Page 1124 - Equine Clinical Medicine, Surgery and Reproduction, 2nd Edition

Nervous system 1099

VetBooks.ir serum calcium levels usually returning to normal in dioxins. There are few credible reports of exposure
of horses to toxic levels of the parent compounds, but
24 hours. Affected horses are very susceptible to car-
neurotoxicity related to exposure to dioxins has been
diac damage, which is often permanent.
reported. Dioxin is present in waste oil sludge, which
ORGANOPHOSPHATE POISONING has previously been used to keep down dust in rid-
ing arenas. Of 85 horses exposed to dioxins in this
Organophosphate poisoning in horses is usually due way, 48 died over weeks to years. In addition to clini-
to overzealous application of insecticides, acaricides cal signs of emaciation, colic and laminitis, cerebral
or anthelmintics, or accidental contamination of oedema was noted at necropsy. Chronic accumula-
feed or water. The clinical signs that are seen are the tion of chlorinated hydrocarbons may cause a wide
result of inhibition of acetylcholinesterase activity, array of central and peripheral neuropathies, in addi-
leading to uncontrolled muscarinic, nicotinic and tion to degeneration of other tissues.
CNS effects of acetylcholine. Hyperexcitability, fre-
quent urination, sweating, muscle tremors and colic MYCOTOXICOSIS
are followed by weakness, recumbency and death
due to respiratory insufficiency. Mycotoxins are secondary metabolites of moulds
Treatment consists of atropine (0.1 mg/kg, half produced during mould growth in grains or forages.
slowly i/v and half i/m) and pralidoxime chloride They are not produced consistently and may result
(2-PAM) (20 mg/kg i/m). Skin washing and saline from stresses or limitations to mould growth, such
purgation can be used to remove residual organo- as unusual temperature, humidity or drought. There
phosphates. Horses are said to be highly resistant to is little correlation between spore counts or fun-
delayed neurotoxicity caused by organophosphates, gal growth amount and mycotoxin contamination.
but transient laryngeal paralysis has been reported. Similarly, the absence of mould growth does not
mean that a food or forage is free from mycotoxins
CHLORINATED HYDROCARBONS (Fig. 10.55). Mycotoxins can persist in mould-free
feed because the common mycotoxins are resistant
Chlorinated hydrocarbons were once widely used to the temperatures and drying that destroy fungi.
in industry, horticulture and agriculture. They are There are many mycotoxins that are of concern,
now highly restricted because of environmental per- including aflatoxin, fumonisin and ergot alkaloids.
sistence of the parent compound and contaminating The syndromes of ryegrass staggers, paspalum

10.55
Fig. 10.55 Mycotoxicosis. This
horse was one of six affected horses
on a farm. When asked to walk the
animals had a marked hypermetric gait
of the hindlimbs only. The animals
would always stand with all four limbs
underneath themselves. A new batch of
hay had recently been purchased and
mould was noted in two of the bales that
were fed to these horses. The horses
improved with supportive care within
72 hours. No residual neurological effects
were noted and mycotoxin ingestion was
presumed.