Lesson 7:Principles of Disease and EpidemiologyMarch 3, 2014

Pathology• Pathology- is the scientific study of disease

▫Pathogen—any microbe that can cause a disease

• Divided into three different parts:▫Etiology-investigates the causes or the origins of

disease (what causes the disease?)▫Pathogenesis-studies the manner in which disease

develops (how does pathogen grow and survive in the body?)

▫Pathophysiology-Affects of the disease on the human body (How does the body react to the pathogen?)

Infection vs. Disease

•Infection- is the invasion or colonization of the body by microorganisms

•Disease-occurs when an infection results in any change in an animals state of health

•Do all infections result in disease????▫No!!!! Poliovirus may infect humans but

signs of “disease” may not be present

Normal Microbiota• In normal circumstances, animals are free

from microorganisms in utero.

•Most Microbes do not infect animals until birth▫Exceptions are T.O.R.C.H. (Taxoplasma, others,

rubella, cytomegalovirus, herpes)▫One of the first microbes encountered during

birth are lactobacillus spp. Lactobacilli become the predominant organisms

in the gastrointestinal tract

Microbes in Action:Lactose Intolerance• Lactose intolerance comes from the inability to break

down the dissacharide lactose (glucose and galactose)▫ Insufficient levels of the enzyme lactase

• Lactobacillus spp.▫ Gram positive rod shaped bacteria▫ In humans, are found in the vagina and the

gastrointestinal tract of humans▫ Converts the dissacharide lactose to lactic acid

Prevents the production of copious amounts of gas (carbon dioxide and methane)

Unabsorbed sugars and fermentation products also raise the osmotic pressure of the colon, resulting in an increased flow of water into the bowels

Normal Microbiota• In normal circumstances, animals are free from

microorganisms in utero.• Microbes begin to colonize animals during the

birthing process▫One of the first microbes encountered during birth

are lactobacillus spp. Lactobacilli become the predominant organisms in the

gastrointestinal tract• Other microbes are introduced into the body from

the environment through breathing and by feeding▫Table 4.1 (page 404) lists common microbiota by

body regions

Normal Microbiota•Human body contains more bacteria than

human cells▫Typical body contains 1013 body cells but has

1014 bacterial cells (ten times more bacteria than cells)

•The Human Microbiome Project set forth to analyze the microbiomes of the human body to determine the relationship b/w changes in human microbiome and human health and disease

Normal Microbiota

•The microbiota of the body is thought to be of two varieties:▫Normal microbiota/flora—microbes that

reside permanently within our bodies▫Transient microbiota—microbes that

reside for only a short period of time within the body and disappear (cleared)

•Neither produce disease under normal conditions

Normal Microbiota

•The location of normal flora in the body depends on several factors▫Temperature▫pH▫Oxygen availability▫Salinity▫Sunlight

Normal Microbiota

•Certain factors affect the distribution and composition of a humans natural microbiota▫Age▫Diet▫Health status▫Personal hygiene▫Stress

Microbiota/Host Interactions

•Normal flora can benefit the host by preventing the growth of harmful bacteria (microbial antagonism/competitive exclusion)▫Competition for nutrients▫Producing substances that inhibit pathogen

growth▫Changing the environment (pH and oxygen

levels)

Microbes in Action:Yeast Infection•The normal microbiota of a woman’s vagina

maintains an acidic pH (~4)•Acidic pH inhibits the growth of the yeast

Candida albicans•Disruption of the normal flora causes a

change in pH (pH of vagina becomes neutral)▫Antibiotics, excessive douching, use of

deodorants•The neutral pH allows the C. albicans to

flourish causing a “yeast infection”

Symbiosis• The relationship between the normal flora and the host

is called symbiosis▫ Commensalism—symbiotic relationship where only one

of the organisms benefits and the other is unaffected Staphylococcus epidermidis uses nutrients found in

secretions▫ Mutualism—symbiotic relationship that benefits both

organisms E. coli uses nutrients but provide us with vitamins K and B

▫ Parasitism—one organism benefits by deriving nutrients at the expense of another organism F. tularenesis uses up the host cells amino acids and lipids

Opportunistic Pathogens•Opportunistic pathogens—organisms that

ordinarily do not cause disease but in a particular environment may cause disease▫Location can cause a bacteria of the normal

flora to become pathogenic E. coli harmlessly resides in the large intestine

but if it infects the urinary bladder it can cause disease

▫Lowered host resistance AIDS patients become more susceptible to

pneumonia (Pneumocystis spp.)

