L18 Infectious Disease epidemiology

Question 1

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Importance of infectious disease epidemiology

• Assessing changes in the pattern of infectious disease
• Especially geographically or changes in seasonal patterns
  
  • E.g. the spread of Ebola
• Discovery of new/emerging infections
• Discovery of new sub-types
  
  • E.g. early detection of HIV
• Potential links between infection and some chronic diseases
  
  • E.g. H. Pylori and peptic ulcers

Question 2

Infectious diseases - An infectious disease is an illness due to a specific infectious (biological) agent or its ____1____ products capable of being directly or indirectly transmitted from ____2____ to man, from ____3____ to man, from animal to animal, or from the ____4____ (through air, water, food, etc..) to man. Also known as a ‘communicable disease’ or a ‘transmissible disease’

Answer 2

1. Toxic
2. Man
3. Animal
4. Environment

Question 3

Infection - Infection is the ____1____ and development or multiplication of an infectious agent in the body of man or animals. An infection does not always cause illness or ____2____. There are several levels of infection:

• Colonization (S. aureus in skin and normal nasopharynx)
• Subclinical or unapparent infection (polio)
• ____3____ infection (virus of herpes simplex)

Answer 3

1. Entry
2. Symptoms
3. Latent

Question 4

Define contagious disease.

Answer 4

A disease that is transmitted through contact between individuals.

Question 5

Define a host.

Answer 5

A person or an animal that affords ‘subsistence or lodgment’ to an infectious agent under natural conditions.

Question 6

Define vector of infection.

E.g.?

Answer 6

Any living carrier that transports an infectious agent from an infected individual, or its wastes, to a susceptible individual or its food or immediate surroundings.

E.g. an insect.

Question 7

Define reservoir.

Answer 7

Any person, animal, arthropod, plant, soil, or substance, or a combination of these, in which an infectious agent normally lives and multiplies, on which it depends primarily for survival, and where it reproduces itself in such a manner that it can be transmitted to a susceptible host.

• It is the natural habitat of the infectious agent.

Question 8

Define incidence of infectious diseases.

Answer 8

Incidence of an infectious disease: number of new cases in a given time period expressed as percent infected per year (cumulative incidence) or number per person time of observation (incidence density).

Question 9

Define prevalence of infectious diseases.

When is prevalence of no interest? E.g.?

When is prevalence of interest? E.g.?

Answer 9

Prevalence of an infectious disease - number of cases at a given time expressed as a percent at a given time.

Of little interest if an infectious disease is of short duration (E.g. measles) or quickly fatal.

Of interest if an infectious disease is of long duration (E.g. chronic hepatitis B).

Question 10

Define endemic.

E.g.? (2)

Answer 10

Refers to the constant presence of a disease or infectious agent within a given geographic area or population group, but at a relatively low frequency.

Chicken pox - Occurs at a fairly predictable rate in young children Malaria - Endemic in some parts of Africa

Question 11

Define epidemic.

E.g.?

Answer 11

The unusual occurrence in a community of disease, specific health related behaviour, or other health related events clearly in excess of expected occurrence. Usually, a sudden severe outbreak of disease within a region, or within a group of individuals.

E.g. HIV in intravenous drug users.

Question 12

Define pandemic.

E.g.?

Answer 12

A pandemic is when an epidemic becomes more widespread.

E.g. 1918 flu pandemic.

Question 13

Define an exotic disease.

E.g.?

Answer 13

Exotic diseases are those which are imported into a country in which they do not otherwise occur.

E.g. rabies in the UK, Ebola outside of the usual infection areas.

Question 14

Define zoonosis.

E.g.?

Answer 14

An infection that is transmissible, under natural conditions, directly from vertebrate animals to man.

E.g. rabies, plague, bovine tuberculosis, Lyme disease, Bubonic plague, Ebola, influenza, salmonellosis.

Question 15

Define sporadic infection.

E.g.?

Answer 15

Cases of sporadic infection occur irregularly, haphazardly from time to time, and generally infrequently. Tend to be rare and separated widely in time and place. So much so that they show no or little connection with each other, nor a recognisable common source of infection.

E.g. polio, meningococcal meningitis, tetanus.

Question 16

A sporadic disease could be the starting point of an ____1____ when the conditions are favourable for its spread. E.g. Creutzfeldt-Jakob disease - can occur as a sporadic form • Can also be an inherited disorder, or be acquired by exposure to ____2____ tissue.

