CASE NOTES
Oxalate Toxicity
Paul Freeman, District Veterinarian, Casino RLPB
Posted Flock & Herd February 2011
There are two main categories of poisoning due to oxalates in plant
- Problems associated with soluble oxalates
- Problems associated with insoluble oxalates
1. Soluble oxalates
Species affected
Sheep, cattle
Sources
A wide variety of plants can contain high concentrations of soluble oxalates mainly as the potassium salt.
Plants - sorrel, dock, soursob, pigweed
Tropical grasses - Setaria, buffel grass, panics
Risk Factors
- The rumen degrades oxalates and its efficiency increases over time so animals are able to adapt to increasing oxalate levels over time
- The calcium level in the diet can also affect the degradation rate
- Highest oxalate levels are found in lush plants so recent growth or fertilizer use can increase the risk.
Pathogenesis
Within the rumen soluble oxalates can either be absorbed intact or form insoluble complexes with calcium which can either be shed in the faeces or lodge in the rumen wall where they may cause a chemical rumenitis.
When soluble oxalates are absorbed into the blood stream they bind strongly with calcium forming insoluble calcium oxalates.
Some of there complexes can cause vascular damage and some pass to the kidneys and are either passed in the urine or lodge in renal tubules causing damage. High circulating levels of soluble oxalates can also cause pulmonary oedema.
Symptoms
- Non parturient sternal recumbency clinically indistinguishable from milk fever
- Weight loss anorexia if renal damage
- Respiratory distress if get pulmonary oedema
Diagnosis
- History — access to oxalate sources
- Clin path — hypocalcaemia, increased creatinine and urea
- Histo - nephrosis, vasculitis rumenitis
Treatment
Calcium parenterally
Prognosis
Poor if recumbent for over 24 hours or if renal damage has occurred.
2. Insoluble oxalates
- ‘Bighead’ or Nutritional secondary hyperparathyroidism
Species
Horses and donkeys.
Risk factors
Grazing pure or near pure swards of tropical grasses with high levels of insoluble oxalates as the druse or prismatic crystal formation.
Pathogenesis
Calcium absorption in horses occurs mainly in the duodenum and calcium oxalate crystals in horses are broken down in the large intestine with the calcium released from degradation primarily excreted in the faeces. The resulting calcium deficiency results in excess parathormone and mobilization of calcium from bones to maintain blood Ca levels.
This leads to osteodystrophy.
Symptoms
Shifting lameness, stiffness, weight loss
Swelling of mandible and/or maxilla (‘bighead’)
Treatment
Add mineral mix with Ca: P ratio of 2:1 for about 3-6 months
Remove from at risk pasture
Prevention
Intermittent grazing on at risk pastures
Supplementary mineral mix at half therapeutic doses.
- Buccal irritation and stomatitis
Species affected
Herbivores
Young dogs and cats
Caused by irritation of buccal mucosa by Calcium oxalate crystals in the ‘raphides’ form
These crystals are present in a small number of plants species (monsterio delicio, cunjevois, umbrella trees, philodendron, arum lily )
Case histories of Oxalate poisoning in cattle on Setaria
Setaria is an African origin tropical grass that is widely used on the north coast because of its productivity and persistence in wet conditions. It produces a large biomass and thrives in a wide variety of soil types.
Case # 1
70 cow dairy herd with 7 cows recumbent after 3 hours grazing on Setaria that had last been fertilized 6 weeks previously.
The cows had been grazing ‘Poona peas’ for 3 days prior to being placed on the Setaria.
The outcome was that 2 cows died and 5 recovered after treatment with calcium parenterally.
Case # 2
80 cow dairy herd with 2 cows dead after 24 hours grazing on recently fertilized Setaria.
The cows had been grazing Lucerne prior to being on the Setaria.
Comment
- Both Poona peas and Lucerne are high in calcium and this would have decreased the efficiency of calcium absorption from the gut. When the cattle were moved to Setaria the efficiency of calcium absorption by the gut from Setaria as it was digested would have been low.
- In the second case recent fertilizer use would have meant that the oxalate level in the pasture was at the maximum level.
- In both cases the rumen did not have time to adapt to oxalates.
References
Mckenzie R., 2002. Toxicology for Australian veterinarians: plant toxins — oxalates pp 28-46. CD available from the author