Transmission, entry and colonisation strategies of pathogens

Question 1

What must a pathogen do in order to cause a disease?

Answer 1

Enter a host and attach to a target

Spread from the site of infection

Multiply within the host

Evade host defences

Be shed from the body (Transmit to another host)

Cause damage

Question 2

Transmission occurs via 3 main mechanisms: What is Contact?

Answer 2

Transmission which is either Direct, indirect or airborne

Question 3

Transmission occurs via 3 main mechanisms: What is Disease vehicles?

Answer 3

Medium in which disease can spread

Includes Blood, water, food, soil and air

Question 4

Transmission occurs via 3 main mechanisms: What are Vectors?

Answer 4

This includes Insects which transmit through bites, faeces or from body or mouth parts

Also includes Mechanical or biological

Often reservoirs of disease agents

Question 5

Explain contract transmission through direct contact

Answer 5

Skin, mucous membrane, puncture wounds

Human to human or animal to human

Animals- Zoonoses/ Zoonosis
Animal disease transmitted to humans- Anthrax, Plague

Question 6

Explain contract transmission through indirect contact

Answer 6

Fomites- inanimate objects that contribute to transmission

Question 7

Explain contract transmission through Airborne (droplets)

Answer 7

Coughing, sneezing and laughing

Question 8

What form of transmission is the most effective?

Answer 8

Human to Human

Question 9

What are the commonest diseases spread by?

Answer 9

The Respiratory route

The faecal-oral route

The venereal route

Question 10

What is the Effectiveness of transmission influenced by?

Answer 10

–Number of organisms shed

–Environmental stability

–Infectious dose – linked to relative disease causing potential (virulence), site of infection

Question 11

How does shedding affect transmission?

Answer 11

The more organisms shed the better the chance of transmission occurring

–Many mechanisms evolved – Cholera

Question 12

Stability depends on?

Answer 12

Species and stress

Question 13

The most resistant to desiccation remain?

Answer 13

Infectious

Question 14

Less environmentally resistant pathogens rely on?

Answer 14

Direct transmission and vehicles

Question 15

The infectious dose depends on?

Answer 15

species, site of infection and host status

Question 16

Dose required for Anthrax (Bacillus anthracis) ?

Answer 16

8000 – 80000 spores

Question 17

Dose required for Salmonella enteritidis (salmonellosis) ?

Answer 17

1,000,000 cells

Question 18

Dose required for Shigella dysinteriae (shigellosis) ?

Answer 18

10 cells

Question 19

Dose required for Coxiella burnettii (Q fever)?

Answer 19

1 cell

Question 20

Dose required for Fransicella tularensis (tularemia)?

Answer 20

< 1 cell

Question 21

Infection via respiratory tract much more?

Answer 21

Acute

Question 22

What are the many portals of entry?

Answer 22

• Mucous membranes - GI, nasopharynx, eyes, ears, genitals
• Broken skin
• Glands

Question 23

What must an organism do once transmitted?

Answer 23

Enter and attach, and evade before colonisation and disease is produced

Question 24

What entry sites can also be exit sites?

Answer 24

• Secretions

* Skin – skin scales, blood

Question 25

What are the entry sites of pathogens?

Answer 25

Eye, Ear, nose, mouth, broken skin, mammary glands, Placenta, vagina, urethra and anus

Question 26

What are the exit sites of pathogens

Answer 26

Eye (tears), Ear (wax), Nose (secretion), mouth (saliva, sputum), Broken skin (blood), Skin (flakes), Seminal vesicle (Semen), Anus (feces), Urethra (urine).

In females mammary glands (milk), vagina (secretions, blood).

Question 27

Explain Skin in relation to entry and exit of pathogens

Answer 27

–Very good barrier to infection
•Normal flora – competition and antimicrobials
•Dry and low pH

–Entry through
•Cuts, abrasions, bites, puncture wounds, hair folicles
•Burns particularly prone to infection

–Also a site of transmission
•Disease vehicle - skin scales – 108 scales/day
•Beware the dreaded shedders!!!!

