Page 1019 - Equine Clinical Medicine, Surgery and Reproduction, 2nd Edition

994 CHAPTER 8

VetBooks.ir Electrocardiography results are variable, ranging overload and cellular damage or death. Acutely, this
may result in cell dysfunction. Over time, myode-
from no abnormalities to premature ventricu-
lar contractions and ventricular tachycardia.
Echocardiography may be unremarkable, although generation, loss of myocytes and replacement with
fibrosis may occur. Decreased cardiac function may
a reduction in ventricular function may be present. result. Additionally, arrhythmias are potentiated due
to regional variation in signal conduction and block-
Management age, resulting in aberrant conduction.
Underlying disease should be diagnosed and treated
if possible. Supportive therapy is indicated. This Clinical presentation
may include anti-inflammatory therapy with flu- Clinical signs are variable and dependent in part on
nixin meglumine (0.5–1.0 mg/kg i/v or p/o q12 h) or the extent and duration of exposure to toxins. Signs
dexamethasone (0.05–0.2 mg/kg i/v or i/m q24 h). range from mild depression to sudden death. Ataxia,
Corticosteroids should be administered with caution weakness, intermittent profuse sweating, difficulty
if active viral infection is suspected. Broad-spectrum rising and eventual recumbency are common in
antimicrobials may be of benefit if bacterial infection acute toxicity. Congestive heart failure, tachycar-
is suspected. Strict stall rest is indicated to limit car- dia and arrhythmias have been documented. Severe
diac workload. This period of rest should continue dyspnoea relating to either cardiogenic pulmonary
until clinical signs have resolved. Horses should be oedema or diaphragm failure may occur. Death in
re-examined prior to return to any form of exer- acute cases occurs within 24–48 hours of exposure.
cise because of the risk of syncope associated with Hepatic, renal and skeletal muscle abnormalities
arrhythmias. Exercise intolerance may persist due to may also be present. The onset of clinical signs may
myocardial damage. occur immediately post exposure but can be delayed
for weeks. Delayed signs may occur in horses that
TOXIC MYOCARDITIS recover from the acute phase or have experienced
subacute or chronic low-level exposure. Subacute
Definition/overview signs include depression, cardiac arrhythmia, tachy-
The most commonly reported cause for toxin-induced cardia and heart failure. Poor performance may be a
myocardial disease in horses is exposure to ionophores. result of acute intoxication and may be the sole clini-
Ionophores are antibiotics that facilitate the move- cal sign.
ment of cations across biological membranes. These
agents are used as growth promoters in cattle feeds Differential diagnosis
and as coccidiostats in poultry feeds. Horses are exqui- Viral myocarditis, cantharidin toxicosis, idiopathic
sitely sensitive to ionophores and exposure to minute cardiomyopathy and vitamin D toxicosis may pro-
amounts can cause clinical signs. The LD50s for duce similar signs.
salinomycin, monensin and lasalocid are 0.6 mg/kg,
2–3 mg/kg and 21.5 mg/kg, respectively, in horses. Diagnosis
Cantharidin (blister beetle) toxicosis and exposure Toxin-associated disease should be suspected when
to plants containing cardiac glycosides (e.g. oleander, multiple animals are affected. Recent feeding of
milkweed, foxglove) may also cause similar signs. a new source or new shipment of feed should be
queried and, if present, provides further suspicion.
Aetiology/pathophysiology The feed should be inspected, and samples saved.
Toxic insults to the myocardium may result in acute Failure of several animals to consume feeds should
cellular necrosis, leading to fibrosis. The mechanism also prompt investigation of the feed. Definitive
of action of the toxins is variable. Ionophores damage diagnosis is based on detection within the feed of
cells by altering cation distribution across the cell ionophores or cardiac glycosides or the detection of
membrane. Facilitation of movement of cations such blister beetles within the hay. Gastric content analy-
as calcium into the cardiac cell may result in calcium sis is important in animals that die suddenly.