Lecture 26: Control of infections Flashcards
Monday 2nd December 2024
What are the main aims of intervention of infection?
Control
Elimination
Eradication
Extinction
Control…
maintains the parasite population to an acceptable level
Elimination…
zero incidence in a defined geographical area (local eradication).
Eradication…
zero incidence worldwide
Extinction…
infectious agent no longer exists in nature or in lab.
What are the intervention options
- Preventing transmission
- Intervening after transmission (to prevent further transmission)
Preventing transmission…
- Mass (random) or targeted vaccination. e.g. smallpox
- by risk group e.g. childhood vaccines MMR
- Spatial vaccination, e.g ring vaccination - FMD
- Reduction in contact
- by handwashing, condom use, environmental sanitation
Intervening after transmission (to prevent further transmission)
- Infectiousness curtailment - tracing & isolation, or culling
- e.g. SARS, hospital MRSA (humans)
- FMD, BSE, avian influenza (animals)
Give the intervention equations
Re = S x c x p x D
Re = S x R0
S =
susceptible proportion
D =
duration infectious
c =
contact rate
p =
probability of transmission
c x p =
effective contact
What is the logic of interventions?
- Reduce number Susceptible
- Reduce time infectious (reduce D)
- Reduce contact (reduce c x p; isolation)
Where does vaccination move susceptibles?
directly into the Immune class
How many (much) should be vaccinated?
Pc = 1 - 1 / R0
What is Pc?
Pc is the (minimum) proportion of individuals you need to vaccinate
What is the long term strategy for controlling infectious disease?
Herd immunity
How many individuals in the general population need to be immunised for herd immunity to be effective?
75%
What is the proportion of the population that needs to be vaccinated dependant on?
R0
Is herd immuity reachable without intervention?
No. Active immunisation is required for herd immunity to be reached.
An infection spreads in a population and has sufficient replenishment of susceptibles – endemicity.
An infection spreads in a population and has sufficient replenishment of susceptibles – endemicity.
Partial immunity on an individual level
Partial immunity may be considered at the individual level – a partial immune response.
A pathogen that generated total herd immunity would go extinct. Which opposes what we know about evolution.
A pathogen that generated total herd immunity would go extinct. Which opposes what we know about evolution.
Vaccination – a history
Edward Jenner an English physician observed that mildmaids who had previously caught cowpox did not later catch smallpox.
In 1798 he innoculated his gardener’s 8 year old son, James Phipps with pus from a cowpox blister of a milkmaid.
Later he inoculated James multiple times with smallpox. James did not become diseased – he was immune.
Jenner had created the worlds first vaccine, and it was for a very significant disease.
The word vaccination is derived from the Latin word vaccinus – ‘of or derived from a cow’. Originally the word only for smallpox vaccine.
What is the cause of smallpox?
Variola major (90%) or Variola minor (10%)
How is smallpox transmitted?
“prolonged face-to-face contact” – essentially from nose and mouth droplets, also from sneezing and coughing. Sores and scabs were contagious.
What is the R0 for smallpox?
3-6
What is the mortality rate for smallpox?
Mortality Rate:
Major 35%
Minor 1%
In the 20th century, how many smallpx deaths were there?
300 – 500 million deaths
How many estimated new cases of smallpox were there per year in the 1950s?
estimated 50 million new cases per year
How many estimated new cases of smallpox were there per year in 1967?
15 million contracted the disease, 2 million died
Smallpox 1959…
mass vaccination initiated; 80% coverage
Smallpox 1969…
eliminated in Africa except Nigeria
Smallpox 1971…
UK/USA ceased vaccination; 12 countries still endemic
Smallpox 1972…
major epidemics in India, Pakistan and Bangladesh
1973…
response: surveillance & containment
When did the WHO declare smallpox as eradicated?
In 1980
When was the first ever eradicated disease?
1980 (smallpox)
what does vaccination coverage depend on?
demography
What is the only human disease that has been eradicated?
Smallpox, but it is not extinct as it still exists in labs.
R0 = 1 + (L / A)
R0 = 1 + (L / A)
Is it true that the measles virus has been eliminated locally, but not eradicated?
Yes
What properties does measles share with smallpox?
No animal reservoir
Safe, cheap, effective vaccine available
High disease-related morbidity/mortality therefore high compliance
How does measles differ from smallpox?
Transmitted more readily
R0 substantially higher
Highly infectious though not as virulent
Wakefield paper and smallpox
- Wakefield falsely claimed that MMR caused autism
- So % immunisation decreased
- The number of measles cases per year increased back into the thousands.
