Epidemiology of infectious diseases .pptx

Epidemiology of infectious disease

1. Chapter 8 EPIDEMIOLOGY OF INFECTIOUS DISEASES Universities Press 3-6-747/1/A & 3-6-754/1, Himayatnagar Hyderabad 500 029 (A.P.), India Email: [email protected] [email protected] Phone: [phone] Part I General Microbiology

2. Dr Sonal Saxena, MD Director Professor and Head of the Department of Microbiology Maulana Azad Medical College, New Delhi and Dr Amala A Andrews, MD Maulana Azad Medical College, New Delhi Universities Press Pvt Ltd.

3. EPIDEMIOLOGY OF INFECTIOUS DISEASES The st udy of the epidemiology of an infectious disease helps in: Understanding its occurrence, course and transmission Planning control measures INFECTION Pathogen: An organism that causes a disease Pathogenesis : Process by which it produces the disease T riad involved in the epidemiology of infection Host → microbe → environment Universities Press Pvt Ltd.

4. INFECTION Communicable disease: Causative microbial agent has the ability to spread from one host to another Communicability : Ability of a microbe to spread from one host to another Infectivity: Ability of an infectious agent to cause a new infection in a susceptible host Contagious disease : Infections that are highly communicable ; pathogen is shed in secretions (e.g., measles and cholera) Zoonotic diseases: Transmitted from animals to humans (e.g., rabies) Universities Press Pvt Ltd.

5. INFECTION Non-communicable infectious disease: Disease that does not spread from one person to another ( e.g., tetanus) Primary infection: Initial infection by a pathogen in a host Re-infection: Subsequent infection by the same organism (e.g., malaria) Secondary infection: A new pathogen sets up an infection in a host whose resistance is lowered by a pre-existing infectious disease Focal infection : Infection or sepsis at localised sites (e.g., appendicitis) Universities Press Pvt Ltd.

6. INFECTION Cross-infection: A patient with an existing infection or disease acquires a new infection from another host/external so urce Cross-infections occurring in hospitals: N osocomial infections/healthcare-associated infections [HAI] Iatrogenic infection: Infections resulting from investigative, therapeutic or other procedures, which are physician-induced Endogenous or exogenous infection: Refer to the source of infection Inapparent infection (subclinical infection): Infection in which the clinical effects are not apparent Universities Press Pvt Ltd.

7. INFECTION Atypical infection : Absence of typical or characteristic clinical manifestations of that particular infectious disease Latent infection : Pathogens remain in the tissues in a latent or hidden form, proliferating and producing clinical disease when the host’s resistance is lowered Colonisation : Organisms live on the host’s body without causing infection and become pathogenic if the host’s immunity is compromised Infestation : Organisms live parasitically on the host, producing harmful effects ; ectoparasites , helminths Universities Press Pvt Ltd.

8. SOURCES OF INFECTION 1. Other humans 2. Animals 3. Insects 4. Inanimate objects 5. Environment Universities Press Pvt Ltd.

9. SOURCES OF INFECTION 1. Humans as sources of infection Pathogen may originate from a patient or a carrier Carrier: A person who harbours the pathogenic microorganism without suffering any ill effect on account of it - Healthy carrier : Harbours the pathogen, but no disease - Convalescent carrier : Has recovered from the disease, but harbours the pathogen - Temporary carriers : Convalescent carriers in whom the microorganisms persist for less than six months Universities Press Pvt Ltd.

10. SOURCES OF INFECTION - Chronic carrier : In whom the infection may last for several years - Contact carrier : A person who acquires the pathogen from a patient - Paradoxical carrier : Who acquires a pathogen from another carrier 2. Animals as the source of infection R eservoir hosts : Asymptomatic animals maintain the microbe in their system and act as reservoirs of human infections Zoonoses : Infectious diseases transmitted from animals to humans - Bacterial (e.g., plague from rats) , viral (e.g., rabies from dogs) , protozoal (e.g., toxoplasmosis from cats) , helminthic (e.g., hydatid disease from dogs) Universities Press Pvt Ltd.

11. SOURCES OF INFECTION Fig. 8.1 Infections transmitted by animals and insects Universities Press Pvt Ltd.

12. SOURCES OF INFECTION 3. Insects as the sources of infection A rthropod-borne diseases : Bloodsucking insects may transmit pathogens to human b eings — vectors Mechanical vectors: The pathogen is transmitted mechanically by the vector Biological vectors: The pathogen multiplies in the body of the vector, often undergoing part of its developmental cycle in the vector’s body 4. Transmission through inanimate objects Mechanical transfer of microbial agents can occur Through fomites, medical equipment and instruments, etc. 5. Infections acquired from the environment Soil and water Food Inanimate objects Universities Press Pvt Ltd.