The Affect of Disease on the Body•Each disease results in altered body

structure and function and is indicated by several kinds of evidence▫Symptoms—changes in body function

experienced by the patient▫Signs—objective changes in the body that

the physician can observe and measure▫Syndrome—a specific group of

symptoms/signs that always accompany a particular disease

•Diseases are often classified in terms of how they behave within the host and within a given population▫Communicable Disease—diseases that

are spread from one host to another Diseases that spread EASILY are said to be

contagious

▫Noncommunicable Disease—disease that is not spread from host to host

Classifying Infectious Diseases

•The classification of diseases are based on several criteria▫Occurrence of a Disease

▫Severity or Duration of a Disease

▫Extent of Host Involvement

Disease Occurrence•Disease incidence is the number of people with

a disease during a particular time period▫Indicator of how well the disease spreads▫Incidence of AIDS in the US in 2007 was 56,300

•Disease prevalence is the number of people in a population who develop a disease at a specified time, regardless of when it appeared▫“how many cases altogether”▫Prevalence of AIDS in the US in 2007 was

1,185,000

What is the Incidence of AIDS in the World in 1991? What is the Prevalence?

•How frequently and where a disease occurs also help to classify the disease▫Sporadic disease occurs occasionally ▫Endemic diseases that are specific to a

certain area/location (i.e. African-sleeping sickness)

▫Epidemic are diseases that affect many people in a given area in a relatively short period of time

▫Pandemic refers to an epidemic disease that occurs worldwide

Severity or Duration of Disease•Another way of classifying a disease is based

on how severe the disease is or how long the disease persists within the body▫Acute disease is one that develops rapidly but

clears quickly▫Chronic diseases are diseases that develop

more slowly but persists or recurs over long periods of time

▫Latent disease is one where the pathogen remains inactive for a long time but can become active to produce symptoms of the disease

Location of Infection• Infections can also be classified based on the

extent or location that the host’s body is affected▫Local infection is one where the invading

microorganisms are restricted to a small area of the body (boils or abscesses)

▫Systemic infection—microorganisms and their products are spread throughout the body (measles or chicken pox)

▫Focal infection—local infection that may affect the progression of disease at another site (periodontitis leading to diabetes or cardiovascular disease)

Extent of Host Involvement•The body’s response (immune system) to

microbes can also affect the severity of the disease

•Septicemia/Sepsis (blood poisoning) is a systemic infection arising from the multiplication of microbes in the blood▫Bacteremia—presence of bacteria in the blood▫Viremia—presence of viruses in the blood▫Toxemia—presence of toxins in the blood

Systemic Inflammation Response Syndrome (SIRS)•Defined as having 2 or more of the

following▫Fever of 100.4 or less than 96.8 (Farenheit) ▫Heart rate >90 beats per minute (bpm)▫Respiratory rate >20 breaths per minute▫Abnormal WBC count (>12,000/µL or

<4,000/µL)•SIRS can lead to MODS (multiple organ

dysfunction syndrome)▫Process is not known but often debated

•The state of host resistance also determines the extent of infections▫Primary infections—is an acute infection

that causes the illness▫Secondary infections—is an infection that

is caused by an opportunistic pathogen after the primary infection has weakened the immune system

▫Subclinical infections—is an infection that does not cause a noticeable illness

Herd Immunity•The rate in which a disease spreads depends

on the immunity of the population•People that are immuned (natural or

unnatural) act as a barrier for certain diseases▫This barrier helps to reduce the occurrence of

the disease•Herd immunity is when many people of a

particular community is immune to a disease therefore reducing the spread of that disease

PRINCIPLES OF DISEASE AND EPIDEMIOLOGYPART II

Overview

•When a pathogen enters into our body there is a series of events that occur leading up to disease development▫Reservoir for the infectious pathogens▫Transmission of pathogen to host

Direct transmission Indirect transmission

▫Invasion of pathogen into the host cells▫Pathogenesis (pathogens growth/survival

within the host cells)

•REMEMBER THE OCCURRENCE OF DISEASE ULTIMATELY DEPENDS ON THE RESISTANCE OF THE HOST TO THE GROWTH/SURVIVAL OF THE PATHOGEN!!!!