Answer 16

1. Epidemic
2. Brain

Question 17

Creutzfeldt-Jakob Disease (CJD)

• Rare and fatal ____1____ disease
• Very ____2____ incubation period
• No test to identify asymptomatic infection
• Prions stick to surfaces
• Transmission by ____3____, ____3____ products and surgery

Answer 17

1. Neurodegenerative
2. Long
3. Blood

Question 18

‘High quality ______ is the cornerstone of infectious disease prevention and control’ (WHO, 2004)

Answer 18

Surveillance

Question 19

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Surveillance Surveillance Concepts

• Passive
  
  • Routine information sources
  • Automatic reporting
    
    • E.g. mandatory lab reports
• Active
  
  • Special effort to contact healthcare providers to collect data and confirm diagnoses
  • Public health response may be needed, e.g. contact tracing which was very useful in the recent Ebola outbreak in Nigeria

Enhanced Surveillance

• Additional information collected for each case
  
  • Capture of clinical information
    
    • E.g. AIDS diagnoses in HIV+ patients
  • Capture of risk factor information
    
    • E.g. demographic and risk taking behaviour related to HIV
    • Injecting drug use in tetanus patients
  • Merging data from different sources
    
    • Maximises case ascertainment
  • Merging results from microbiological characterisation to routine reports
    
    • E.g. Salmonella infection

Purposes of Surveillance

• Describe the disease and its’ importance
• Monitor secular trends
• Detection of outbreaks and emerging epidemics
• Monitor changes in infectious agents
• Provide an evidence base for policy and guidance information
• Monitor control and prevention measures
• Generate hypotheses and stimulate public health research
• Provide information for modelling studies

Sentinel Surveillance

• ‘Sample Surveillance’
• Crucial that the sampling is representative
  
  • E.g. only in a sample of STI clinics in England &Wales, but needs to represent the entire population at risk
• Nil return needed
• Clinicians/labs must report that no cases have occurred in the period of interest for nil return to be treated as zero.

Question 20

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Example: The influence of early infections on hearing at age 61-63 years: the Newcastle Thousand Families Study

Hearing

• Although a major public health issue, it is often under-recognised
• Almost 300 million people are affected by hearing loss worldwide
• Hearing loss has physical, emotional, mental and social consequences for individuals and economic consequences for both individuals and society
• Over 50 % of childhood hearing loss and 75% of adult hearing loss have no known cause

Early life risk factors

• Early growth is suggested to influence later hearing status, although early socio-economic status (SES) may be more important.
• Also evidence for impacts of numerous childhood infections on later hearing loss
• Little known about the potential for long-term effects
  
  • A number of the infections thought to be risk factors are vaccine preventable, but currently increasing in incidence

Study aims

• To investigate whether birth weight or socio-economic status (SES) at birth is associated with hearing function over 60 years later
• To investigate whether childhood infections also play a long-term role
  
  • Addressed using data from the Newcastle Thousand Families Study
    
    • A cohort pre-vaccination and pre-personal headphone noise exposure

The Newcastle Thousand Families Study:

• All 1142 babies born in Newcastle upon Tyne during May and June 1947
  
  • Birth weight as recorded by midwives; standardised for gestational age and sex.
  • Social class at birth
• Prospectively collected information in childhood
  
  • Childhood infections
• Hearing measured at age 61-63 years in 346 study members
• Assessed by a trained research nurse using a bilateral air conduction audiogram
• Measurements at 7 frequencies between 250Hz and 8kHz in octave steps and at 3kHz and 6kHz
  
  • Following British Society of Audiology guidelines
• Individual hearing function was calculated as a weighted mean between ‘best’ and ‘worst’ ear measurement in the ratio 4:1
• Any hearing aids were removed during the assessment
• Unable to use a sound-proofed room, but no bias suggested

Additional data

• Questionnaire used to obtain additional data at same time as hearing assessment included:
  
  • Exposure to occupational noise
  • Lifetime socio-economic status

Results

• 334 individuals with hearing data and complete data in childhood
• Hearing function ranged from 0 to 130 dBHL across all frequencies
• Females generally had better hearing than males

Results for infections

• Associations varied depending on type of infection (in the first 5 years) and frequency level
  
  • Those that had tonsillitis, otorrhea, or 2 or more episodes of bronchitis in the first 5 years, or 2 or more severe respiratory infections in the first year were more likely to have worse hearing at age 61-63 years
  • Associations independent of early SES and occupational noise

Discussion

• Hearing function was measured using pure tone audiometry, so unable to determine if any hearing loss is conductive or senso-nerual
  
  • With the age of the participants, likely to be mostly senso-neural
  • Lack of association with infections such as measles likely to reflect the pre-vaccination status of the cohort and almost complete coverage in terms of infection
• This suggests that some of the antecedents of impaired hearing may lie early in life, and that some insults to hearing may have lifelong effects.