The Epidemiology of Infectious Disease

1. The Epidemiology of Infectious Disease 

2. Introduction

3. I. The Science of Epidemiology • Epidemiology: • The field of science concerned with the circumstances under which diseases occur • An epidemiologist works in this field • Factors under investigation: • Incidence (morbidity rate) and spread of infectious and non-infectious diseases • Prevention and control of infectious and non-infectious diseases • Effects of diseases on populations and individuals within a population (measured by death rate = mortality rate)

4. Basic terms used in epidemiology • Sporadic disease • Occurs irregularly and only occasionally in a population • Example: Typhoid fever (Salmonella typhi) • Endemic disease • Occurs at regular intervals but at low levels • Example: Common cold (Rhinovirus) • Hyperendemic disease • When occurrence frequency rises, but not to epidemic proportions • Example: Common cold in the winter months

5. Epidemic • Sharp increase in the incidence above the predicted/expected level • Reservoir • Natural location of the organism • Can be animate or inanimate location • Examples: • Rabies – Dogs, foxes, raccoons (zoonoses) • Neisseria meningitidis (meningitis) – Humans • Malaria – Humans • Cryptococcus – Bird guano

6. Source • Immediate location from which infectious agent has been transmitted • Examples: • Neisseria gonorrhea • Source = humans • Reservoir = humans • Salmonella typhi • Source = food and water • Reservoir = humans • Hepatitis C • Source = transfusion, blood products • Reservoir = humans

7. Carriers • Hosts that harbor a pathogen without clinical symptoms and are capable of transmitting the infectious agent (sometimes unknowlingly) • Carrier state may be short (transient) or long-term (chronic carrier – e.g. tuberculosis, herpes, hepatitis B, typhoid) • Carrier state may also occur during: • Incubation period (before symptoms appear) • Convalescent period (recovery)

8. Vector • A biological or inanimate source that contributes to the transmission of an infectioius agent from one host to another • Examples: • Arthropods • Mosquitos – Malaria, West Nile Virus • Ticks – Lyme disease • Fleas – Bubonic plague • Flies - Trachoma

9. Birds • Parrots – Psittacosis • Pigeons – Cryptococcus • Lower vertebrates • Frogs and turtles – Salmonella • Inanimate objects (fomites) • Sporothrix schenkii (sporotrichosis)

10. Index case - the first case in an epidemic • Outbreak - an epidemic-like increase in frequency, but in a very limited (focal) segment of the population • Rapid increase, usually localized • Example: Legionnaire’s disease • Pandemic - a long-term increase in frequency in a large (usually worldwide or continental) population • Disease frequency rises on a large scale geographically

11. Epizootology • Deals with animal diseases affecting animal populations • Enzootic = moderate incidence • Epizootic = rapid increase • Panzootic = wide spread incidence • Zoonoses = if transferable to humans

12. II. Equations for Determining Frequency of Disease

13. Statistical Analyses • The mathematics of collection, organization, and interpretation of numerical data (rate acoomparisons, chi-square, SEMs) • Used by state public health lab, CDC, WHO and USPHS

14. Morbidity - the number of new cases in a specific time period per unit of population • # new cases within a specified period x 100 #individuals in a population Indicator of new cases – critical for controlling spread of disease

15. Prevalence - number of individuals infected at any one time per unit of population

16. Mortality - number of deaths from a disease per number of cases of the disease • # deaths ascribed to disease x 100 # individuals affected by disease Proportion of all deaths assigned to a single cause

17. III. The Epidemiology of Infectious Disease

18. Recognition of an Infectious Disease in a Population

19. Factors Affecting the Cycle of Disease • Causative agent • Source/reservoir • Method of Transmission • Influence of host or environment in the spread of the disease

20. Goal of the Epidemiologist • Control the spread (dissemination) • Eliminate etiological agent

21. Surveillance and Data Collection for Control • Calculation of morbidity and mortality rates • Case studies • Field studies • Review clinical records and lab reports • Investigate source, reservoir and vectors • Review treatments/success rates • Employ demographic data to track the movement of disease

22. Signs versus symptoms: • Sign = observable or measurable change in body function • Diarrhea, rash, fever, vomiting • Symptom = subjective • Pain, appetite loss, lethargy, depression

23.  Disease syndrome - a set of signs and symptoms that is characteristic of a disease

24. Phases of Infectious Disease Life Cycle • Incubation period • Variable length • Prior to development of signs of symptoms • Prodromal stage • Beginning of signs and symptoms • Often infectious/contagious • Innate immune response “kick in” (first line of defenses)

25. Illness stage • Most severe phase • Clear evidence of signs and symptoms • Acquired immune responses begin • Humoral – Antibodies and complement • Cell-mediated – T cells instruct destruction of infected cells of destruction of intracellular bacteria • Decline stage • Alleviation of signs and symptoms • Recover/convalescence

26. IV. Two Major Types of Epidemic

27. Common Source Epidemic • Sharp increase to a peak, then a rapid resolution • Associated with common contaminated source • Examples • Food poisoning (food) • Legionnaire’s disease (water – air conditioning)

28. Propagated Epidemic • Extended rise with a gradual resolution • Frequently observed when one individual = source • Gradual dissemination • All susceptible individuals succumb • Examples • Mumps, chickenpox • # susceptible individuals eventually decreases due to acquired immunity • Agent loses the ability to disseminate through the population