Predisposing Factors to Disease

•Several factors other than the pathogen and the host’s resistance towards the invasion/growth/survival of the pathogen dictate the occurrence of disease

•Predisposing factors—makes the body more susceptible to a disease’s progression

• Some predisposing factors for disease include:▫Gender

Women contract UTIs more frequently then men Men have higher rates of pneumonia contraction

▫Genetics Sickle cell anemia

▫Climate/Weather Respiratory diseases increase during the winter

months▫Age

Parkinson’s disease more prevalent in people over 50

•Inadequate nutrition, fatigue, environment, habits, occupations, pre-existing illness, and emotional distress all are considered to be predisposing factors

•Usually difficult to link the importance of one of these factors to a particular disease▫Lactose intolerance is linked to several

disposing factors

Development of Disease

•After a pathogenic infection there is a systematic series of steps that follows▫These steps are referred to as the

development of disease

• Incubation Period▫Defined as the interval of time between the initial

infection and the development of signs/symptoms of that disease

▫ Incubation period can vary depending several factors Infecting microorganisms Virulence (how pathogenic the pathogen is) Number of infecting microorganisms Host resistance to the pathogen

▫Table 15.1 page 431 has a listing of different pathogens and the length of their incubation times

•Prodromal Period▫Follows the incubation period in some

diseases and is characterized by early or mild symptoms of the disease

▫Relatively short period of time

• Period of Illness▫Most severe stage of the disease due to exponential

growth of the pathogen▫Characterized by the exhibition of overt signs and

signals of a disease Fevers Myalgia (muscle pain) Photophobia (sensitivity to light) Pharyngitis (sore throat) Lymphadenopathy (swelling of the lymph nodes)

▫ If a patient’s immune system does not overcome the rapid growth of the pathogen, they die in this stage

•Period of Decline▫In this stage, the host immune system

begins the eradication of the pathogen (clearing of pathogen)

▫Signs and symptoms of the disease begin to subside

▫During this stage, the patient is vulnerable to secondary infections!!!!

•Period of Convalescence▫Final stage of disease progression▫Host defense (immune system) have mostly

cleared the infecting pathogen▫Patient regains strength and the body

returns to the pre-diseased state

•People can easily spread disease during the Period of Illness stage▫Pathogen is most abundant.

•Patients can also spread disease during the Period of Decline and Convalescence stage as well▫Pathogens are still present. Sometimes the

pathogen is never cleared.

Stages of Disease

Spread of Infection

•In order to be infected with a pathogen, we first must come into contact with that pathogen

▫Where do these pathogens reside????

•Reservoirs of Infection—the place where a pathogen lives ▫This can be a living organism

Humans Animals

▫Inanimate objects also serve as reservoirs Swimming pools and gardens

•Human Reservoirs▫Principal living reservoir of human disease▫Carriers are people who harbor pathogens

and can transmit it to others▫People with no signs/symptoms can also

spread disease

•Why are people that show no signs most dangerous to spread disease?

•Animal Reservoirs▫Both wild and domestic animals are living

reservoirs of disease▫Any diseases that normally occur in animals

and are capable of infecting humans are called zoonoses

▫Some common diseases from animals are Rabies (bats, dogs, raccoons, squirrels) Lyme disease (mice and other rodents carry

ticks) Toxoplasmosis (cats)

• Non-Living Reservoirs • Most common is soil and water

▫Soil Soil where animal feces are used as fertilizer are

prime reservoirs Fungi Clostridium botulinum Clostridium tetani

▫Water Contaminated water from waste run-offs Vibrio cholerae

• Food can serve as a reservoir (ill-prepared food)

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Disease Transmission

•The passing of the pathogen from the reservoir to a susceptible host is called transmission

•Pathogens can be transmitted from the reservoir to the host via three routes:▫Contact▫Vehicles▫Vectors

Contact Transmission• Direct contact transmission—transmission of a

pathogen by physical contact with carrier “person-to person transmission”

• Indirect contact transmission—transmission of a pathogen to a host via an inanimate object

Fomite—non-living object involved in spread of pathogen (ex. doorknobs, drinking cups, money)

• Droplet transmission—spread of pathogen via mucous (ex. spit from sneezing, coughing, talking)▫Travel less than 1 meter from the reservoir (not

considered AIRBORNE!!!!

Vehicle Transmission• Transmission of pathogens through a medium

Medium can be water, food, or dust▫Waterborne transmission—pathogens are usually

spread by contaminated water▫Foodborne transmission—incompletely cooked,

poorly stored, prepared under unsanitary conditions▫Airborne transmission—spread of pathogens through

dust. (Staph and Strep spp. survive on dust particles) Travel more than 1 meter from reservoir to host.

• What are some other mediums in which pathogens can be transmitted?

•Vectors are animals that carry pathogens from one host to another

•Two ways that vectors can transmit pathogens▫Mechanical transmission▫Biological transmission

• Mechanical transmission—is the passive transport of pathogen via insects’ body parts

• Biological transmission—complex process that can be broken down into three parts Vector bites the diseased

individual and ingests blood (bloodmeal)

The pathogen in the blood begins to grow inside the vectors gut

The vector bites another individual and spreads the pathogen

▫ Passed via vomit or feces