The enteroviruses were originally classified into four groups: polioviruses, coxsackie A, coxsackie B, and echoviruses. This was based on the physical structure of the viruses, the tissue cultures in which they would grow, and their pathogenesis in humans and experimental animals. Today, they are classified based on the virus genomic structure.
The enterovirus genus has five species of human enteroviruses: poliovirus and human enteroviruses A, B, C, and D. Virus serotypes use their original name given before the current reclassification. Although it appears strange at first glance, coxsackie A9, coxsackie B4, echovirus 6, and EV-69 are all listed under the species "human enterovirus B." Viruses within a species can recombine to produce viable hybrids.
Another useful way to group enteroviruses is by the receptors that are used. All the coxsackieviruses group B use the "coxsackievirus and adenovirus receptor" (CAR), but some of the echoviruses use integrin molecules. To some extent, the receptor used affects disease as it dictates where in the body the virus can go.
For further information on the classification of enteroviruses see the Univeral Virus Database of the International Committee on the Taxonomy of Viruses (ICTVdB) website. Clink on the link and search on "enteroviruses." http://www.ncbi.nlm.nih.gov/ICTVdb/Ictv/index.htm
Challenges to Classification: Relentless Mutation and Recombination
Enteroviruses evolve relentlessly through mutation and recombination as they circulate in human and animal hosts. (Domingo, 2008) The genetic flexibility and continuous variation of enteroviruses renders them as candidates for agents of emerging infectious disease. (Palacios, 2005)
"The behavior of enteroviruses can be considered extreme as a Darwinian system because the combined high rates of mutation and recombination produce an explosive adaptive capacity. This is reflected in an ever increasing identification of variant enteroviruses with new disease manifestations." (Domingo, 2008)
Could enteroviruses be associated with diseases that have unknown causes?
Recent findings of enterovirus disease associations may lead to speculation that more diseases could have links to enteroviruses.
"As suggested more than two decades ago by J.J. Holland and colleagues, variant forms of viruses not easily recognizable by standard diagnostic procedures could underlie several forms of persistent infections associated with chronic disease (Holland et al. 1982). It would be interesting to use new genome screening procedures to try to identify atypical forms of enteroviruses that could be associated with diseases that, for the moment, are of unknown etiology. (Domingo, 2008)