2. Disease transmission and the application of disease spread models
Disease spread involves the transmission of a pathogen (or a microbe with pathogenic potential) from one individual to another. In some cases, the event during which transmission occurred can be identified (for example, a sexually transmitted disease such as Campylobacter fetus venerealis in cattle, or a disease such as rabies in which the bite was observed). However, unequivocal identification of the transmission event is rare; most diseases have several routes of transmission, and microbes, by definition, are usually invisible to the naked eye. Typically, an individual becomes infected, and a range of time, places and methods by which transmission could have occurred are identified, with the certainty of these estimates dependent on the epidemiology of the disease and the information available about the individual case. Identification of transmission events in wildlife populations − which might be rarely observed or hide from people − is even more challenging.
When considering the epidemiology of infectious diseases, it is useful to think in terms of the ‘infectious diseases triad’ which includes features about the pathogen, hosts, and the environment. As well as describing the distribution of disease in terms of where and in whom it occurs, these three domains determine the situations in which ‘effective contact’ can occur. Effective contact is the situation in which disease could successfully transmit between two individuals if one were infected and the other susceptible. The least complex infectious disease triad combination involves pathogens with a single host and a limited or non-existent environmental stage. Examples include herpes viruses, such as infectious bovine rhinotracheitis virus and equine herpes virus 1, and lente viruses such as caprine arthritis encephalitis virus. Combinations of factors – and hence the range of possible circumstances, or routes, in which effective contact could occur – increase if the pathogen can survive in the environment (for example, foot and mouth disease virus on fomites, foodborne bacteria such asSalmonella spp, and the spore-forming bacteria that cause clostridial diseases), there is a mechanical or biological vector (for example, Stomoxys flies and porcine reproductive and respiratory syndrome virus, and ornithodorid ticks and African swine fever virus), and there is more than one host (for example, many parasitic diseases, and zoonoses). In any disease transmission event between two individuals (one infected, the other susceptible), there is both a probability of a route of transmission given the combination of factors that could have occurred, and a probability that pathogen transfer and infection then occurred.
Disease spread models are a useful tool to generate insights about how the dynamics of a particular disease in a population can be influenced by control methods. At a population level, models are useful to explore how disease spreads, given the probabilities of the range of effective contact possibilities and subsequent transmission and infection. The simplest disease spread model can be defined in terms of only 2 states in which an individual can be either susceptible or infected, and therefore, the population is divided between these two states. In this simple model, the assumption is that once an individual is infected, they remain infected. The model is dynamic, because the risk of infection for individuals who are susceptible (S) depends on the relative proportion or number of infected individuals. To start with, when there are very few infected individuals, the overall risk of infection is low, and the rate of transition of the population from S to I is low. However, this rate increases as the number in I grows, and then decreases again as the number of S falls. Unpacking the mathematical basis of this dynamic change in rate of infection demonstrates the importance of defining effective contact in disease spread models, and the type of field data that is required for parameterisation.