3/11/2023 0 Comments Density dependent factorsWe’ve partially answered Question 1! But before we can go any further with our transmission equations, we need to figure out why the c and c’ are different. So the reason that B =/= B’ is also because c =/= c’. Furthermore, remember the transmission rates B and B’? Well, B = c*v and B’ = c’ * v. (I’m using Begon et al.’s (2002) variables for these rates.) So, we can say that:Īs you can see, the reason that λ =/= λ’ is because c =/= c’. That is, FOI is the product of three probabilities/rates: 1) the probability/rate that a contact happens between individuals in the population (c or c’), 2) the probability/rate that a given contact is with an infected individual (I/N), and 3) the probability/rate that a contact between an S and an I individual results in successful transmission of the parasite (v). I like to call the force of infection a ‘conglomerate’ rate because it is really a combination of other rates. Where λ and λ’ are the force of infection terms. Another way to look at those transmission equations is: FOI is a rate that is pretty easy to measure in some systems you just stick susceptible animals out in the field and count how many get infected per unit time. When we look at those dI/dt equations, we’re asking, “What is the rate at which we increase the number of infected individuals in the population?” Or, equivalently, “What is the per capita rate at which susceptible individuals get infected?” That rate has a special name – it is the force of infection (FOI). This is a good time to introduce the force of infection, which is something that I have previously blogged about without explaining it very well.
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