VIRUSES

    A virus is a very small infectious agent that has a simple composition making it unable for a virus to grow and replicate outside of a host cell.  This makes a virus an obligate parasite as it relies on the host cell for energy to grow and replicate.  

Virus Morphology:
    The genetic make up a virus can be either DNA or RNA.  DNA viruses insert their genetic material right into the host cell’s DNA and replicate in the nucleus.  The RNA virus must first make DNA from the RNA in the cytoplasm using the host cell’s machinery before it can insert its DNA into the host cell’s.  Once the DNA has been inserted it is replicated.


    The genetic material is surrounded by a protein coat called a capsid that is made up of capsomere subunits.  Capsomere subunits may be made up of one or a number of different proteins.  The stability of the virus to environmental factors varies and is associated with an extra lipid based membrane called an envelope.  Naked viruses, which lack this extra membranous layer, are more hardy and are not as easily inactivated as enveloped viruses which do have this membranous layer.   The viral surface also contains attachment proteins with attached to receptor sites on the host cell.2  

naked virus    eveloped virus2



The Viral Life Cycle:
The life cycle of a virus has 4 main phases:

Attachment
Interaction between surface proteins on the virus and its specific receptors on the host cell.  This is the first step of infection. 
If the virus does not attach, infection does not occur. 
Entry and Uncoating Uncoating of the virus usually occurs in vesicles with then release the genome.
Replication RNA viruses usually replicate in the cytoplasm and DNA virus replicate in the nucleas of the host cell. 
DNA viruses insert their genetic material right into the host cell’s DNA.  The RNA virus must first make DNA from the
RNA using the host cell’s “machinery” before it can insert its DNA into the host cell’s.  Once the DNA has been inserted it is ready to be replicated.1
Assembly and Release 
The genome and capsids (protein coat) are synthesized and assembled.  The virus is released from the cell via lysis of the cell or budding off of the cell. 3


<>What is a vaccine?
    A vaccine is a manufactured biological preparation that can fool an animal’s immune system into thinking that it has been exposed to an agent that causes disease.  The vaccine must be killed or attenuated to avoid causing disease, but it still has to possess particular epitopes to deceive the body’s immune system into mounting a response. 15  The two most common types are Modified Live Virus and Killed Vaccines. 4

Different Types of Vaccinations:


1) Modified live virus vaccines (MLV) (Attenuated):

    This type of vaccine contains living viral organisms that have gone through a process to render them less virulent in order to not cause disease in the vaccinated animal.  The steps to making a MLV include collecting the virus from a diseased animal, grow that virus in a series of abnormal host cells so that it changes or attenuates.  After several passages through the abnormal cells, the less virulent virus is administered back to an animal to see if it still has the potential to cause disease.  If the virus does not cause disease, the vaccine manufacturer must check to see if it is still capable of causing an animal to create antibodies to it.  If both of these criteria are met, then the vaccine should be successful pending clinical trials.4  It is this type of vaccine that requires reconstitution with an approved liquid, usually sterile saline, in order to administer it to the animal.15

2)  Inactivated or killed vaccines

    This type of vaccine is easier to develop because virulence after growth is not a problem as a virus is collected directly from an outbreak and inactivated so that it no longer has the capability to cause disease.  It is inactivated by chemical physical methods, most commonly by using formalin and ultraviolet light respectively.  The major concern when producing this type is that there may be a loss of epitopes.  This loss may not allow the animal to mount the appropriate immune response and may be the reason behind the decreased efficacy compared to the MLV vaccine. 4  Adjuvant is frequently added to this type of vaccination to heighten the immune response and to hold the killed virus in the injection site.15

3)  Genetically Altered vaccines

    This type of vaccine is made by genetically altering the virus.  This is usually accomplished through generating mutations to decrease virulence or viral growth characteristics. 4  A common method is to grow the virus at an ambient temperature that is outside of the normal body temperature range of the host.  This causes the virus to mutate so that it grows best in that temperature, and not within the host’s body. 9 Therefore, the virus is still live, it is just modified. 4

4)  Autogenous Vaccines

    This type of vaccine is made privately for use by veterinarians and cattle owners.  These vaccines do not fall under federal guidelines, therefore they can not be sold to other farms.  The viruses for these vaccines are usually derived from specific cultures submitted from a particular farm in order to combat a pathogen that is not able to be controlled with conventional vaccinations.  It is important to know that these vaccines are not tested for safety or efficacy like the ones mentioned above.

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