Equine infectious anemia (EIA), also known as swamp fever, is an infectious and potentially fatal viral disease that affects all equines. The infection is blood-borne and transmitted by blood-feeding insects carrying the virus. Researchers from the University of Kentucky, in conjunction with researchers at the University of Pittsburgh and Istituto Zooprofilattico Sperimentale delle Regioni Lazioe Toscana in Rome, Italy, have been looking into options for more accurate EIA diagnosis. After comparing results from current testing methods, the researchers recommend a three-tiered approach to testing for EIA.
In the 1970s, Leroy Coggins, DVM, invented the first reliable assay to diagnose EIA—the commonly known and popular Coggins test that determines EIA antibody presence in a blood sample. For decades the Coggins test has been required in movement of all horses and is currently the gold standard as a serological diagnosis of EIA. A negative Coggins test is usually required for a horse to be imported from another state or country. A positive test result, however, requires that the horse be euthanized or quarantined for the rest of its life, significantly affecting the equine industry.
Testing has expanded to about two million samples each year. The good news is that veterinarians rarely find animals with clinical signs associated with the infection. The major problem is primarily the hard-to-find and unapparent carriers of the virus.
Optimized assays to detect the disease have therefore become increasingly important to the horse industry. Since the Coggins test’s introduction, scientists have developed several new enzyme-linked immunosorbent assay (ELISA) tests, some, which have proven to have fewer false negatives than the Coggins, said Charles Issel, DVM, PhD, a professor at the UK Gluck Equine Research Center.
“Data from our studies has shown the benefits of using the combined strengths of the Coggins test, several enzyme-linked immunosorbent assay tests, and an immunoblot test to diagnose EIA,” Issel said. “In combination with the immunoblot, ELISA assays identified up to 20% more cases of EIA.”
The studies involved samples from experimental studies at UK, which were tested using Coggins and ELISA test formats, as well as a group of mules naturally exposed to EIA. These results indicated that some of the serum samples that were negative with the Coggins tested positive with several commercially available ELISA assays as well as with immunoblots. These animals were also found positive for genetic material of the virus (they are proven carriers).
A national surveillance program conducted in Italy from 2007-2010 provided 96,468 blood samples for validation of the Competition-ELISA (C-ELISA) assay. Overall, 331 of the 96,468 samples proved positive in C-ELISA, but only 124 of those were interpreted as positive by the Coggins test. Therefore, the 207 samples testing positive with the C-ELISA, but negative with the Coggins test, were investigated further with immunoblots in conjunction with meticulous evaluation of history information for each horse.
Examination results showed that 182 samples had tested falsely positive on the C-ELISA. In comparison, 25 samples were confirmed to be false negative with the Coggins test. The researchers thus documented 20% more cases of EIA using a three-tiered laboratory system, the C-ELISA assay, the Coggins test, and the immunoblot.
“Based on this, we would suggest minimizing false negative cases by adopting the three-tiered system: ELISA first, instead of the Coggins, and then use the Coggins to confirm a positive ELISA to catch as many carriers of the virus as possible,” Issel said.
According to Issel, veterinarians should always perform the immunoblot if the results from the two tests do not agree, because the ELISAs have higher rates of false positive results than Coggins tests.
“By combining the strengths of the two test formats, we can improve the accuracy of our surveillance,” Issel said. “Some states, however, only use the ELISA test in their initial testing of animals because the ELISA, as documented here, has fewer false negative results.”