Research Accomplishment Reports 2010

Control of Equine Infectious Anemia (EIA)

C.J. Issel, R.F. Cook, S.J. Cook
Department of Veterinary Sciences

 

Non-Technical Summary

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.

2010 Project Description

We have collaborated with USDA, state regulatory, and international bodies to explore new initiatives to improve delivery of accurate diagnostics for EIA. We continue to expand our analysis of EIA virus genetic diversity by establishing international collaborations with scientists in Brazil, Argentina and several European countries with an initial focus on gag gene sequences. Studies conducted in Italy and in this laboratory have demonstrated the PCR protocol currently recommended by the OIE is not suitable for detection of most European and American EIAV strains.

Fortunately, new information from different laboratories, including our own, have enabled recognition of relatively conserved "islands" in the genomes of geographically distinct EIAV isolates. This, in turn, has permitted the design and constant refinement of PCR based methods capable of detecting EIAV isolates from many countries without the need for prior nucleotide sequence characterization. Furthermore, phylogenetic analysis of gag gene sequences obtained using these PCR techniques have demonstrated that there are at least 3 separate genotypes currently circulating in Central Europe.

As horses were reintroduced into the New World from Europe, it is perhaps surprising that only one of these genotypes is present among the published sequences from Argentina, Canada, Idaho, Texas and Wyoming, or new sequences generated in this laboratory from Pennsylvania, Florida and Shackleford Banks, North Carolina. Furthermore, this greatly expanded sequence analysis has confirmed the observation that although extensive gag gene variation occurs between geographically distinct isolates, it is not uniformly distributed. Viral sequences encoding the capsid antigen (p26) are the most conserved showing only 12 to 15% variation while more than 50% variation occurs in those sequences encoding the late antigen (p9).

Following our seminal attenuated vaccine studies demonstrating the critical role that variation in viral envelope (env) sequences play in inducing clinically protective responses, we have investigated the evolution of these sequences in vaccine strains prior to challenge. These studies have demonstrated the population of env sequences in horses immunized with the attenuated genetic deletion mutant D9 increase in both diversity and divergence compared to the original env gene. Although diversity of the env sequence population had no significant effects on efficacy, protection afforded by D9 was significantly associated with divergence from the env sequence present in the original vaccine strain as a consequence of its progressive evolution during the six-month pre-challenge period.

In tandem, our studies are using 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. Initial results are encouraging and show that specific viral peptides are correlated with the responses measured in individual horses. The video production "EIA: refining our approach" is available for distribution at mms://ocbmtcwmp.usda.gov/content/aphis/EIA final HDCAP.wmv

2010 Impact

The control of EIA in 2010 is effective in most developed countries and in the tested populations. Our biggest challenge, internationally, is to better understand the risk of EIA from the reservoir of EIA in untested equids and to deliver/improve diagnostics where appropriate. 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. The challenge to discover approaches that will enable induction of protective immune responses in outbred human and equine populations remains a major impediment. The collaborative studies with scientists in other countries are 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.

In summary, the study of gag gene sequences among different EIAV isolates has demonstrated:

(1) Although additional refinements may be required, PCR-based tests for detection of most if not all EIAV strains currently in circulation are feasible.

(2) These direct detection methods will be valuable for screening populations that have recently been exposed to EIA before the development of humoral immune responses.

(3) Important inroads have been made to determine the extent of genetic diversity among naturally occurring EIAV isolates, information that is crucial to the design of diagnostic reagents and the next generation of vaccines.

(4) The fact that extensive variation occurs between different viral isolates in p9 while the antigen is extensively conserved over prolonged time-periods in individually infected equids, make the sequences that encode it ideal for molecular epidemiological investigations to establish the source and extent of EIA outbreaks.

As an example, by analysis of p9 sequences we were able to present very compelling evidence confirming the commonly held belief that the 2006 EIA outbreaks in Italy and Ireland were linked to a common source of infected material. Our studies in D9 vaccinated horses have demonstrated for the first time a correlation between protective efficacy and the extent of vaccine virus env gene evolution.

As a result of variation associated with lentiviruses, it has been proposed that vaccines should comprise multiple immunogens that reflect the extent of this diversity. However, rather than administering single doses of a complex multivalent vaccine our data suggests it might be better to present these antigens sequentially in the context of multiple serial presentations thereby simulating env gene evolution.

2010 Publications

Craigo, J.K., Barnes, S., Cook, S.J., Issel, C.J., and Montelaro, R.C. 2010. Divergence not diversity of an attenuated equine lentivirus vaccine strain correlates with protection from disease. Vaccine 28:8095-8104.