29. V. Herd Immunity • Resistance a population acquires as a whole to infectious disease • The number of individuals that must be immune to prevent an epidemic outbreak of a disease is a function of: • Infectivity of the disease (I) • Duration of the disease (D) • Proportion of susceptible individuals in the population (S) • When 70% of individuals in a population are immune, the propagation from individual to individual is not sustained and epidemics do not occur

30. Opportunity for contact and transmission decreases as the number of immune individuals increases • Susceptible individuals benefit from an indirect immunity (not self-made immunity)

31. Acquisition of Herd Immunity through Immunization • Immunization of large numbers of susceptible individuals in a population can induce herd immunity • Necessary to achieve a balance between immune and susceptibles • Dynamic • Births, deaths, migratory patterns • Immune individuals can become susceptible again if the pathogen mutates (antigenic shift or antigenic drift)

32. VI. Antigenic Shift and Antigenic Drift Caused by Mutations • Major genetic changes in a pathogen = Antigenic Shift • Too great to be the result of simple mutations • Example: Influenza strains derived from mixing of different influenza serovars • Can occur between animal and human virus (e.g. human and avian influenza) • Co-infection of same cell • Genomes recombine (8 RNA strands/genome) • Mixing of gene pools, addition of new genes • New serovar is generated • No resistance in the population

33. Influenza pandemic outbreak of 1918 (“Swine Flu”) • Killed 20-40 million people • In the Far East, animal hosts for influenza viruses (ducks, chickens and pork) live close together and close to humans • Other examples: • 1957 – “Asian Flu” • 1968 – “Hong Kong Flu” • 1977 – “Russian Flu” • 1997 – All chickens killed in Hong Kong, 4 deaths, new strain in chickens

34. Antigenic Drift • Minor genetic changes affecting critical epitopes • Point mutations in nucleic acids can cause single amino acids to change in a protein • Gradual and cumulative • Therefore, major changes are apparent only with time • Herd immunity will decrease as the number of susceptible individuals increases above a threshold density

35. Example – Influenza virus – Types A, B and C (B and C are more stable) • Inside of the virion • Nucleoprotein • Matrix protein (under the envelope) • Outside of the virion • Hemagglutinin spikes (HA) • Neuraminidase spike (NA)

36. RNA viruses have high rates of spontaneous mutations because RNA synthesis is not proof-read as well as DNA synthesis  error prone (~1 base change per replication) • RNA viruses can adapt quickly to new environments • Point mutations in NA and HA change the antigenic structure • Influenza A changes antigenic makeup often  so vaccines become ineffective

37. VII. The Infectious Disease Cycle: Story of a Disease - Links in the infectious disease chain

38. Agent responsible • What pathogen caused the disease? • Epidemiologists must determine the etiology (cause) of a disease • Koch’s postulates (or modifications of them) are used if possible • The clinical microbiology laboratory plays an important role in the isolation and identification of the pathogen • Communicable disease - one that can be transmitted from one host to another • Transmittable?

39. Source or reservoir of pathogen • Inanimate or animate • Human or non-human • Carriers

40. Carrier - an infected individual who is a potential source of infection for others • Active carrier - a carrier with an overt clinical case of the disease • Convalescent carrier - an individual who has recovered from the disease but continues to harbor large numbers of the pathogen • Healthy carrier - an individual who harbors the pathogen but is not ill

41. Incubatory carrier - an individual who harbors the pathogen but is not yet ill • Casual (acute, transient) carriers - any of the above carriers who harbor the pathogen for a brief period (hours, days, or weeks) • Chronic carriers - any of the above carriers who harbor the pathogen for long periods (months, years, or life)

42. Route of transmission to susceptible host • Airborne • Direct contact • Indirect contact • Vehicle • Vectors

43. How was the pathogen transmitted? • Airborne - suspended in air; travels a meter or more • Droplet nuclei - may come from sneezing, coughing, or vocalization • Dust particles - may be important in airborne transmission because microorganisms adhere readily to dust

44. Contact - touching between source and host • Direct (person-to-person) - physical interaction between infected person and host • Indirect - involves an intermediate, such as eating utensils, thermometers, dishes, glasses, and bedding • Droplets - large particles that travel less than one meter through the air

45. Vehicle (fomite) - food and water, as well as those intermediates described for indirect contact • Vector-borne - living transmitters, such as arthropods or vertebrates • External (mechanical) transmission - passive carriage of the pathogen on the body of the vector with no growth of the organism during transmission • Internal transmission - carried within the vector • Harborage - organism does not undergo morphological or physiological changes within the vector • Biologic - organism undergoes morphological or physiological changes within the vector

46. Immune status of host – susceptible? • Depends on defense mechanisms of the host and the pathogenicity of the organism • Release of pathogen • Active escape - movement of organism to portal of exit • Passive escape - excretion in feces, urine, droplets, saliva, or desquamated cells • Virulence and mode of transmission

47. Virulence and the Mode of Transmission

48. A virus that is spread by direct contact (e.g., rhinoviruses) cannot afford to make the host so ill it cannot be spread effectively • A virus that is vector-borne can afford to be highly virulent • Pathogens that do not survive well outside the host and that do not use a vector are likely to be less virulent while pathogens that can survive for long periods of time outside the host tend to be more virulent

49. VIII. The Emergence of New Diseases and the Resurgence of Old Diseases

50. New diseases have emerged in the past few decades such as AIDS, Hepatitis C and E, hantavirus, Lyme disease, Legionnaire’s disease, toxic shock E. coli 0157:H7, cryptosporidiosis and others • Systematic epidemiology focuses on the ecological and social factors that influence the development, emergence and resurgence (TB, diphtheria) of disease