Virulence

Virulence
n., [ˈvɪɹələns]
Definition: degree to which an organism can cause disease

Table of Contents

Virulence Definition

In biology, virulence is defined as the degree to which a pathogenic organism can cause disease. Etymologically, the term came from Latin vīrulentus, meaning “full of poison”, “toxin”. A related word, virulent, is a derived word that is used to denote a pathogen as extremely toxic. Synonyms: virulency.

What is the difference between pathogenicity and virulence?

Virulence is related to pathogenicity in the sense that its meaning is correlated to the manifestation of a disease. However, pathogenicity, in particular, is defined as the ability of a pathogen to cause disease. An organism that harms its host and causes the disease is referred to as a pathogen. The capability to produce disease is associated with the inherent characteristics of the organism in an effort to survive inside the host. Conversely, virulence refers to the degree of pathogenicity of a particular organism. (Ref.1) A virulent pathogen is one that causes damage to its host to an extent that is significantly greater than those caused by a non-pathogenic organism.

What is a virulence factor definition?

Pathogenic organisms have a different breadth of virulence. For example, a strain of bacteria may be more virulent than the other strains of the same species. The virulence of a pathogen is often correlated with the so-called virulence factors. A virulence factor is defined as the factor that enables an organism to invade a host and cause disease. It also determines the extent of damage to the host. These factors may be secretory, membrane-associated, or cytosolic in nature. (Ref.2)

An example of a virulence factor is the ability of microbes to multiply within their host cells. In microbiology, these factors are considered vital to epidemiology, particularly, when tracking a novel pathogenic strain. That is because the strain is often highly virulent, and therefore more detrimental, even fatal, to its host. Some of the virulence factors that researchers look into are the route of entry into its host, the pathobiological machinery employed, and its effects on the host’s immune response. Viral virulence factors, for instance, are chiefly proteins that are incited by the infective virus to be produced by the host’s own protein machinery. Bacterial virulence factors are likewise proteins that are coded for by their own genes or by plasmids that they acquired via horizontal gene transfer. The damage may be compounded by the host’s overly reactive immune response when the immune cells are so triggered by the presence of these virulence factors that they tend to damage the host cells in an effort to counter the infection. These virulence factors are, therefore, one of the major targets in medical research that intend to create new treatments and vaccines.

What is an example of a virulent?

Human immunodeficiency virus or HIV is an example of a virulent virus. It is the causative agent of AIDS. It is virulent because it employs mechanisms for evading the host immune cells. For instance, it infects the T-helper cell, one of the immune cells. Thus, the immune response of the host is already reduced and compromised.

Another example is lyssavirus that causes rabies. It enters and hijacks muscle cells, and then travels to the nervous system through the neuromuscular junctions. (Ref.3) Thus, it is particularly described as neurovirulent, i.e. for being able to cause disease in the nervous system.

As for bacteria, examples are the two human pathogens: Mycobacterium tuberculosis (causative agent of tuberculosis) and Bacillus anthracis (causative agent of anthrax).

Try to answer the quiz below to check what you have learned so far about virulence.

References

Pathogenicity vs Virulence. (2020). Retrieved from Tulane.edu website: https://www.tulane.edu/~wiser/protozoology/notes/Path.html
‌Sharma, A. K., Dhasmana, N., Dubey, N., Kumar, N., Gangwal, A., Gupta, M., & Singh, Y. (2016). Bacterial Virulence Factors: Secreted for Survival. Indian Journal of Microbiology, 57(1), 1–10. https://doi.org/10.1007/s12088-016-0625-1
Gonzaga, M. V. (2019, May 24). Rabies pathobiology and its RNA virus agent – Lyssavirus – Biology Online Article. Biology Articles, Tutorials & Dictionary Online. https://www.biologyonline.com/rabies-pathobiology-and-its-rna-virus-agent-lyssavirus