EPIDEM: patterns of disease

epidemic curve

epidemic curve

a graph of an epidemic represented with the number of new cases of a disease, with the number of new cases on the vertical axis and calendar time on the horizontal

factors affecting the shape of the curve

incubation period, infectivity of the agent, susceptible animals, distance between animals

threshold level

when a minimum density of susceptible animals theoretically is required to allow a contact-transmitted epidemic to commence

kendall’s threshold theorem

illustrates mathematically when a minimum density of susceptible animals is required to allow a contact-transmitted epidemic to commence

critical community size

determines whether or not new susceptible individuals are recruited at a high enough rate for infection to persist

basic reproductive number

frequently defined as the ability of an infectious disease to spread in a population, where the average number of secondary cases caused by one typical infectious individual during its entire infectious period in an entirely susceptible population

transmission rate

number of secondary cases that occur per unit time; where it should be equal to the product of the transmission risk and the contact rate

dissemination rate (DR)

is the propensity of infection to spread to other herds. It represents the average number of uninfected herds (or premises) to which an infectious agent is delivered by each infected herd.

estimated dissemination rate (EDR)

calculated from the observed outbreaks of disease

common-source epidemic

in which all cases are infected from a source that is common to all individuals.

point-source

common-source epidemic if the period of exposure is brief

propagating epidemic

an epidemic caused by an infectious agent in which initial (i.e., primary) cases excrete the agent, and thus infect susceptible individuals, which constitute secondary cases

index case

one of the primary cases that is first to come to the attention of the investigators

reed-frost model

a basic model where the shape of the epidemic curve in a propagating epidemic in a defined population can be mathematically modelled

wave train

Some epidemics – notably those caused by viruses – occur as a series of outbreaks, which can be considered as a series of epidemic waves

Short-term trends

typical epidemics, which already have been discussed

Cyclical trends

associated with regular, periodic fluctuations in the level of disease occurrence. They are associated with periodic changes in the size of the susceptible host population and/or effective contact, and may produce recurrent epidemics or endemic pulsations

seasonal trend

a special case of a cyclical trend, where the periodic fluctuations in disease incidence are related to particular seasons

long-term (secular) trends

occur over a long period of time and represent a long-term interaction between host and parasite

time series analysis

used in epidemiology to detect temporal trends; a record of events that occur over a period of time; cases of disease are typical events. The events are plotted as points on a graph, with ‘time’ along the horizontal axis

contagious or clustered spatial pattern

spatial trend when an infectious disease propagates through a population

sporadic outbreak

when clustering of cases is distributed randomly

Space–time clustering

an interaction between the places of onset and the times of onset of a disease; cases that are close in space tending to be close in time