Disease, Disease Transmission, and Epidemiology

Chapter 14

Table 14.1 The Three Types of Symbiotic Relationships

Symbiotic Relationships Between Microbes and Their Hosts

• Normal Microbiota in Hosts– Organisms that colonize the body’s surfaces

without normally causing disease– Two types

• Resident microbiota• Transient microbiota

Symbiotic Relationships Between Microbes and Their Hosts

• Normal Microbiota in Hosts– Acquisition of normal microbiota

• Development in uterus is free of microorganisms• Microbiota begin to develop during birthing

process• Much of your resident microbiota is established

during first month of life

Symbiotic Relationships Between Microbes and Their Hosts

• How Normal Microbiota Become Opportunistic Pathogens– Opportunistic pathogens

• Normal microbiota that cause disease under certain circumstances

– Conditions that provide opportunities for pathogens• Introduction of normal microbiota into unusual site

in body (cuts and abrasions)• Immune suppression (radiation therapy)• Changes in the normal microbiota (antibiotic use)

What is a disease?

• Infection– When contaminating organism evades body’s

external defenses and multiplies in the host– Most infections are eliminated by the bodies

defenses• Disease

– Infections may lead to disease– Results when normal body functions are altered

• Causation of Disease– Germ theory of disease

• Disease caused by infections of pathogenic microorganisms

– Robert Koch developed a set of postulates to prove a particular pathogen causes a particular disease

• Exceptions to Koch’s postulates– HIV/AIDS

» Not ethical– Syphilis

» Not culturable in laboratory setting– Mycobacterium leprae

» Never been grown in lab

Infectious Disease

Symptoms and Signs

• Symptoms– Subjective characteristics of disease felt only by the

patient• Signs

– Objective manifestations of disease observed or measured by others

• Asymptomatic– Infections that lack symptoms but may still have signs of

infection and can be transmitted

Classification of Infectious Diseases

• Terms used to classify infectious disease– Acute disease (rapid onset, short period of time ex.

Common cold)– Chronic disease (develop slowly, continual or

recurrent ex. Hepatitis C, TB)– Latent disease (pathogen remains inactive for a long

period of time before becoming active ex. shingles)

• The Stages of Infectious Disease– Many infectious diseases have five stages

following infection• Incubation period (time between infection and first symptoms

and signs of disease)• Prodromal period (short time, mild symptoms)• Illness (severe stage, signs and symptoms most evident)• Decline (gradual decline of signs and symptoms as body returns

to normal due to immune response or drug treatment)• Convalescence (patient recovers no signs or symptoms)

– Patient can be infectious at any stage of disease depending on the causative agent

Figure 14.10 The stages of infectious diseases

Incubationperiod

(no signs orsymptoms)

Prodromalperiod(vague,general

symptoms)

Illness(most severe signs

and symptoms)

Decline(declining signsand symptoms)

Convalescence(no signs orsymptoms)

Time

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How do we acquire a disease causing organism?

• Reservoir of infection– Sites where pathogens are maintained as a source of

infection• Reservoir may or may not be harmed by pathogen

• Most pathogens cannot survive for long outside their host– A successful pathogen needs to overcome the innate and adaptive

defenses long enough for the pathogen to multiply and exit the host

• It is a disadvantage for the pathogen to actually kill a host, since the opportunity to be transmitted may be limited and it loses the source of nutrients

Reservoirs of infection

• Three types of reservoirs– Human reservoirs– Animal reservoirs– Nonliving reservoirs

• Human reservoirs– The principle living reservoir- the human body– People with signs and symptoms may transmit– Infected individuals who are asymptomatic

but infective to others (before or after onset of illness)

• Carriers• individuals remain asymptomatic and infective for

years (ex. Tuberculosis, syphilis, and AIDS)

• Animal Reservoirs– Zoonoses or Zoonotic diseases (150 diseases)

• Diseases naturally spread from animal host to humans

– Acquire zoonoses through various routes• Direct contact with animal or its waste (E. coli

O157:H7)• Animal bites (Rabies)• Insect bites (Lyme Disease)

• Nonliving Reservoirs – Soil, water, and food can be reservoirs of

infection• Presence of microorganisms often due to

contamination by feces or urine• Can be naturally occurring in environment (Tetanus

and botulism)• Water (Example: parasitic eggs or E. coli)• Meats and vegetables

Movement of a pathogen into a host

• Portals of Entry – Sites through which pathogens enter the

body– Four major pathways

• Skin• Mucous membranes• Placenta• Parenteral route

BrokenskinInsectbite

Anus

Ear

Conjunctivaof eye

Mouth

Placenta

VaginaPenis

Urethra

Censored

• Portals of Entry – Skin

• Outer layer of dead skin cells acts as a barrier to pathogens

• Some pathogens can enter through openings or cuts

• Others burrow into or digest outer layers of skin

• Portals of Entry – Mucous membranes

• Line the body cavities that are open to the environment

• Respiratory tract is the most common site of entry– Entry is through the nose, mouth, or eyes

