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Porcine Reproductive and Respiratory Syndrome (view source)

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==Aetiology==
 
==Aetiology==
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The virus that causes PRRS is an aterivirus, within the Arteriviridae family. The arteriviruses are the only genus contained within the Arterivirdae family, and other members include the agent responsible for equine viral arteris and lactate dehydrogenase-elevating virus of mice. These viruses are antigenically distinct from each other. Arteriviruses are enveloped viruses of 45 6o 80 nm in diameterand appear spherical due to the shape of the envelope. However, the positive-sense single-stranded RNA genome is contained within The virus is enveloped and ranges in size from 45 to 80 mm. Inactivation is possible after treatment with ether or chloroform; however, the virus is very stable under freezing conditions, retaining its infectivity for 4 mo at -70°C. As the temperature rises, infectivity is reduced (15-20 min at 56°C).
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The virus that causes PRRS is an aterivirus, within the Arteriviridae family. The arteriviruses are the only genus contained within the Arterivirdae family, and other members include the agent responsible for equine viral arteris and lactate dehydrogenase-elevating virus of mice. These viruses are antigenically distinct from each other. Arteriviruses are enveloped viruses of 45 to 80 nm in diameter with a spherical appearance due to the shape of the envelope. However, the positive-sense single-stranded RNA genome is contained within an icosohedral nucleocapsid.  The lipoprotein envelope has ring-like structures on its surface, but there are no gross surfae spikes. Arteriviruses have a tropism for macrophages and endothelial cells, where they reproduce in the cytoplasms. The genome is 13Kb in length, and is infectious itself without the capsid or envelope. The viruses are host-specific, and persistent infections are established after invasion. Inactivation is possible after treatment with ether or chloroform; however, the virus is very stable under freezing conditions, retaining its infectivity for 4 mo at -70°C. As the temperature rises, infectivity is reduced (15-20 min at 56°C).
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This family of enveloped, positive-sense, single-stranded RNA viruses, established in 1996, was classified formerly in the Togaviridae family. It has only one genus, Arterivirus, whose virus species are antigenically distinct from each other.
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Viral Characteristics
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    * These positive sense, single-stranded RNA viruses are medium in size (50 - 70 nm) have a spherical appearance due to the envelope, but the nucleocapsid is icosahedral in shape (see Fig. 25.1).
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    * They possess a lipoprotein envelope with ring-like structures on the surface, but no gross surface spikes.
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    * They replicate in the cytoplasm of macrophages and endothelial cells.
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    * The genome is 13K nucleotides in length, has a 5'-methylguanosine cap and a 3' poly A tail of approximately 50 nucleotides. The genome alone is infectious.
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    * Individual viruses are antigenically distinct and host specific; they establish persistent infections
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          o Porcine Respiratory and Reproduction Syndrome
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          o Lactate Dehydrogenase-elevating Virus of Mice
      
==Transmission and Epidemiology==
 
==Transmission and Epidemiology==
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