Equine viral arteritis test under the microscope

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Concerns have been raised that cases of equine viral arteritis (EVA), a disease that can cause foals to be aborted or born weak, may be at risk of going undetected in Britain. A recent incident suggests that some of the tests less experienced laboratories are using to detect the disease are not stringent enough.

A horse owner in Devon, David Paddon, questioned the reliability of the new commercially available test, known as ELISA, after receiving contradictory lab results for a broodmare.

Paddon’s vet Marcus Langer explains: “The mare has shown a low level of antibodies [with the traditional test] for the past three years. This year the ELISA test came up negative, so I asked the lab to send it away to do the traditional test, which showed the significant level of antibodies. My concern is that the ELISA is not sensitive enough and like any test that is not totally accurate, may have serious implications.”

EVA is contagious and is spread via the respiratory system or, more usually, by artificial insemination or natural covering. Infected animals may show flu-like signs but some appear normal.

Infected stallions can “shed” the virus in their semen, and, because there is no effective treatment, a lifelong “shedder” becomes worthless.

A positive antibody test may indicate current or previous infection or earlier vaccination against the disease. The presence of antibodies does not necessarily mean that an animal is infectious to other horses.

Screening has traditionally been done using a Virus Neutralisation (VN) test, recommended by the World Organisation for Animal Health (OIE) and available at only three labs in the UK. A new ELISA test kit is now more widely available, but is not yet validated by the OIE.

Dr Richard Newton of the Animal Health Trust, which performs both VN and ELISA tests, says: “The VN test is not without its problems, and there has been pressure from the equine industry to introduce a quicker, more reliable test, which is why the ELISA has been developed. The European OIE EVA reference laboratory is presently evaluating all available EVA ELISA tests.

“Our concern relates to the use of commercial test kits that have not been properly validated because unpublished data suggests considerable variation in sensitivity between ELISAs.”

Horse & Hound asked Dr Peter Webb of Axiom in Devon, the lab which tested David Paddon’s mare, whether the failure to identify low levels of EVA antibodies could potentially lead to horses in the early stages of infection going undiagnosed.

“Theoretically, yes,” he replied. “But that’s the same with any serological test [involving blood serums].

“This horse was below the level detected by the ELISA, which means it is not worthy of further investigation. When we [sent off for] the more sensitive test, [antibody levels] showed up as being very low.”

Reproduction expert Jonathan Pycock, who wrote recently of the dangers of EVA in H&H, says: “EVA is a tremendous concern to the breeding industry. If it arrived in the UK it would spread very quickly.”

EVA occurs globally and although there have been few recent cases in the UK, where it is notifiable, there was an outbreak in Ireland last year. EVA testing is highly regulated in the thoroughbred industry, but unless a horse is being exported or imported (when it must, by law, be tested by DEFRA), there are no strict rules for the rest of Britain’s horses.

This news story was first published in Horse & Hound (30 September, ’04)

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