April 7, 2009 1

Back to Basics, 2009POPULATION HEALTH (1): Infectious Diseases and Outbreak Investigation

N Birkett, MDEpidemiology & Community Medicine

Other resources available on Individual & Population Health web site

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Infectious Disease SummaryN. Birkett, Back-to-Basics, 2009

• Material relates to MCC objective 78.5

• Terminology

• Nature of diseases

• Outbreaks/epidemics– Identification– Methods of control

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Foodborne Botulism Clostridium Perfringens Salmonellosis Shigellosis Staphylococcal disease Traveler’s disease Trichinosis

Water & Foodborne Amebiasis Cholera Giardiasis Legionellosis E Coli

Person-to-person spread• Aseptic Meningitis• Viral hepatitis• Respiratory Infections

(influenza)• Herpes Simplex• Streptococcal disease

(rheumatic fever)• Tuberculosis• Leprosy

Infections: Sources and agents (1)

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Arthropod Borne• Encephalitis (West Nile)• Lyme Disease• Malaria• Plague• Rocky Mountain Spotted

Fever

Sexually Transmitted• HIV/AIDS• Gonorrhea• Syphilis• Chlamydia Trachomatis

Vaccine preventable• Chickenpox• Diphtheria• Measles• Mumps• Poliomyelitis• Tetanus

Infections: Sources and agents (2)

Zoonotic• Psittacosis• Q fever• Rabies• Hantavirus

Opportunistic fungal/fungal• Coccidioidomycosis• Candidiasis

Prions• Kuru• vCJD

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Terminology (1)

• Infectivity– The ability of an agent to invade and multiply

in a host (an infection).– Dose of organism required to establish

infection in 50% of animals.

• Pathogenicity– The ability of an agent to produce clinically

apparent illness.

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Terminology (2)

• Virulence– The proportion of clinical cases which produce severe

disease and/or permanent sequelae.

• Immunogenicity– The ability of an agent to produce specific immunity

against the agent– Can be produced in general body or within specific

sites such as the GI tract.– Determines the ability of an agent to re-infect a host

• Measles vs. gonorrhea

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Terminology (3)

• Reservoir– Living organisms or inanimate matter in which

infectious agent normally lives and multiplies• Fomites (Vehicle)

– Inanimate objects contaminated with infectious agent (not the reservoir). Example would be toys in a daycare centre.

• Vector– An animate source of an infectious agent. The vector

may be infected with the organism (e.g. mosquitoes and malaria) or just be a mechanical carrier (e.g. flies). There is disagreement about whether vectors are restricted to insects or can also include small mammals.

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Terminology (4)

• Zoonoses– Diseases transmitted to humans from animals (e.g.

anthrax)• Carriers

– An infected person without apparent clinical disease who remains infectious (e.g. Typhoid Mary)

• Index Case– The person (case) who brings the infection to the

attention of the medical community or the public agency. Sometimes used to refer to the person who brings the infection into a community. This will often (but not always) be the same person.

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Terminology (5)

• Attack Rate– The probability that people will get ill from the

disease. Usually applied in an outbreak situation.

• Secondary Attack Rate– Probability of infection in a closed group who are at

risk but excluding the index case(s). Formula is:

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Terminology (6)

• Case Fatality Rate (CFR)– The probability of death in people with an

infection.

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Pathogenic Mechanisms (1)

• Direct Tissue Invasion

• Production of Toxins

• Allergic Host Reaction

• Resistant/latent infection (carriers)

• Enhancement of host susceptibility to drugs (e.g. Reye’s syndrome and ASA).

• Immune Suppression

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Reservoirs (examples of transmission patterns)

• Human Human Human

• Animal Animal Animal

Human

Vector

Animal

Vector

Human

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Mechanisms of Spread (1)

• kissing• sexual intercourse• hand shaking• droplets• spores in soil

• Vehicle borne• fomites (e.g. toys)• food• IV fluid• Vector borne• mechanical (e.g. soiled

feet of insect)• biological (e.g. malaria)• Airborne• dust• droplet nuclei

Direct transmission Indirect transmission

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Epidemics (1)• Epidemic (now often called ‘outbreak’)

– the occurrence in a community or region of cases of a disease/condition/behaviour clearly in excess of normal expectancy

• Endemic– the occurrence of a disease/condition at a relatively

constant level in a given setting• Pandemic

– an epidemic covering a very wide area and affecting a large proportion of the population

• Pathogen– Infectious and non-infectious substance capable of

producing tissue damage or initiating a process which can lead to a disease.

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Epidemics (2)

• Common conditions increasing likelihood of an epidemic– The introduction of a new pathogen or an

increased amount of, or a change in the virulence of, a pathogen.

– An adequate number of exposed and susceptible persons.

– An effective means of transmission between the source of the pathogen and the susceptible person.

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Epidemics (3)• Incubation period and causal agent

Time frame ExamplesHours Food toxins

Heavy metals

Days Bacterial infections

Salmonella / cholera

Weeks Measles / mumps / Hep A

Months Hep B / Rabies

Years Kuru / cancer

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Epidemics (4)

• Types of epidemics– Common source

• Point source• Ongoing exposure• Need not be geographically localized

– Propagated/progressive– Mixed

• Epidemic curve• Spot maps• Note that epidemics can arise from behaviour as

well as from traditional infectious sources.

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Epidemic Curves: point source (1)

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Epidemic Curves: propagated (2)10 days

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Factors Influencing Spread of Disease in a Population (1)

• Period of infectivity in relationship to symptoms– Includes consideration of carrier states

• Herd immunity• Type of spread

– Person to person– Common vehicle– Vector-borne– Zoonoses

• Transmission mechanics– Consider sexual vs. droplet spread

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Develop disease

Immune

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Epidemic Control (1)

Twin goals:• Understand the cause, etc.• Minimize the impact to the affected communityGoals can conflict:• need to collect full information base• need to take action in absence of full informationEffective and clear communication with general public

is essential• SARS outbreak• designate one spokesperson• regular press briefings

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Actions for ‘acute’ outbreak control

Isolation• separation of infected persons or animals from others during the

period of communicability– usually isolate for at least two incubation periods.

Quarantine• restrictions on the activities of well people who (may) have been

exposed to a communicable disease during its period of communicability.– active surveillance is an alternative– usually quarantine for at least two incubation periods.– More controversial than isolation since it affects people who are not

currently ill (and may never get ill).Immunization• passive or active. Passive (IGG) is more useful for acute outbreaks.Chemoprophylaxis

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General approaches to outbreak control (1)

Reduce host susceptibility• Immunization (active and passive)• nutrition• improved income, etc.

Interrupt transmission of the agent• Quarantine/isolation• Case treatment• contract tracing• inspections• environmental clean-up• animal population control

– rabies vaccination of wild animals– insect spraying– monitor for animal infections

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General approaches to outbreak control (2)

Inactivate agent• water purification; chlorination

Personal hygiene measures• hand washing (#1 strategy)• protective clothing (masks, gowns)• avoid at risk situations

Family/community measures• preventing sexual abuse of children leads to reduction in

STDs• Needle exchange and related programmes.

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Surveillance‘the continuing scrutiny of all aspects of occurrence

and spread of disease that we pertinent to effective control’

• Reportable diseases.• Sentinel practices• Animal/water surveys• Environmental monitoring• Mortality (vital statistics)• Provincial laboratory tests• Epidemic investigations• Disease registries• CIHI and related data.