Pseudorabies
Pseudorabies
A Microbial Biorealm page on the genus Pseudorabies
Classification
Viruses; dsDNA viruses, no RNA stage; Herpesvirales; Herpesviridae; Alphaherpesvirinae; Varicellovirus; Suid herpesvirus 1 (1).
Description and significance
Pseudorabies, also known as Aujeszky's disease, is an acute, frequently fatal disease found worldwide that affects swine. Other domestic animals are also affected including cattle, sheep, cats, dogs, and goats. Wild animals that are affected include raccoons, opossums, skunks, and rodents. This virus emerged as a major pathogen in the United States in the 1960s because of the emergence of more virulent strains affecting swine populations. This virus is also referred to as "mad itch", and characteristics of infection are similar to those of rabies (2).
This virus has been reported in humans few times, and does not pose a large risk to humans. Only one serotype of pseudorabies virus is recognized, but strain differences have been identified (2). Mass vaccination of all pigs on a farm generally occur to alleviate clinical signs in pigs. Regular vaccinations have controlled the virus since transmission to other animals is unlikely once vaccinated. Pseudorabies has been successfully removed from most areas of the United States (2).
Genome structure
Pseudorabies virus has double stranded genomic DNA. The linear viral DNA are circular which serve as the template for DNA synthesis. These cells perform rolling-circle DNA replication. Herpesviruses encode many of the enzymes required for viral DNA replication. Likewise, these virus genomes encode several enzymes for nucleotide metabolism (3).
The Psuedorabies DNA sequence (GenBank accession number BU001744) is 143,461 bases long. There are 73 genes. Some splicing occurs in immediate-early or latency genes. There is high G and C content in these genomes (68-74%). PRV genes UL16, UL17, and UL15 were sequenced and are very similar to those of other alphaherpesviruses (4).
Cell structure, metabolism & life cycle
Pseudorabies is a herpesvirus and all of these viruses have double-stranded DNA and are 200-250 nm in size. The mature virion consists of four morphologically distinct components. (1) The central core contains the DNA. (2) The DNA is surrounded by an icosahedral capsid to form a nucleocapsid. (3) The capsid is embedded in a tegument, a protein matrix. (4) The lipid membrane surrounds the tegument and has many viral glycoproteins (4).
Pseudrabies cells attach to cells, and the glycoproteins allow the capsid and tegument into the cytoplasm of the attacked cell. After reaching the nuclear pore, the viral DNA is released into the other cell's nucleus. DNA replication, transcription, and translation occur within the targetted cell. Viral progeny can be detected within 4 to 5 hours. Proteins are synthesized mostly between 1 and 4 hours post infection (4). The virus may last up to 7 hours in the air, and 2-4 days on grass, soil, feed, water, and straw. The virus can be killed in UV light and in high temperatures above 37°C (2).
Ecology (including pathogenesis)
Pseudorabies is a virus that attacks the brains of animals. This pathogen is transmitted via nose-to nose or fecal-oral contact. Pseudorabies can be indirectly transmitted through inhalation, as it may exist in the air for up to 7 hours in adequate levels of humidity. The virus can travel several miles while airborne. It may last for 2 days in grass, soil, feces, and corn; 3 days in hog feed; and 4 days in straw bedding (2).
The affected animal will display tremors and paddling, respiratory disease, and inhibition of aveolar macrophages, increasing susceptibility to bacterial infections. Weight loss, sneezing, and dyspnea are common symptoms. The virus spreads through the lymph nodes where it continues to replicate. Pseudorabies spreads through nervous tissue to the brain where it primarily replicates. The virus may also spread to the lungs, spleen, embryos, and luteal cells(2). Viral maturation is rapid; all viral replication stages may be complete 18 hours after onset of infection (3).
Interesting feature
Pseudorabies virus has become used as a model system. Researches use this virus to study alphaherpesviruses regarding their viral life cycles and their interactions with the host. In particular, researchers study virus attachment, entry, replication, assembly, intracellular tracking, and egress. Mechanisms of neuroinvasion, transneuronal spread, and host immunes responses are also being researched (3).
Researchers are using pseudorabies because this virus can cause infection in a variety of animals and is not dangerous to humans. This virus grows well and is easy to work with in laboratory (3).
References
(1)http://www.ncbi.nlm.nih.gov/Taxonomy/Browser/wwwtax.cgi?mode=Info&id=101947&lvl=3&lin=f&keep=1&srchmode=1&unlock. "Pseudorabies Virus Ea". NCBI.
(2)http://www.merckvetmanual.com/mvm/index.jsp?cfile=htm/bc/102200.htm The Merck Veterinary Manual.
(3)http://www.ncbi.nlm.nih.gov/pmc/articles/PMC1197806/?tool=pmcentrez Pomeranz,L., Reynolds, A., and Hengartner, C. "Molecular Biology of Pseudorabies Virus: Impact on Neurovirology and Veterinary Medicine". Microbiology and Molecular Biology Reviews. 2005. Volume 69. p. 462-500.
(4)http://www.ncbi.nlm.nih.gov/pmc/articles/PMC303424/?tool=pmcentrez Klupp, B., Hengartner, C., Mettenleiter, T., and Enquist, L. "Complete, Annotated Sequence of the Pseudorabies Virus Genome". Journal of Virology. 2004. Volume 78. p. 424-440.
