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2. Components of Infectious disease cycle.pptx [Repaired].pptx

  1. 1. Communicable diseases epidemiology (Infectious disease cycle) BY: Agumas Fentahun (BSC in HO, MPH in Epidemiology) 1
  2. 2. Communicable diseases epidemiology (Infectious disease cycle) 1/26/2023 By: Agumas F.
  3. 3. Communicable Disease (Infectious Disease)  Is an illness due to a specific infectious agent or its toxic products that arises through transmission of that agent or its products from an infected person, animal, or reservoir and transmit to a susceptible host, either directly or indirectly through an intermediate plant or animal host, vector, or the inanimate environment. 1/26/2023 By: Agumas F.
  4. 4. The causes of disease can be classified in to two 1. Primary causes (Etiologic Agent) are the factors which are necessary for a disease to occur, in whose absence the disease will not occur 2. Risk factors (contributing, predisposing, or aggravating factors).  The etiology of a disease is the sum total of all the factors (primary causes and risk factors) 1/26/2023 By: Agumas F.
  5. 5. Infectious disease cycle Infectious diseases result from the interaction of I. Infectious(Etiologic)agent II. Susceptible host/reservoir III. Environment that brings the host and the agent together. I. Etiologic Agent: is an infectious micro-organism-  virus,  bacteria,  parasite, or other microbe 1/26/2023 By: Agumas F.
  6. 6. II. Host: Host factors influence individual's exposure, susceptibility or response to a causative agent. For example Age,Sex, Race  Socioeconomic status Behaviors (smoking, drug abuse, lifestyle, sexual practices and contraception, eating habits) affect exposure. Infectious disease cycle 1/26/2023 By: Agumas F.
  7. 7. III. Environment: Environmental factors are extrinsic factors which affect the agent and the opportunity for exposure.  Physical ,  Biologic,  Socioeconomic factors Infectious disease cycle 1/26/2023 By: Agumas F.
  8. 8.  Physical factors such as geology, climate, and physical surrounding (e.g., maternal waiting home, hospital)  Biologic factors such as insects that transmit the agent  Socioeconomic factors such as crowding, sanitation, and the availability of health services. 1/26/2023 By: Agumas F.
  9. 9. The epidemiologic triad or triangle is the traditional model of infectious disease causation. Epidemiologic triad has three components:  an agent,  a susceptible host, and  an environment that brings the host and agent together. Infectious disease cycle 1/26/2023 By: Agumas F.
  10. 10. Examples of causes of disease by host, agent and environmental factors. Host factors Agent factors Environmental factors Age Sex Previous disability Behaviour Genetic inheritance Height Weight Virulence of organisms Serotype of organisms Antibiotic resistance Cigarette-tar content Home overcrowding Air pollution Workplace Hygiene Weather Water composition Food contamination Animal contact 1/26/2023 By: Agumas F.
  11. 11. Chain of Infection  Infection implies that the agent has achieved entry and begun to develop or multiply, whether or not the process leads to disease.  Chain of infection is a model used to understand the infection process which has different components  Understanding the characteristics of each link provides insight for methods to prevent the spread of infection. 1/26/2023 By: Agumas F.
  12. 12. Chain of infection  Components of Chain of Infection  Causative Agent  Reservoir host  Portal of exit  Mode of transmission  Portal of entry  Susceptible host (final link in chain of infection) 1/26/2023 By: Agumas F.
  13. 13. Chain of infection 1. Causative Agent  Is a particle which range from small viral particles to complex multi cellular organisms (parasites)  Characterized by its Size Chemical character Antigenic makeup Ability to survive out side the host Ability to produce toxin 1/26/2023 By: Agumas F.
  14. 14. Chain of infection Host –agent interaction  Host –agent interaction is characterized by Infectivity Pathogen city Virulence Immunogenicity 14
  15. 15. Chain of infection Spectrum of infectious disease  The range of disease from asymptomatic to illness with mild to severe or fatal condition  If many individuals are equally exposed to infectious agent  Some may not be infected at all  Some are infected but develops no or mild symptoms  Some may be infected and develop severe symptoms  Others may die as a result of their infection 15
  16. 16. Chain of infection Spectrum of infectious disease….  Terms commonly used to explain the spectrum of infectious disease are infectivity, Pathogenicity and virulence  Infectivity  refers to the ability of an organism to invade and infect the host  Measured based on the proportion of exposed persons who become infected or  The ability of the agent to invade and multiply in the host (the ability to produce infection ) 16
  17. 17. Chain of infection Spectrum of infectious disease…  Pathogenicity  refers to the ability to cause disease  Measured by proportion of infected individuals who develop clinically apparent disease, or  the ability to produce clinically apparent infection  Virulence  Refers to the ability to cause serious complications or death  Measured by the proportion of clinically apparent cases that are develop severe or fatal clinical disease 17
  18. 18. Chain of infection Spectrum of infectious disease…  Immunogenicity: the infection ability to produce specific immunity  The spectrum of disease can be affected by: strain of the organism, dose of agent, route of infection, host immunity and influence of treatment 18
  19. 19. chain of infection Pathogenic mechanisms of disease  Direct tissue invasion(ex. bacillary dysentery/ shigella species mainly)  Production of toxin(e.g Tetanus/ clostridium tetani)  Immunologic enhancement or allergic reaction  Persistent /latent infection  Enhancement of host susceptibility to drugs  Immune suppression(HIV) 1/26/2023 By: Agumas F.
  20. 20. Chain of infection 2. Reservoirs host  The reservoir of an agent is the habitat in which an infectious agent normally lives, grows, and multiplies.  Thus , human reservoirs may be persons with symptomatic illness, or carriers.  A carrier is a person without apparent disease who is capable of transmitting the agent to others 1/26/2023 By: Agumas F.
