Agave americana

Abstract

DISTRIBUTION: Originally from South America. Planted for drought feeding, etc. Widespread.

BOTANICAL DESCRIPTION: General: These plants are used as fences or garden subjects and occasionally fed to animals as fodder in times of drought. The fibre can also be used for making ropes, etc., but is inferior to that of Agave sisalina. Leaves: The stem is thick and short and bears very large wax-coated leaves arranged like a rosette. Because very little carbon dioxide is taken in, few stomata are present in the thick, often waxy cuticles, which give the plants further protection against harsh sunlight and excessive transpiration. The edges of the leaves have spines. Flowers: An enormous flower stem (up to 8m high) starts to grow when the plant has accumulated enough reserve material in a period of 5 - 60 years or longer.

TOXIC PRINCIPLE: Contains soluble oxalates.

SYNDROMES: Soluble oxalate poisoning, Primary nephropathy. Systems affected: Urogenital system.

CLINICAL SIGNS: Acute poisoning: • Hypocalcaemia phase: -soon after intake, 2-6 hours - Weakness - Paresis to paralysis, semi-comatose, “milk fever” signs. - Head thrown back onto shoulder - Bradycardia - Mortalities Treatment of these symptoms with Ca-borogluconate gives good results and animals may recover. • Kidney failure phase: Following day to few days later due to blockage and damage of tubuli by Ca-oxalate crystals resulting in: - Uraemia: BUN and creatinine increase - Oliguria or anuria Treatment of very little value - irreversible condition.

EPIDEMIOLOGY: • Acute poisoning happens where: - unadapted animals suddenly eat a relatively large amount of oxalate containing plants and the oxalates are absorbed into the circulation - excessive large amounts of oxalates are absorbed in adapted animals which are not able to detoxify all the oxalates in the rumen (e.g. large amounts during droughts) • Chronic effect characterized by: Calcium deficiency resulting in: - bone abnormality, - poor milk production and - poor growth. N.B. Kidney- and bladder stones where oxalates can play a role amongst other things.

NECROPSY: Macroscopical findings: • Hypocalcaemia: - Nothing significant, - haemorrhages. • Nephrosis and Uraemia: - Ascites, hydrothorax, perirenal and subcutaneous oedema. - Kidneys pale, oedematous, swollen - nephrosis. - Ammonia and urea odour (uraemia). - Haemorrhages in different organs. - Oedema and haemorrhages in rumen.

HISTOPATHOLOGY: Typical oxalate crystals in kidney tubules (seen under polarized light) with signs of kidney damage.

TREATMENT: Control: • Avoid sudden exposure to oxalate containing plants or intake of large quantities • Avoid oxalate containing plants as the only food. • Feed Ca2+ in the form of dicalcium phosphate as a lick (25% or more with salt) or mixed in the supplementary feed.