Veterinary Epidemiology and Zoonosis

AGENT DETERMINANTS

Virulence and pathogenecity

• Infectious agents vary in their ability to infect and to induce disease in animals.
• Ability to infect depends upon the susceptibility of the host and also the immune status of the host.
• Ability to induce disease is expressed in terms of virulence and pathogenecity.

Virulence

• Virulence is the ability of an infectious agent to cause disease in a particular host in terms of severity.
• Highly virulent organisms are often associated with high case fatality rate.

Pathogenecity

• It refers to the quality of disease induction. Pathogenecity can also be defined as the ratio of number of individuals developing clinical illness to the numbers exposed to infection.
• A highly pathogenic organism induces clinical disease in large proportions of animals.
• Virulence and pathogenecity are commonly intrinsic characters of an infectious agent.
• Pathogenecity is sometimes used as a synonym for virulence.
• Virulence and pathogenecity are either phenotypically or genotypically conditioned.
• Phenotypic changes are transient and are lost in the succeeding generations.
• Genotypic changes result from a change in DNA and RNA of the microbial genome.
• Genotypic changes in an infectious agent can result from mutation, recombination, conjugation, transduction and transformation.

Mutation

• It is an alteration in the sequence of nucleic acids in the genome of a cell or virus particle.
• There may be either point mutation of one base or deletion mutation where segments of genome are removed.

Recombination

• It is the reassortment of segments of a genome that occurs when two microbes exchange genetic material.
• Major changes are referred to as 'antigenic shift' and minor changes as 'antigenic drift'.
• Conjugation involves transmission of genetic material from one bacterium to another by conjugal mechanism (i.e. by touch) through a sex pilus.
• This is common in E.coli, Salmonella, Proteus, Shigella and Pasteurella species.

Transduction

• It is the transfer of small portion genome from one bacterium to another by a bactriophage.
• It occurs in Shigella, Pseudomonas and Proteus spp.

Transformation

• It is transmission of naked nucleic acid between bacteria without contact.