  21. 21. Chain of infection Some disease of human reservoir are  Most bacterial and viral diseases  Most staphylococcal and streptococcal infections  STDs , etc  Some disease transmit from animal to human beings and they are named as zoonotic diseases/zoonoses 1/26/2023 By: Agumas F.
  22. 22. Chain of infection  Asymptomatic carriers : transmit the infection without ever showing signs of the disease  Incubatory carriers : transmit the infection by shedding the agent before the onset of clinical manifestations  Convalescent carriers : transmit the infection after the time of recovery from the disease  Chronic carriers : transmit the infection for a long period of time, or even indefinitely. 1/26/2023 By: Agumas F.
  23. 23. Chain of infection  The importance of carriers in the transmission of disease depends on their:  1) number  2) detect ability  3) mobility  4) chronicity. 1/26/2023 By: Agumas F.
  24. 24. Chain of infection 3. Portal of exit  is the way the infectious agent leaves the reservoir host and these includes all body secretions and discharges 4. Mode of transmission  is the various mechanisms by which agents are conveyed to a susceptible host  Direct Transmission:- immediate transfer of the agent from a reservoir to a susceptible host by direct contact or droplet spread.  Direct contact ,  Direct projection,  Transplacental route 1/26/2023 By: Agumas F.
  25. 25. Chain of infection  Indirect Transmission:- an agent is carried from reservoir to a susceptible host by  suspended air particles or  animate (vector mosquitoes, fleas, ticks...) or  inanimate (vehicle-food, water, biologic products) intermediaries.  Example: ⎯ Vehicle-born: food, water, towels, ⎯ Vector-borne: insect animals, ... ⎯ Airborne: dust, droplets 1/26/2023 By: Agumas F.
  26. 26. Chain of infection 5. Portal of entry Is through which that an infectious agent enters a susceptible host For Example:  Nasal _ Common cold  Vaginal – Sexually transmitting diseases  Injury site _ Tetanus  Skin – Hookworm infection  Respiratory –Tuberculosis 1/26/2023 By: Agumas F.
  27. 27. Chain of infection 6. Susceptible host  The chain of infection may be interrupted if the agent does not find a susceptible host. 1/26/2023 By: Agumas F.
  28. 28. The susceptible human host  Defn:- A person lacking sufficient resistance to a particular pathogenic agent to prevent disease if exposed.  The concept of host susceptibility or resistance can be seen at two levels, individual and that of the population. 28
  29. 29. At the individual level  The state of the individual host at any given time is the result of interaction of genetic endowment with the environment over the entire life span. Examples:  Genetic factors: sex, blood type, ethnicity etc.  Environmental factors: immunity acquired as a result of past infection 29
  30. 30. At the community level  Host resistance at the community (population) level is called herd immunity.  Herd immunity can be defined as the resistance of a community (group) to invasion and spread of an infectious agent, based on the immunity of a high proportion of individuals in the community.  This may occur if a high proportion of individuals in a population is resistant to the agent. 30
  31. 31. At the community level…  The high proportion of immunes prevents transmission by highly decreasing the probability of contact between reservoirs and susceptible hosts.  Through such herd immunity, immune persons limit the spread of the infection to the relatively few who are susceptible by reducing the probability of contact between infected and susceptible persons. 31
  32. 32. Conditions under which herd immunity best functions  Single reservoir (the human host): If there is other source of infection it can transmit the infection to susceptible hosts.  Direct transmission (direct contact or direct projection): Herd immunity is less effective for diseases with efficient airborne transmission. 32
  33. 33. Conditions…  No shedding of agents by immune hosts (no carrier state).  Uniform distribution of immunes: Unfortunately, susceptible usually happen to live in clusters or pockets because of socioeconomic, religious, or geographic factors. 33
  34. 34. Conditions… • No overcrowding: Overcrowding also increases the likelihood of contact between reservoirs and susceptible hosts. • Total immunity: Partially immune hosts may continue to shed the agent, and hence increase the likelihood of bringing the infection to susceptible hosts. • However, these conditions for the operation of herd immunity are seldom fulfilled. 34
  35. 35. Infection versus Disease  Exposure does not necessarily lead to infection.  Infection is defined as the entry and development or multiplication of an infectious agent in a host, whether or not this process results in disease.  Infection may remain asymptomatic or may appear as a disease. 35
  36. 36. Chain of Infection … Implications (of chain of infection) for public health  provides a basis for determining appropriate control measures  Eg: -treating patients with antibiotics -Decontaminating equipments -isolation of someone with infection, or counseling persons to avoid the specific type of contact associated with transmission -Using bed nets, gloves, etc 36
  37. 37. Time course of an infectious disease  Pre-patent period  Incubation period  Communicable period  Latent period 1/26/2023 By: Agumas F.
  38. 38. Time course of an infectious disease  Pre-patent period- the time interval between biological onset and the time of first shedding of the agent  Incubation period- the interval between biological onset and clinical onset  Communicable period- the time interval during the agent is shed by the host  Latent period –the interval between recovery and relapse in clinical disease 1/26/2023 By: Agumas F.
  39. 39. 1/26/2023 By: Agumas F.
  40. 40. Infection vs disease  Infection -is the condition in which the causative agent invaded the host and multiplied in the host  Disease -is a condition in which clinically apparent onset of the problem was observed 1/26/2023 By: Agumas F.
  41. 41. Thank you! 1/26/2023 Getasew M. 41

Notas do Editor

  • The shedding of disease agent in the discharge or excretions.
  • Endowment = gift = donation
  • Seldom = rarely = not often
  • Detaching