Global Health - Infectious Disease Flashcards
What are 3 origins of human infections?
- Inherited from our ancestors
- Acquired from wildlife
- Acquired from livestock
What is the fraction of transmission by animal reservoirs and why?
60-70% (2 and 3)
Animal reservoirs = zoonosis
Humans increasingly involved due to interference with animals and because we are a dominant species
What are some world changes in transmission of infectious disease?
- Increasing population density
Faster transmission
Each transmission event is opportunity for increased rate of evolution - Migration of people allows faster transmission as environment is changing for virus very quickly so must adapt to selection pressure
- Airplane travel takes -24hrs (incubation period for most microorganisms)
What is epidemiology?
ability to quantify disease occurrence in populations
What is a case?
person with the disease, health disorder of suffering from the event of interest
How is prevalence calculated?
- frequency of disease in a population at a point in time
- it is a proportion: number of cases/number of people in population
What does prevalence measure?
Measures burden of disease in a population from a cross sectional study
It compares the burden of chronic disease between populations
Prevalence = ALL CASES
What does incidence measure?
Number of new cases of disease within specific time interval
It is a probability/risk that individual will develop disease in that specific time period
Incidence = NEW CASES
What does prevalence depend on?
Number of new cases (incidence) and time they remain cases before death/recovery
How is incidence calculated?
define time period
define total population at risk (denominator)
need an accurate test to define a true cases
What is the challenge of designing a test to define a true case?
Easy for infections with short time period
But chronic disease (HIV hard to find positive antibody test that can identify new infection)
How does incidence influence policy makers?
When incidence near 0, outbreak over
How can incidence stay same but prevalence increase?
People with condition are kept alive e.g. HIV treatment
What is mortality?
number of deaths from specific disease condition in given time period
deaths from disease in given time period/population at start of time period
What does it mean if the mortality = incidence?
Epidemic is stable
What is the contradiction with reduced mortality with HIV?
More people alive with antiretroviral therapy
Mortality reduces - people alive longer
Transmit virus to sexual partners, babies causing incidence to rise
Prevalence also increases as people not dying
Explain the relationship between prevalence and mortality?
Define prevalence and mortality
Is treatment confers survival benefit (ART for those living with HIV), mortality falls and prevalence increases
For rapidly fatal diseases (Ebola) rapid mortality, large number of cases but low prevalence
What is morbidity?
number of cases of ill health, complications and side effects attributed to particular condition over particular time period
What is the difference between mortality and morbidity?
morbidity - state of being diseased/unhealthy i.e. incidence of ill health in population
mortality - number of people who died i.e. incidence of deaths in population
Unit of measurement for morbidity?
Scores/predicted morbidity assigned to ill patients (APACHE II, SAPS II/III, GCS, PIM2, SOFA)
Help decide treatment/medicine that should be administered
Useful to compare two sets of patients/different time points in hospitals
Unit of measurement for mortality?
Expressed as number of deaths/1000 per year
Where is the highest death rate?
Swaziland 30.83
Also high in Angola, Lesotho, Sierra Leone, Zambia
What are the leading causes of death?
Heart disease Cerebro-vascular diseases Respiratory infections HIV-AIDS COPD
What about the leading causes of mortality in third world countries?
58% reported due to malnutrition/deficiencies
What are the leading causes of infectious disease?
6 Lower respiratory infections HIV/AIDS Diarrhoeal diseases TB Malaria Measles
Where are infectious diseases the leading cause of death?
Sub-saharan Africa
What is the importance of age standardised death rates?
The standardised mortality rate is a weighted average of the age-specific mortality rates per 100 000 persons, where the weights are the proportions of persons in the corresponding age groups of the WHO standard population.
Measure how many people die each year and numbers help authorities determine whether they’re focusing on right public health actions to reduce numbers of preventable deaths and disease
SAR event (2003)
originated from bat in central asia
West Nile Virus (1999)
originated from birds (crows/rooks), transmitted by mosquitoes (arbovirus)
high infection rate
low serious morbidity cases
vaccine in clinical trials
HIV 1 - RNA virus
Epidemics increasing linearly, should level off
Integrase inhibitors used
RNA viruses have no proofreading mechanism which increases the chance of mutation and evolutionary progression
Malaria
generation of genetic variability - mutation and recombination of different games, plasmid transfer
Plasmodium virus has multiple surface antigenic proteins. Each is synthesised different making it harder to target with vaccine
MERS virus
originated from camels
high incidence in Middle East
Ebola
Slow characteristic of transmission
Symptoms before highly infectious so patient can be placed in isolation
Highly contagious, rapidly fatal but can be prevented
Spread via direct contact with bodily fluids, contaminated surfaces
Frequent transmission to health care workers
Public health measures for Ebola?
Identify, isolate and follow up contacts
Effective contact tracing and isolation for 21 days post contact
Zika virus
- initially isolated from rhesus monkeys
- it is a flavivirus, transmitted by mosquitoes (arbovirus), also sexually transmitted
linked to children with microcephaly born to infected mothers
risk of infection in first trimester 1/100 to 1/100 over all trimesters
risk of Gullian-Barre syndrome in infected adults 4/100000
How to indicate emergence of new infectious disease?
- Indication from unusual clusters of morbidity/mortality in space and time
- Identify aetiology (cause) - looking for homogeny in cases/study results worldwide
- Develop diagnostic tests e.g. blood test, use antibodies, use QPCR by amplifying parts of virus to deterine genetic sequencing
- Follow routes of infection
- Ensure public aware of new information to introduce wide spread initiatives to avoid disease/take precaution
What is the basic reproductive number?
Ro
= average number of secondary cases from the emergence of a single primary case
It determines incidence and prevalence of infection
If less than 1, extinction
More than more, suggests epidemic, increase transmission
What factors affect Ro?
- duration of incubation period
- peak infectiousness
- how quickly infectiousness decays
- acquired immunity important
- population data determines human movement and how this may affect disease progression
What is the incubation period?
the period between exposure to an infection and the appearance of the first symptoms
What is the effective reproduction number?
number of infections caused by each new case occurring at time, t
Explain graph of rate of new infections against time
Rate increase slowly initially then adopts exponential steepness
This tails off as it exhausts the number of people susceptible to the virus/bacteria, downgrade in rate of infections. Vaccines also decrease rate.
Level of infection rises again as new mutation, selected advantage may initiate new epidemic
What is the policy for the prevention and control of influenza?
- Minimise morbidity and mortality with fixed or variable budget
- Buy time for vaccine development
- Minimise epidemic duration, impact on economy
- Minimise peak prevalence below defined level and avoid collapse of healthcare systems
What are the cause of much morbidity and mortality?
Neglected tropical diseases (NTDs)
Examples of protozoan infections
Leishmaniasis African Trypanosomiasis (sleeping sickness)
Examples of Helminth Infections
Schistosomiasis Lymphatic Filariasis (Elephantiasis) Onchocerciasis (River Blindness) Hookworm - soil transmitted Dracunculiasis - Guinea worm
Examples of bacterial infections
leprosy
trachoma
buruli ulcer
