2. The Chain Model of
Communicable Diseases
Dr. Ashish V. Jawarkar
3. The Chain Model of
Communicable Diseases
Infectious agent
Reservoirs and/or sources
– human
– animal
– Environment
Portal(s) of exit:
– Respiratory tract
– GI tract
– Genital/urinary tract
– Breaks in skin
Dr. Ashish V. Jawarkar
4. The Chain Model of
Communicable Diseases
Modes
of transmission
– Direct contact
– Indirect contact
Portals
of entry
Susceptible
hosts
Dr. Ashish V. Jawarkar
5. Modes of Transmission
Direct
Transmission
– Direct Contact
– Droplet
Indirect
Transmission
Vertical
transmission (mother to
– Vector-borne
infant)
Dr. Ashish V. Jawarkar
7. Transmission of Infections by
Respiratory Aerosols
•
Droplets: land directly on mucosal lining of nose,
mouth, eyes of nearby persons or can be inhaled.
Highest exposures within 3-6 feet.
•
Airborne: aerosols become smaller by evaporation;
small aerosols (≤ 10 microns) remain suspended for
longer periods, if inhaled travel deep into the lungs.
•
Contact: Aerosols/ secretions contaminate nearby
surface. Touch surfaces can infect self or others.
Relative contribution of three routes varies with agent.
Dr. Ashish V. Jawarkar
8. Modes of Transmission via
Infectious Respiratory Secretions
Droplet: meningococcal meningitis, rubella,
pertussis, common cold, SARS, influenza*
Airborne: tuberculosis, measles, varicella,
smallpox, SARS, avian influenza
Indirect contact: (fomite) RSV, SARS
*Influenza traditionally droplet, increasing evidence
for airborne component
Dr. Ashish V. Jawarkar
10. Infection Control in a Health Care
Setting
Basic principles
Standard precautions
Transmission-based precautions
Seasonal influenza in health care settings
Vaccination of HCWs
TB screening of HCWs
Proper donning and doffing
Choose your PPE
Dr. Ashish V. Jawarkar
11. Basic Principles
All body fluids are potentially infectious (except
sweat)
– blood and blood-tinged fluids including openwounds
– stool, urine, vomit, respiratory secretions,
saliva, semen, vaginal secretions, breast milk,
other body fluids such as pericardial and
synovial fluids
Minimize exposure to potentially infectious body
fluids
Infection control measures designed to “break the
chain” of transmission
Dr. Ashish V. Jawarkar
12. Standard Precautions in Health
Care Settings
1.
2.
Appropriate hand hygiene
Barrier protective equipment:
–
–
2.
if splash, splatter, or sprays can be
reasonably anticipated
choose appropriate PPE as needed:
gloves, gown, mask, eye protection
(face shield, goggles)
Proper use and handling of
patient care equipment
Dr. Ashish V. Jawarkar
13. Standard Precautions in Health
Care Settings
4.
5.
6.
7.
8.
9.
Proper environmental cleaning and
disinfection
Proper Handling of Linen
Adherence to Bloodborne
Pathogens Standards
Proper patient placement
Respiratory Hygiene/Cough
Etiquette
Safe injection practices
Dr. Ashish V. Jawarkar
14. Expanded Isolation Precautions:
Transmission-based Standards
When standard precautions are
not enough
Additional measures based on
mode of transmission
Contact Precautions
Droplet Precautions
Airborne Precautions
Dr. Ashish V. Jawarkar
15. Transmission-Based Precautions:
Contact Precautions
For known or suspected infections
that represent an increased risk of
spread by direct or indirect
contact with the patient or the
patient’s environment
Dr. Ashish V. Jawarkar
16. Transmission-Based Precautions:
Contact Precautions
Personal Protective Equipment
Gown & Gloves for all patient interactions
Don PPE on entry, discard before exiting
room. (in addition to Standard
Precautions)
Examples: MRSA, C difficile, Norovirus, other
GI pathogens, RSV, antibiotic-resistant
pathogens
Dr. Ashish V. Jawarkar
17. Transmission-Based Precautions:
Droplet Precautions
Single room preferred, no special ventilation
Patient: Mask if transport necessary. Instruct
on respiratory hygiene/cough etiquette
HCWs wear surgical or procedure mask within
6 feet of patient. Eye protection if splash, spray
anticipated
(in addition to Standard Precautions)
Dr. Ashish V. Jawarkar
18. Transmission-Based Precautions:
Airborne Precautions
Airborne Infection Isolation Room (AIIR) if available
Patient: Mask if transport necessary (as tolerated).
