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Control of Equine Infectious Anemia (EIA)
C.J. Issel, R.F. Cook, S.J. Cook
Department of Veterinary Sciences
Equine infectious anemia virus (EIAV) is a lentivirus and a close relative of the human immunodeficiency virus. Many isolates of EIAV have high disease-inducing potential even though they have been isolated from inapparent carriers of the virus. The ability of the virus to induce disease is directly related to viral and host factors. Our research is directed at defining the viral determinants of disease pathogenesis, as they are amenable to analysis and modification. Once the key genetic determinants are identified, rational approaches to removal of these genes may lead to effective immunization regimens.
By the same token, some viral genes may code for structural proteins parts of which may be recognized as dominant by equid hosts. The result may be a massive response to the dominant epitopes that are not protective while precluding or preventing effective immune responses to subdominant epitopes, which by themselves could be protective.
These research priorities are important for their own merit in horses, but also for their comparative value as a model for HIV. Control of equine infectious anemia requires an understanding of basic virus and host factors in protective immunity.
2009 Project Description
We have collaborated with USDA and state regulatory bodies to explore new initiatives to better deliver accurate diagnostics for EIA and to refine our approach to better identify the remaining reservoirs of the infection in the United States. We have refined and extended our analysis of EIA virus genetic diversity by establishing international collaborations with scientists in Ireland, Italy and Brazil and characterized isolates from the United States.
These results revealed greater than expected genetic diversity in EIAV. More importantly it has been possible to use this information to design broadly reactive PCR-based tests for the detection of viral genetic material in suspected cases of EIA. The need for new broadly reactive PCR primers is exemplified by the fact that primers based solely on North American strains as recommended by The OIE for detection of EIAV sequences have proven to be completely ineffective in Italy.
To date a conventional nested RT-PCR system has been produced that can be employed either as a straightforward diagnostic tool or to amplify complete viral gag gene fragments for subsequent nucleotide sequencing and epidemiological analysis. This assay has been tested successfully in Italy, Ireland, and in our laboratory using samples from the US and Brazil.
In addition to characterization of the 5' regions of the EIAV genome we have also begun to examine viral envelope (env) genes because of the critical role that variation in these sequences play in inducing clinically protective responses. To better understand the relationships between env gene diversity and vaccine induced clinical protection we are currently testing chimeric virus strains with exchanged surface unit (SU) protein segments. We are developing, in collaboration with Dr D Horohov (Gluck Equine Research Center), novel in vivo methods to analyze the cellular immune responses of EIAV-infected and immunized horses with the eventual aim of identifying the protective determinants in the system. In addition to evaluating attenuated vaccines it was also proposed to utilize the inherent genetic flexibility of DNA-based vaccines to investigate modifying immunodominant immune responses to EIAV. These investigations are evaluating the use of exogenous and endogenous enhancers to increase the intensity and duration of immunity.
The control of EIA in 2009 is effective in most developed countries and in the tested populations. Our biggest challenge, internationally, is to understand better the risk of EIA from the reservoir of EIA in untested equids. Thus our efforts at addressing that within the US initially has merit and is being effective, albeit slowly. Understanding and managing the political and social impediments to such changes is problematic.
The fact the global AIDS epidemic continues unabated highlights the urgent requirement for effective vaccines against lentiviruses. EIAV, in addition to being a significant health problem for horses throughout the world, provides a powerful model system for the development of lentiviral vaccines. Our recent experiments have demonstrate for the first time that Th and CTL responses against certain epitopes within the SU glycoprotein can confer protection, even against infection with heterologous viruses.
The next challenge will be to discover approaches that will enable induction of these responses in all horses or ponies. The collaborative studies with Irish Italian and now Brazilian scientists on the molecular characterization of European EIAV strains is yielding valuable new insights into the extent of diversity within field isolates of this virus. These studies have demonstrated that genetic variation is much greater than previously thought, even in regions of the genome such as the gag that were originally thought to be very highly conserved.
Almost all of the structural models and mechanistic predictions for EIAV proteins have been based on single virus strains, many of which have been adapted in the laboratory to replicate in atypical host cells. The wealth of new sequence information based on wild-type field isolates has enabled us to test some of these structural and mechanistic predictions. For example, we have shown that despite an overall variation of 25% in amino acid content between strains the five alpha-helical configuration of the viral matrix protein (p15) is maintained and that despite greater than 50% variation in the viral late antigen (p9) the YPDL late assembly domain is universally conserved.
However, variation is observed in p15 amino acids predicted to be highly conserved because of proposed interactions with host cell phosphatidylinositol 4,5-diphosphate and most surprisingly extensive changes are seen in the major homology domain of the capsid antigen (p26) that is believed to be essential for multimerization of gag antigens. As more of this type of information becomes available it will, in addition to increasing our fundamental understanding about the molecular biology of this lentivirus, be used to refine molecular diagnostic techniques and develop improved vaccines.
Issel, C.J. and Sadlier, M. 2009. Reducing the risks of infection in veterinary practices: Recent lessons learned with Equine Infectious Anemia (EIA). Article #14553 on The Horse.com http://www.thehorse.com/ViewArticle.aspxID=14553
Cook, R.F. and Issel, C.J. 2009. Equine infectious anemia. Pp 56-71, in Infectious Diseases of the Horse, Edited by TS Mair and RE Hutchinson, Eq Vet Journal Ltd, Cambridgeshire, UK.
Craigo JK, Barnes S, Zhang B, Cook SJ, Issel CJ, Montelaro RC. 2009. A Pennsylvanian-derived EIAV field isolate reveals much higher levels of subtype variability than originally reported for the lentivirus family. Retrovirology 6:95. Available as an Open Access article at: http://www.retrovirology.com/content/6/1/95.