• Gastrointestinal tract may be route of entry– Must survive the acidic pH of the stomach

• Portals of Entry – Placenta

• Typically forms effective barrier to pathogens• Pathogens may cross the placenta and infect the

fetus– Can cause spontaneous abortion, birth defects,

premature birth– Listeria (lunch meats)

• Portals of Entry – Parenteral route

• Not a true portal of entry– A way portals of entry can be bypassed

• Mechanically depositing pathogens directly into tissues beneath the skin or mucous membranes

– Hypodermic needles– Thorns, nails, etc..

The Movement of Pathogens Out of a reservoir: Portal of Exit

• Pathogens leave host through portals of exit• Many portals of exit are the same as portals of

entry• Pathogens often leave hosts in materials the

body secretes or excretes

Figure 14.11 Portals of exit

Ear (earwax)

Broken skin(blood)

Skin(flakes)

Anus(feces)

Urethra(urine)

Seminal vesicles(semen and lubricating secretions)

Eyes(tears)

Nose (secretions)

Mouth(saliva, sputum)

In females:Mammary glands(milk, secretions)

Vagina(secretions, blood)Censored

Censored

Microbial Mechanisms of Pathogenesis

• Pathogenicity- the ability of a microorganism to cause disease

• Pathogenesis- The mechanism a microorganism uses to cause disease

• Virulence- the relative ability of a pathogen to infect a host and cause disease.

A pathogens goal

– Enter a host– Adhere to host cells– Gain access to nutrients– Evade detection and removal by immune system

in order to colonize– Leave host

Virulence factors

• Traits a pathogen uses to:– Enter a host

• Portals of entry

Virulence factors– Adhere to host cells

• Adhesion factors– Bacteria and viruses have surface proteins that enable them to bind to

complementary host receptors» Adhesins on bacteria

• Fimbriae» Attachment proteins on viruses

– Capsule– Biofilm production

• Host specific interactions – Adhesins and receptors determines the specificity of pathogens for particular

hosts– Neisseria gonorrhoeae

» Has adhesins on its fimbriae that adhere to cells lining urethra and vagina of humans, can not infect other hosts

» Can attach to sperm cells as they swim by!

Virulence factors

• Traits a pathogen uses to:– Gain access to nutrients

• Produce extracellular enzymes– Dissolve structural chemicals in the body – To further invade and gain access to nutrients

• Produce Toxins– Exotoxins

» Secreted by pathogen to destroy host cells • Cytotoxins-Kill host cells or affect their function• Neurotoxins- interfere with nerve function• Enterotoxins- affect cells lining gastrointestinal tract

• Membrane damaging toxins– Disrupt plasma membranes– Hemolysins

• Blood cells

• Endotoxins– Also called lipid A– Lipid portion of Gram negative LPS– Released when G – cells die naturally, divide, or

are digested by phagocytic cells– Cause fever, inflammation, diarrhea, shock

Virulence factors• Traits a pathogen uses to:

– Escape detection and removal by immune systems• Antiphagocytic factors

– Capsules» Capsules are composed of chemicals found naturally in the body

• Immune cells do not recognize them as foreign antigen» Capsules are slippery which makes it difficult for phagocytes to adhere

– Antiphagocytic chemicals» Preventing the fusion of lysosomes with phagocytic vesicles.

• Pathogen is able to survive inside phagocyte

– Avoiding Antibodies» Secretion of proteases that break down antibodies

• IgA in body secretions bind to bacterial adhesins on pili

» Rapid turn over of pili» Antigenic variation- genetically alter the type of pili made» Fc Receptors

– Gonorrhea produce chemicals which prevents fusion of lysosome with phagosome and secretes a protease which breaks down IgA in mucus.

• Leave host to “find” new host– Portal of exit

Modes of Infectious Disease Transmission

• Transmission is from a reservoir or a portal of exit to another host’s portal of entry

• Three groups of transmission– Contact transmission

• Direct, indirect, or droplet

– Vehicle transmission• Airborne, waterborne, or foodborne

– Vector transmission• Biological or mechanical

Contact Transmission

Direct Contact• One person physically touches another

– Handshake– Sexual intercourse– Fecal-oral

Contact Transmission

Indirect Contact• Transfer of pathogens via inanimate

objects (called fomites)– Clothing– Doorknobs– Glasses and eating utensils

Ex. Scabies: Bedding serves as fomite involved in transmission

Contact Transmission: Droplet

• Transmission of a pathogen through inhalation of respiratory droplets. – Large microbe saturated droplets fall to the ground

no farther than 3 feet from release.• Close proximity necessary

– Coughing– Sneezing

• Examples of diseases: – Cold, Flu

Vehicle Transmission: Airborne • Transmission of a pathogen that is suspended in the

air and travels through the air to another host• Very difficult to control!• Remains suspended indefinitely