Health care workers (HCWs):
N95 respirator prior to entry into room, discarded after exit.
Higher level respirators for aerosol-gen procedure. Careful
attention to proper putting on & taking off (don/doff)
respirator, including seal check.
Hand hygiene before & after don/doff.
Alert others if need to transfer
(in addition to Standard Precautions)
Dr. Ashish V. Jawarkar
19. Seasonal Influenza in Healthcare
Settings: Isolation Precautions
For
aerosol-generating procedures:
N95 respirator + standard
precautions (gown, gloves, goggles
for spray/splash)
Dr. Ashish V. Jawarkar
21. Vaccination of HCWs
Protect patients, protect yourself and other HCWs
CDC recommends
– Measles, mumps, rubella (MMR): vaccinate unless
documentation of immunity or previous
vaccination
– Varicella (chicken pox): vaccinate unless
documentation of immunity or previous
vaccination
– Tdap
– Yearly influenza vaccination
– Hepatitis B: vaccinate unless documentation of
previous vaccination
Dr. Ashish V. Jawarkar
22. Tuberculosis Screening
for Health Care Workers
TB
screening at hire and then
annually for all licensed healthcare
facilities (e.g., acute care hospitals,
skilled nursing facilities, primary
care clinics)
Dr. Ashish V. Jawarkar
23. Sequence for Donning PPE
1.
Gown
2.
Mask or Respirator
www.cdc.gov/ncidod/dhqp/ppe.html
Dr. Ashish V. Jawarkar
24. Sequence for Donning PPE
3.
Goggles/Face Shield
4.
Gloves
www.cdc.gov/ncidod/dhqp/ppe.html
Dr. Ashish V. Jawarkar
25. Sequence for Removal of PPE
1.
Gloves
www.cdc.gov/ncidod/dhqp/ppe.html
Dr. Ashish V. Jawarkar
26. Sequence for Removal of PPE
2.
Goggles/Face Shield
www.cdc.gov/ncidod/dhqp/ppe.html
Dr. Ashish V. Jawarkar
27. Sequence for Removal of PPE
3.
Gown
www.cdc.gov/ncidod/dhqp/ppe.html
Dr. Ashish V. Jawarkar
28. Sequence for Removal of PPE
4.
Mask or Respirator
www.cdc.gov/ncidod/dhqp/ppe.html
Dr. Ashish V. Jawarkar
29. What Type of PPE Would You
Wear?
Giving a bed bath?
– Generally none
Suctioning
oral secretions?
– Gloves and mask/goggles or a face
shield – sometimes gown
www.cdc.gov/ncidod/dhqp/ppe.html
Dr. Ashish V. Jawarkar
30. What Type of PPE Would You
Wear?
Transporting
chair?
a patient in a wheel
– Generally none required
Responding
to an emergency where
blood is spurting?
– Gloves, fluid-resistant gown,
mask/goggles
www.cdc.gov/ncidod/dhqp/ppe.html
Dr. Ashish V. Jawarkar
31. What Type of PPE Would You
Wear?
Taking vital signs
– Generally none
Drawing
blood from a vein?
– Gloves
www.cdc.gov/ncidod/dhqp/ppe.html
Dr. Ashish V. Jawarkar
32. What Type of PPE Would You
Wear?
Cleaning
an incontinent patient with
diarrhea?
– Gown, gloves
Taking
vitals on a patient with
suspect TB?
