Infections and infectious diseases

Pathology: the study of disease

Etiology: focuses on the cause of a disease

Pathogenesis: the development of a disease

Infection: invasion of the body by pathogens

Disease (morbidity): any change from a state of health

Pathogenicity: a microorganism’s ability to cause disease

Virulence: a measure (degree) of pathogenicity

Terminology

Symbiosis indicates the relationship between the normal microbiota and host

In mutualism, both organisms benefit

In commensalism, one organism benefits, and the other is unaffected

In parasitism, the parasite

benefits at the damaging expense of the host

Human host and microorganisms

Any parasite that causes disease is called a pathogen

Normal microbiota of the human body

Normal microbiota Colonize skin and mucous membranes of the human body without normally causing disease

Resident microbiota

permanently colonize the host

Transient microbiota may be present for days, weeks, or months

Members of the normal microbiota can become opportunistic pathogens or opportunists

Members of the normal microbiota gain access

to other body sites

Escherichia coli (colon/mutualistic) can

cause urinary tract infections (urethra)

Immune-compromised/suppression

(disease, malnutrition, stress, chemotherapy, immunosuppressive

drugs)

AIDS patients and fungal infections (HIV infections

suppresses immune system functions) – Candidiasis

Opportunistic members of the normal microbiota

The normal microbiota protect the host Occupying niches that pathogens might occupy (nutrients) Producing acids (vagina) Producing bacteriocins that inhibit other bacteria

This is known as microbial antagonism or competitive

exclusion

Changes in the normal microbiota can lead to opportunistic infections caused by members of the normal microbiota Long-term antimicrobial treatment can cause changes in

the normal vaginal microbiota, leading to Candida albicans infections

Reservoirs of infectious diseases

Reservoirs are sites where pathogens are maintained as a source of infection

Human reservoirs Animal reservoirs and zoonoses Nonliving reservoirs

What is the source of pathogens?

Human reservoirs

People with active disease (signs/symptoms of a disease) can transmit it

Carriers People with asymptomatic disease or latent disease

are a source of infection Gonorrhea Typhoid fever Syphilis Tuberculosis AIDS

Reservoirs of infectious diseases

Animal reservoirs (zoonoses)

Both wild and domestic animals are reservoirs of pathogens Infectious diseases that spread from wild or domestic animals

to humans are called zoonoses

Transmission of zoonosis to humans Direct contact with the infected animals or their waste Consuming infected animal products Contaminated food, water or air Bloodsucking arthropods (vectors)

Usually, humans are a dead-end host for zoonotic pathogens Zoonoses that involve bloodsucking arthropods can be

transmitted back to animal hosts

Nonliving reservoirs

Nonliving reservoirs include water, soil, foods (improperly prepared or stored foods)

Water contaminated by human/animal feces Vibrio cholerae Salmonella

Soil

Clostridium botulinum C. tetani

How do pathogens enter the human body?

Pathogens enter the body at several sites called portals of entry

Three major types Skin

A barrier to pathogens if it remains intact Mucous membranes

Linings of the respiratory, gastrointestinal, urinary, and reproductive tracts, conjunctiva

Placenta

Typically an effective barrier to most pathogens

Parenteral route: not a true portal of entry but allows pathogens to circumvent the portals of entry

Pathogens gain direct access to tissues beneath the skin and mucous membranes

Punctures by a nail or hypodermic needle

Sometimes included in this route:

breaks in the skin: stab wounds, bites, surgery

Parenteral route

How do pathogens enter the human body?

The pathogen must Be transmitted to a susceptible

host

For an infectious disease to occur

There must be a reservoir of infection as a source of pathogen and a route of exit

Enter the host and attach to cells of the host (ligands) Multiply sufficiently to adversely affect the body (causing

damages to the host) To do so the pathogen must overcome the defense of the

host

Predisposing Factors

Predisposing factors make the body more susceptible to disease Inadequate nutrition Fatigue Age Gender Lifestyle Chemotherapy Emotional disturbances

Pathogens affect the host “producing virulence factors”

Virulence factors allow pathogens to enter a host, adhere to host cells, or escape detection/killing by the immune system

Examples of virulence factors

Virulence factors Type

Ligands Adhesins, attachment proteins

Extracellular enzymes

Hyaluronidase, collagenase, coagulase, kinases

Toxins

Endotoxins (Lipid A) Exotoxins (cytotoxins, neurotoxins, enterotoxins)

Anti-phagocytic factors Capsule, leukocidins

Some bacterial pathogens attach to the surface they colonize and to each other to form a biofilm

Biofilms

Dental plaque

Extent of infections

Local infection: Pathogens are limited to a relatively small area of the body (boils)

Systemic infection: An infection throughout the body spread by the blood or lymph (measles)

Focal infection: Infection that “functions” as a source of pathogen(s) to infections at other sites of the body (teeth)

Type of infections

Primary infection: Acute infection that causes the initial illness

Secondary infection: Often opportunistic infections after a primary (predisposing) infection – HIV and fungal infections

Asymptomatic or subclinical infections: No noticeable signs or symptoms – hepatitis

Extent of Host Involvement

Septicemia Systemic infection (blood)

