Zearalenone Toxicosis
Basic Information 
Definition
Zearalenone is a nonsteroidal estrogenic mycotoxin produced by numerous Fusarium molds—including Fusarium graminearum (Gibberella zeae), Fusarium culmorum, Fusarium verticilliodes, Fusarium avenaceum, Fusarium tricinctum, Fusarium oxysporum, and Fusarium nivale—that can contaminate corn and grains and occasionally hays, straws, and silages.
Epidemiology
Species, Age, Sex
The most adverse effects have been observed in females. Horses are not considered a sensitive species to zearalenone; however, young horses are generally more sensitive than older animals, and cycling animals are more sensitive than pregnant animals.
Risk Factors
• Low toxicity except for reproductive effects. Adverse effects are proportional to the zearalenone concentration in the diet. Zearalenone is a stable compound in the environment and not readily altered.
• Cycling mares dosed orally with purified zearalenone, which approximated 1 mg/kg (ppm) in feed, for 8 to 10 days had no changes in luteal or follicular activity.
• One report of enlarged uteri and vulvas and vaginal prolapses in mares and flaccid genitalia in stallions fed barley straw and corn screenings for 30 days; approximately 2.7 mg/kg zearalenone was detected in feed by thin-layer chromatography.
Geography and Seasonality
• Occurs worldwide, especially in northern temperate climates and in high moisture grains and hays containing greater than 22% moisture.
• Zearalenone production is favored by alternating warm days (20°–25° C) and cool nights (7° C) during corn maturation and harvesting and in the fall haying during wet, cold conditions. Zearalenone can be found particularly in stored ear corn and in maturing corn after hail storms.
Clinical Presentation
Disease Forms/Subtypes
Hyperestrogenism includes clinical signs of prolonged estrus, anestrus, infertility, delayed return to estrus, increased mammary or udder development and abnormal lactation, swollen external genitalia, and atrophy of ovaries or testicles.

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