops more slowly but will likely last a longer period of time (Ex: TB and mononucleosis).

– Subacute is between acute and chronic, while latent indicates a disease where the causitive agent remains dormant.

– Another way to classify is by the extent of host involvement.

• Local infections are confined to a small area of the body (Ex: boils and abscesses).

• Systemic infections spread throughout the body through the lymph or blood vessels.

• A local infection that moves to other specific parts of the body is called a focal infection.

• Bacteremia is bacteria in the blood while multiplying bacteria in the blood is septicemia.

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• A primary infection is an acute infection that causes an initial illness while secondary infection results from an opportunistic pathogen after the primary infection has weakened the body’s defenses.

– Ex: Pneumocystis pneumonia as a consequence of AIDS.

• Subclinical infections do not cause any noticible illness (Ex: polio virus or hepatitus A virus can be carried and not cause illness).

• Patterns of Disease- Definite sequence of events leading to infection and disease.

– Reservoir à Susceptible host à Invasion à pathogenesis.

– Predisposing factors.

• A predisposing factor, can affect the occurrence of a disease by making the body more susceptible.

– Can be related to gender (Ex: urinary tract infections vs. meningitis) or genetics (Ex: sickle-cell anemia).

– Weather, fatigue, age, environment, stresses etc. can also be predisposing factors.

– The development of disease.

• Once a microorganism overcomes the defenses of the host development of disease follows a sequence.

• The incubation period is the time interval between the initial infection and the first appearance of signs or symptoms (may depend on host resistance) (Table 15.1).

• The predromal period is a short period following incubation and is characterized by mild symptoms (Ex: malaise).

• Period of illness is when the disease is the most acute, when the patient exhibits the most overt signs and symptoms (patient death usually occurs during this period).

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• Period of convalescence is the period of recovery.

– People can serve as reservoirs and spread disease all the way through convalescence (some diseases not all).

• The Spread of Infection.

– Reservoirs of infection.

• A reservoir of infection provides a pathogen with adequate conditions for survival and multiplications well as an opportunity for transmission.

– Human reservoirs- May be people with signs and symptoms of the disease or they may have no signs or symptoms (carriers).

– Animal reservoirs- Animals can transmit disease to humans (zoonoses).

» Ex: rabies and lyme disease.

» About 150 zoonoses (Table 14.2) are known and can occur via direct contact (dead or alive), feces, contaminated food or water, air, or insect vectors.

– Nonliving reservoirs- Soil and water are two major reservoirs (Ex: Clostridium botulinum and C. tetani from soil via horse and cattle feces).

– Transmission of Disease- Three routes contact, vehicles and vectors.

• Contact transmission- is the spread of a disease through direct contact, indirect contact or droplet transmission.

– Direct contact transmission involves no intermediate and involves physical contact.

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– Indirect contact transmission involves a nonliving intermediate called a fomite.

» Syringes, bedding, drinking fountains, doorknobs etc.

– Droplet transmission is the transmission through droplets in the air, by sneezing or coughing (not airborne only travel a short distance, 1m).

• Vehicle transmission is transmission through water, air or food (other media include blood and body fluids).

– Waterborne transmission- pathogens spread by water contaminated with untreated or poorly treated sewage (Ex: Cholera).

– Food-borne transmission is due to improperly handled/prepared food (Ex: food poisoning and tapeworms).

– Airborne transmission refers to the spread of agents of infection by droplet nuclei in dust (must travel > 1 m).

» Small droplets from sneezing/coughing may remain airborne long enough also dust particles (Ex: Staph. and Strep.).

• Vector transmission includes mechanical and biological transmission (most common is the arthropod).

– Mechanical is passive transport where the pathogen is on the insect and is passed to food or water.

– Biological transmission is an active process usually through the insect biting the potential infected individual.

» Ex: The insect bites an infected animal or person then the pathogen reproduces in the insect and then the pathogen is transmitted to the next organism bitten.

– Portals of Exit- Just as microbes enter an organism at a preferred site they also exit at a preferred site.

• Portals of exit are often related to the part of the body infected.

– Ex: most common are the respiratory tract or the gastrointestinal tract.

