Disease, Disease Transmission, and Epidemiology

Chapter 14

Symbiotic Relationships Between Microbes and Their Hosts

• Types of symbiosis– Mutualism– Commensalism– Parasitism

Table 14.1 The Three Types of Symbiotic Relationships

Figure 14.1 Mutualism as shown by wood-eating termites

Symbiotic Relationships Between Microbes and Their Hosts

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

without normally causing disease– Two types

• __________microbiota• __________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•

• •

What is a disease?

• Infection–

– Most infections are eliminated by the bodies defenses

• Disease– Infections may lead to disease–

• 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 lab

The Nature of Infectious Disease

• Manifestations of Disease: Symptoms and Signs

• Symptoms–

• Signs–

• 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– _________disease (rapid onset, short period of time

ex. Common cold)– _________disease (develop slowly, continual or

recurrent ex. Hepatitis C, TB)– _________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• ____________period (time between infection and first symptoms

and signs of disease)• _____________period (short time, mild symptoms)• ____________(severe stage, signs and symptoms most evident)• ____________(gradual decline of signs and symptoms as body

returns to normal due to immune response or drug treatment)• _____________(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

Num

ber o

f mic

roor

gani

sms

orin

tens

ity o

f sig

ns o

r sym

ptom

s

How do we acquire a disease causing organism?

• 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

• Reservoirs of infection–

• Reservoir may or may not be harmed by pathogen• Three types of reservoirs

– __________reservoirs– __________reservoirs– __________reservoirs

• Human reservoirs– The principle living reservoir- the human body–

– Infected individuals who are ___________ but infective to others (before or after onset of illness)

– Some individuals remain asymptomatic and infective for years (ex. Tuberculosis, syphilis, and AIDS)

•

• Animal Reservoirs– __________or Zoonotic diseases (150 diseases)

• Diseases naturally spread from animal host to humans

– Acquire zoonoses through various routes• _____________with animal or its waste (E. coli

O157:H7)• __________animals (tape worm)• ____________(Rabies)• ____________(Lyme Disease)

• Nonliving Reservoirs – _______________________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

The Movement of Pathogens Out of a reservoir: ______________

• 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)

Movement of a pathogen into a host

• Portals of Entry – Sites through which pathogens enter the

body– Four major pathways

• • • •

BrokenskinInsectbite

Anus

Ear

Conjunctivaof eye

Mouth

PlacentaVaginaPenis

Urethra

• 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..

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 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

• –

– Toxin causes illness not the living organisms– Staphylococcus aureus and Clostridium botulinum

Vehicle Transmission: Food

• –

• 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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– –

Vector Transmission

1. Biological Vector• An organism transmits and serves as

host.•

– Ex: Malaria: Part of life cycle of protozoan, Plasmodium, occurs inside the Anopheles mosquito

2. Mechanical Vector

– 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

•

Example: • Plague in U.S.

Endemic •

• Endemic disease can lead to epidemic

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

Year Reported Cases*2000 7,8672001 7,5802002 9,7712003 11,6472004 25,8272005 25,6162006 15,6322007 10,4542008 13,2782009 16,8582010 27,5502011* 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!

•

• CDC reports that on average, health care workers wash their hands before interacting with patients only __________________

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

Emerging Disease

• What is an emerging Disease?

• Examples:– West Nile virus– HIV/AIDS– Cat Scratch Fever

Factors that contribute to disease emergence

• Microbial evolution• Advances in technology• Mass production, widespread distribution and

importation of food• Climate Change

Reemerging Diseases

• Resurgence of old diseases due to:– – –

Diseases include:TB, Pertussis

Tuberculosis reemerging in US

• Mycobacterium tuberculosis– Causes an infection of the lower respiratory system– Inhalation of airborne bacteria from infected

persons– 9 million people each year develop TB

• 2 million deaths• About 2 billion people world wide are infected• 14,000 cases annually in US usually from foreign born

individuals. Increasing at rate of about 2% each year.

Pertussis commonly called whooping cough

• Bordetella pertussis• Causes an infection of the lower respiratory system• Before DTP vaccination 50% of individuals would develop

pertussis.• Vaccination significantly decreased this disease

– Since 1980s number of cases has increased significantly affecting adults and adolescents.• Individuals are not up to date on booster shots which are

intended to boost immunity• Immunity to the DTP vaccine declines after a few years

– No protection by age 12