Virus Epidemiology Basics PDF

Epidemiology of Viruses Figure 13.4  Structure and Classification of Animal Viruses Groupings based on route of transmission – Disease-causing viruses often grouped by route of transmission – Enteric viruses Generally transmitted via fecal-oral route Often cause g...

Epidemiology of Viruses Figure 13.4  Structure and Classification of Animal Viruses Groupings based on route of transmission – Disease-causing viruses often grouped by route of transmission – Enteric viruses Generally transmitted via fecal-oral route Often cause gastroenteritis – Some can cause systemic disease – Respiratory viruses Usually inhaled via infected respiratory droplets Generally remain localized in respiratory tract – Zoonotic viruses Transmitted from animal to human via animal vector – Sexually transmitted viruses Can causes lesions on genitalia or cause systemic infections  Interactions of Animal Viruses with Their Host Outcome of infection of eukaryotic cells depends on factors independent of cell – defense mechanisms of host Viruses may develop relationships with normal hosts – No obvious disease or damage is caused to host State of balanced pathogenicity – Relationships divided into two categories Acute Persistent  Interactions of Animal Viruses with Their Host Acute infections – Usually short in duration – Host may develop long-lasting immunity – Result in productive infections Produce large number of viruses during replication – Disease symptoms result from tissue damage and infection of new cells  Interactions of Animal Viruses with Their Host Acute infections – Reproductive cycle of animal virus can be compared to virulent bacteriophage – Essential steps include Attachment Entry into susceptible cell Targeting site of reproduction Uncoating of virion – Removing protein coat exposing nucleic acid Replication of nucleic acid and protein Maturation Cell lysis Spreading within host Shedding outside host Transmission to next host Fig. 14.5, p.417 from Microbiology, Eighth Edition by Gerard J. Tortora, Berdell R. Funke and Christine L. Case. Copyright © 2004 by Pearson Education, Inc. Reprinted with permission. Acute Infections Influenza Virus Kinetics Influenza Viral Kinetics  Interactions of Animal Viruses with Their Host Persistent infections – Viruses continually present in host Release from infected cell via budding – Can be divided into three categories Latent infections Chronic infections Slow infections – Categories distinguished by detection of virus during period of persistence  Interactions of Animal Viruses with Their Host Persistent infections – Latent infections Infection is followed by symptomless period, then reactivation Infectious particles not detected until reactivation Symptoms of reactivation and initial disease may differ Example – Herpes simplex viruses 1 and 2 (HSV1 and HSV2) – Shingles (zoster)  Interactions of Animal Viruses with Their Host Chronic infections – Infectious virus can be detected at all times – Disease may be present or absent during extended times or may develop late – Best known example Hepatitis B – a.k.a serum hepatitis  Patterns of Serologic and Molecular Markers in HBV Infection. Ganem D, Prince AM. N Engl J Med 2004;350:1118-1129.  Interactions of Animal Viruses with Their Host Slow infections – Infectious agent gradually increases in amount over long period of time No significant symptoms apparent during this time – Two groups of infectious agents cause slow infections Retroviruses which includes HIV  Write One minute paper Describe the differences between acute and persistent viral infections What distinguishes? – Latent – Chronic – Slow Introduction to Epidemiology  Pathology, Infection, and Disease Pathology: The study of disease Etiology: The study of the cause of a disease Pathogenesis: The development of disease Infection: Colonization of the body by pathogens Disease: An abnormal state in which the body is not functioning normally Classifying Infectious Diseases Symptom: A change in body function that is felt by a patient as a result of disease Sign: A change in a body that can be measured or observed as a result of disease Syndrome: A specific group of signs and symptoms that accompany a disease Classifying Infectious Diseases Communicable disease: A disease that is spread from one host to another Contagious disease: A disease that is easily spread from one host to another Noncommunicable disease: A disease that is not transmitted from one host to another  Occurrence of a Disease Incidence: Fraction of a population that contracts a disease during a specific time ( cases per year) = rate of spread Prevalence: Fraction of a population having a specific disease at a given time (cases per 100,000 or a %) = instantaneous measure Sporadic disease: Disease that occurs occasionally in a population  Occurrence of a Disease Endemic disease: Disease constantly present in a population Epidemic disease: A disease that appears above the normal endemic levels Outbreak: Disease acquired by many hosts in a given area in a short time, often localized May spread and become an epidemic Pandemic disease: Worldwide  Example of Disease threshold Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 