chromosome, a bacteriophage, a plasmid, or some other unit. These virulence factors include:

Adherence. Some bacteria have specialized structures called pili that attach to the intestinal epithelium of the host and permit replication before the infecting cells are swept away.

Invasion. These factors permit bacteria to gain entry into the cell, where they replicate in a protected environment.

Capsules. Some bacteria, such as Pneumococcus, form an outer capsule of viscous polysaccharide gel that increases their virulence and resistance to phagocytosis and destruction.

Endotoxins. Gram-negative bacteria have an outer membrane that consists of lipopolysaccharides, or fat- and sugar-based compounds, that may damage or kill the host.

Exotoxins. These are proteins that are poisonous to cells and often have specific cell targets.

Siderophores. Some bacteria produce these iron-binding proteins that serve to increase virulence by consuming iron needed by eukaryotic cells, which are the type of nucleus-containing cells that comprise humans.

Viruses, with a rare exception, do not have bacterial-like virulence factors or toxins. Being intracellular, viruses disrupt the body’s function either by direct destructive effect on target cells and organs or by inducing pathogenic host responses. The cell can be looked on as a factory. A virus may use up the cell’s energy; shut off the synthesis of required materials; compete for the cell’s ribosomes, which are necessary for building proteins; or compete for the cell’s polymerases and inhibit its innate defense system. Some viruses have the capacity to integrate into the cell’s genome and thus cause indirect damage, leading, for instance, to malignancy. As with bacterial virulence factors, the elements responsible for viral virulence affect both zoonotic and nonzoonotic agents equally.

Examples of Pathogenesis and Virulence in Zoonoses

Zoonotic diseases offer some interesting insights into pathogenesis and virulence, as well as to the links between them. Consider two examples, one a bacterium and the other a virus.


Anthrax is one of the more interesting zoonoses from the point of view of a link between pathogenesis and virulence. The virulence is directly related to the site of entry of the bacterium.

Bacillus anthracis is a gram-positive organism that infects cattle and

The National Academies of Sciences, Engineering, and Medicine
500 Fifth St. N.W. | Washington, D.C. 20001

Copyright © National Academy of Sciences. All rights reserved.
Terms of Use and Privacy Statement