13. ROUTES OF TRANSMISSION OF INFECTION 1. Direct contact 2. Indirect contact : Through fomites 3. Inhalation i ) Large drops of secretions fall to the ground and dry there. Pathogens resistant to drying may remain viable in the dust and act as sources of infection ii) Small droplets under 0.1 mm in diameter— droplet nuclei (usually 1–10 µ m in diameter) 4. Ingestion 5. Inoculation : Directly into the tissues through a breach in the skin and soft tissue 6. Congenital/vertical transmission: Pathogens cross the placental barrier and infect the fetus in utero 7. Iatrogenic and laboratory infections including HAIs Universities Press Pvt Ltd.

14. HOST-PARASITE INTERACTION When an individual is exposed to a pathogen, the outcome depends on the interaction between the host and parasite (pathogen) Fig. 8.2 Outcome of host-pathogen interaction Universities Press Pvt Ltd.

15. MICROBIAL FACTORS PREDISPOSING TO INFECTION Pathogenicity: Ability of a microorganism to produce disease Virulence: Lethal property of the microorganism Exaltation: This is an enhancement of virulence Produced in vitro by serial passage in susceptible hosts Attenuation: This is a reduction of virulence Achieved by passage or culturing Universities Press Pvt Ltd.

16. MICROBIAL FACTORS PREDISPOSING TO INFECTION Factors influencing virulence 1. Adhesion: The attachment of the bacteria/parasite to body surfaces First step in pathogenesis Adhesins: Adhesive structures on the surface of bacteria Present as organised structures (fimbriae or pili)/colonisation factors Are virulence factors 2. Invasiveness: A bility of a pathogen to spread in the host tissues after establishing infection 3. Toxigenicity: Bacteria produce two types of toxins—exotoxins and endotoxins Exotoxins - Heat-labile proteins - Highly potent in minute amounts - Exotoxins treated with formaldehyde yield toxoids - Generally formed by gram-positive bacteria Endotoxins - Heat-stable lipopolysaccharides (LPS) - An integral part of the cell wall of gram-negative bacteria - They cannot be toxoided - They are poor antigens - Their toxicity is not completely neutralised by homologous antibodies Universities Press Pvt Ltd.

17. MICROBIAL FACTORS PREDISPOSING TO INFECTION 4. Genes: Coding for virulence factors — in plasmids and bacteriophages 5. Microbial products: Contribute to virulence by inhibiting the mechanisms of host resistance Coagulase ( Staphylococcus aureus ) : Prevents phagocytosis Fibrinolysins and hyaluronidases : Promote the spread of infections Leucocidins : Damage polymorphonuclear leukocytes Hemolysins : Capable of destroying erythrocytes Fungal toxins and parasitic enzymes and toxins 6. Bacterial appendages: Capsule and Vi antigen—withstand phagocytosis Universities Press Pvt Ltd.

18. MICROBIAL FACTORS PREDISPOSING TO INFECTION 7. Biofilms: Well-organised microcolonies of bacteria enclosed in self-produced extracellular polymer matrices known as glycocalyx Adherent/clinging/submerged biofilms Monomicrobial biofilms/polymicrobial biofilms These polymers act as a barrier to the action of antibiotics 8. Infecting dose: Minimum infecting dose (MID): The minimum number of bacteria required to produce clinical evidence of infection Minimum lethal dose (MLD): The minimum number of bacteria required to cause death of a susceptible animal under standard conditions 9. Route of infection: Some bacteria initiate infection following entry by any mode Others can survive and multiply only when introduced by the optimal routes Universities Press Pvt Ltd.

19. EPIDEMIOLOGY OF INFECTIONS Sporadic infection: Occurs i n a single individual due to some predisposing factor in the individual Outbreak : A cluster of cases occur within a geographic area Endemic infection: An infection that is prevalent within a localised area Epidemic infection : Affects many persons simultaneously in a geographically defined area Prosodemic infection: Slow-spreading infection transmitted from person to person Universities Press Pvt Ltd.

20. EPIDEMIOLOGY OF INFECTIONS Pandemic infection : Affects populations in regions, countries and continents across the globe Pandemic risk assessment factors i ) Properties of the pathogen (agent) ii) Attributes of the population (host) iii) Epidemiology of the pathogen (external environment) Management of pandemics Conducted p er the guidelines of the World Health Organization Universities Press Pvt Ltd.

21. Management of pandemics i ) Planning and coordination ii) Situation monitoring and assessment iii) Reducing the spread of disease iv) Continuity of healthcare provision v) Communication Ring vaccination: Process of vaccinating and monitoring a ring of people around each infected individual Universities Press Pvt Ltd.