Diagnosing Grass Sickness

Equine grass sickness is so named because it occurs in the spring in pastured horses which are eager to eat plentiful green grass. Its cause is unknown, but the result is destruction of the nerves of the gastrointestinal system, which is often rapidly fatal. While extremely rare in North America, grass sickness is a problem in Scotland, northern England, Switzerland, and Australia. There is no diagnostic test for grass sickness, which has prompted researchers, including a group from Midlothian, Scotland, to investigate whether urinalysis can help diagnose equine grass sickness.

Their results indicated that horses with acute grass sickness had significantly higher specific gravity of the urine than control horses. Urine specific gravity is commonly used to monitor hydration along with other indicators. Dehydration changes the ratio of water to solids, hence the specific gravity of urine. Specific gravity likely increases as horses become dehydrated and urine becomes more concentrated. Acute grass sickness also resulted in increased protein in the urine, increased creatinine, and decreased urine pH (increased acidity). Increased protein might be the result of rapid tissue breakdown and rapid weight loss (called cachexia) seen with this disease. Increased creatinine can be due to muscle breakdown or to significant dehydration. Finally, low urine pH was likely the product of metabolic acidosis, a condition that can develop when a horse's massive gastrointestinal tract becomes immobilized. So, while any of these findings alone are not diagnostic for grass sickness, taken together with signs of grass sickness, they are strongly suggestive of the disease.

Fintl, C.; Milne, E.; McGorum, B.C. Veterinary Record, 151, 721-724, 2002.

About the Author

Susan Piscopo, DVM, PhD

Susan Piscopo, DVM, PhD, is a free-lance writer in the biomedical sciences. She practiced veterinary medicine in North Carolina before accepting a fellowship to pursue a PhD in physiology at North Carolina State University. She lives in northern New Jersey with her husband and two sons.

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