» Another important exit is the urogenital tract (Ex: STDs).

• Nosocomial Infections.

– Infections acquired at a hospital, nursing home or other health care facility.

– 5-15% of all patients acquire a nosocomial infection (CDC).

– Nosocomial infections result because of the interaction of three factors (Fig. 14.9):

• 1) Microbes in the hospital environment (Table 14.4).

– S. aureus, P. aeruginosa, E. coli, and Enterococcus spp..

• 2) The weakened state of the potential host, compromised host.

• 3) Chain of transmission in the hospital.

– Direct contact: Staff to patient and patient to patient.

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– Control of nosocomial infections.

• Reduce the numbers of organisms by using aseptic techniques.

– Education, hand washing, isolation rooms and isolation wards.

• According to the CDC hand washing is the single most important means of preventing the spread of infection.

– 1997 study showed that hand washing only occurred 27% of the time before interacting with patients in one long-term care facility.

– Also as low as 31% in one emergency department.

• Emerging infectious diseases (EID).

– Infectious diseases that are new or changing (Table 14.6).

– Increased incidence in the near past or potential in the near future.

– Could be a virus, fungus, protozoan, bacterium or helminth.

– There are several criteria for identifying an EID.

• Presents clearly distinguished symptoms, recognized because of improved diagnostic techniques, local disease becomes widespread, rare disease becomes common, mid disease more severe or increase in life span allows a slow disease to develop.

– A variety of factors contribute to the emergence of new infectious diseases:

• 1) A new serovar (Ex. Vibrio cholera 0139)

• 2) Widespread unwarranted use of antibiotics and pesticides.

• 3) Global warming may increase the distribution and survival of reservoirs and vectors (Ex: Hantavirus and Malaria).

• 4)

• 5) Animal control measures (Ex: Lyme disease due to increased deer populations)

• 6) Previously unrecognized infections as a result of changing environment and natural disasters (Ex: Coccidioidomycosis after the Northridge earthquake and Venezualan Hemorrhagic fever due to clearing the rain forest.

• 7) Failures in public health measures and policy.

– CDC, NIH and WHO have plans to address issues relating to EID their priorities include:

• 1) Early detection, prompt investigation and monitoring of EID and the pathogens that cause them.

• 2) Expand basic and applied research on ecological and environmental factors that influence EID.

• 3) Enhance the communication of public health information and prompt implementation of prevention strategies regarding EIDs.

• 4) Establish plans to monitor and control EIDs worldwide.

– There is even a new publication, Emerging Infectious Diseases.

• Epidemiology is the science of studying when and where diseases occur and how they are transmitted.

– Modern epidemiology began in the mid-1800s by three studies that looked at infection rates and were able to identify a cause (how it was transmitted).

– Epidemiologists are concerned with the above plus factors and patterns concerning the individuals affected (Ex: DM and Melioidosis) and methods for controlling disease.

– Descriptive Epidemiology entails collecting all data that describe the occurrence of the disease under study.

• Generally retrospective and includes data collection on affected individuals and the place and period of disease occurrence.

– Analytical Epidemiology analyzes a particular disease to determine a cause.

• With case control method the epidemiologist looks for factors that might have preceded the disease.

• With the cohort method the epidemiologist studies two populations one that has contact with the agent causing disease and another group that has not.

– Experimental Epidemiology begins with a hypothesis about a particular disease and leads to experiments to test the hypothesis on a group of people (Ex: drug effectiveness)

– Case reporting- requires health care workers to report specific diseases to local, state and national health officials (Ex: AIDS, gonorrhea and typhoid fever).

• Leads to establishing a chain of transmission, once a chain is established it can be interrupted.

– This may lead to slowing or stopping the spread of the disease.

– The centers for disease control and prevention (CDC).

• Branch of the U.S. public health service that is the center of epidemiological information.

• Issues a publication called Morbidity and Mortality Weekly Report (MMWR).

– Morbidity is the incidence of a disease and mortality is the number of deaths from a disease.

– MMWR includes reports on the rates of morbidity and mortality, case histories of interest status summaries of diseases of interest.