Transmission & Outbreaks Week 2 PDF

Transmission & Outbreaks Week 2 Objectives Review basic concepts of the outbreak transmission dynamics Use examples to discuss the links in the chain of transmission for specific pathologies Content Transmission dynamics – Disease transmission dynamics R0 – Outbreak trans...

Transmission & Outbreaks Week 2 Objectives Review basic concepts of the outbreak transmission dynamics Use examples to discuss the links in the chain of transmission for specific pathologies Content Transmission dynamics – Disease transmission dynamics R0 – Outbreak transmission dynamics Herd immunity Epidemiological triad Incubation period and latency Mode of transmission Reservoirs Transmission dynamics Transmission dynamics Disease transmission Outbreak transmission dynamics dynamics Description of incubation and Descriptions of the links in the symptomatic periods; latency disease spread chain and infectivity Disease transmission dynamics End of the disease Cure Death Onset of Immune Infects Moment of infection symptoms Carrier Transmits Susceptible host Incubation Symptomatic Time Latency Infectivity or transmissibility Outbreak transmission dynamics Links in the Chain of Infectious Disease Spread Portal of Potential Agent Source Transmission Portal of exit entry host Basic Reproduction Number (R0) x.60 = An estimate of the speed with which a x 1.0 = disease can spread within a population. This figure is highly useful within the population group from which data is taken. x 2.0 = 1 infection x R0 = New infections x 3.5 = Ridenhour B, Kowalik J, y Shay D. El número reproductivo básico (R0): consideraciones para su aplicación en la salud pública. Am J Public Health. 2018 December; 108(Suppl 6): S455–S465. Disponible en: 10.2105/AJPH.2013.301704s Key: Infected Infectious 2º Susceptible 2º Key: R0 = 2 Infectious Transmission Susceptible No Transmission T(0) T(1) T(2) Some estimated R0 values Disease Geographical location Period R0 Diphtheria New York, USA 1918–19 4–5 Maryland, USA 1908–17 4–5 Scarlet fever Maryland, USA 1908–17 7–8 New York, USA 1818–19 5–6 Pennsylvania, USA 1910–16 6–7 Mumps Baltimore, USA 1943 7–8 England and Wales 1960–80 11–14 Netherlands 1970–80 11–14 Gregory E. Glass. Measuring Disease Dynamics in Populations: Characterizing the Likelihood of Control. On line course. Johns Hopkins University Some estimated R0 values Disease Geographical location Period R0 Rubella England and Wales 1960–70 6–7 East Germany 1970–7 6–7 Czechoslovakia 1970–7 8–9 Poland 1970–7 11–12 Gambia 1976 15–16 HIV (Type I) England and Wales (homosexual men) 1981–5 2–5 Nairobi, Kenya (female sex workers) 1981–5 11–12 Kampala, Uganda (heterosexuals) 1985–7 10–11 Gregory E. Glass. Measuring Disease Dynamics in Populations: Characterizing the Likelihood of Control. On line course. Johns Hopkins University Determining factors in the occurrence of disease outbreaks The number of If the total The balance is cases of disease population is very delicate within a immune, there (herd population will be no immunity), and depends on a outbreaks. if it breaks balance between down, those who are outbreaks may susceptible (at occur. risk) and those who are not susceptible (not at risk). Herd immunity Definition: a If a high number of The cutoff point is Important concept: group of people’s inhabitants is calculated for each Vaccination resistance to immune, the disease. campaigns seek to disease, due to ENTIRE population reach the cutoff the fact that a high is immune (the point (i.e., higher number of probability of than 90%). individuals in that transmission is group is immune. extremely low). Herd immunity levels for selected vaccine-preventable diseases Immunization Levels Disease R0 Herd Immunity 1999 1997–98 19–35 months Preschool Difteria 6–7 85% * 83% 97% Measles 12–18 83–94% 92% 96% Mumps 4–7 75–86% 92% 97% Pertusis 12–17 92–94% 83% * 97% Polio 5–7 80–86% 90% 97% Rubella 6–7 83–85% 92% 97% Smallpox 5–7 80–85% – – * 4 doses Adapted from Epid Rev 1993;15: 265-302, Am J Prev Med 2001; 20 (4S): 88-153, MMWR 2000; 49 (SS-9); 27-38 Conditions for herd immunity to function The only reservoir must The infection must be human generate robust (long- term) immunity The population must mix Outbreaks can occur at random when immunity is below the cutoff point Epidemiological triad Many diseases are better explained Susceptible using the infectious disease model. Host Vector Gordis: Epidemiology, 4th Edition. Copyright © 2008 by Saunders, an Agent Environment imprint of Elsevier, Inc. All rights reserved. Agent Every element that acts as a determining cause and is capable of disrupting the health-disease balance. It might be: – Biological (bacteria, virus) – Chemical (toxins) – Physical (radiation, toxic gases) Agent features Pathogenicity The agent’s ability to generate disease (attack rate) Contagiousness The agent’s ability to spread (secondary attack rate) Infectivity The agent’s ability to infect The agent’s ability to trigger serious disease or death Virulence of its host (mortality rate) The agent’s ability to spread within the individual Invasive capacity following infiltration. Host The organism impacted by the agent Incubation period The period