Question 28

Explain Respiratory tract in relation to entry and exit of pathogens

Answer 28

–Approx 10,000 viable organisms inhaled/day

–Removed by entrapment in mucous and swallowing
•Aerodynamic baffle design - Nasal turbinates
•Ciliated epithelium - Mid/upper respiratory tree

–In the deep lung only clearance achieved by alveolar macrophages

Question 29

To colonise the respiratory tract a pathogen must?

Answer 29

–Adhere strongly

• Specific adhesins (ligands) that bind to host epithelial cell receptors
• Influenza virus – Haemaglutanin – sialic acid residues

–Inhibit clearance mechanisms

• Muco-ciliary escalator clears particulates
• Pseudomonas aeruginosa – uncoordinated movement

–Resist macrophage killing

•Mycobacterium tuberculosis

Question 30

Explain transmission from the respiratory tract

Answer 30

–Respiratory pathogens damage respiratory tract
•Mucous production
•Sneezing
•Coughing
•All due to chemicals produced by the immune system

–Results in microbial contamination of surfaces and infectious aerosol production

• Creates fomites
• Survival better in humid conditions
• Transmission better in enclosed spaces

Question 31

Explain the gastrointestinal tract in relation to entry and exit of pathogens

Answer 31

•The Gastrointestinal (GI) tract

–Adherence to host cell receptors is very important

• Must bind and multiply in large numbers – Salmonella enteritidis – 1 billion per g of faeces
• E. coli – adhesins on pili and fimbrae

–Invasion of underlying tissues
•Shigella dysenteriae – invade immune tissues

–Bile and acid resistance
•Helicobacter pylori – lives in the stomach

–Production of diarrhoea is an advantage •Faecal - oral transmission

Question 32

The gastrointestinal tract: What causes “food poisoning”?

Answer 32

A consequence of microbial growth in or on foods
•Ingestion of large amounts of organisms – colonisation
•Ingestion of preformed microbial toxins

–Clostridium botulinum – canned foods

–Bacillus cereus – rice

–Aspergilli – badly stored nuts and pulses

Question 33

Explain transmission from the GI tract

Answer 33

•Transmission from the GI tract
–Occurs in unsanitary conditions
–Faecal – oral spread
•Hand washing important

–Water treatment, sanitation and purification reduced incidence in 1st world

•Water borne disease still common in third world

Question 34

Explain the unrinary tract in relation to entry and exit of pathogens

Answer 34

–Two main types of infection
•Urinary tract infections (UTI’s)
•Sexually transmitted diseases (STD’s)

–Pathogens have special adaptations that aid in attachment
•Resist flushing action of urine

•Posses adhesins that bind to urethral epithelial cells
–Neisseria gonorrhoea – pillus adhesins

•Invade brakes in the skin and membranes–Treponema pallidium - syphilis

Question 35

What causes Urinary tract infections?

Answer 35

–UTI’s are caused by transmission of enteric organisms to urinary tract

• Poor personal hygiene
• Catheters
• Lead to cystitis, urethritis, kidney infections
• Common in hospitals
• Women more prone
• Escherichia coli (most common in UK)

Question 36

Pathogens causing STD’s and UTI’s are poorly transmitted, why?

Answer 36

–Low environmental resistance and persistence

–Rely on direct forms of transmission
•Touching
•Sexual intercourse

–Those that form blisters, growths or puss are transmitted the best!

Question 37

Explain blood in relation to entry and exit of pathogens

Answer 37

–Type of disease vehicle – new hosts

–Spread within the body – within host

–Pathogens in the blood can enter and exit a host by parenteral routes
•Breaks in skin barrier

–Blood sucking insects – Vectors – malaria, plague, typhus

–Sharing needles–Contaminated blood products and surgery
»iatrogenic infection

–Mother to baby – Transplacental transmission•Mostly viruses