A treating veterinarian in southwestern Virginia still isn’t sure what made three of his clients’ horses sick this spring while generating widespread concern on an Internet message board. Titer results searching for a definitive diagnosis or rule-out on the bacterial disease listeriosis came back inconclusive in late May. But Thach Winslow, DVM, of Blacksburg, Va., the practitioner that treated the horses, says listeriosis cannot be ruled out as a differential diagnosis in the cases.

The owner of the affected animals, John Holland, had dubbed the illness in question “Whisper Syndrome,” after he lost a horse by that name following rapid onset of inappetence, neurologic signs, and colitis. Necropsy results on the horse were inconclusive. Hundreds of horse owners signed onto a message board Holland devoted to the topic in March with concerns there might be a new disease affecting horses. Winslow and other veterinarians responded to the concerns saying that they didn’t think a new disease or any sort of epidemic was taking place, but that owners should keep an eye on their horses for signs of illness. For background information, see www.TheHorse.com/ViewArticle.aspx?id=5584 and www.TheHorse.com/ViewArticle.aspx?id=5588.

Blood titers were run on samples from four horses on the premises taken while the cases were resolving and several weeks after the horses were better. Winslow also had comparative titers run on another horse that wasn’t on the premise (thus serving as a control). Titer results are only run to a certain dilution, and up to the cutoff dilution level, the animals had similar titer results for listeria. The samples were re-run at higher dilutions, and the results were suspiciously high for the three horses exposed to round bales. These titers however, are not conclusive, but do help to build a case for listeria. Winslow explained, “The results could be circumstantial…they were certainly not conclusive that listeria is involved, but there was certainly enough evidence there to keep it on the suspicious list.

“There probably will never be a true diagnosis on those horses,” he added. “However, it definitely raises an eyebrow enough that similar cases in the future should probably be considered listeria as a differential to rule out, hopefully by brain culture.” He said veterinarians wouldn’t be able to know more unless other cases turn up that seem similar enough for pursuing listeria diagnostics