Animal Viruses

By Regina Bailey, About.com Guide

Animal Viruses

At one time or another, we have all most likely been infected with a virus. The common cold and chicken pox are two common ailments caused by animal viruses. Animal viruses are intracellular obligate parasites, meaning that they rely on the host animal cell completely for reproduction. They use the host's cellular components to replicate, then leave the host cell to infect other cells.

Viruses gain entry into host cells via several sites such as the skin, gastrointestinal tract, and respiratory tract. Once an infection has occurred, the virus may replicate in host cells at the site of infection or they may also spread to other locations. Animal viruses typically spread throughout the body mainly by way of the bloodstream, but can also be spread via the nervous system. Viruses have several methods to counter host immune system responses. Some viruses, like HIV, destroy immune system cells. Other viruses, such as influenza viruses, experience changes in their genes leading to antigenic drift or antigenic shift. In antigenic drift, viral genes mutate altering virus surface proteins. This results in the development of a new virus strain that may not be recognized by host antibodies. Antibodies connect to specific virus antigens to identify them as 'invaders' that must be destroyed. While antigenic drift happens gradually over time, antigenetic shift occurs rapidly. In antigenetic shift, a new virus subtype is produced through the combination of genes from different viral strains. Antigenetic shifts are associated with pandemics as host populations have no immunity to the new viral strain.

Viral Infection Types

Animal viruses cause various types of infection. In lytic infections, the virus will break open or lyse the host cell, resulting in the destruction of the host cell. Other viruses may cause persistent infections. In this type of infection, the virus may go dormant and be reactivated at a later time. The host cell may or may not be destroyed. Some viruses can cause persistent infection in different organs and tissues at the same time. Latent infections are a type of persistent infection in which the appearance of disease symptoms does not happen immediately, but follows after a period of time. The virus responsible for the latent infection is reactivated at some later point, usually prompted by some type of event such as infection of the host by another virus or physiological changes in the host. HIV, Human Herpesviruses 6 and 7, and the Epstein-Barr Virus are examples of persistent virus infections that are associated with the immune system. Oncogenic viral infections cause changes in host cells, tuning them into tumor cells. These cancer viruses alter or transform cell properties leading to abnormal cell growth.

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