Bacteremia Bacteria in the blood

Toxemia Toxins in the blood

Viremia Viruses in the blood

Manifestation of a disease

A disease causes changes in structures/functions of the body Symptom: A change in body function that is felt by a

patient such as pain or malaise (discomfort) Sign: A change in the body that can be measured or

observed by a physician such as fever, swelling, redness, diarrhea

Diagnosis

Involves an evaluation of signs, symptoms, and results of laboratory tests

The Etiology of an infectious disease

Koch’s postulates help determine the etiology of an infectious disease, which allows treatment and prevention

Exceptions to Koch’s postulates to establish a single cause for certain infections such as pneumonia Ethical considerations

The Stages of an infectious disease

Many infectious diseases have five stages

Severity or Duration of a Disease

Acute disease

Disease develops rapidly/short time (common cold)

Chronic disease

Disease develops slowly, it is likely to continue or recur for long periods

(tuberculosis, hepatitis)

Subacute disease

Intermediate between acute and chronic

(bacterial endocarditis-streptococci)

Latent disease

A pathogen remains inactive for a long period before becoming active to

produce signs/symptoms of the disease (herpes/shingles - varicella virus)

Communicable disease: Any disease that is spread from one host to another (genital herpes)

Contagious disease: A communicable disease that is easily spread from one host to another (chickenpox)

Noncommunicable disease: A disease that is not spread from one host to another Clostridium tetani - abrasions or wounds

Types of Infectious Diseases

Pathogens must leave the infected patients in order to infect other hosts

Portals of exit Pathogens often leave the infected host via bodily secretions and

excretions

Transmission of an infectious disease

Transmission of a pathogen from a reservoir or a portal of exit to another host’s portal of entry can occur through contact, vehicles, vectors

Contact Direct

Involves person-to-person contact, sexual intercourse (gonorrhea), and transfer across the placenta

Animal to person contact, zoonosis (rabies/ring worm)

Indirect

Spread by fomites (non-living objects) - medical equipment, needles (HIV virus)

Transmission of an infectious disease - Contact

Droplets (Distance < 1m) Transmission via droplets (droplets of mucus) Cold and flu viruses

Transmission of an infectious disease

Vehicle transmission is the transmission of a pathogen by a medium Air, water or foods Blood or other bodily fluids handled outside the body

Airborne transmission

Transmission of an infectious disease

Transmission of pathogens via aerosols (Distance >1m)

Aerosols from sneezing and coughing , air conditioning systems, flaming inoculating loops

Mycobacterium tuberculosis, Legionella pneumophila Fungal spores

Foodborne transmission

Waterborne transmission

Transmission of an infectious disease

Fecal-oral route of infection is a major source of disease in the world Spread of gastrointestinal

diseases (giardiasis, cholera)

Contamination of foods with feces and pathogens is another example of fecal-oral transmission

Food inadequately processed, undercooked, poorly refrigerated

Vectors

Arthropods carry pathogens from one host to another Fleas, ticks, mosquitoes, mites, bloodsucking flies

Transmit disease by 2 general methods:

Mechanical transmission: (passive process) - Arthropod carries pathogen on feet or other body parts

Biological transmission: (active process) - Pathogen reproduces within the vector and it is generally transmitted through a bite

Foodborne infections Salmonella, Shigella, E. coli

Tick Mite Flea Lice

Fly Mosquito Bug

Incidence Fraction of a population that contracts a disease during a specific

time (new cases)

Prevalence Fraction of a population having a specific disease at a given time

(total number of cases)

Occurrence and frequency of a disease - Epidemiology

Sporadic disease: Disease that occurs occasionally in a population

Endemic disease: Disease constantly present in a population (common cold)

Epidemic disease: Disease acquired by many people in a given area in a short time (greater frequency than usual)

Pandemic disease: Worldwide epidemic (influenza/AIDS!?)

Occurrence and frequency of a disease - Epidemiology

Occurrence and frequency of a disease - Epidemiology

Nosocomial Infections

Nosocomial Infections are acquired in hospitals, nursing homes or other health care facilities

Nosocomial diseases cause thousands deaths annually in the US

Interaction between

three factors poses a significant risk of nosocomial infections

Nosocomial Infections

Microorganisms involved in nosocomial Infections

Microorganisms involved in nosocomial Infections

Clostridium difficile Long-term antimicrobial

therapy in hospitalized patients

Excessive growth of C. difficile (a transient microbe of the colon)

Nosocomial diarrhea Inflammation, bloody stools,

and formation of lesions in the colon - pseudomembranous colitis Lesions: connective tissue,

dying leukocytes, dead colon cells

Nosocomial infections are classified as

Exogenous Pathogens acquired from the health care environment

Endogenous

Pathogens from the normal microbiota within the patient

Iatrogenic Associated with modern medical procedures such as

catheters, invasive diagnostic procedures, and surgery

Nosocomial Infections

The Center for Disease Control and Prevention (CDC)

The CDC publishes Morbidity and Mortality Weekly Report (MMWR) , which indicates the number of cases of the nationally notifiable diseases

Their occurrence must be reported to the CDC