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.