Equine foamy virus (EFV), also called foamy virus (FV), is virus in the genus Equispumavirus. It shares similarities, with respect to replication, with lentiviruses. EFV, along with other FVs are from the family Retroviridae and subfamily Spumaretrovirinae. Spumarivuses, such as EFV, are complicated retroviruses that have been characterized in many animals including nonhuman primates, cattle, cats. Additionally, these viruses have been identified in animals that most often carry lentiviruses.
The name foamy virus can be attributed to the foamy appearance of the cells upon rapid lysation and syncytium formation, vacuolization and cellular death, which is also known as a cytopathic effect (CPE).
EFV has been found to display a similar capsid structure along with exhibiting similar functionality to other foamy viruses, yet differs in some respects.
Viral classification
Equine foamy virus is a single-stranded RNA-RT virus. It is classified in the genus Equispumavirus, subfamily Spumaretrovirinae and family Retroviridae. Foamy viruses are the only viruses of the Retroviridae that reside in the subfamily Spumaretrovirinae. The remainder of the viruses from the family Retroviridae are classified under the subfamily Orthoretrovirinae, which include alpharetroviruses, betaretroviruses, gammaretroviruses, deltaretroviruses, epsilonretroviruses and lentiviruses. The separation of foamy viruses under a different subfamily was determined in 2002 by the International Committee on Taxonomy of Viruses (ICTV) based on a difference in replication pathways, thus making a clear distinctive quality of foamy viruses from other Retroviridae.
EFV has characteristics of viruses from other genera in Spumaretrovirinae. EFV is similar to viruses from the paraetrovirus genus because of their formations of a pol mRNA and their infectivity of viral DNA. Although not fully researched, EFV has been found to share several distinct qualities with lentiviruses including transmission, natural hosts, and replication
Identification
Foamy viruses were first discovered and described in 1954 in kidney cultures of simians and were first isolated in 1955. From then on, a variety of foamy viruses were isolated from several species including cats, cows, monkeys, humans. With the discovery and isolation of foamy viruses in other natural hosts such as bovine, feline, and simian, the hypothesis that an equine foamy virus might exist was explored in the late 1900s to early 2000s
Blood samples of naturally infected horses were taken to determine the existence of an equine foamy virus. Because foamy viruses have been linked to lifelong yet not pathogenic diseases, the presence of antibodies without the presence of a disease or illness is a key indicator of a foamy virus infection. Since antibodies against foamy viruses are not often found in animals three to five months of age due to passive immunity, the blood samples were only taken from sexually mature horses and horses who had surpassed the range of passive immunity and were in close living quarters with horses that had the virus
Genome
Through nucleotide sequence analyses and molecular cloning, EFV was compared to other foamy viruses regarding the organization of their genome along with the ultrastructure. While the overall organization of EFV is similar to other foamy viruses, it was found to share minor similarities to primate foamy viruses. EFV is most similar sequentially to bovine foamy virus (BFV), yet it still only shares 40% of its sequence with it
Size
Equine foamy virus, like other foamy viruses, has been reported to have a rather large genome. Foamy virus genomes can range anywhere from 12 to 30 kb in length. The genome length of EFV specifically has not been recorded
Genes and proteins
Within this genome, classical structural genes common to FVs are present such as gag, env, and pol. Sequence analysis has also led to the discovery that while EFV shares most of the same features as other foamy viruses, it differs slightly with respect to the noncoding regions, the gag gene, the pol gene, the env gene and the regulatory region
In addition to the encoded retroviral genes pol, gag and env, the genome of EFV also encodes for two more genes: tas and bet. Both of the two additional genes, although bet has been under researched, play a strong role in replication. One of EFVs two promoters is transactivated by the protein made from tas, therefore, tas is required for replication of the entire genome to occur