Numinbah horse sickness is caused by ingestion of Eupatorium adenophorum (Ageratina adenophora)
. E. adenophorum
affects the horse's respiratory system, resulting in reduced performance and increased respiratory effort in horses.
Horses must consume large quantities of E. adenophorum
for an extended period of time (several weeks to months) for clinical signs to appear. Most cases have been reported in Australia and New Zealand during the summer time, following several months of consuming the plant in the spring.