1.2. Definitions: pathogenicity vs virulence; incidence vs prevalence
The terms ‘Infectivity’, ‘Pathogenicity’, ‘Virulence’ and ‘Transmissibility’ are often used interchangeably, which has led to efforts to tighten and standardize their definition and adapt them to our improved understanding of host-pathogen interactions (Casadevall and Pirofski, 1999; 2001). The same is true for the terms ‘Incidence’ and ‘Prevalence’ in surveys and epidemiology. Here are their definitions:
- Prevalence: The proportion of a population that is infected, or diseased, at any one time.
- Incidence: This is the risk of new infection during a specified time. It is globally related to prevalence as a function of time: prevalence = incidence x time
- Infectivity: This refers to the ability of a microorganism to invade and replicate in a host tissue, whether the microbe is pathogenic or not.
- Pathogenicity: This is a qualitative trait, referring to the inherent, genetic capacity of a microorganism to cause disease, mediated by specific virulence factors. Whether or not it does so, is the result of the specific host-pathogen interactions.
- Virulence This is a quantitative trait, representing the extent of the pathology caused by a microorganism. Virulence is therefore a trait expressing the interaction between a pathogen and its host. Its definition has been re-assessed recently (Casadevall and Pirofski, 1999; 2001), in view of the significant influence of the host’s immunological condition on the extent of the damage (i.e. virulence) caused by a pathogen.
Virulence is usually correlated to the pathogen’s capacity to multiply in the host (Casadevall and Pirofski, 2001) represented, for example, by the virion titre when symptoms appear (Figure 2) or the rate of multiplication. It can also be affected by host and environmental factors, such as the transmission route or type of tissue/life-stage infected. For example, a pathogen may be virulent when infecting one type of tissue and non-virulent when infecting a different tissue (Casadevall and Pirofski, 1999). Virulence is therefore dependent on the nature of the infection.
Virulence is also a relative trait, referring to the differences in the degree of pathology caused by strains of the same pathogen, or differences in the efficiency with which different strains can cause symptoms (Pirofski and Casadevall, 2012). For example, a pathogen strain that requires few particles to produce disease symptoms (strain-A in Figure 2) would be more virulent than a strain that requires many particles to produce the same symptoms (strain-B in Figure 2).
Since virulence is a quantitative measure, methods have been developed to quantify the relative contributions of different virulence factors to a phenotype (McClelland et al., 2006). - Transmissibility: This refers to the efficiency with which a pathogen is transmitted to naïve hosts. There are valid arguments that at epidemiological level, transmissibility could be considered a component of virulence (Figure 2). The relationship between transmission and virulence is a major topic in pathogen-host evolutionary theory (e.g. Ebert and Bull, 2003) and has been discussed within the context of honey bee colony structure (Fries and Camazine, 2001) and honey bee virus transmission (de Miranda and Genersch, 2010).
Fig. 2. Diagram describing the Log-linear relationship between virus concentration (X-axis) and virulence (Y-axis), represented by the degree/probability of Pathology (PPathology) or the efficiency/probability of Transmission (PTransmission). Other variables can also be plotted on the Y-axis. Image © J R de Miranda.