Reflects the time This period is between needed by the influenced by infection and microorganism the infective the onset of to replicate dose received symptoms itself enough to reach the number of copies necessary to cause the disease Phases of the disease Diseases have The Thus, the different asymptomatic severity and phases: clinical disease is VERY transmissibility (symptoms) IMPORTANT in of an infection and subclinical epidemiology varies based (no symptoms) and plays a key on the agent role in (and host) transmission Asymptomatic phase Pre-clinical Subclinical Persistent Latent (chronic) Symptoms The host does have not yet not and will not The infection The begun but will present persists for microorganism’s progress symptoms. years (or may genetic material Diagnosed by be lifelong) becomes antibodies incorporated with the host and remains there without reproducing The iceberg of disease severity Cellular response Host response Classic severe Visible effect disease Formation of inclusion bodies or Clinical cellular transformation or cellular dysfunction Mild or moderate disease Viral multiplication without visible changes Asymptomatic or incomplete viral infection Subclinical disease Unseen changes maturation Exposure without Exposure but cell attachment or no infection entry Types of infection based on severity Class A: Frequent Subclinical infection Example: Tuberculosis (of 100 cases) Subclinical Mild Moderate Severe Death Types of infection based on severity Class B: Frequent symptomatic infection with few deaths Example: Measles (of 100 cases) Moderate Severe Subclinical Mild Death Types of infection based on severity Class C: Usually fatal infection Example: Rabies (of 100 cases) Death Severe (with timely intervention) Factors that impact the severity of a disease This is related to: – The microorganism’s virulence – The host organ where it multiplies – The host’s immune response Factors that influence the host’s susceptibility Genetic history Nutritional state Immunological features Prior exposure to the agent Immunization Factors associated with human disease Host features Types of agents Environmental factors Age Biological: Temperature Sex Bacteria, virus Moisture Race Chemical: Altitude Occupation Poison, alcohol, smoke Overcrowding Genetic profile Physical: Water Marital status Trauma, radiation, fire Food Family history Nutritional: Radiation Prior disease Excess or deficit Air pollution Immune status Noise Mode of transmission Mechanism through which the outbreak’s causal agent spreads from a source or reservoir to a host Portal of Potential Agent Source Transmission Portal of exit entry host Classification Dependent on an Dependent on the intermediary for mode of transmission transmission between individuals Vertical Horizontal Via direct contact Via indirect contact transmission transmission Person to person: Shared vehicle Contact with skin, (contaminated mucus membranes, water) or vector or secretions Portal of entry and exit The path through which an infectious agent leaves the reservoir or enters the host Portal of Potential Agent Source Transmission Portal of exit entry host Portal of entry and exit Nose and mouth Injury to the skin and Other secretions mucus membranes Broken skin Urinary meatus Ano-rectal genitals Portal of entry: Modes of transmission Respiratory tract Skin/mucous Bloodstream membranes Digestive tract Placental barrier Genital tract Reservoir A population of living things that chronically harbor a disease germ The natural habitat where the causal agent lives and on which it depends to survive, develop, and multiply Types: – Human – Animal – Inanimate object (exceptionally). i.e., Clostridium tetani Human reservoir: state of the carrier The subject harbors the microorganism but is not infected Serological studies will be negative There are no signs of clinical disease The reservoir may infect, but to a lesser degree This state may last for weeks, months, or years Diseases with a human reservoir HIV/AIDS Dengue Hepatitis A, B, C virus Measles Others Animal reservoir The importance of animal reservoirs during an outbreak will depend on their level of contact with humans. Animal reservoirs for the same agent may vary depending on the geographical area. Zoonosis: diseases transmitted from animals to humans – Leptospirosis – Rabies – Hantavirus Mixed reservoir Either humans or animals may be a source of contagion: – Influenza – Yellow fever Reservoir and source: differences RESERVOIR SOURCE OF CONTAMINATION SOURCE OF INFECTION Is a living thing Generally, are inanimate Multiplication and Multiplication takes place over reproduction take place over a limited period of time a more or less long period of Majority of biological agents time must quickly move to a new This allows for the spread of susceptible host biological agents The agents may remain for a prolonged period, up to and including the entire life of the reservoir © Pan American Health Organization, 2023 Property of the Pan American Health Organization. Reproduced with permission pursuant to the Creative Commons Attribution-NonCommercial-NoDerivs 3.0 IGO license (CC BY-NC-ND 3.0 IGO). 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