Diclazuril Injectable for EPM in Horses
Equine protozoal myeloencephalitis (EPM) continues to frustrate North American horse owners and veterinarians as one of the most common neurologic diseases in horses–and one of the most challenging to diagnose and treat.1 (EPM) is caused by the protozoan Sarcocystis neurona. The clinical syndrome caused by the disease can have a variety of presentations. Typically, horses present with asymmetric ataxia and weakness defined by multifocal central nervous system lesions. Successful management of this disease has been frustrated by failure of treatment success and by relapse after withdrawal of therapy.2
Background
EPM is the leading infectious neurologic disease of American horses. Classified as a “New World disease,” EPM is the most common infectious neurological disease of horses in the Western hemisphere. First discovered in the 1960s, the causative organism (S. neurona) was not identified until the 1980s. Later, researchers determined that S. neurona has a two-host life cycle. To complete its life cycle, this organism requires a definitive host which, in this case happens to be the opossum. When these scavenge the remains of intermediate hosts (such as raccoons, skunks, cats, or armadillos), the S. neurona sarcocysts mature to their infective stage (sporocysts), which the opossum passes in its feces.
Horses, which are generally considered "dead-end hosts" (meaning they typically can’t pass the protozoa on to other animals), contract the disease by ingesting infected matter, often grass or hay contaminated with opossum feces containing S. neurona sporocysts.1 Sarcocystis enter the asexual stage of the life cycle in the horse, ultimately forming muscle cysts (sarcocysts). This is referred to as equine muscular sarcocystosis or EMS.3 Toxins are sometimes released from degenerating cysts in horses, which produce signs that can mimic EPM.
Clinical Signs of EPM
Traditionally, the prognosis for EPM has been poor, with many cases ending in euthanasia of the affected horse. Although some horses with EPM have irreparable damage that therapy cannot remedy, researchers have developed several treatment options that can ease the effects of the disease and allow many affected horses a longer, healthier life.1
Symptoms of EPM can be consistent with other neurologic diseases, but once a clinical infection has manifested, signs include:
- Asymmetric muscle atrophy (wasting);
- Cranial nerve dysfunction;
- Ataxia (incoordination); and
- Dysphagia (difficulty swallowing)1
Veterinarians usually begin the diagnostic process with a neurologic exam. A presumptive diagnosis can be made with the neurologic exam alone, however most veterinarians continue the process by running a blood or cerebrospinal (CSF) test to detect an antibody response against the parasite. A blood test can only determine whether the horse has been exposed to the causative agent; it cannot determine whether the animal will develop a clinical infection.
Prevention and Treatment Regimens
Apart from controlling measures to reduce horses’ risk for contracting EPM (such as securing feed and water sources from opossum fecal contamination), prophylactic medication has gained wide acceptance among horse owners and managers, and in the veterinary community. Favorable reports have been noted for prophylaxis using drugs like diclazuril and ponazuril, although this strategy remains an extra-label protocol. At one point, an EPM vaccine from was given a conditional license for five years, but was subsequently taken off the market and is no longer available.4
Diclazuril, given as low-dose topdressing, has been shown to significantly reduce the prevalence of the S. neurona pathogen in blood as compared to horses not receiving the medication. The recommended dosage for diclazuril is 1mg/kg body weight fed over a 28-day course, like ponzuril, and its alfalfa-based pellet used as a top-dressing on the horse’s normal grain ration makes it preferable to a paste or injection.2
In horses exhibiting clinical signs of EPM, the injectable form of diclazuril is preferred by veterinarians. Diclazuril is typically labeled at 1mg/kg body weight, but was shown in a study to have higher relapse rates at that level than other drugs. Commonly compounded diclazuril rates are 5.0 to 15.0 mg/kg body weight. In Compendium Equine Jan/2008, Dr. Robert MacKay suggests treating relapses using diclazuril at 7mg/kg body weight; however, this protocol is extra-label.3
1Larson, E. Equine Protozoal Myeloencephalitis: Past and Present. The Horse, Aug. 4 2011.
2Bentz, B.G., et. al. Diclazuril and equine protozoal myeloencephalitis (EPM): a clinical report. Equine vet. Educ. (2000) 12 (4) 195-200.
3Fenger, C.K. (1997) Equine protozoal myeloencephalitis: findings from a retrospective study. International Equine Neurology Conference. pp 14-15.
5Loving, N., DVM. EPM Prevention. The Horse, Apr 1, 2008.
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