"Yes, an equine lung infection can absolutely be undetectable during the early or subclinical stages. Because horses possess massive respiratory reserves, they often mask symptoms until the disease is advanced. In many cases, mild or stealth infections only become obvious during peak athletic exertion."

-Tufts School of Veterinary Medicine

"Yes, an early-stage lung infection—often falling under the umbrella of Equine Asthma (formerly Inflammatory Airway Disease) or mild pneumonia—will negatively affect a Standardbred's running performance. Even in the early stages, it significantly impairs their racing ability."

- Harrisonburg Animal Hospital

OR could this be the first step in trying to preserve its breeding value? Can you say mysterious career ending 'INJURY' to come?
Wouldn't be the first time. Hope I'm wrong but just saying, don't be shocked.