Age and Internal Structure

Abstract

Age plays an important role in the dynamics of disease transmission. Often the degree of susceptibility of an individual to a pathogen is dependent on the individual’s age, and contact rates among various parts of the population which may lead to the transfer of infections also depend on age. However, for horizontally transferred diseases it is often a valid approximation to neglect age as an independent parameter and to treat the population as being uniform in age. In fact, Ludwig and Walters [1985] argue that, in certain population models, it is possible that the added requirements for information in an age-structured model may lead to a degradation in the estimation of parameters from available data. In contrast, age enters as a fundamental parameter in vertically transmitted diseases, and it is always necessary to take age into account in one way or another in order to produce a valid model. There are at least two reasons for this. First, it is only the newborn, that is individuals of age zero, that are susceptible to this mode of disease transmission. Consequently, the newborn must be taken into account in some way that distinguishes them from the rest of the population. Second, in a number of cases of vertically transmitted diseases, the pathogenic agent builds up in an infected female as time goes by, and hence, so does the probability of transferring the infection to her offspring. Other effects of the infection, such as increased mortality or decreased fertility, can also depend on the length of the infection, hence on age. So, some means of tracking the internal state of infected fertile individuals needs to be incorporated in the model. Age is the simplest such internal variable. As we shall see later, other variables describing the internal structure of the population may need to be introduced. For an example of estimation of parameters from age-specific data, see Dietz and Schenzle [1985].