Foxtail Grass Poisoning in Horses

Foxtail grass is native to Asia and Europe, but can be found anywhere in the world now. This weed is a tall grass, about three feet tall, that grows in bunches with large fuzzy seed pods on the tips that look like foxtails. It contains a high level of soluble calcium oxalates, which causes a blistering rash anywhere it touches including the mouth, tongue, lips, and esophagus. It also causes a calcium deficiency that can make the bones weak and damages the kidneys. Tubular necrosis can occur when the calcium oxalate crystals embed themselves in the renal tubes if treatment is not received right away.

Foxtail grass poisoning is caused by the consumption of Setaria grass and causes blistering and calcium deficiency sometimes referred to as big head syndrome. This creates a buildup of soluble calcium oxalate crystals in the kidneys, which may cause serious kidney problems. All parts of the plant are toxic and contain large amounts of calcium oxalates, which can cause acute or chronic poisoning. Acute poisoning is from eating a large amount of foxtail grass in a short period of time and chronic poisoning happens when your horse consumes small or moderate amounts of foxtail grass over a period of two to six months.

The signs of foxtail grass poisoning depend on whether it is acute or chronic poisoning. Acute poisoning produces signs of poisoning right away like skin irritation, colic, and abscesses in the mouth. Chronic poisoning is not as easy to identify as it is gradual and causes more mild symptoms but progresses to more serious side effects like muscle tremors, swelling of the face, depression, and kidney damage.

Acute Poisoning

• Stomach irritation
• Abscesses
• Irritation to mouth, nose, ears, and skin

Chronic Poisoning

• Muscle tremors
• Depression
• Lethargy
• Face and head swelling (big head syndrome)
• Loose teeth and broken bones from lack of calcium
• Secondary hyperparathyroidism (weak muscles, staggering)
• Kidney damage (lack of urination, fluid build-up in abdomen, dark urine, abdominal pain)

 Types

The scientific name for foxtail is setaria spp. from the Poaceae family. There are over 100 different species and types, but the most common include:

• Bristly foxtail (Setaria Verticillata)
• Yellow foxtail (Setaria Glauca)
• Knotroot foxtail (Setaria Geniculata)
• Green foxtail (Setaria Viridis)
• Giant foxtail (Setaria Faberi)

Soluble calcium oxalate crystals bind to magnesium, calcium, and iron, which makes them unavailable to the rest of the body. It can produce oral abscesses, blisters, and colic due to the jagged edges of the crystals becoming embedded in the tissues. If the condition is chronic, the buildup of toxins will clog up the renal tubes and cause kidney damage and possible kidney failure.

The most obvious sign of foxtail grass poisoning are the lesions on the mouth, face, and throat, but this is not conclusive so the veterinarian will need to perform a complete and thorough physical examination and diagnostic tests to rule out other illnesses. The examination should include vital signs, a check of the skin from head to tail, overall health condition, lameness evaluation, palpation and auscultation of the major muscles and organs. Be sure to let the veterinarian know if you have given your horse any medications (whether prescription or otherwise) and a copy of your horse’s medical records and immunizations would be extremely helpful. During the examination, the veterinarian will be assessing your horse’s stature and behavior as well. 

Some of the tests your horse needs include a serum biochemistry analysis, which will show low magnesium and calcium, and increased levels of (lactate dehydrogenase) LDH, (aspartate aminotransferase) AST, (alanine aminotransferase) ALT, creatinine, phosphates, blood urea nitrogen (BUN), and metabolic acidosis. In addition, a CBC is done to check for abnormal white or red blood cell count and the urine will contain oxalate crystals, casts, and protein. The veterinarian will most likely do an endoscopy to check your horse’s esophagus and upper airway for inflammation and lesions.

This is done by inserting a lighted tube called an endoscope into your horse’s throat while your horse is sedated. Radiographs (x-rays) can pinpoint areas of calcium crystals around the heart, brain, lungs, and kidneys. An ultrasound, MRI, and CT scan will give the veterinarian an idea of how much damage has been done to the vital organs.

Treating foxtail grass poisoning includes getting as much of the poison out of your horse’s system as possible, soothing the lesions, and increasing the calcium levels.

Removing the Poisons

To detoxify your horse, charcoal is given by mouth to adhere to the toxins so they are not absorbed. Afterward, a gastric lavage is done to rinse away the remaining toxins and plant particles with warm saline through a nasogastric tube inserted into the digestive tract. Intravenous (IV) fluids will be given to flush the kidneys and prevent dehydration.

Medication  

Topical medication will be given to soothe the lesions and stomach protectants to ease the pain in the digestive system. Calcium supplements to increase your horse’s available calcium. NSAIDS will probably be given to help with pain and inflammation.

Observation

If there are any serious kidney problems, the veterinarian may want to keep your horse overnight for observation.

Provide your horse with calcium supplements and a healthy diet with plenty of fresh water. Be sure to get rid of the foxtail grass and watch for signs of foxtail grass in your horse’s hay. Follow the veterinarian’s instructions and call if you have any questions or concerns.