– N95 respirator
www.cdc.gov/ncidod/dhqp/ppe.html
Dr. Ashish V. Jawarkar
33. Controlling the Spread of
Aerosol Transmissible
Diseases in Health Care
Settings
Breaking the Chain
Dr. Ashish V. Jawarkar
34. Aerosol Transmissible Diseases in
Health Care and Public Safety Settings
Droplet
Airborne
– Meningococcal
meningitis
– Pertussis
– Mumps
– Rubella (German
measles)
– Strep pharyngitis
– Influenza
– Tuberculosis
– Varicella
(chickenpox)
– Measles
– SARS
– Avian influenza
– Smallpox
– Influenza
Dr. Ashish V. Jawarkar
35. Hierarchy of Infection Prevention and
Control Measures
Elimination of Potential
Exposures
Protects
most
people
Engineering Controls
Administrative
Controls
PPE
Protects
only the
wearer
Dr. Ashish V. Jawarkar
36. Hierarchy of Control Technologies
• Goal is to reduce exposures to a
hazard
Order
in which these elements are
selected to control exposure is
important
–
–
–
–
Elimination of Potential Exposures
Engineering controls
Administrative and work practice controls
Personal protective V. Jawarkar
equipment/apparel
Dr. Ashish
38. Engineering Controls
Physically
separates the employee
from the hazard
Does not require employee
compliance to be effective
Examples:
– physical barriers at triage
– airborne infection isolation room for
patients with known or suspect airborne
infectious diseases
Dr. Ashish V. Jawarkar
39. Administrative Controls/
Workplace Practices
Policies, procedures, and programs that minimize
intensity or duration of exposure
– Examples:
signs on door of an airborne isolation room
triage, mask symptomatic patient
provide tissues/ masks/hand sanitizer to
public
Standard procedures/ behaviors in caring for
patients e.g. hand hygiene, HCW vaccination
Only as good as enforcement
Dr. Ashish V. Jawarkar
40. Personal Protective Equipment
Lowest
level of hierarchy - requires
employee compliance for efficacy
Means higher elements of hierarchy
fail to adequately protect employee
May involve use of gowns, gloves,
eye/splash protection or respirators
Last line of defense
Dr. Ashish V. Jawarkar
41. Face Masks vs. N95 Respirators
Loose fitting, not designed to
filter out small aerosols
Place on coughing patient
(source control)
HCW should wear mask to
– protect patient during
certain procedures (e.g.,
surgery, LP)
– protect HCW
droplet precautions
Mask + goggles for
anticipated
spray/splash
Tight fitting respirator,
designed to filter the air
Protects the wearer
HCW should wear when
concerned about
transmission by airborne
route
Dr. Ashish V. Jawarkar
The chain model of communicable diseases is a model to understand the factors involved in the spread of infectious diseases. There are 6 links in the chain. All these steps are necessary to spread a “contagious or communicable disease”.
To stop the spread of disease, one or more of these links must be broken.
Source: Epidemiologic Methods for the Study of Infectious Diseases, Oxford University Press 2001
The first link in the chain of infection is the infectious agent.
Often called microbes or “germs”. The main agents are bacteria, viruses, fungi, or parasites
The second link or reservoir are “hiding places” – the usual places where the microbe can live, grow and multiply.
Reservoirs can be:
Human (both acute clinical cases who are infectious and carriers). Example: infectious (measles, influenza) and carriers (Hepatitis B, Salmonella typhi)
Animal (zoonotic diseases) Examples: tularemia, rabies
Environmental: soil: tetanus, cocci; water: legionnaire’s
The third link is portal of exit: routes by which the infectious agents escapes the human or animal reservoirs. Some diseases may have several portals of exit.
Examples: respiratory tract (nasal/respiratory secretions); GI tract (saliva, vomitus, stool); GU tract (urine, semen, vaginal secretions; Breaks in skin: (skin rash, needle sticks, bites of mosquitos)
The fourth link is modes of transmission: We’ll discuss more about these in the next slide.
The fifth link is portals of entry: Same as exit portal or may differ.
An finally, susceptible host(s) – susceptible either due to lack of immunity to the infectious agent or immune system compromised
Communicable Diseases can be transmitted by two different mechanisms: direct transmission and indirect transmission.
Direct transmission occurs through:
Contact with infectious body fluids of human/animal. For human to human transmission this is usually through direct contact (e.g. touching, kissing, biting, sexual intercourse).
Direct contact is one important route of transmission for HIV and Hepatitis B.
Humans can contract rabies by direct contact with infectious saliva through the bite of a rabid dog or bat.
Large droplets produced by sneezing, coughing, or even talking (direct transmission). Transmission is by direct spray over a few feet before the droplets fall to the ground (e.g., Pertussis and meningococcal infection).
Indirect transmission can occur through:
Common vehicle that is contaminated (e.g., food, water, fomites or inanimate objects like the doorknob), or biologic products (e.g., blood).
Vector-borne (e.g., malaria, WNV. Lyme disease)
Airborne (e.g., inhalation route).
Airborne transmission refers to spread of infectious aerosols, spores, contaminated dust through the air causing diseases primarily by inhalation. Most of us know that TB is transmitted by the airborne route.
Hanta virus pulmonary syndrome is another disease transmitted by the airborne route.
Small rodents can carry hanta virus and excrete the virus in their urine.
Sweeping in enclosed areas with rodent infestations can stir up contaminated dust particles which, if inhaled, can cause a life-threatening acute respiratory disease syndrome in humans.
