Page 1125 - Equine Clinical Medicine, Surgery and Reproduction, 2nd Edition

1100 CHAPTER 10

VetBooks.ir staggers and leucoencephalomalacia are examples of Clinical presentation
Affected horses show signs consistent with action-
mycotoxicosis in horses and are discussed below.
TREMORGENIC MYCOTOXICOSIS related tremors and symmetrical vestibular ataxia.
Initially, there is a diffuse, intermittent, mild
(RYEGRASS STAGGERS, muscle tremor that progresses to varying degrees
PASPALUM STAGGERS) of ataxia, with a head nod and a wide-based rock-
ing stance. An uncoordinated and swaying gait is
Definition/overview common and affected animals may stumble and
The ingestion of several tremorgenic mycotoxins fall. If undisturbed, recumbent horses usually
can result in syndromes that are characterised by recover and regain their feet within a short time.
staggering. Ryegrass staggers is the best described Excitement and blindfolding markedly exacerbate
and has been reported in the USA, Australia, New the signs. The condition itself is not usually fatal,
Zealand and Europe. but accidents associated with falling have resulted
in death.
Aetiology/pathophysiology
Perennial ryegrass (Lolium perenne) is the major forage Differential diagnosis
supporting the endophytic growth of Neotyphodium Fluphenazine reaction; reserpine reaction; EPM;
lolii, which produces tremorgenic mycotoxins known WNV encephalitis; lead toxicosis; equine LMN
as lolitrems A, B, C and D. Lolitrem B is considered disease; trauma; hyperammonaemia; metabolic
the major toxin and is characterised by an indole moi- derangement (hypocalcaemia); spinal cord impinge-
ety. The toxin is highest in the seed, and the syndrome ment secondary to neoplasia.
is most commonly seen in the late summer or autumn,
as the concentration of lolitrems varies seasonally. Diagnosis
Paspalum staggers results from ingestion of dal- Ruling out other potential neurological disorders
lis grass (Paspalum dilatatum) that has been invaded and/or the detection of these toxins in the pasture
by Claviceps paspali, which subsequently produces an environment are used for diagnosis.
ergot sclerotium. The sclerotium contains alkaloids
produced by C. paspali and includes indole com- Management
pounds, which are derivatives of lysergic acid and There is no antidote for either perennial ryegrass
known as paspalitrems. Inhibition of GABA, which staggers or paspalum staggers, but removal from
is one of several inhibitory neurotransmitters, is the suspect forage results in recovery in 3–7 days
the probable mechanism of toxicosis. Dallis grass is or 1–3 weeks, respectively. The animals should be
grown in the south central and south-eastern regions placed in a quiet secure place during recovery and
of the USA and the sclerotia may be consumed on provided with nursing care.
pasture or in hay.
Staggers has occurred in Florida in horses fed Prognosis
coastal Bermuda grass hay and may be comparable to The prognosis is good provided the source of the
Bermuda grass tremor of ruminants. A fungal trem- toxin is removed and supportive care is provided.
orgen is thought to be responsible. With chronic ryegrass staggers, residual defects
Tremorgenic mycotoxins are associated with from degenerative CNS lesions may be present.
reduced effects of inhibitory amino acids, which
result in increased pre-synaptic neurotransmit- LEUCOENCEPHALOMALACIA
ter release and prolonged depolarisation. Synaptic
transmission at the motor end plate is facilitated, pro- Definition/overview
viding a probable mechanism for the uncontrolled Leucoencephalomalacia is a highly fatal neurological
tremors and incoordination observed clinically. disease caused by ingestion of mycotoxins.