Since 1974 we have been interested in understanding the impact of EIA virus infections on equid populations. Before accurate testing for EIA was available (before 1972), we had no way to know the extent of inapparent infections. Then, it was not uncommon to find EIAV infected horses at major congregation points. Today, the mortality and morbidity losses from EIA are extremely low (almost nonexistent) in countries and areas where testing and control programs have been adopted. When equids are found positive on serologic tests, the usual reaction of horseowners and animal disease regulators is generally an overreaction based on the potential for disease, which fortunately is seldom realized.
In field sites in Louisiana, Oklahoma, North Carolina and Florida, we have been fortunate to have had cooperation from state animal health officials to be able to study samples from EIAV infected horses over extended periods of time. In some of these cases, the horses were reproductively active and in others, uninfected horses served as sentinels for virus transmission. In the majority of cases, the horses were selected because they were found positive on serologic tests for EIA but they had no known history of clinical disease ascribed to EIA.
The unpredictable nature of EIA has been verified in these situations. In the majority of cases, transmission was documented at a low rate, with foals of test-positive dams appearing to be at lower risk of infection than adults in the same situations with relatively high insect vector pressures (Issel, C.J. et al. 1985). Overall, the rate of infection in foals of test-positive mares after natural weaning practices in field situations was much lower than expected (<5%; McConnico, R.S. et al. 2000, Issel, C.J.,et al. 2001 ). This is in stark contrast to reports of high infection rates and lower foal production where the mares had been recently infected or where clinical signs of EIA were evident.
At the FRIENDS Ranch in Florida, we have monitored an assembled group of test-positive inapparent-carrier horses at a Florida Department of Agriculture approved quarantine site for more than 30 years. In several cases, horses that had been positive were reported as negative on routine AGID (Coggins) tests when submitted without history of EIA. These observations verified and extended observations at our field sites in Louisiana and helped convince of the need for more sensitive diagnostic tests, if our goal is to find all of the EIAV-infected horses. In one case, after a bout of laminitis a mare that had been "falsely negative" for more than 5 years became an unequivocal strong positive reactor, again demonstrating the unpredictable nature of EIA. We are indebted to Debbie Beye-Barwick and her team at FRIENDS for their continued support and for providing images used on this web site. A gallery of photos showing the FRIENDS horses and some of their uses at the quarantine site can be seen here.
The Foundation for Shackleford Horses (FSH) in North Carolina has operated a quarantine site where test-positive horses from Shackleford Banks were relocated under permit from the North Carolina Department of Agriculture in 1997. For the first 10 years, occasional illnesses were observed but were not considered to be EIA-related. In 2008, a series of acute episodes ascribed to EIA were noted in several horses at the site. The signs observed and the clinical pathology noted were consistent with the chronic form of EIA and the horses with serious and protracted signs were euthanized. We thank Carolyn Mason of the FSH for her outstanding photo documentation of the Shackleford Banks horses and her willingness to share her images on this web site.
In both cases (FRIENDS and FSH), these are unique and invaluable resources for studying the long term consequences of infection with EIA and we appreciate the efforts of both groups and their state regulators. Samples from these horses are being used currently to refine PCR methods for detection of EIAV strains in studies led by Dr. Frank Cook in our laboratory.
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Updated on: February 17, 2010