10 8 Percent of deaths 6 4 Epidemic threshold Seasonal (endemic) baseline Actual percentage of total deaths in the population 0 5 15 25 35 45 5 15 25 35 45 5 15 25 35 45 5 15 25 35 45 5 Year 1 Year 2 Year 3 Year 4 Weeks of the year Types of Epidemics Common Source – Point source Continuous or originated E.g., multiple from Contaminated exposure cantaloupes exposure to E.g. Contaminated water supply common agent Propagated – E.g., person to Classroom, person football game, festivals spread Seasonal – E.g., Influenza, E.g. Diarrheal arise based colds, diseases in on seasonal pneumonia in summer or cool seasons rainy season activities Cyclical – in repeating Changes in E.g. syphilis 10 pathogen or cycles over host year cycle time  Examples – of epidemic Common-source epidemic: rapid rise in cases suggests exposure to single source of pathogen – Propagated epidemic: slow rise in cases suggests contagious disease spreading in population; first case is called index case – Seasonal: Change with climate Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Respiratory syncytial virus Campylobacter Common-source out break Propagated epidemic Number of new cases Presence of disease Jan. June Jan. June Jan. June Jan. Year 1 Year 2 Year 3 1 2 3 4 5 6 7 Days Month  Catch up How can you differentiate different epidemics? – Common source Point source or continuous – Propagated – Seasonal – Cyclic or periodic  Severity or Duration of a Disease Acute disease: Symptoms develop rapidly Chronic disease: Disease develops slowly Subacute disease: Symptoms between acute and chronic Latent disease: Disease with a period of no symptoms when the causative agent is inactive  Extent of Host Involvement Local infection: Pathogens are limited to a small area of the body Systemic infection: An infection throughout the body Focal infection: Systemic infection that began as a local infection Example of epidemic/outbre aks Listeria monocytogenes – Gram neg bacterium – Associated with elderly, pregnant women and newborns – Grows well, but slowly in fridge multistate outbreak – Linked to contaminated cantaloupes – 28 deaths, 133 hospitalized – More probably not diagnosed Any ongoing Epidemics?  Epi Curve Persons infected with the outbreak-associated strains of Listeria monocytogenes, by date of illness onset (Figure A) and date of clinical specimen collection (Figure B), 2011* http://www.cdc.gov/listeria/outbreaks/cantaloupes-jensen-farms/102511/epi.html Review The Stages of a Disease Figure 14.5  Reservoirs of Infection Reservoir - Continual sources of infection – Human: AIDS, gonorrhea Carriers may have inapparent infections or latent diseases – Animal: Rabies, Lyme disease Some zoonoses may be transmitted to humans – Nonliving: Botulism, tetanus, Pseudomonas Soil Water  Principles of Epidemiology Transmission – Successful pathogen must be passed from reservoir to next susceptible host – Transmission of pathogen via contact with food, water, or living agent is termed horizontal transmission – Transmission via transfer of pathogen from mother to fetus or child through breast feeding is termed vertical transmission  Spread of Disease Direct contact Occurs when one person physically touches another – Can range from simple contact to intimate contact In some cases direct contact is primary route of transmission Hands are main vehicle of contact transmission Handwashing physically removes organisms – Important in preventing direct contact transmission Pathogens that do not survive for extended periods in the environment usually spread by direct contact  Spread of Disease Droplet transmission- Aerosols Microbe-laden respiratory droplets generally fall to the ground within three feet of release – Aerosols travel farther People in close proximity can inhale infected droplets, spreading disease via droplet transmission Droplet transmission considered direct transmission because of the close range required for transmission Figure 14.6d  Spread of Disease Indirect contact Involves transmission of pathogens via inanimate objects or fomites – Usually clothing, tabletops, doorknobs, and drinking glasses Organisms on hands or fingers of carrier can be transferred to objects and picked up by another individual Handwashing important control measure  Transmission of Disease Contact – Direct: Requires close association between infected and susceptible host – Indirect: Spread by fomites – Droplet: Transmission via airborne droplets  Vectors Usually Arthropods, especially fleas, ticks, and mosquitoes Transmit disease by 2 general methods: – Mechanical transmission: Arthropod carries pathogen on feet – Biological transmission: Pathogen reproduces in vector Vectors Figures 14.8, 12.30  Why are carriers important reservoirs of infection? 14-12 How are zoonoses transmitted to humans? 14-13 Give an example of contact transmission, indirect transmission, mechanical transmission, and biological transmission. 14-14  Epidemiology tools Notifiable Diseases – CDC National Notifiable Disease Surveillance system Surveys – Passive – Active - Sentinel Surveys Establish sentinel or catchment sites Animal sentinels Vector Surveys  Next Time Read chapter 16 Overview of Coronavirus metrics Start immune system