Animal Viruses

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Animal Viruses

Sleeping child with chickenpox
Mieke Dalle/ Photographer's Choice/ Getty Images

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. Examples of viruses that infect humans include chickenpox, measles, influenza, HIV, and herpes.

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.

How Viruses Counter Your Immune System

Viruses have several methods to counter host immune system responses. Some viruses, like HIV, destroy white blood 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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Animal Virus Types

Measles Virus
Measles Virus Particle. CDC

Animal Virus Types

There are several types of animal viruses. They are commonly grouped into families according to the type of genetic material present in the virus. Animal virus types include:

  • Double-Stranded DNA

    Double-stranded DNA viruses usually have a polyhedral or complex structure. Examples include: Papilloma (cervical cancer and warts), Herpes (simplex I and II), Epstein-Barr virus (mononucleosis) and Variola (smallpox).
     
  • Single-Stranded DNA

    Single-stranded DNA viruses usually have a polyhedral structure and depend on adenoviruses for parts of their growth.
     
  • Double-Stranded RNA

    Double-stranded RNA viruses usually have a polyhedral structure with the diarrhea viruses being a common example.
     
  • Single-Stranded RNA

    Single-stranded RNA viruses are usually of two subtypes: those that can serve as messenger RNA (mRNA) and those that serve as a template for mRNA. Examples include: Ebola viruses, Rhinovirus (common cold), HIV, rabies virus and influenza viruses.

Animal Virus Vaccines

Vaccines are made from harmless variants of viruses to stimulate an immune defense against the 'real' virus. While vaccines have all but eliminated some illnesses such as smallpox, they are usually preventative in nature. They can help prevent an infection, but do not work after the fact. Once a person has been infected with a virus, little if anything can be done to cure a viral infection. The only thing that can be done is to treat the disease symptoms.