Two horses have been removed from the showgrounds after testing positive for equine influenza virus at the FEI show jumping event in La Baule, France. The horses were not showing any signs of illness but were positive on PCR testing (presumably from a nasal or nasopharyngeal swab) and were sent home, along with four other horses taken care of by grooms of the two test-positive horses. Organizers are also planning to isolate all the horses that were in the same stable block as the positive horses, but they will stay on the showgrounds.

Authorities were monitoring horses at this show after a confirmed case of equine influenza was identified at a horse in another recent horse show in France. The article states that "precautionary testing on some horse was undertaken" but it’s unclear how horses were selected for testing.  Was it random? Were horses that had been to the other show tested?  Had the horses tested been in contact with those other infected horses?

Testing of all horses is an aggressive (and expensive) approach, but equine influenza is an ever-present risk at shows and it’s highly transmissible, so it may have been decided that the time and effort was worth it to test at least some of the animals.

The two positive horses were healthy and may not develop influenza. The test that was used is highly sensitive and detects viral RNA. That means if the test is positive there is actually flu virus present in the horse, as opposed to blood tests that detect antibodies to the virus, which simply determine whether the horse has been exposed. Viral PCR tests can also pick up dead virus, but it’s unlikely that a horse would have RNA from dead influenza in its nose in the absence of an infection. 

Since these horses are currently healthy, there are a few possible explanations:

• They are incubating flu and will get sick soon. Horses can start shedding the virus before they get sick, which complicates influenza control.
• They could have had flu and recovered. However, since horses only shed the virus for a short time and it would be very unlikely for the horses to have been sick and recovered, and still be shedding the virus.
• The horses might be infected but developed a very mild or clinically unapparent infection. They might have had enough inherent or vaccine-induced immunity to prevent disease from occurring but not enough to prevent the virus from growing. Flu vaccines are designed to reduce the incidence and severity of disease, not viral shedding,
• It could have been a false positive lab test.

Is testing and exclusion of healthy test-positive horses overkill?

• No, I don’t think so. It’s certainly aggressive, but it is a reasonable approach to dealing with a concerning and highly transmissible infectious disease, although even so it won’t eliminate all infectious disease concerns. People whose horses are excluded probably don’t like it, but hopefully they see the greater good, meaning "what goes around comes around" (or more specifically when dealing with disease control "what doesn’t go around doesn’t come around," and hopefully their horses will have a lower chance of being infected in the future because of similar measures taken by other people).