– Coughing– Sneezing

• Examples of diseases: – Tuberculosis, Measles

• Usually digestive tract diseases• Contamination can occur in a variety of different ways• Animals can harbor pathogens

– Salmonella– E. coli O157:H7

• Inadvertently added during food preparation– No handwashing

• Staphylococcus aureus

– Cross-contamination• Cutting board used on meat and vegetables for your salad• Meat and vegetable processing facilities

Vehicle Transmission: Food

• Foodborne intoxication– Ingestion of an exotoxin produced by an organism

growing in a food product.– Toxin causes illness not the living organisms– Staphylococcus aureus and Clostridium botulinum

Vehicle Transmission: Food

• Foodborne infection– Requires the consumption of living organisms

• Undercooked hamburger or chicken

– Symptoms do not usually appear for at least 1 day• Diarrhea

– Thorough cooking before consumption will kill the organisms

– E. coli O157:H7– Salmonella

Vehicle Transmission: Food

• Can involve large numbers of people• 1993 Cryptosporidium parvum (intestinal

parasite)– Watery diarrhea 10-15L a day– Infectious dose: less than 10 organisms– Milwaukee, WI

• Infected ~400,000 people from municipal water source

Vehicle Transmission: Water

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–Biological Vector–Mechanical Vector

Vector Transmission

1. Biological Vector• An organism transmits and serves as

host.• Vector involved in life cycle of

pathogen– Ex: Malaria: Part of life cycle of protozoan,

Plasmodium, occurs inside the Anopheles mosquito

2. Mechanical VectorVector transmits disease causing organism

through mechanical contact – Ex: Trachoma (Blindness): Chlamydia

trachomatis carried on feet of fly from infected person's eye to eye of new host

Epidemiology

• Epidemiology focuses on the effect of a pathogen in a population – The study of where and when diseases occur and how they are

transmitted in a population.• Why is this field important?

– Earth’s population is becoming over populated. – Humans are relying more on mass food production and distribution – Travel to other countries very readily. – Leads to higher incidence, number of new cases of a disease in a

population) – Prevalence, the total number of cases, new and already existing in

a population

• Sporadic• Endemic• Outbreak• Epidemic• Pandemic

Occurrence of an infectious disease can be classified in terms of geographic distribution and frequency.

Sporadic

• A few cases randomly distributed geographically

Example: • Plague in U.S.

Endemic • A native disease that prevails continuously in a

geographic region• Endemic disease can lead to epidemic

Example: • Pertussis in U.S.• Common cold• influenza

Year Reported Cases*

2000 7,867

2001 7,580

2002 9,771

2003 11,647

2004 25,827

2005 25,616

2006 15,632

2007 10,454

2008 13,278

2009 16,858

2010 27,550

2011* 18,719

*Total reported cases include those with unknown age.

• Implies a cluster of cases occurring during a brief period of time and attacking a specific population, usually food borne

• E. coli O157:H7 from contaminated food sources

Outbreak

Seventy-seven people sick and one died in a salmonella outbreak caused by contaminated ground turkey:Centers for Disease Control 2011

Epidemic • Affecting an unusually large number of

individuals within given region or population • Epidemic may lead to pandemic• H1N1 was epidemic in US in 2009

• Pandemic- epidemic on morethan one continent– Example 2009 H1N1, also known as Swine Flu, breaks

out and quickly spreads to more than 70 countries.

Pandemic • A disease affecting an increased proportion of

the population over a wide geographic area, most often worldwide

Example:• AIDS

• Local and state level health departments rely on doctors and hospitals to report infectious diseases.

• In some cases the disease occurrence is nationally recorded to the Center for Disease Control (CDC). – Weekly updates can be viewed of the notable

diseases in the Morbidity and Mortality Weekly Report (MMWR).

Epidemiology of Infectious Diseases

• Hospital Epidemiology: Nosocomial Infections– Control of nosocomial infections

• Disinfection, good housekeeping, bathing, sterile procedures, and HAND WASHING!

• Hand washing is the most effective way to reduce nosocomial infections

• CDC reports that on average, health care workers wash their hands before interacting with patients only 40% of the time.

Figure 14.20 The interplay of factors that result in nosocomial infections

Presence ofmicroorganisms in

hospital environment

Immunocompromisedpatients

Transmission ofpathogens betweenstaff and patients

and among patients

Nosocomialinfection

Factors that influence nosocomial infections:>>>>>

Infection can result from any one of these factors but usually it is a product of all three