Heaves in horses resembles the condition farmer’s lung in people, where hypersensitivities develop to common antigens in the environment, particularly certain molds and mold spores. The thermophilic fungi, those that grow best in warm temperatures, are particularly incriminated. Dr. Robert Wright, with the Ontario Ministry of Agriculture, Food and Rural Affairs, has been looking at mold levels in hays sold for horses. He considers good horse hay to have no more than 25,000 CFU (colony forming units) of molds, of which 10,000 CFU or less will typically be thermophilic molds. However, he reports finding levels of 25,000 to 100,000 fairly commonly.
Wright also has been studying mold growth in hays treated with the preservative propionic acid and finds thermophilic mold levels of 2,500,000 CFU or higher in these hays. Propionic acid is typically used to help preserve large bales and/or when hay must be baled at a moisture level higher than normal. By using copper pipes to study what is going on deep inside these bales, he has found that starting at five days after baling they go through cycles of heating (sometimes called ”sweats”), which favor the growth of thermophilic molds. This means that the preservative-treated hays you may think are safest for your horse with heaves are actually at risk of having high concentrations of the very types of molds that could be the most dangerous.
Wright feels it could be important to better characterize exactly which molds are involved in heaves/RAO in horses. This could allow us to avoid grazing and harvesting at times when mold growths are at their peak in the grasses. For example, he mentions a species of mold called Puccinia. This mold has different plant preferences at different stages of its life cycle and Wright has noticed heavy growth of it on orchardgrass in late summer.