Vertical transmission: This is from mother to child, often in utero or during childbirth (also referred to as perinatal infection). It occurs more rarely via breast milk. Example: HIV, Hepatitis B and Syphilis.
Some diseases are transmitted by multiple routes:
Malaria, West Nile virus- mosquito (vector), transfusions (vehicle; rare), transplacental (direct; rare)
As we heard earlier today
Infectious aerosols are generated when an some ill person sneezes, coughs or speaks. For some types of infections (such as chickenpox or measles), a person can be infectious even before the onset of symptoms.
Expulsion of infectious material into the air through sneezing, coughing or through aerosol-generating medical procedures such as sputum induction creates large droplets and smaller aerosols. Even speaking and breathing can generate smaller amounts of aerosols routes of transmission of respiratory infections
Bacterial or viral laden droplets (10-100 ) can land on the mucous membranes of the nose, mouth eyes or nearby persons or inhaled
Highest exposures within 3-6 feet.
Once in contact with the mucosal surface, an infection can be established in an innocent bystander who is not immune.
Airborne smaller aerosols created during cough or after evaporation of larger aerosols can remain suspended in the air for longer periods and can be inhaled deeply in the lungs.
Respiratory secretions contaminate a surface and if transferred to the hands, may then lead to self-inoculation or infection of others
For instance, TB is transmitted by the airborne route. Household contacts are the most susceptible, but those sharing the same airspace in close quarters (e.g. in airplanes) are also at increased risk of infection.
Measles is highly infectious. Transmission is primarily person to person by large droplets. However, airborne transmission has occurred in a closed area (e.g., doctor’s exam room) up to 2 hours after a person with measles has occupied the area.
Some infections can be transmitted by multiple routes
RSV – a common respiratory illness in early childhood that causes infant wheezing or bronchiolitis. The virus can survive on non-porous surfaces for hours and some studies have shown that direct transmission by contact with contaminated surfaces is the most common mode of transmission in health care settings
Influenza: traditionally droplet; increasing evidence for airborne component. The flu virus can survive on hands for 15 minutes and 2-48 hours on surfaces depending on the surface material, temperature and relative humidity.
Cal/OSHA ATD Standard, Appendix A for list of Aerosol Transmissible Diseases (ATDs) which require airborne and droplet precautions
Will find varying concentration of infectious agent in certain body fluids
CDC: 2007 Guideline for Isolation Precautions: Preventing Transmission of Infectious Agents in Healthcare Settings
http://www.cdc.gov/hicpac/2007IP/2007isolationPrecautions.html
Diseases may be spread through inhalation of small infectious particles, including respiratory droplets that become smaller in size due to evaporation.
Airborne Infection Isolation Rooms are intended to prevent transmission of infectious agents suspended in the air that remain infectious over long distances. Requirements include:
Increased ventilation rate
Air exhausted directly to the outside or HEPA filtration on exhaust
H1N1 AIIR not necessary (near aerosol) not infectious over long distances
Measles is highly communicable and secondary cases have been documented in health care settings over an hour after a measles case left the doctor’s office
PH recommends that in general exam rooms should not be used for 2 hours
Facility must have respiratory protection program (education, fit-testing, user seal checks in place)
Respirator should be donned prior to entry into room and discarded after exit
Single room preferred; alternative is cohorting
Patient should be transported with surgical mask
CDC describes a hierarchy of infection prevention and control measures ranked in order from overall effectiveness in controlling disease in a population (protecting most of the people to protecting only the wearer).
The most effective measure is eliminating potential exposures. Examples: patients with mild influenza-like illness stay home, policy to not allow ill visitors.
Engineering controls do not require an individual employee implement the control. Examples: installing partitions in triage areas and other public spaces to reduce exposures by shielding personnel and other patients; use of negative pressure rooms for aerosol generating procedures.
Administrative controls are work practices and policies that prevent exposures. Effectiveness is dependent on consistent implementation. Examples: vaccination; masks for symptomatic patients; and promoting respiratory hygiene and cough etiquette.
Personal protective equipment (PPE) is a last line of defense for individuals against hazards that cannot otherwise be eliminated or controlled. PPE will not be effective if adherence is incomplete or when exposures to infectious patients or ill co-workers are unrecognized.
Again, in health care settings, a patient with suspect ATD should wear a mask when around others
HCW should wear surgical or procedural mask to protect the patient when performing certain procedures such as insertion of central lines/ lumbar punctures & epidurals
HCW should wear a mask to protect his/her self against ATDs or as part of PPE for anticipated spray or splash
http://www.cdc.gov/Features/MasksRespirators