What is ineffective curtailment?
Intervening after transmission has occured
Describe Infectiveness Curtailment
Surveillance for infected
Trace contacts
Isolate/vaccinate contacts
Tracing focuses on potential secondary/ tertiary cases
Spatially explicit form is “ring vaccination/culling” – every individual within a fixed radius.
Give an example of Isolation by Ring CULLING
- E.g. 2003 Dutch avian influenza A virus epidemic (H7N7)
- R0 = 5.8
- 30 million birds slaughtered in the Netherlands and Belgium
- From 1145 farms (255 +ve for infection)
- Ring culling in 1km zones around farms
- Movement restrictions in affected regions (10km - creating isolation)
Summary (part 1)
- Vaccination and Herd Immunity central to Disease Prevention
- Age of onset and life span can change R0
-R0 = 1 + (L / A)
- Following outbreak, identification of 1st and 2nd ring contacts essential for control
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What are the different vector groups?
Mosquitoes (Anopheles, Aedes, Culex)
Blackflies (Simulium spp)
Tsetse flies (Glossina spp)
Triatomine bugs (Triatoma spp)
Sandflies (Phlebotomines)
Anthroponotic…
human-arthropod-human
What is an example of an Anthroponotic disease?
Malaria: p. falciparum MOSQUITOES
Zoonotic…
animal-arthropod-human
What is an eample of a zoonotic disease?
Zoonotic visceral leishmaniasis (ZVL) SANDFLIES
What is a virus with a complex pattern of transmission?
West Nile virus
Describe west nile virus
A complex host – virus relationship
Birds are primary amplifier hosts
Migratory birds have a role in distribution
Mosquitoes (Culex) vector
What is Vectorial Capacity (C)
“the average number of potentially infective bites that will be delivered by all the vectors feeding upon a single host in 1 day” hence, has units “per day”
How is vectoral capacity calculated?
R0 = C x d
where d = the duration of host infectiousness (in days)
What is vectoral capacity particularly sensitive to?
- The vector biting rate per day (a2) (2 bites needed).
- The proportion of bloodmeals taken on the host (host choice, h2)
- The daily vector survival rate (p)
- The latent period of the agent inside the vector (“extrinsic incubation period”) (n = EIP)
What is the equation for the sensitivity of the vectorial capacity ?
in notes
Give some examples of vector control
- Human bait traps (e.g. Insecticide Treated Net)
Anopheles gambiae (African malaria vector) - Non-human bait traps
Anopheles stephensi (Asian malaria vector) - Urban breeding site source reduction
Aedes aegypti (yellow fever & dengue vector) - Rural drainage of breeding sites
Culex tritaeniorhynchus (Japanese Encephalitis vector)
What do non-human bait traps do?
They attract vectors to something that’s harmful to them i.e tiny targets for Tsetse’s, which attract and kill them
What does Tsetse cause?
African sleeping sickness
Old vector control methods are…
Old vector control methods are prohibitively expensive.
What are the problems with old vector control methods?
- Old vector control methods are prohibitively expensive.
- Controlled by case detection and treatment but this often only reaches less than 75% of the population.
- Therefore, reactive intervention rather than proactive.
Human bait and human protection
- Insecticide Treated (bed)Nets (ITNs)
- May provide herd immunity if high coverage
- Protects against multiple vector species
- Effects on vector include:
- Increased vector mortality
- Excito-repellency (avoiding the ITNs)
- Increased zoophagy
How do GM mosquitoes work?
- GM males mate with wild females. Their offspring don’t survive, so this ends the malaria cycle.
- The larvae are treated with tetracyline, so that more males that have the GM gene are sterile.
Describe Wolbachia
- Endosymbiotic bacteria in many insect species
- Mosquitoes infected with this are less able to trabsmit viruses, as viral replication is disrupted
- Also decreases population size in mosquitoes
Summary
Control can be gained by…
Vaccination
But there are issues for change in disease presentation
Tracing
Quarantine / isolation / culling
Change in behaviour
Vector dynamics and the climate
- Climate change brings about extreme weather events
- vector borne diseases are sensitive to climate change, as environmental parasites change
Are cases of dengue virus underreported?
Yes
Describe BTV
- Vecrot borne
- Sub tropicqa
- The range of the vector has geograohically increased
- Outbreaks further north- carried by another vector whwch became competent in spreading infection.
