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Transcript
Training Manual on Wildlife Diseases and Surveillance
VII. The basic reproductive number (‘R’) - A measure of
pathogen transmission
The concept of the basic reproductive number for a pathogen, symbolised by ‘R’ or sometimes
‘R0’, is one of the most important concepts in disease ecology. The basic reproductive number of
a pathogen is the number of new infections that will occur when an infected individual is
introduced to a population.
R0 is used to symbolise the basic reproductive number of a pathogen in the very
particular situation when an infected individual is introduced into a population of individuals
which have not been exposed previously to the pathogen and are totally susceptible to infection.
As an infection spreads, however, some individuals in the population recover and are
immune, and the value of R changes. Since R0 is a value when the entire population is assumed to
be susceptible, the symbol Reff, (effective value of R), or just R, sometimes is used to represent
the true value of R at a particular time and place.
R is easiest to measure and to think about for pathogens that cause acute infections of a
relatively short duration, such as small pox, measles, influenza, and Newcastle disease viruses.
A population of animals or people which is infected with such acute pathogens will consist of
three categories of individuals:
– susceptible to infection
– infected
– recovered and immune.
An individual animal (or person) starts out as susceptible to infection. If it becomes
infected due to transmission of the pathogen from an infected individual, it either dies or
recovers, and, if it recovers, is then immune to further infection.
R gives a numerical description of how a pathogen is transmitted within a population of
host animals or people. If R = 1, then the number of infected individuals in the population will
not change over time. Each infected individual will, on average, transmit the pathogen to only
one other individual. For a pathogen to persist in a population, R must be 1 or greater (R > 1). If
R is less than one, (R < 1), there will be fewer and fewer infected individuals after each cycle of
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