Page 1098 - Equine Clinical Medicine, Surgery and Reproduction, 2nd Edition

Nervous system 1073

VetBooks.ir pyrimethamine (1 mg/kg p/o q24 h for 3–6 months) 10.25
and sulphonamides (20–30 mg/kg p/o q12 h for 3–6
months). These sequentially inhibit folic acid syn-
thesis in the protozoa. Originally, pyrimethamine
was administered with trimethoprim/sulphonamide
tablets, but many compounding pharmacies now
produce sulphadiazine/pyrimethamine solutions/
suspensions. It is recommended that the horse is
not fed for 1 hour before and after administration.
These folate-inhibiting drugs are associated with
adverse effects including bone marrow suppres-
sion and anorexia. Folate-inhibiting drug combina-
tions should not be administered to pregnant mares
because they are teratogenic and have been associ-
ated with fatal congenital disease. Fig. 10.25 Gross lesions of EPM in the spinal
Another class of drug, named benzeneazeto- cord of a horse. Haemorrhagic regions are visible.
nitriles, are effective against a broad spectrum of (Photo courtesy FT Bain)
protozoal parasites affecting avian and mammalian
species. The activity of these drugs against S. neu-
rona in horses has been demonstrated in several field TRYPANOSOMIASIS
studies. Diclazuril and ponazuril are licensed for use
against EPM in the USA. Trypanosomes are blood-borne protozoa that cause
Antioxidant therapy with vitamin E (10,000 IU disease in many mammals, including horses. There
p/o q24 h for 30 days) can be used as adjunctive are numerous species with different modes of trans-
treatment on the presumption that the damaged mission and strains with differing virulence. Tsetse
neurons may be susceptible to oxidative stress. fly-transmitted trypanosomes cause disease in Africa.
Anti-inflammatory therapy (e.g. flunixin meglu- Trypanosoma evansi is transmitted by haematophagous
mine, 1 mg/kg i/v or p/o q12–24 h or dexametha- flies and vampire bats, resulting in surra, an important
sone, 0.1 mg/kg q12–24 h) is recommended in the disease of economic importance in Asia and South
first few days of treatment with an anticoccidial America. Dourine is caused by T. equiperdum in Asia,
drug to minimise worsening of neurological signs South Africa, India and Russia and is regarded as a
associated with death of the parasite. sexually transmitted disease, although arthropod vec-
Practical preventive measures include not feeding tors may also play a role in transmission. This is the
from the ground, avoiding ground water consump- only trypanosome that is vertically transmitted. This
tion by offering an alternative fresh water source and disease has been eradicated from North America.
preventing access of wildlife to pasture and stables. Clinical signs of trypanosomiasis are variable,
An EPM vaccine is available, but currently there and there may be a considerable period of subclini-
are no scientific data supporting the efficacy of the cal infection. Affected horses may show pyrexia,
vaccine. anaemia, weight loss and lymphadenopathy, and the
disease is often fatal. Signs of meningoencephalomy-
Prognosis elitis that can be seen include muscle atrophy, facial
The prognosis can be good if ataxia of grade 2 or nerve paralysis and limb ataxia and weakness that is
less is present. If the animal becomes recumbent, worse in the hindlimbs. Treatment involves the use
the prognosis is considered poor. Some animals of various trypanosomides, but resistance may be
recover from the initial event but have sustained encountered. Treatment of animals with dourine is
permanent nerve injury resulting in residual nerve not recommended, because they may continue to be
deficits. a reservoir of infection to others.