African horse sickness

The World Organisation for Animal Health (OIE) hopes to help to stop the spread of the African horse sickness (AHS), a deadly disease of horses, donkeys, and mules. It is an OIE-listed disease and is one of the six animal diseases that the OIE includes in its procedure for official recognition of a country’s disease‐free or risk status.

Spread by biting midges, AHS has a mortality rate of up to 90%. As such, containing it from further spread is essential. A key element in determining if AHS virus has been or is circulating in an equine population is blood testing; this helps veterinarians and reseaarchers ascertain which horses—and which countries—have been exposed to the virus. The blood test used must meet extremely high standards for sensitivity and specificity, and it must be implemented and results interpreted stringently and verified in a rigorous, multiple step process, said Javier Castillo-Olivares, LV, MSc, PhD, MRCVS, a vaccinology research scientist in the University of Cambridge Department of Veterinary Medicine, and visiting reader in vaccinology in the University of Surrey School of Veterinary Medicine Faculty of Health & Medical Sciences, in Guildford, both in the U.K.

Castillo-Olivares led an international consortium of laboratories with the aim of validating an AHS ELISA (enzyme-linked immunosorbent assay) blood test in a study supported by the International Horse Sports Confederation, which is engaged in a public-private partnership with the OIE.

“Specifically, our group of laboratories worked on a project to take the VP7 blocking ELISA to stage 3 of the OIE Validation Pathway,” he said. Essentially, what this means is that the researchers showed this AHS test—the “VP7 blocking ELISA” —to be highly reliable and they confirmed that it works in exactly the same way every time, regardless of where in the world it is used.

“It was important to demonstrate that the test performed very well in any laboratory,” Castillo-Olivares said. “We demonstrated this.”

Castillo-Olivares said the VP7 blocking ELISA had already been commercialised and used in diagnostic laboratories in Europe and elsewhere for determining AHS status. In fact, it had already been recommended for use by the OIE. The test however, had “not [yet been] subjected to this level of scrutiny before the recent validation procedure was completed,” he explained. Given the AHS situation worldwide and the importance of the equestrian industry, reaching those standards is fundamental, he said.

“AHS should be a notifiable disease in every country, is a high-priority disease for the (European Untion), and has a profound negative effect on the equine industry in countries where the disease exists,” Castillo-Olivares said. “Furthermore, AHS is the only equine infectious disease for which the OIE officially recognises disease freedom status for countries or zones so it was important that the international scientific community and veterinary authorities were reassured of the quality of the test.

“AHS is of tremendous importance in international trade of horses, and diagnostic tests are critical to ensuring that this trade is not disrupted or results in the inadvertent introduction of a foreign disease into a territory,” he continued. “For this it is important to use diagnostic methods that are fully validated.”

In their study, Castillo-Olivares and his fellow researchers tested more than 180 serum samples in 10 different laboratories. The samples came from equids from various parts of the world, all having different origins and vaccination and exposure statuses. The researchers found that the VP7 blocking ELISA had very high sensitivity and specificity in all laboratories.

Castillo-Olivares said the VP7 blocking ELISA for AHS can be useful for:

  • Making a retrospective AHS diagnosis (detecting antibodies from a horse that may have had the disease);
  • Demonstrating that a population of horses is disease-free, by testing a number of horses and seeing if they have antibodies against AHS. If they do, the country cannot be declared AHS-free unless there is an epidemiological explanation for the seropositivity of the population);
  • Pre-export testing; and
  • Disease monitoring and control in endemic countries.

The test’s final step in the OIE Validation Pathway could soon be undertaken, said Castillo-Olivares, although that will require additional funding that has not yet been acquired.

“The VP7 blocking ELISA has been shown to be reproducible, complying with the OIE Standards, and it is expected that it will be recognized as validated to Stage 3 of the OIE Validation Pathway,” he said. “Recognition of this status is a long process, but I am confident it will eventually be gained.

“More work is needed to make this test even more useful and to take the assay to stage 4 of the OIE validation pathway, thereby extending its application and improving surveillance by adapting the test to determine not only the presence or absence of AHSV antibodies but also to quantify the levels in serum of those antibodies,” he added. A major benefit of this would be the ability to distinguish between animals that had been vaccinated and those that had been naturally exposed to the virus. “This will make this test even more valuable, but it can only be achieved through further funding.”

The study, “Assessment of reproducibility of a VP7 Blocking ELISA diagnostic test for African horse sickness,” was published in Transboundary and Emerging Diseases.