Ambrosia Mexicana Poisoning in Horses

Ambrosia Mexicana Poisoning in Horses - Symptoms, Causes, Diagnosis, Treatment, Recovery, Management, Cost
Ambrosia Mexicana Poisoning in Horses - Symptoms, Causes, Diagnosis, Treatment, Recovery, Management, Cost

What is Ambrosia Mexicana Poisoning?

Although edible during certain times, it is the environmental factors during their growth phase that can turn the Ambrosia Mexicana plant into a lethal meal for your horse. Normally the toxins contained in the plant are too small to do any harm, but it is the plant's ability to uptake and hold excess nutrients (especially nitrates) during adverse growing conditions that turn them into a problem. This accumulation of nitrates combined with your hungry horse grazing in a  drought-ridden pasture becomes a real problem and poisoning can occur rapidly.

The Ambrosia Mexicana plant is a member of the Chenopodium family, and contains oxalates, nitrates and cyanogenic glycosides and can be extremely toxic under the right circumstances.

Symptoms of Ambrosia Mexicana Poisoning in Horses

At first, there may be no signs at all of the illness, but on reaching a certain level of accumulated toxin, the symptoms rapidly accelerate, and treatment becomes vital.

  • Difficulty with breathing
  • Gasping for air 
  • Diarrhea from gastrointestinal upset caused by the toxin build-up
  • Blood becomes dark red or brown from the lack of oxygen which affects it
  • Lethargy
  • Depression
  • General weakness
  • Uneasy gait and stance
  • Muscle tremors and seizures 
  • Irregular heartbeat 
  • Coma
  • Death 


  • The ambrosia Mexicana is but one of the large family of the Chenopodium family, and poisoning from this plant is rare among horses
  • Under normal circumstances, this plant can be consumed by your horse in small amounts and not harm them as horses have a certain level of tolerance to nitrates
  • Adverse growing conditions or plant stress causes the immediate accumulation of nitrates in the plant which changes the nature of the plant
  • The elevated levels of nitrate then affect your horse, and if consumption has been heavy, the poisoning will be rapid


Causes of Ambrosia Mexicana Poisoning in Horses

  • Poisoning occurs as a buildup of nitrates start to accumulate within your horse's system and begins to affect their body functions
  • Excessive amounts of nitrates in your horse causes gastrointestinal irritation, leading to colic and diarrhea 
  • Conversion within the body to the more toxic nitrite anion form causes more damage to tissues 
  • Injury to the red blood cells from the excess nitrate impacts upon the ability of the blood cells to carry oxygen 
  • As a result of lack of oxygen, your horse shows the classic symptoms of nitrate poisoning as listed above


Diagnosis of Ambrosia Mexicana Poisoning in Horses

Diagnosis is  based on your horse’s behavior, and as you know them so well, you will be alerted to any changes in their characteristics if you are observant. It will depend on the amount of Ambrosia Mexicana that your horse has eaten as to how severe the symptoms are. It will also depend on the season and what environmental factors have affected the plant's growth. While normally a plant that is not classified as one of the top toxic plants, its character can change and a buildup of nitrates in the plant can elevate its status considerably.

It is always advisable to call your veterinarian as your horse can react suddenly to this condition and the sooner the specialist can examine your horse, the better. Ambrosia Mexicana can be very similar to arsenic poisoning yet they are treated differently. If you know what your horse ate, save a sample and show the veterinarian who will speed up the treatment. Otherwise, he will run a series of blood work on your horse to determine what is affecting his health (nitrate levels may be indicated). Once the diagnosis is established, he will be able to act effectively and promptly.



Treatment of Ambrosia Mexicana Poisoning in Horses

Treatment depends on the reaction and severity of the poisoning. It is thought that intravenous injections with methylene blue are required for successful treatment, but have no substantiated proof that this is the case. However, your veterinarian is the person to ask, as he will have the latest updates on treatments available. Sometimes a gastric lavage and continued flushing of the stomach to prevent further absorption of the poison from the ingested plant may help, but your specialist is the best person to advise you. Removing your horse from an Ambrosia Mexicana infested pasture is important, and ensuring other pasture is clean from noxious weeds through an effective maintenance plan for effective grazing.

Be aware of your hay rations, where it was made and check it for contamination from poisonous plants. Check that the hay is in tip top condition and not musty or moldy because that can affect the health of your horse. Prevention is always better than trying to cure a condition that has already taken hold. Keep in mind that your horse may not exhibit any symptoms at first, but then as the poison takes a firm hold and moves to the internal organs it will be so much harder to treat than spending a few hours doing pasture maintenance.



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Recovery of Ambrosia Mexicana Poisoning in Horses

Prevention is always the better option, and in times of adverse weather conditions, it pays to be vigilant in regards to pasture maintenance. Regularly walk around your grazing fields to ensure your horse is getting the best feed possible. If you identify any noxious weeds that are making a move into the pasture lands, it is vital to eradicate it as soon as possible, either by physically digging it up roots or by using specific sprays targeted towards the type of plant that it is.

If you are going to be spraying, it is advisable to move your horse to another paddock to prevent it being affected by any spraying. Remember that dried toxic weeds often retain their potency once dried, so checking through the hay and finding where it came from may unearth the cause of the poisoning. Dried plants can inflict as much damage as fresh plants.



Ambrosia Mexicana Poisoning Questions and Advice from Veterinary Professionals

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