Microbial Infections PDF

8. MICROBIAL INFECTIONS 8.1. INFECTIOUS DISEASES 8.1.1. SUMMARY OF INJURIES AND DISEASES - Types of injury and response: - Cause of diseases: 2  8.1.2. INTRODUCTION TO COMMUNICABLE DISEASES - Microbiology: - The study of...

8. MICROBIAL INFECTIONS 8.1. INFECTIOUS DISEASES 8.1.1. SUMMARY OF INJURIES AND DISEASES - Types of injury and response: - Cause of diseases: 2  8.1.2. INTRODUCTION TO COMMUNICABLE DISEASES - Microbiology: - The study of organisms which are usually small in size, simple in structure (and neither plants nor animals except in the case of parasites and algae) - Communicable diseases: - Communicable diseases can be spread, directly or indirectly, from one person to another person, or from other species to humans - Communicable diseases are caused by pathogenic microorganisms, such as bacteria, viruses, fungi, parasites or prion - “Communicable diseases” is used interchangeably with “infectious diseases” - Definition of an “infectious agent”: - Koch’s postulate (1876): - The parasite occurs in every case of the disease in question and under circumstances which can account for the pathological changes and clinical course of the disease - The parasite occurs in no other disease as a fortuitous and nonpathogenic parasite - After being fully isolated from the body and repeatedly grown in pure culture the parasite can induce the disease anew - The parasite can be re-isolated from an experimentally inoculated host - Limitations: - Possible state of asymptomatic infections - Not all microbes can be isolated/ cultured - Not all microbes can be experimentally inoculated in suitable animal models to reproduce the infection - Classification of infectious agents (microbes): 3  1. Parasites: - Parasites include single-celled (protozoa) or multicellular organisms (helminth) - They live on or in a host organism and get its food from or at the expense of its host 2. Fungi: - Fungi can be single-celled (yeast) or multicellular organisms (filamentous moulds) 3. Bacteria: - Bacteria are single-celled organisms - The cell structure is simpler than that of other organisms as there is no nucleus or membrane bound organelles 4. Viruses: - A virus is made up of a core of genetic material, either DNA or RNA, packed within an organic particle - Viruses invade living cells and use their host's metabolic processes to produce a new generation of viral particles 5. Prions: - Prions are misfolded proteins that can induce abnormal folding of specific normal cellular proteins - Prions are autocatalytic, e.g. in JCD [Molecular techniques: include structural microscopy, x-ray diffraction] 4 - Modes of transmission: 1. Direct: a. Direct contact transmission: - The infectious agent is passed from one skin/mucosal surface to another skin/mucosal surface (eg. kissing or sexual intercourse) b. Droplet transmission: - The infectious agent is spread via large aerosols that travel within a short-distance (due to gravity) to another mucosal surface 2. Indirect: a. Airborne transmission: - The infectious agent is spread via fine-size dust or droplet nuclei that may remain suspended in air for an extended period of time b. Vehicle transmission: - The infectious agent is spread via food, water, blood, or fomites (inanimate objects, e.g. door knobs, lift buttons) - A vehicle may passively carry a pathogen or provide an environment in which the agent multiplies or produces toxin c. Vector-borne transmission: - The infectious agent is spread by mosquitoes, fleas, or ticks. A vector may carry an infectious agent purely mechanically or may support replication or development of the infectious agent - An infectious agent may be transmitted via multiple transmission modes: - E.g. a respiratory virus: droplet, airborne, and vehicle (fomite) transmission - E.g. Zika virus: direct contact and vector-borne transmission 5  8.1.3. PUBLIC HEALTH SURVEILLANCE - Targets of public health intervention: - Controlling or eliminating infectious agents at the source of transmission: - Discard food that has been contaminated with Salmonella typhimurium - Quarantine and treatment of infected subjects - Protecting portals of entry: - Wearing surgical masks and face shields to prevent droplet transmission - Using bed nets to prevent transmission of malaria - Increasing host's defences: - Vaccination - Public health surveillance: - Public health surveillance is the ongoing, systematic collection, analysis, interpretation, and dissemination of data regarding a health-related event for use in public health action to reduce morbidity and mortality and to improve health - Purposes of surveillance systems: - Measure the burden of a disease - Detect and monitor outbreaks - Probe epidemiologic investigation - Guide the planning and implementation of public health programs - Evaluate interventions or public policies - Prioritise the allocation of health resources - The ultimate goal is to produce information to guide public health actions - Types of surveillance system: - Data sources: - Vital statistics, e.g. birth rate and mortality - Statistics on communicable diseases: - Statutory notifiable diseases – medical practitioners are required by law to notify the Director of Health of any suspected or confirmed case of notifiable infectious diseases (currently there are 55 such diseases) 6  - Sentinel surveillance (e.g. general out-patient clinics/ private medical practitioners may report weekly consultation rate of influenza-like illness) - Statistics on Antimicrobial Resistance Control, e.g. antimicrobial use and resistance - Statistics on Laboratory Surveillance, e.g. detection of pathogens from respiratory specimens - Registries (e.g. cancer, immunisation) - E.g. Sentinel surveillance for influenza-like illness: - Defined as fever >38ºC and cough/sore throat) - Consultation rate at sentinel GOPC and PMP: - Helps inform the CHP of the number of people in a population who have acquired respiratory infections and show symptoms of influenza-like illness - Multiple pathogens may cause the same symptoms, so the measurement is not specific enough to reflect the number of influenza patients in the community - E.g. Detection of influenza viruses in respiratory specimens: - By obtaining detection frequency - Percentage of respiratory specimens tested positive for influenza viruses by subtypes - More specific and accurate for influenza virus as the target pathogen - Take more resources to generate such a data, including efforts to collect specimens and to perform specific laboratory testing 7  8.1.4. DISEASE OUTBREAKS AND EPIDEMICS - Disease outbreaks: - A situation when diseases or health events occur at a greater frequency than normally expected within a specified period and place - The occurrence of a greater number of disease cases than normally occurs in the same place during the same period as in past years (e.g. influenza epidemics) - A cluster of cases of the same disease occurs that can be linked to the same exposure (e.g. group of guests who developed gastroenteritis) - A single case of disease that has never occurred before or might have a significant implication for public health policy and practice (e.g. first report of a child infected with H5N1) - Categorisation: 1. Outbreak/ Epidemic: - The occurrence of an illness or specific health-related behaviour/events that is clearly in excess of normal expectancy in a community or region 2. Pandemic: - An epidemic occurring worldwide or over a very wide area, crossing international boundaries and usually affecting a large number of people 3. Endemic: - The constant presence of a disease or infectious agent within a given geographic area or population group [Communicable diseases cause outbreaks which have the potential to become pandemics] - Zoonosis: - Pathogens that infect animals cross the species barrier and infect humans - A majority of zoonotic diseases cannot be transmitted effectively from human to human 8  - Epidemic curve: - An epidemic curve shows the progression of illness and the magnitude of an outbreak over time - It is generated by plotting the number of cases by the date of illness onset - During ongoing outbreak investigations, the epidemic curve is updated as new data become available - Therefore, there is an inherent delay between the date that an illness starts and the date that the case is reported to public health authorities - By monitoring the epidemic curve, an investigator may get information on the size, time trend (ongoing or decreasing), and the pattern of spread of an outbreak - Basic reproduction number (R0): - The average number of secondary infections caused by an infected individual in a naive population of susceptible hosts - It is an estimate of contagiousness that is a function of human behaviour and biological characteristics of pathogens - An outbreak is expected to continue if R0 has a value >1 and to end if R0 is